LEEM,SPLEEM 9.LEEM,SPLEEMandSPELEEMincluding: low-energy electron microscopy (LEEM), its extension...

LEEM, SPLEEM487

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9. LEEM, SPLEEM and SPELEEM

Ernst Bauer

This chapter discusses some of the most impor-tant imaging methods with low-energy electrons,including: low-energy electron microscopy (LEEM),its extension to spin-polarized low-energy elec-tron microscopy (SPLEEM), and its combinationwith spectroscopic photoemission and low-energyelectron microscopy (SPELEEM). Other imagingmethods mentioned only briefly in the chapterinclude ultraviolet photoemission electron mi-croscopy (UVPEEM), mirror electron microscopy(MEM), low-electron energy loss microscopy(LEELM), and Auger electron emission microscopy(AEEM). The instruments used in these imagingmethods allow imaging not only in real spacebut also in reciprocal space, such as low-energyelectron diffraction (LEED) and angle-resolvedphotoelectron spectroscopy (ARPES in SPELEEM).The combination of these methods with comple-mentary high-lateral-resolution methods rendersimaging with low-energy electrons a comprehen-sive surface analysis tool.

9.1 Electron Beam–Specimen Interactions. 488

9.2 Instrumentation ................................ 492

9.3 Electron Optics . .................................. 497

9.4 Contrast . ............................................ 500

9.5 Applications. ...................................... 5049.5.1 The Si(111) Surface ................................ 5059.5.2 Si(100)................................................ 5069.5.3 Other Elemental Semiconductor

Surfaces ............................................. 5069.5.4 Thin Films on Semiconductors.............. 5079.5.5 Wide-Band Semiconductors ................ 5089.5.6 Metal Surfaces .................................... 5099.5.7 Metal Layers on Metals ........................ 5099.5.8 Reactions on Metal Surfaces ................. 5109.5.9 Oxides and Nitrides ............................. 511

9.6 Spin-Polarized LEEM (SPLEEM) ............. 512

9.7 SPELEEM . ............................................ 516

References ................................................... 523

Low-energy electron microscopy (LEEM) is an imag-ing method that makes use of elastically backscat-tered electrons with energies below about 100 eV, fre-quently < 10 eV. In contrast to transmission electronmicroscopy (TEM), which generally works with elec-trons in the 100 keV range where backscattering isnegligible, the backscattering cross sections for low-energy electrons are large enough to be useful forsurface imaging. This was already evident in the clas-sical diffraction experiments of Davisson and Ger-mer [9.1], but it would be 35 years before the useof slow diffracted electrons for surface imaging wassuggested [9.2], and another 23 years before convinc-ing images could be published [9.3]. Thus, althoughdiffraction of slow electrons and imaging with slowemitted electrons with resolution in the micrometerrange were demonstrated [9.4] before TEM reachedsubmicron resolution [9.5], LEEM became a viableimaging method only much later.

The reason for this late appearance of LEEMin electron microscopy is twofold: (1) For LEEM,well-defined surfaces are necessary, which in generalrequires ultrahigh vacuum (UHV) and efficient sur-face cleaning procedures. Although these have beenavailable for some time in glass systems, such as inFarnsworth’s low-energy electron diffraction (LEED)systems [9.6–8], glass systems are not very user-friendly, and metal UHV technology did not come intowidespread use until the beginning of the 1960s. In fact,the first display-type LEED system that heralded therevival of LEED was a glass system [9.9, 10], as wasthe first unsuccessful model of a LEEM system [9.2].(2) There was a widespread belief within the elec-tron microscopy community, based on the fundamentaltheoretical work of Recknagel on emission electron mi-croscopy [9.11], that the chromatic aberration of theobjective lens would limit the resolution to such an ex-tent as to make LEEM unattractive. This, of course,

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488 Part A Electron and Ion Microscopy

was a misunderstanding, as already pointed out in theearly phase of the development of LEEM [9.12, 13].In the decades since then, LEEM has slowly developedinto a powerful surface imaging technique, as recountedelsewhere [9.14].

Over the past 10 years, developments have been fo-cused on the combination of LEEM with x-ray-inducedphotoemission electron microscopy (XPEEM), whichresulted in the spectroscopic photoemission and low-energy electron microscope (SPELEEM) [9.15, 16],and on the correction of the aberrations of the objec-tive lens [9.17]. Despite these efforts, however, LEEMstill lags behind TEM in the technological state of theart.

This chapter reviews the basics of LEEM and itsapplications through the period up to about 2005. Morerecent developments are discussed in reviews listed at

the end of this chapter and in Chap. 11. A related imag-ing method, spin-polarized LEEM (SPLEEM), whichprovides magnetic information, will also be discussed.Other methods, including mirror electron microscopy(MEM), which provides information mainly on thelocal surface potential; Auger electron emission mi-croscopy (AEEM), which provides chemical informa-tion; electron energy loss microscopy (EELM), whichprovides some electronic information; and secondaryelectron emission microscopy (SEEM), will be men-tioned only briefly, as they have been used much lessfrequently, although they are also useful. Photoelectronemission microscopy (PEEM), in particular XPEEM,is included only in connection with the discussion ofSPELEEM, because it is the subject of another chapter.The sections on SPLEEM and SPELEEM describe thepresent state of the art.

9.1 Electron Beam–Specimen Interactions

To understand the possibilities and limitations of LEEMand the associated techniques, a fundamental under-standing of the interactions of slow electrons with con-densed matter is necessary. The following interactionsmust be taken into account: elastic scattering, inelas-tic scattering, and quasielastic scattering (phonon and,in magnetic materials, magnon scattering). The interac-tions of slow electrons with matter are vastly differentfrom those of fast electrons. In ferromagnetic materi-als, they also depend upon the relative orientation ofthe spin of the incident electrons and the electrons inthe matter.

Consider first elastic scattering. Because of its lowvelocity v D p

2E=m, the interaction time of a slowelectron is much longer than that of the fast electronsused in TEM, and an n-electron atom may no longer beconsidered undisturbed, but becomes an nC 1-electronsystem during the interaction. Therefore, the incidentelectron experiences the temporary excitations of then-electron atom. This can be taken into account byadding a correlation potential to the potential of theground-state n-electron atom. Similarly, the repulsiveinteractions between electrons with the same spin dueto the Pauli principle cause a spin-dependent potentialthat also has to be added. As a consequence, the scat-tering of slow electrons by the atoms that constitute thecondensedmatter can no longer be described by the firstBorn approximation, which assumes a static atom in theground state and which is a good approximation at highenergies. Instead, a partial wave analysis is necessary,taking into account the exchange and correlation poten-tials [9.18, 19]. No calculations of this type are available

for condensed atoms, whose potentials are truncatedby overlap with the neighbor atoms. Fortunately, themagnitudes of the correlation and exchange potentialdecrease rapidly with energy, so they may be neglectedin the energy range of conventional LEED studies (usu-ally above 30 eV). In LEEM, however, they should betaken into account. This is a formidable task, and onethat has not yet been mastered. Therefore, only some re-sults of partial wave analysis calculations for truncatedground-state potentials will be given here.

In partial wave analysis [9.20], the incident planewave and the outgoing scattered wave are expanded intospherical harmonics centered at the atom, and the phasedifferences �l between the incident and outgoing par-tial waves are calculated. In the nonrelativistic case, thescattering amplitude is given by

f .�; k/ D 1

2ik

X.2lC 1/Œexp.2i�l � 1/�Pl.cos �/ ;

(9.1)

with the sum over l extending from zero to infinity. k isthe wave number, and the Pl’s are Legendre polynomi-als. The intensity distribution of the scattered electronsas a function of scattering angle � and energy E � p

k isthen simply � jf .�; k/j2. Figure 9.1 shows the angulardistribution of the scattered intensity of 50 eV electrons,calculated in this manner for realistic solid-state Ag, Al,and Cu atomic potentials [9.18].

Here we see that not only is the total scatteringcross section of Cu (Z D 29) smaller than that of Al(Z D 13), but its backscattering cross section is as well,

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LEEM, SPLEEM and SPELEEM 9.1 Electron Beam–Specimen Interactions 489Part

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I a

Fig. 9.1 Angular distribution of 50 eV electrons elasticallyscattered from Ag, Al, and Cu atoms in the solid state. Af-ter [9.18]

QR

E

Fig. 9.2 Energy dependence of the backscattering intoa 30ı cone around the backward direction for Ag, Al, Cu,and W atoms in the solid state. After [9.19]

and Al scatters nearly as strongly as the much heavierAg atom (Z D 47). The scattering is, however, stronglyenergy-dependent. This is illustrated for the scatter-ing into a 30ı cone around the backward direction inFig. 9.2 [9.19], which shows that at very low energies,Cu scatters nearly as strongly as W (Z D 74), while Aland Ag scatter much more weakly in the backward di-

a) R E

E

E

b) k a

E

Fig. 9.3a,b Normal incidence specular reflectivity R ofa W(110) surface (a) and band structure along the surfacenormal (b). After [9.21]

rection. It should be noted that the zero of the energy isthe inner potential resulting from the overlap of the freeatom potentials, so that the maxima of the W and Cubackscattering cross sections are just around the vac-uum level.

In condensed matter, the electrons are, of course,scattered not only within one atom but also by the atomssurrounding it, causing strong multiple scattering. Thisis taken into account in LEED in the dynamic theoryof electron diffraction [9.22, 23]. Another way to lookat the problem of scattering in a periodic system is interms of the band structure theory, if we assume thatthe nC 1-electron system (n crystal electrons C in-cident electron) does not differ significantly from then-electron system (Koopmans’ theorem [9.24]). Thenthe 180ı backscattering from a single crystal surfaceis determined by the band structure E.k/ perpendicu-lar to the surface. This is illustrated in Fig. 9.3 for theW(110) surface [9.21]. The band structure in the [110](�N) direction has a wide band gap between about 1and 6 eV above the vacuum level. An electron incidentin this direction, therefore, does not find allowed states

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in the crystal and forms an evanescent wave. The extinc-tion length of this electron wave in the crystal is quiteshort in the center of the gap, only about two mono-layers [9.25], so that the electron is reflected before itis significantly attenuated by inelastic scattering. This,together with the strong backscattering cross section,causes the high reflectivity at about 2�3 eV. The secondreflectivity peak is due to the low density of states in thecrystal, as indicated by the steep bands. The band struc-ture influence is strongly orientation-dependent. Forexample, on theW(100) surface, the band gap is locatedbetween 3 and 5 eV above the vacuum level [9.21, 26],which causes a pronounced reflectivity peak at about4 eV. This is preceded by a deep reflectivity minimum,which is caused by the strong inelastic scattering ofthe electron that could otherwise penetrate deeply intothe crystal. A second reflectivity peak occurs around8 eV, where the density of states in the crystal is small.This simple picture neglects the influence of surface ef-fects. For quantitative agreement between experimentand theory, the surface barrier [9.27], surface reso-nances [9.21], and reconstruction have to be taken intoaccount. For LEEM, these details are not important, atleast at the present state of the art, because they mainlydetermine the reflected intensity and have little influ-ence on the contrast.

In general, the main factor determining the highsurface sensitivity of LEEM is not the influence ofthe band structure and elastic scattering, but the strongattenuation of slow electrons by inelastic scattering. In-elastic scattering is due to single-electron excitations(electron–hole pair creation) and collective-electron ex-citations (plasmon creation). In the energy range ofLEEM, single-electron excitations mainly involve va-lence band and weakly bound outer-shell core electrons.The universal inelastic mean free path (IMFP) curvestypically found in the literature are of rather limitedvalue at the low energies used, because they do nottake into account the differences in the electronic struc-ture of the various materials. Therefore, only somegeneral features will be discussed and some specificexamples will be given. In materials that may be de-scribed approximately by a free electron gas embeddedin a homogeneous background of equal charge (jel-lium model), the IMFP is a function of k=kF (kF Fermiwave number), with the electron density as parame-ter [9.18, 19, 28]. As an example, the attenuation length� D IMFP�1 of Al, for which the free electron approx-imation is good, is shown in Fig. 9.4 together with theattenuation coefficient � due to elastic backscattering,assuming a random distribution of Al atoms with bulkdensity (randium model) [9.29, 30]. The initial rise in� until the volume plasmon creation threshold at ET D17:5 eV (above vacuum level) is due to single-electron

E

Fig. 9.4 Energy dependence of the attenuation coeffi-cients �; � of slow electrons in Al by inelastic scatteringand elastic backscattering, respectively. After [9.29]

excitations. The maximum of � and the correspond-ing minimum of about 0:3 nm of the IMPF at about37 eV is mainly due to plasmon losses. Figure 9.4 alsoshows that attenuation by elastic backscattering is muchweaker above ET than that by inelastic scattering.

For most metals, the jellium approximation is notuseful, particularly for transition and noble metals. Forexample, in contrast to jellium, transition metals havea high density of unoccupied states just above the Fermilevel into which excitations can occur. The deviationfrom jellium can be partially taken into account by re-placing the ! dependence in the Lindhard dielectricfunction "L.!; q/, which is used in the jellium calcu-lations, with that obtained in the experiment for zeromomentum q transfer, that is, by optical data for whichq D 0. Results of such calculations [9.31] typically giveminimum IMFPs in metals of 0:3�0:5 nm at energiesbetween 30 and 120 eV for Mg and Au, respectively,with a rapid increase at low energies to values as highas 2:4 nm in Si and 3:5 nm in W at 10 eV, for exam-ple. Figure 9.5 illustrates the agreement between theoryand experiment that can be obtained in this approx-imation [9.32]. The deviations below E�EF D 5 eVare irrelevant for LEEM because the work function ofAu is about 5 eV; those above 50 eV are probably dueto inaccurate 5p and 4f ionization cross sections. Theagreement is surprisingly good considering that the qdependence of " has been approximated by simple ex-pressions and that correlation and exchange have notbeen taken into account. At energies below several tensof electronvolts, these are of comparable importancefor inelastic and elastic scattering, in particular the in-fluence of the detailed band structure and nondirect(q ¤ 0) transitions [9.33]. For example, inclusion of ex-

LEEM, SPLEEM and SPELEEM 9.1 Electron Beam–Specimen Interactions 491Part

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E E

Fig. 9.5 Energy dependence of the inelastic mean freepath of electrons in Au. The points and dots are experi-mental data and the dashed and solid lines are theoreticaldata using different approximations. After [9.32]

change in the dielectric model of the IMFP gives IMFPvalues that are larger by a factor of 1.3 or more thanwithout exchange [9.34].

The IMFPs calculated in this approximation for in-sulators are even larger, such as 6 nm at 10 eV forKCl [9.31]. For several groups of insulators with largeband gaps (condensed noble gases, N2, and organicdielectrics such as benzene or methane), no elec-tronic excitations are possible at low energies. Herethe (quasi)elastic mean free path (EMFP) determinesthe sampling depth. EMFP measurements in the energyrange of 2�15 eV give EMFPs up to 10 nm [9.35]. Anexample is shown for solid Xe in Fig. 9.6 [9.36]. Thusthe mean free path (MFP may be very long at low ener-gies (� 10 eV), while at energies between about 30 and100 eV, depending upon the material, it may be onlya few tenths of a nanometer. The large MFPs at verylow energies are, however, not found frequently.

They depend strongly on the density of unoccu-pied states into which bound electrons can be excited,as is clearly evident in an insulator with wide bandgaps [9.35]. These are extreme cases inasmuch as theirband gaps are so large that the density of unoccupiedstates becomes significant only at several electronvoltsabove the vacuum level. At the other extreme are transi-tion metals with their unfilled d bands with high densityof states just above the Fermi level. Here the IMFPs arevery short, as seen in Fig. 9.7 [9.37], in which the recip-rocal values of the IMFPs are plotted as a function of thenumber of d holes. The values shown are for electronswith energies between 5 and 10 eV above the Fermi en-

E

Fig. 9.6 Energy dependence of the elastic mean free pathof slow electrons in solid Xe at T D 45K. After [9.36]

Fig. 9.7 Reciprocal inelastic mean free path in nm�1 ofelectrons with energies between 5 and 10 eV above theFermi level as a function of the number of holes in the 3dand 4d shells. After [9.37]

ergy. For Fe, the IMFP is only about 0:5 nm, and forGd only 0:25 nm. In ferromagnetic materials, the den-sity of unoccupied states differs between majority andminority spin states, which causes corresponding differ-ences in the excitation probabilities. Calculations thattake this into account show a significant difference be-tween the IMFPs of incident electrons with majorityand minority spin, as seen in Fig. 9.8 [9.38]. The zeroof the energy is the Fermi energy; the work functionof Fe is 4:5 eV, so that 1 eV above the vacuum level,the IMFPs are only 0:6 and 0:2 nm for majority and mi-

PartA|9.2


E

Fig. 9.8 Energy dependence of the inelastic mean freepath of majority and minority spin electrons in Fe. Af-ter [9.38]

nority spin electrons, respectively. At 10 eV above thevacuum level, the corresponding values are 0:45 and0:3 nm, which are much lower than the 1:6 nm obtainedfrom the dielectric theory discussed above. For more in-formation see [9.39].

While the knowledge of reliable absolute numbersfor elastic and inelastic mean free paths at low energyis still limited, the influence of phonon and magnon ex-citation on the effective sampling depth is much lesswell understood. Both processes involve only small en-ergy losses up to several hundred million electronvolts,but can occur with large momentum transfer. In LEEM,only the electrons in a diffraction spot and its immediateenvironment contribute to the image formation. There-fore, energy losses with momentum transfer larger thanthat determined by the radius of the contrast aperturecause an attenuation of the intensity contributing to theimage formation. At relatively low temperatures, thiscan be taken into account by the Debye–Waller fac-tor. At higher temperatures, multi-phonon and -magnon

excitations occur, causing increased attenuation, whichmay be described by an anharmonic Debye–Wallerfactor [9.40]. In this conventional description of theinfluence of phonons on the scattering from surfaces,there is no thickness dependence. However, in thinfilms, the number of electrons scattered outside the con-trast aperture increases with increasing thickness, so aneffective attenuation coefficient could be defined. Thishas not been done to date for several reasons:

1. There are no numbers for the cross sections forphonon and magnon scattering that could be com-pared with those for inelastic scattering.

2. With increasing temperature, there is frequentlyatomic disordering that causes diffuse scattering.

3. At high temperatures, thin films usually break upinto three-dimensional (3-D) crystals before attenu-ation by these processes becomes significant.

A rough idea of the influence of thermal vibra-tions on the attenuation length may be obtained fromthe analysis of LEED patterns from Cu single-crystalsurfaces. At 50 eV, the total attenuation length, whichincludes elastic backscattering, inelastic scattering, andphonon scattering, from the (111) surface is 0:33 nm at300K versus 0:34 nm at 0K [9.41]. The difference iswell within the limits of error of the values, so at leastat this energy phonon scattering does not limit the sam-pling depth.

According to the present state of understanding, thesampling depth of LEEM and SPLEEM is determinedprimarily by inelastic and elastic backscattering. De-pending upon the energy and electronic structure of thematerial, the sampling depth may be as small as a fewtenths of a nanometer, for example, around the plasmonexcitation maximum, in transition metals with a highdensity of unoccupied states above the Fermi level, orin band gaps along the k direction normal to the sur-face. On the other hand, sampling depths as large asseveral nanometers can occur at very low energies, forexample, in insulators and free-electron-like metals. Inmany cases this enables tuning of the sampling depthby proper choice of the energy, which makes LEEMand SPLEEM ideal for imaging of surfaces and thinfilms.

9.2 Instrumentation

The electron optics of a LEEM/SPLEEM instrumentin the imaging section is essentially the same as thatin emission electron microscopes, which date back tothe 1930s. In these microscopes, the specimen is the

cathode of a so-called cathode lens in which the slowemitted electrons are accelerated in a high field to thefirst of several image-forming electrodes of an elec-trostatic lens or to the entrance of a magnetic lens.

LEEM, SPLEEM and SPELEEM 9.2 Instrumentation 493Part

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This lens is the objective lens of the microscope, whichproduces a primary image with fast electrons. The sub-sequent electron optics is basically the same as in TEM.In fact, the first objective lens used in LEEM wasa modified version of an electrostatic triode lens devel-oped for PEEM [9.42].

To perform LEEM with such a system, fast elec-trons have to be injected from the high-energy side ofthe objective lens along its optical axis. In the cathodelens, they are decelerated to the desired low energy atthe specimen. To be able to produce an image, this inci-dent beammust be separated from the reflected beam bya beam divider. As a consequence, a LEEM or SPLEEMsystem has a bent optical axis. A schematic of the firstinstrument is shown in Fig. 9.9 [9.3]. The beam separa-tor deflects the incident beam from a field emission gunthat is focused by two quadrupoles, the deflection field,and optionally by a collimator lens into the back focalplane of the objective lens. They reach the specimen onparallel trajectories with an energy that is determined bythe adjustable potential difference between field emitterand specimen. The specimen is imaged by the elas-tically reflected electrons into the center of the beamseparator, and its diffraction pattern into the back fo-cal plane of the objective lens where the angle-limitingcontrast aperture is located. The astigmatism of the ob-jective lens is corrected with a magnetic stigmator. Theprimary image in the center of the beam separator is im-aged with a magnetic intermediate lens and a projectivelens onto the final screen, and the diffraction pattern byadjusting the focal length of the intermediate lens. Theelectrostatic filter lens was originally intended to fil-ter out secondary and inelastically scattered electrons,but was later removed because the dispersive proper-ties of the magnetic beam separator were found to besufficient to eliminate them from the image. A pair ofmultichannel plates enhances the image intensity on

Fig. 9.9 Schematic of the first LEEMinstrument. After [9.3]

the fluorescent screen, allowing observation and im-age recording with a video camera at very low beamcurrents. Both illumination and imaging columns areequipped with deflectors and stigmators, some of whichare indicated. Emission microscopy is possible withthermionic emission by heating the specimen, with pho-toelectrons generated by a 100W high-pressure Hg arclamp, and secondary electrons using an auxiliary elec-tron gun.

While Fig. 9.9 shows the principle of the LEEMsystem, more recent instruments differ considerably indetail. For example, a transfer lens, which transfers thediffraction pattern from the back focal plane of the ob-jective lens toward the front of the intermediate lens,is now inserted just behind the beam separator, so thatthe illuminating beam does not have to pass through thecontrast aperture as in the original design. The elec-trostatic triode lens is now replaced by lenses withbetter resolution, such as the electrostatic tetrode lens,the magnetic diode lens, or the magnetic triode lens,which will be briefly discussed below. The beam sepa-rator has been improved in a variety of ways, includingthe use of close-packed or separated multiple mag-netic prisms, concentric square or round pole pieces,or a Wien filter, resulting in deflection angles rangingfrom 16 to 90ı, compared to the original 60ı. In addi-tion, energy filters have been added to instruments sothat they can also be used for AEEM, low electron en-ergy loss microscopy (LEELM), energy-filtered SEEM,and spectroscopic PEEM.

Before discussing these components of a LEEMinstrument, a short account of the various designs isappropriate. The first major development after the orig-inal instrument and a similar one [9.43] also useda beam separator with 60ı deflection but with close-packed multiple magnetic prisms and only magneticlenses, including the objective lens, a LaB6 cathode,

PartA|9.2


and a transfer lens so that the contrast aperture couldbe placed behind the beam separator [9.44]. The mag-netic prism on the illumination side of the instrumentcan be excited differently from that on the exit side sothat a higher beam energy is possible than on the imag-ing side, which allows AEEM when an energy filteris added to the instrument. The dense packing of themagnetic lenses together with the shielding of the beamseparator and the specimen region makes the magneticshielding used in the more open earlier instruments un-necessary. The addition of an energy filter allows notonly AEEM but in combination with synchrotron ra-diation also spectroscopic x-ray PEEM [9.14, 45, 46].This instrument, shown in Fig. 9.10, was built by theArbeitsgruppe Bauer at the Technical University (TU)Clausthal in the early 1990s [9.47]. It is a precursorof the commercial instrument. The version with energyfilter, the SPELEEM, which is presently the most ver-satile instrument, has been described repeatedly [9.48]in connection with PEEM. The cross section of an-other LEEM instrument (Fig. 9.11) [9.49] is shownhere. Similar to the previous instrument (Fig. 9.10),it has densely packed magnetic lenses both in the il-lumination (top) and in the imaging (bottom) section.However, it uses a beam separator with 90ı deflec-tion that is incorporated in the vacuum system (center).The objective lens (right side) is a magnetic diode andconsists of two sections with opposing image rotation.A commercial cold field emission electron gun (top)with an energy spread of about 0:25 eV enables a the-oretical resolution of 0:4 nm at 10 eV. The right side is

Fig. 9.10 The final noncommercial LEEM instrumentbuilt by the Arbeitsgruppe Bauer at the TU Clausthal,Germany, for Arizona State University. Left and rightfront: illumination and imaging column, respectively. Cen-ter: beam separator. Center back: specimen chamber.Right back: preparation chamber and specimen exchange.From [9.47] ©IOP Publishing. Reproduced with permis-sion. All rights reserved

the specimen chamber and a preparation chamber plusairlock. This instrument presently holds the resolutionrecord (0:5 nm) among the instruments without aberra-tion correction.

The instruments described above are freestanding.There has long been a desire to add a LEEM instru-ment to existing UHV systems. To be practical, suchLEEM systems must be much smaller and have smalldeflection angles. Two solutions were chosen: one usesa simple beam separator with a small deflection angle(10ı) [9.50], and the other uses three deflections by45ı [9.51]. In both cases, the instrument is mountedon an 800 diameter UHV flange and can be attached toa UHV system via a 600 diameter UHV flange. All lensesare electrostatic, including the electrostatic tetrode lens.In contrast to magnetic lenses, electrostatic lenses canbe easily floated at high voltage. Therefore, the com-plete electron optics can be at high voltage so that thespecimen can be near ground potential, whereas in themagnetic lens systems the specimen is at high voltage.Because of the compact design, high-voltage insula-tion requirements allow final energies of only 5 keV, incontrast to the 15�20 keV used in the larger systems.Only the extraction electrode of the tetrode lens canbe increased to 15 kV. External fields are screened byinternal �-metal screening. Figure 9.12 shows the me-chanical configuration of one of these flange-on LEEMinstruments [9.50]. Its overall length is 60 cm and itsweight is about 20 kg.

Several other LEEM instrument designs have beenproposed and in part realized. One design [9.52] in-cluded some interesting features such as a combinedmagnetic-electrostatic beam separator that allows dif-ferent energies to be used in the illumination andimaging beams, an electrostatic tetrode combined witha Schwarzschild-type optical mirror objective, whichfocuses UV radiation onto the specimen for PEEM,and a 70ı spherical condenser for electron energy fil-tering. Unfortunately, it never came into operation. Inanother design that is used in a commercial instru-ment [9.53], the beam separation is achieved by a Wienfilter, into which the illumination beam enters at anangle of 36ı, while the imaging beam runs along itsoptical axis. A second Wien filter is used as an en-ergy filter. This instrument has been used mainly forPEEM, MEM, and metastable impact electron emis-sion microscopy (MIEEM). Apparently, the difficultyin aligning the incident beam normal to the surfaceand keeping the imaging beam on-axis makes system-atic LEEM studies with acceptable resolution difficult.A similar problem must be overcome in an instrumentin which the beam separator is replaced by a W sin-gle crystal [9.54]. This crystal is tilted 45ı against theoptical axis and reflects a low-energy electron beam

LEEM, SPLEEM and SPELEEM 9.2 Instrumentation 495Part

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Fig. 9.11 Cross section of a LEEMinstrument with a 90ı separator.The illumination column with thefield emission gun is on top and theimaging column is at the bottom.The right side shows the specimenchamber, airlock, and part of thepumping system. After [9.49]

Fig. 9.12 Schematic of the mechanicalconfiguration of a flange-on LEEMsystem. After [9.50]

from a side-mounted gun along the optical axis. Ex-act normal incidence on the sample requires that thereflector is on the optical axis, which would obstructthe imaging beam. This instrument, called a double

reflection electron emission microscope (DREEM), isalso commercially available. Other LEEM instrumentshave been built as well, but their design has not beenpublished.

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All instruments discussed thus far suffer from thelarge chromatic and spherical aberrations of the objec-tive lens. That these aberrations can be corrected withan electron mirror has been known for some time, butonly during the past decade have efforts been madeto develop aberration-corrected instruments. Simultane-ously, however, the aberrations of the beam separatormust be corrected. This has led to the design of a com-plex system [9.55, 56] which has already been realizedin the so-called spectromicroscope for all relevant tech-niques (SMART) [9.17, 57, 58], schematically shownin Fig. 9.13. Similar to the SPELEEM system, thisinstrument is designed for a wide range of operationmodes [9.59], one of which is LEEM. The center-piece is the beam separator, to which the illuminationsystem, a field emission gun, the objective lens, and

Fig. 9.13 Schematic of the electron-optical configuration of the SMART system. After [9.57]

the mirror corrector are attached via field lenses (L).Beyond the beam separator, five electrostatic lensestransfer either the diffraction pattern in the plane ofthe contrast aperture between L3 and L4 or the pri-mary image at various magnifications into the entranceof the �-type energy filter, where the field-limitingaperture is located. With the energy selection slit in-serted, the resulting projective lens system produces anenergy-filtered image or diffraction pattern on the chan-nel plate-fluorescent screen unit, which is coupled tothe CCD (charge coupled device) camera using a fiber-optic coupler. Numerous deflectors (Di) enable precisealignment, and several n-pole elements (n D 2; 6; 12)are used for the correction of residual aberrations. Theinstrument operates at 15 kV and is expected to havea resolution in LEEM of 1 nm at 10 eV.

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9.3 Electron Optics

Unless there are resolution-limiting aberrations of thebeam separator, the energy filter, other optical compo-nents or vibrations, electromagnetic fields, charging, orother disturbances, the chromatic and spherical aber-rations of the objective lens determine the resolution.The aberrations of the accelerating field of the cath-ode lens cause resolution limit values that are muchhigher than those in transmission microscopy. Thesevalues can be calculated analytically by assuming thatthe lens may be separated into a homogeneous field infront of the specimen, which produces a virtual im-age behind the specimen, and an einzel lens, whichproduces a real image of the virtual image. Realisticcalculations must consider the cathode lens as a unit,

E z

E z

E z

B z

r z

r z

r z

a)

b)

c)

Fig. 9.14a–c Schematic configurations of LEEM cathodelenses: (a) electrostatic triode, (b) electrostatic tetrode, and(c) magnetic triode. One-half of the electrodes/pole pieces,the electron energy E.z/, and the electron ray path r.z/ areshown. After [9.60]

and such calculations have been made by many au-thors. A comparison of the resolution obtainable withan electrostatic triode, electrostatic tetrode, and mag-netic triode lens (Fig. 9.14) shows that the electrostatictetrode and the magnetic triode lenses are much bet-ter than the original triode lens, because in the latterthe field strength at the sample is low under focusingconditions (1:18�0:52 versus 10 kV=mm in the othertwo lens types) (Fig. 9.15) [9.60]. The data are for theoptimal aperture, which is determined by minimizingthe contributions of the aberration disks due to diffrac-tion at the angle-limiting aperture, and chromatic andspherical aberrations. These contributions can be seenin Fig. 9.16, together with the radius of the optimalaperture. From these figures it is clear that the mag-netic triode is superior to the electrostatic tetrode inboth resolution and transmission. Transmission doesnot play an important role in LEEM but is important inXPEEM with photoelectrons and secondary electrons,which have a wide angular distribution. Today, the mag-netic diode, which differs from the triode only in thatboth pole shoes are at the same potential, is the stan-dard in the best LEEM instruments without aberrationcorrection, while the electrostatic tetrode is the domainof the smaller, purely electrostatic LEEM instruments.In the multimethod instruments [9.15–17, 44, 57, 58],combined electrostatic–magnetic cathode lenses, suchas the magnetic triode, are useful because they allow thefield strength at the specimen to be varied. Such a lens,with a design somewhat different from that shown inFig. 9.14, is used in the SMART instrument. Becausethe chromatic and spherical aberrations of the objec-tive lens are corrected in this instrument by the mirror

Fig. 9.15 Comparison of the resolution ı of the lensesshown in Fig. 9.14 for a final energy of 20 keV, an energyspread of 0:5 eV, and optimized aperture. After [9.60]

PartA|9.3


a) r

b) r

c) r

C

C E E

Fig. 9.16a–c Energy dependence of the contributionsof the spherical, chromatic, and diffraction aberrations(dashed, dotted, and dash–dotted lines) to the resolutionı (solid line) at the optimal aperture with radius r forthe lenses shown in Fig. 9.14: (a) electrostatic triode,(b) electrostatic tetrode, and (c) magnetic triode. In (a), thefield strength has to be changed for forming a real image(1:18 ! 0:58 kV=mm for focus at infinity); in the othertwo lenses, 10 kV=mm has been chosen. Energy spread0:5 eV. After [9.58, 60]

corrector, the largest aberrations are now those of theenergy filter. The resolution improvement obtained bycorrection is shown in Fig. 9.17 as a function of angu-lar acceptance for an initial energy of 10 eV, an energywidth of 2 eV, and a final energy of 15 keV [9.58]. Thedashed lines show the contributions of the aberrationsof the uncorrected lens, and the thin solid lines thoseof the energy filter. For LEEM instruments with LaB6

or field emission guns that have lower energy widths,the resolution is still improved by about a factor of 5.

d

d

d

d

d

d

d

Fig. 9.17 Resolution limit d as a function of the accep-tance angle ˛ without and with correction of the sphericaland chromatic aberrations. Data for the system shown inFig. 9.13 for 10 eV initial energy and 2 eV energy spread.In the uncorrected system, d is limited by the chromaticand spherical aberrations (dashed lines), and in the cor-rected system by higher-order aberrations (solid lines).After [9.58]

The main advantage of aberration correction, the largeincrease in transmission, however, comes to bear onlyin emission microscopy, in particular in AEEM andXPEEM.

The next critical component of a LEEM instrumentis the beam separator. Early beam separators [9.3, 43]aimed only at the reduction of the unidirectional beamdispersion in the deflection plane. This was achievedwith a D-shaped cutout in the round pole pieces [9.61].The focusing action of the fringing field of the magnetwas compensated by magnetic quadrupoles. In con-trast to these first separators, which tried to eliminateits focusing action, later separators made use of it inorder to obtain optimal image and diffraction patterntransfer in close-packed magnetic prism arrays. Theyconsist of an array of inner pole pieces surroundedby a single outer pole piece with different relative ex-citations. Such arrays act almost like round lenses.They transfer image and diffraction planes stigmaticallyand distortion-free to corresponding planes behind theseparator but at different settings [9.62, 63]. A 90ı de-flector with four inner prisms was suggested for theaddition of a mirror corrector [9.63], and a 60ı de-flector with three inner prisms was realized in the firstfully magnetic LEEM instrument [9.44]. If illumina-tion and imaging beam have the same energy as inLEEM—in contrast to the more versatile instruments—then a single inner pole surrounded by an outer pole

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A|9.3

Fig. 9.18 The three fundamental modes of operation of a SPELEEM system. The various sections of the instrument areshown folded into one plane. In imaging and diffraction, the energy selection slit is inserted in the dispersive plane (DP),and the image/diffraction pattern behind the DP is imaged with the projector. The intermediate lens (IL) is used to switchbetween imaging and diffraction, simultaneously with the exchange of the contrast aperture in FPI and the field-limitingaperture in IIP. For fast spectroscopy, both apertures are inserted, the energy selection slit is removed, and the dispersiveplane is imaged by the projector. After [9.15]

ring are sufficient. Both square [9.49, 55, 64, 65] andround 90ı [9.66] separators have been proposed andbuilt. With proper geometry and excitation ratio, astig-matic and distortion-free imaging can be achieved forimage and diffraction planes with the same settings. Foran aberration-corrected microscope, the aberrations ofthese separators would limit the resolution, and have tobe corrected. This is achieved in the highly symmetricbeam separator seen in Fig. 9.13 [9.67–69]. In smallflange-on LEEM instruments, the design of the beamseparator is determined less by minimizing aberrationsthan by geometry and space considerations [9.50, 51].For example, to achieve parallel illumination and imag-ing beams within a minimum of space, the beams weretranslated achromatically [9.70] by magnets with fieldboundaries perpendicular to the beam and electrostaticcylinder lenses in order to achieve stigmatic focus-ing [9.51]. Finally, a Wien filter may also be used asa beam separator [9.71], though with some alignmentdifficulties.

Although the use of electron mirrors for the cor-rection of lens aberrations was proposed many yearsago [9.72, 73] and was suggested for the correction

of the cathode lens aberrations in LEEM and PEEMinstruments in the early 1990s [9.55, 62, 74], seriousefforts have been made only in the past 20 years [9.75–77]. In addition to the first instrument, the SMART,there are now two commercial instruments equippedwith a mirror corrector available.

An energy filter is not needed in simple LEEM in-struments with sufficiently large separator deflectionangles, because at low energies, at which the energyof the secondary electrons differs little from that of theelastically reflected electrons used for the LEEM im-age, the secondary electron intensity is small comparedto the diffracted beam intensity. With increasing energy,when the elastically backscattered intensity decreasesand the secondary electron intensity increases, the dis-persive action of the sector field deflects a sufficientnumber of secondary electrons such that only a smallfraction pass through the contrast aperture. Likewise,inelastic scattering in the high LEEM energy range oc-curs mainly in the forward direction, and so it can beseen in the backward direction only through diffrac-tion, via energy loss either before or after diffraction.Therefore it is a second-order effect. Furthermore, the

PartA|9.4


dispersion of the beam separator and the momentumtransfer in the energy loss ensure that most inelasticallyscattered electrons do not pass through the contrastaperture. In the low LEEM energy range, inelastic scat-tering is lower in the forward direction but in generalis weak, and thus does not contribute noticeably toimage formation. The main advantage in having an en-ergy filter in a LEEM instrument is that it eliminatesthe secondary and inelastically scattered electrons inthe LEED pattern. In many cases this is not impor-tant, but in materials with high secondary electron yield,it improves the LEED pattern dramatically. An energyfilter is also useful for quantitative analysis of the back-ground in LEED patterns.

The main motivation for adding an energy filterto a LEEM instrument is its importance in multi-method techniques such as SPELEEM or SMART. Itallows LEEM to be combined with low electron en-

ergy loss spectroscopy and microscopy. Furthermore,when higher beam energy can be used in the illumi-nation beam than in the imaging beam, Auger electronemission spectroscopy (AEES) and AEEM are possibleas well. These various modes of operation of such aninstrument are schematically shown in Fig. 9.18 [9.15,48]. In SEEM, whether excited by electrons, x-rayphotons, or energetic ion/neutrals, it allows selectionof a narrow energy window from the wide secondaryelectron energy distribution. This leads to a notice-able improvement in resolution, also in the aberration-corrected systems, in which the correction rapidly dete-riorates at low energies with decreasing energy [9.58].A number of different energy filters are used in thesemultimethod instruments: one-hemispherical analyzer,two-hemispherical analyzers, a Wien filter, or an omegafilter. As they are not essential for LEEM, they will notbe discussed here.

9.4 Contrast

For imaging with LEEM, several contrast mechanismsare available, depending upon the specimen to be im-aged. The fundamental contrast is diffraction contrast incrystalline samples or backscattering contrast in amor-phous or fine-grained crystalline materials. The originof the backscattering contrast is evident in Fig. 9.1. Anexample of its consequences is shown in Fig. 9.19. Be-cause of the higher backscattering cross section of Co atthe selected energy, the fine-grained polycrystalline Cosquares appear bright compared to the Si surrounding,which is covered with native oxide. A slight preferredorientation of the Co layer enhances the contrast [9.78].Typically, however, larger crystals or single crystallinelayers with a well-defined orientation are studied. Inthis case, in addition to the specular beam ((00) beam),other diffracted beams may be used for imaging. Anatomically flat single-crystal surface without steps and

Fig. 9.19 Backscatteringcontrast from 20 nm-thick Co squares on a Sisubstrate. The electronenergy is 5:1 eV andthe diameter of thefield of view is 10 m.Reproduced from [9.78],with the permission ofAIP Publishing

other defects produces contrast only when regions withdifferent crystal structures are present. A standard ex-ample is the Si(111) surface when the unreconstructed(1�1) and reconstructed (7�7) structures coexist. Here,both normal and lateral periodicity differ and producestrong contrast (Fig. 9.20a) [9.79]. Si(100) surface re-construction occurs with the formation of dimer rowswhose orientation rotates from terrace to terrace by 90ı,with constant normal periodicity. Here, the two result-ing (2� 1) domains are equivalent at normal incidenceand well-centered aperture. Contrast is obtained by ei-ther tilting the incident beam or shifting the aperturesomewhat in the direction of one of the rows. Us-ing the 1=2 order spot of one of the domains givesmaximum contrast (Fig. 9.20b) [9.80]. Similar domaincontrast can be obtained on all reconstructed surfaceson which reconstruction domains with different az-imuthal orientations exist. All surfaces have steps orstep bunches that will produce another contrast to bedescribed below.

In general, surfaces are heterogeneous not only incrystallography but also in composition and topogra-phy. Compositional differences are usually associatedwith crystallographic differences and produce, togetherwith backscattering differences, diffraction contrast inthe (00) beam. Here also, imaging with nonspecu-lar beams is useful for identifying different coexistingphases, as illustrated in Fig. 9.21 [9.81], which isfrom a Si(111) surface covered with a submonolayerof Au. The three LEEM images are taken with the (00)beam and with nonspecular beams (1=5-order beams)

LEEM, SPLEEM and SPELEEM 9.4 Contrast 501Part

A|9.4S

S

a) b)Fig. 9.20a,b Diffraction contrastfrom Si surfaces. (a) Si(111). (Normalincidence) contrast due to differentnormal periodicity of coexisting (1�1)(dark) and (7� 7) (bright) structure.Electron energy 10 eV. Inset: LEEDpattern. Reprinted from [9.79], withpermission from Elsevier. (b) Si(100).(Oblique incidence) contrast due todifferent azimuthal orientation ofcoexisting (2�1) and (1�2) domains.Electron energy 6 eV. From [9.80]

a) b)

c) d)

Fig. 9.21a–d Phase identification bydark-field imaging. The LEED pattern(a) of the Au submonolayer on Si(111)shows a hexagonal pattern from the.p3� p

3/-R30ı phase and two linearpatterns from the (5� 2) phase. Thebright-field image (b) shows darkregions that are identified as (5� 2)regions by imaging with (1=5, 0),spots (c,d). Electron energy in (a)30 eV and in (b–d) 6 eV. Reprintedfrom [9.81], with permission fromElsevier

of (5�2) superstructure domains. This enables the iden-tification of the dark regions between the bright .

p3�p

3/-R30ı structure regions in the specular image withdifferent (5� 2) domains.

Topography distorts the electric field distribution onthe surface. This causes the usual topographic contrast,which is most evident near zero electron energy. Topog-raphy also produces diffraction contrast. This happenswhen surface elements are inclined against the averagesurface, for example, in small crystals on an otherwiseflat surface. In a LEEM instrument, the positions of theLEED spots from a flat surface do not change with en-ergy as they do in an ordinary LEED system. This isbecause the observed LEED pattern is a magnified im-

age of the LEED pattern in the back focal plane of theobjective lens, where the electrons have a constant highenergy E D .h2=2m/k2 independent of their initial en-ergy. Because the wave number k is proportional to therefractive index n, we have

k sin � D k0 sin �0 ; (9.2)

where � and �0 are the angles of the electron trajecto-ries with the optical axis in the back focal plane andat the surface, respectively. For two-dimensional (2-D)diffraction

k sin �0 D 2 h ; (9.3)

PartA|9.4


where h is the distance of the LEED spot h D .h1; h2/from the (00) beam, which is on the optical axis at nor-mal incidence. Combining the two equations leads to

k sin � D 2 h : (9.4)

Thus the angular distance � of the LEED spot h inthe back focal plane is independent of the initial en-ergy E0 D .h2=2m/k20, and depends only on the finalenergy E D .h2=2m/k2. This is no longer true whenthe surface normal is inclined against the optical axis.Then the specular beam is off-axis, and the diffractedbeams h move toward the specular beam with increas-ing energy. The simple geometric relations at normalincidence, which lead to E0-independent spot positions,are no longer valid, and thus the spots now move inthe back focal plane. An example of these spot move-ments is shown in Fig. 9.22, [9.82]. A simple geometricanalysis enables deduction of the inclination of thesurface.

Faceted surfaces, that is, surfaces on which all sur-face elements are tilted so that no specular beam is onthe optical axis, can be imaged either by selecting an en-ergy at which one of the diffracted spots passes throughthe optical axis or by tilting the illuminating beam orshifting the contrast aperture off axis into one of thespecular beams. For large tilt angles, only the first modeis practical.

Another important contrast mechanism is the inter-ference contrast on flat surfaces with height differences

Fig. 9.22 Drawing of the movements of LEED spots fromfaceted Cu silicide crystallites on a Si(111) surface. Theopen circles are from the Si(111)-ı.7� 7/ structure. Thesmall solid and shaded circles are from the crystallites. In-creasing shading shows the movement of the spots withenergy increasing from 3 to 10:5 eV. After [9.82]

such as atomic steps. The step contrast was alreadyobserved in the early studies (Fig. 9.23) [9.3] and at-tributed to destructive interference between the wavefields reflected from the adjoining terraces within thelateral coherence length (Fig. 9.24a). Detailed modelcalculations based on Fresnel diffraction from two ad-joining straight edges shifted relative to each otherby the step height produce all salient features of thestep contrast [9.83, 84]. Here, some results of the gen-eral theory of image formation by a typical magneticcathode lens will be given [9.85]. In the absence of aber-rations, the reflection of slow electrons from a pointsource would produce an interference pattern that ex-tends far out from the step. This would make imageinterpretation in the presence of several steps diffi-cult. The spherical aberration reduces the range ofthe interference pattern significantly, and the chromaticaberration reduces it to one intensity maximum next tothe step at energy spreads as low as 0:5 eV. The in-tensity distribution around the step depends upon thephase difference between the waves reflected from eachside of the step, that is, upon the step height and thewave length, and upon the defocus. This is illustratedin Fig. 9.25 [9.85] for two phase shifts �� D n (n D0:5; 1) and several defocus values �z� D �z.Cs�/

�1=2,where �z is the geometric defocus, Cs the sphericalaberration constant, and � the wavelength. �z� D 0; 1corresponds to the Gaussian image and to the Scherzerfocus, respectively. For integer n, the step contrast issymmetric and optimal at �z� D 0; for noninteger n itis asymmetric, with the bright edge changing from one

Fig. 9.23 Monatomic steps on an Mo(110) surface. Elec-tron energy 14 eV. Reprinted from [9.3], with permissionfrom Elsevier

LEEM, SPLEEM and SPELEEM 9.4 Contrast 503Part

A|9.4

a) b)Fig. 9.24a,b Conditions for phasecontrast in LEEM. (a) Step contrast.(b) Quantum size contrast. Thepenetration of the electron wave uponreflection is indicted

z z

a) I

x x

b) I

x x

z z

I

x x

I

x x

z z

I

x x

I

x x

Fig. 9.25 Step contrast for the phase differences �� D (a) and �� D 0:5 (b) between the waves reflected from the terracesnext to the step for zero defocus and small positive and negative defocus. Modified from [9.85]

side of the step to the other when the sign of the de-focus changes. Optimal contrast is achieved for slightdefocus.

A third contrast mechanism, the quantum size con-trast, is also based on wave interference, which doesnot necessarily require crystal periodicity. In a thin filmbounded by two parallel surfaces, the wave reflectedfrom the bottom surface can interfere constructivelyor destructively with that reflected from the top sur-face, similar to a Fabry–Pérot interferometer, dependingupon the wavelength �, the thickness t, and the phaseshifts � upon reflection at the surfaces (Fig. 9.24b).

Constructive interference, and therefore enhanced re-flectivity, occurs whenever n.�=2/C� D t, where n isan integer and � is the wavelength in the film, whichdiffers from the vacuum wavelength by the inner poten-tial. As a consequence, regions with different thicknessappear in the image with different brightness. This wasfirst observed in Cu films on Mo(110) [9.86] and hasbeen since studied in detail in several other systemswith the goal of determining the band structure k.E/above the vacuum level [9.83, 87–89] or spin-dependentelectron reflectivity effects [9.90], or to understand spe-cific features in thin-film growth [9.91–93].

PartA|9.5


Fig. 9.26 Quantum size contrastbetween regions with differentthickness of a Fe film on W(110),taken with different electron energies,that is wavelengths. The images in thetop row show the intensity and thoseat the bottom the magnetic signal(exchange asymmetry). Blue and redcorrespond to opposite magnetizationdirections. Reprinted from [9.87],with permission from Elsevier

To determine the band structure, the constructiveinterference condition above is rewritten by replacing� by k D 2 =�, which leads to k.E/t� k.E/�.E/ Dn . The energy-dependent phase term can be elimi-nated by choosing film thickness pairs t1, t2 for whichthis condition is fulfilled (with different n), whichgives a set of k.E/ values. With proper growth condi-tions regions with different thickness can be obtained(Fig. 9.26) [9.87] and analyzed quasisimultaneously.After subtraction of the reflectivity of a thick film,which does not show quantum size effects, the os-cillations of the reflectivity due to constructive anddestructive interference can clearly be seen. This is il-lustrated in Fig. 9.27 [9.88] for a ferromagnetic filmin which the band structures of the majority and mi-nority spin electrons differ by the exchange splitting.The reflectivity curve for the minority spin electrons isshifted relative to that of the majority electrons due tothis splitting and is also damped more strongly due tothe shorter IMFP of the minority electrons mentionedin Sect. 9.1.

R R

Fig. 9.27 Quantum size oscillations of the reflectivity Rof spin-up (circles) and spin-down (crosses) electrons ina six-monolayer-thick Fe film on W(110) as a function ofenergy. After [9.88]

9.5 Applications

The high intensity available in LEEM studies of single-crystal surfaces, which enables rapid image acquisition,and the high surface sensitivity, which strongly accen-tuates the topmost layer in imaging, both discussedin Sect. 9.1, along with the various contrast mecha-nisms described in Sect. 9.4, have made LEEM one ofthe most powerful methods for surface studies, partic-ularly with regard to the thermodynamics of surfacesand the kinetics of surface processes. While most ofthis information came from detailed studies of semi-conductor surfaces, mainly from Si(111) and Si(100)surfaces, important insight into surface processes has

also been obtained from various metal and oxide sur-faces, such as the TiO2(110) surface. The informationobtained from such studies ranges from the chemi-cal potential of adatoms, diffusion across terraces andsteps, anisotropic step free energy, step stiffness, stepmobility, step–step interactions, surface free energy andsurface stress, vacancy exchange between the bulk andthe surface, to nucleation, growth, phase transitions,self-organization, faceting, segregation, oxidation, andother surface and thin-film phenomena. Only some ex-amples can be mentioned in the following subsections.These are organized according to the material, but the

LEEM, SPLEEM and SPELEEM 9.5 Applications 505Part

A|9.5

references will provide access to most of the relevantwork conducted to date. For illustrations, results of theearly exploratory work will be used, because the laterquantitative studies require much more discussion.

9.5.1 The Si(111) Surface

The Si(111) surface is probably the surface most oftenstudied with LEEM, mainly because of its phase tran-sition from the reconstructed (7� 7) to the disordered(1� 1) at 1100 or 1135K, depending upon the author.In precise LEED diffractometer measurements [9.94,95], the superstructure spots disappeared at 1120K,and an intensity fit assuming a continuous transitiongave a critical temperature of 1100˙ 1K. However, nocritical scattering was observed, which put into ques-tion earlier conclusions that the phase transition wassecond-order. The first LEEM measurements [9.79, 80,96] demonstrated without doubt that the transition wasfirst-order, as seen in the nucleation and growth of the(7� 7) structure (Fig. 9.28). The growth rate of the(7� 7) domains was found to increase linearly withundercooling �T , and for �T > 12K, nucleation alsooccurred on the terraces.

The transition was found to be strongly influencedby impurities [9.97]. In particular, the apparent discrep-ancy between LEEM and the preceding LEED studieswas attributable to near-surface contamination duringthe long measurement time near the phase transitionneeded in the quantitative LEED studies. This is il-lustrated in Fig. 9.29 [9.80], which shows that longannealing near the transition temperature completelydestroys the regular domain structure. The LEED pat-terns differ only by a slightly higher background inthe annealed sample, but the transition range is nowmuch wider, similar to that in the LEED studies. Onclean surfaces that have been quenched rapidly andhave completely converted into the (7� 7) structure,many domains of various sizes form. Upon subsequent

a)

b) Fig. 9.28a,b Nucleation and growthof the (7� 7) structure at surfacesteps with different orientationsat low supersaturation. Electronenergy: (a) 10:5 eV and (b) 1:5 eV.From [9.96], with permission ofBunsengesellschaft, Germany

a) b)

Fig. 9.29a,b Comparison of a surface that was annealedfor a long period around the transition temperature (a) andone that was cooled rapidly from 1450K to this tempera-ture (b). Electron energy 10:5 eV. From [9.80]

annealing, they coarsen without preference to certainboundary orientations or number of bounding domainwalls [9.98].

Recent, more detailed studies [9.99–108] haveshed considerable light on the forces and processesinvolved in the phase transition, including adatomdiffusion [9.99, 100, 104], the influence of the sur-face stress difference between the (7� 7) and (1� 1)phases and of long-range interactions on phase coex-istence [9.101], shape [9.105] and distribution of the(7� 7) domains [9.106], and other aspects. There areexcellent reviews on these subjects [9.109, 110] wheredetails may be found. Another phenomenon that hasbeen studied with LEEM and which is closely re-lated to the (1� 1)-to-(7� 7) phase transition uponcooling is the faceting of vicinal (stepped) Si(111) sur-faces [9.111–113]. Other studies have been concernedwith the conditions for step flow growth instead of two-dimensional nucleation from which the parameters thatdetermine the growth kinetics can be derived [9.114–116]. In and Sb surfactants that form a .

p3�p

3/-R30ıstructure were found to either enhance or suppress

PartA|9.5


step flow, respectively [9.117]. An apparently similarboron-induced surface structure ..

p3� p

3/-R30ı-B/has a quite different effect: it causes twinning [9.114].

9.5.2 Si(100)

This surface is the basis of semiconductor technologyand, as such, has attracted particular attention. It wasstudied qualitatively in the early years of LEEM [9.80](Fig. 9.20b) and convincingly showed the nonequiva-lence of the A and B type steps, the lower-energy SAsteps being smooth, while the higher-energy SB stepswere rough. Step migration during sublimation [9.118]and interaction with dislocations formed by plasticdeformation during cooling [9.119] were studied, aswell as the enhancement of one domain upon elasticdeformation [9.80, 119]. Other processes included con-secutive Lochkeim formation during sublimation of flatregions [9.120] and homoepitaxial growth [9.80, 120–122]. From the terrace shape during growth close toequilibrium (Fig. 9.30), a lower limit of the ratio of thestep free energies of SB and SA of ˇB=ˇA � 2:6 at about800K was obtained. In other qualitative work [9.123,124], the step morphology was studied in more detailas a function of miscut angle, which led to a step phasediagram ranging from a hilly phase near zero miscut via

a) b)

c) d)

Fig. 9.30a–d Images from a video taken during the ho-moepitaxial growth of Si(100) at low supersaturationso that growth can occur only from a defect at thelower edge of the image. Electron energy 5 eV. Reprintedfrom [9.121], with permission from Elsevier

single-height wavy steps, to straight steps, to double-height steps at a miscut of about 0:1ı.

In subsequent quantitative studies [9.125–136],comprehensive information was deduced from step andisland shapes and distributions. Many of the results canalso be found in the reviews mentioned above [9.109,110]. These include the determination of the mobil-ity and stiffness of the SA and SB steps [9.125, 129],their free energy [9.125], and the anisotropy of the sur-face stress [9.136], and the extraction of the chemicalpotential, formation energy, and diffusion coefficientsof adatoms from number, area, and distribution oftwo-dimensional islands [9.126, 130, 133]. On the morequalitative side, the fabrication of large step-free re-gions [9.128, 135] and of periodic gratings [9.131, 132,134] by e-beam lithography, reactive ion beam etching,and high-temperature annealing has also contributedconsiderably to the understanding of the surface prop-erties. Other methods of surface modifications havebeen studied as well. Oxygen etches the surface at hightemperature and produces vacancy islands [9.137–139].Arsenic was found to displace Si even on large ter-races, driven by surface stress, causing two-dimensionalisland formation [9.140]. Boron segregation leads tostrong temperature-dependent surface roughening atthe monolayer level, forming a striped [9.141, 142](Fig. 9.31) or triangular-tiled surface structure [9.143–145]. Originally believed to be driven mainly by surfacestress relaxation, it was later shown that a strong reduc-tion in the step free energy of the SA steps was the maindriving force [9.143, 144].

9.5.3 Other Elemental SemiconductorSurfaces

On the silicon-on-insulator (SOI) (100) surface,LEEM was used to determine the dislocation-inducedstrain [9.146]. On the Si(311) surface, the LEEM im-age intensity fluctuations in small surface regions weremeasured during the continuous disordering transitionof the (3� 1) reconstruction around 965K, in order todetermine the critical parameters [9.147].

The first-order transition at about 1005K from thehigh-temperature (1�1) to the low-temperature (16�2)structure of the Si(110) surface and the stress-stabilizedcoexistence of the two phases was the subject of an-other study [9.148]. Similar to the work on Si(111) andSi(100), various surface thermodynamic data have alsobeen obtained for the Si(110) surface from island de-cay measurements [9.149]. Finally, a LEEM study ofthe (2� 1)-to-(1� 1) phase transition above 925K onGe(100) led to the conclusion that the transition wasdue to dimer breakup and roughening [9.150].


A|9.5

a) b)

e) f)

g) h)

c) d)

Fig. 9.31a–h Images from a video taken during the seg-regation and desegregation of B from B-doped Si(100)during cooling and heating. Electron energy 4:2 eV; diam-eter of field of view 7 m (5 cm). From [9.142]

9.5.4 Thin Films on Semiconductors

Because of its importance in the semiconductor indus-try, Ge—or more precisely SiGe on Si(100)—is themost widely studied system to date [9.151–161]. Thissystem initially forms a several-monolayer-thick layerthat is highly strained depending upon alloy composi-tion. From this layer, three-dimensional islands developwith facets that are dependent upon size and composi-

tion, leading to a rich variety of phenomena well suitedfor LEEM studies. The transition from the initial layerto the three-dimensional islands is strain-driven anddoes not require three-dimensional nucleation [9.156,157, 159]. Surface steps play a critical role in the al-loying of Ge with the Si(100) surface [9.160]. Theresults up to 2000 are summarized in an excellentreview [9.161]. The growth of Ge on other Si sur-faces has been studied in much less detail. On the(111) surface, the influence of the surfactant Sb ongrowth was studied [9.162], and on the (311) surfacethe transition from the two-dimensional layer to three-dimensional islands [9.163]. These showed a muchmore complicated facet structure than on Si(100). Fi-nally, the growth of Ge on GaAs(100) was also studiedbriefly [9.164].

The growth of metals on semiconductor surfacescan also be studied very well with LEEM. Most of thework has been done on Si(111) surfaces, with some onSi(100), using video recording of the growth, diffusion,ordering, and disordering processes. Au on Si(111)is a good example: after several two-dimensional su-perstructures have formed with increasing coverage,three-dimensional particles grow that show an interest-ing temperature dependence due to the formation ofan Au-Si eutectic [9.81, 165]. On vicinal Si(100) sur-faces with a miscut of 4ı, adsorption of a monolayerof Au causes pronounced faceting at elevated temper-atures [9.166–169]. The growth of Ag on Si(111) hasalso been studied extensively [9.161, 170–173], in par-ticular the growth shape [9.171, 173], and has been usedto demonstrate that buried interfaces can be imaged viatheir strain fields [9.172]. In the various studies of Agon Si(100) [9.174–176], a strain-induced shape tran-sition of Ag crystals into quantum wires [9.174] andbamboo-like growth [9.177] have been observed. Sub-strate material incorporation into the two-dimensionalsuperstructure [9.176] and the influence of steps onthe domain ordering in it [9.175] have been found.On vicinal Si(100), self-assembled Ag quantum wiresform at high temperatures [9.178]. The growth ofCu on Si(111) has been studied on both the cleanand the hydrogen-passivated surface [9.82, 179–182].Hydrogen termination strongly influences the forma-tion of the two-dimensional (5� 5) superstructure andthe three-dimensional nucleation of Cu silicide. Athigher temperatures, at which hydrogen is desorbed,large Cu3Si crystals form, in a variety of shapes andfacets.

In Al films on Si(111), the two-dimensional phasediagram and the phase transitions between the phaseshave been studied [9.183]. The growth of Pb on Si(111)has been used to obtain an understanding of how inter-

PartA|9.5


a)

b)

Fig. 9.32a,b Images taken during the growth of Pbon Si(111) at 290K with (a) and without Au surfac-tant (b). Electron energy 8 eV. Reprinted with permissionfrom [9.184]. Copyright 2000 by the American PhysicalSociety

factants, that is, substrate surface layers that remain atthe interface during growth, produce quasi-monolayer-by-monolayer growth, using Au and Ag as interfac-tants [9.184–186]. Figure 9.32 illustrates the influenceof an Au interfactant layer on the growth of Pb. Onthe Si(100) surface, Pb grows in h111i-oriented crys-tals on the two-dimensional initial layer. The crystalsfrequently have an ashtray shape due to the large sup-ply of Pb atoms by diffusion on the initial layer [9.187].On vicinal and high-index surfaces of Si, which havebeen faceted by Au adsorption, Pb grows in meso-scopic wires [9.188, 189]. In was found not to wet theSi(111)-(7� 7) surface but to grow at low temperature,monolayer by monolayer, on the Si(111)-.

p3� p

3)-R30ı surface with a variety of superstructures. Three-dimensional crystals that grow at somewhat higher tem-peratures have predominantly (100) orientation, witha surface reconstruction similar to that of the under-lying two-dimensional layer [9.190, 191]. The Si(100)surface is etched at high temperatures by In similar tothe situation mentioned above for Ag; at lower temper-atures a superstructure forms [9.192].

Transition metals are highly reactive with Si andform silicides that are frequently more stable at hightemperatures than Si. An example is Co. When de-posited or annealed at high temperatures, Co layersform more or less hexagonal CoSi2 crystals that showmisfit dislocation contrast when sufficiently thin. Whenheated to temperatures at which Si sublimes, CoSi2-topped hillocks form because of the lower vapor pres-sure of CoSi2 (Fig. 9.33) [9.97, 193]. At lower tem-peratures, the crystals are triangular and act as Coscavengers, cleaning the surrounding surface from Coso that it develops the (7� 7) structure of the clean

Fig. 9.33Hillocks ona Si(111) surfacecaused by CoSi2crystals duringthe sublimationof Si. Reprintedwith permissionfrom [9.97].Copyright 1999,American Vac-uum Society

surface. At very low Co coverages, an interesting ringcluster (RC) phase forms, which has been studied inconsiderable detail [9.194–196]. Ni forms a similarRC phase [9.197]. Another transition metal, Ti, spon-taneously forms Ti silicide nanowires when depositedat about 1120K. Their formation process and stabilityhave been studied in detail [9.198].

Only a few nonmetal films on Si have been studiedwith LEEM: CaF2 and Si nitride. In CaF2 films, studieshave investigated both the complexities of the forma-tion of the first two layers [9.199] and the formationmechanism of the interfacial dislocation network withincreasing film thickness [9.200]. The Si nitride workwas concerned mainly with the formation of the initialepitaxial layer by reaction with NH3 at high tempera-tures, which occurs via nucleation and growth of nitridedomains similar to the (7�7) domains on the clean sur-face [9.201].

9.5.5 Wide-Band Semiconductors

Very little work has been done on these materialswith LEEM. There is a cursory study of the tempera-ture dependence of the structure of the 6H-SiC(0001)surface used as a substrate in the growth of GaN lay-ers [9.202]. The structure of the GaN(0001) surface wasstudied with dark-field imaging, which enabled deter-mination of the surface termination [9.203], similar tothat of the SiC(0001) surface [9.204]. Several papershave demonstrated the importance of the Ga=N ratioin homoepitaxial growth of GaN [9.202, 204, 205], inparticular the necessity of a Ga double layer on top ofthe GaN surface for the growth of films with (0001)surfaces [9.202, 205, 206]. The double layer shows aninteresting phase transition [9.202]. Without an ad-sorbed Ga layer, the GaN films grow rough, with f10N11gand f101N2g facets. GaN growth on 6H-Si(0001) is sim-ilar [9.207], and not as reported (incorrectly) in anotherpaper [9.208], except that three-dimensional crystalsform initially instead of the spiral and step flow growthin homoepitaxial growth on GaN.


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9.5.6 Metal Surfaces

Refractory metal surfaces have been the most popularsubject in LEEM because of their high melting pointand low vapor pressure, allowing experiments to beconducted over a wide temperature range. W and Mocan be easily cleaned by heating in oxygen. The chemi-sorbed oxygen is then flashed off at high temperatures,a procedure that is not successful in Nb and Ta, in whichoxygen goes into solid solution and can be partially re-moved only by lengthy sputter and annealing cycles.The imaging of the step structure on the Mo(110) sur-face (Fig. 9.23) was one of the first demonstrationsof the power of LEEM, and step contrast has beena major tool in the study of surface processes on cleansurfaces. While W(110) and Mo(110) have been usedfrequently as substrates for thin films, and to a muchlesser extent W(100) and W(111), little has been pub-lished about the clean surface, except for a brief study ofthe Mo(110) surface [9.209]. Extensive work has beencarried out, however, on epitaxial Mo(110) [9.210–215]and Nb(110) [9.213, 216–218] layers grown on sap-phire (11N20) surfaces at high temperatures, which afterproper cleaning produces surfaces that are comparablein quality to single-crystal surfaces. One complicationis the interfacial strain and the dislocations introducedupon cooling and thermal cycling by the different ther-mal expansion coefficients between film and substrate.Nevertheless, pure surface quantities such as step stiff-ness could be extracted from such films. In the caseof the Nb(110) films, the situation is complicated bythe residual oxygen. This causes reconstruction andfaceting [9.213, 216, 217], which in themselves are in-teresting processes and are useful for the study ofextended defects [9.218]. Some work has been done onTa(110) films as well [9.213].

Noble metal surfaces have also been the subject ofseveral LEEM studies. For Pt(111) single-crystal sur-faces, step stiffness, step–step interactions, step freeenergy [9.213, 215, 219], and bulk–surface vacancy ex-change [9.220, 221] have been determined. For Pd(111)surfaces, the step stiffness [9.215] was obtained andsputter erosion processes [9.222] were observed. Stud-ies of the island decay on Rh(100) indicated a newsurface diffusion process [9.223]. Step fluctuation spec-troscopy of Au(111) yielded the surface mass diffusioncoefficient and the orientation-dependent step stiff-ness [9.224]. Dark-field imaging of the reconstructedAu(100) surface was used to establish the connectionbetween the reconstruction domains and the step ori-entations (Fig. 9.34) [9.80, 225, 226]. On the Ag(111)surface, a critical island size was found for layer-by-layer growth [9.227]. The Ehrlich–Schwoebel bar-rier, the energy barrier for diffusion across a step,

a) b)

c) d)

Fig. 9.34a–d Au(100) surface. Bright-field image (a) anddark-field images (c,d) taken with the (5� 1) superstruc-ture reflections indicated in the LEED pattern (b). Electronenergy in the images 16 eV. From [9.225]

was deduced from an investigation of the homoepi-taxial growth of Cu on Cu(100) [9.228]. Similar toother fcc(110) surfaces, reconstruction of Pb(110) sur-faces can be observed. A LEEM study of the vari-ous reconstructions, some of them alkali-induced, re-vealed the topography of the various phases and theinfluence of surface defects on the transitions be-tween them [9.229, 230]. Finally, studies of the surfacemorphology of the NiAl(110) surface demonstratedthe importance of bulk diffusion for surface smooth-ing [9.231]. Most experiments on clean surfaces rely onthe step contrast discussed in Sect. 9.4, which illustratesits usefulness.

9.5.7 Metal Layers on Metals

Although the growth of many metals on W(110) andMo(110) has been studied with other methods, LEEMhas been used infrequently, often only in a very cursorymanner. Cu is the most frequently investigated layermaterial, both on Mo [9.86, 122, 165, 232, 233] and W.The growth on the two substrates is similar and has beenstudied in detail on W(110) [9.234]. Layer spacingshave been determined using the quantum size effect dis-cussed in Sect. 9.4 [9.235]. The more qualitative workon Mo revealed an interesting striped phase upon an-nealing at high temperatures, as well as details in thestructural phase transition in the double layer. Layerspacings have also been obtained from the quantum sizeeffect for Ag on W(110) [9.92]. Au has been studiedbriefly on Mo(110) in the submonolayer range, where itforms needle-like crystals [9.122, 233]. The transition

PartA|9.5


from two- to three-dimensional growth of Pd layers onW(110) was the subject of a combined LEEM-XPEEMstudy [9.236]. In other works, the growth of Ag andCu on the Ru(0001) surface served as a demonstra-tion of the influence of substrate steps on the growthof three-dimensional crystals [9.237]. In the metal-on-metal systems discussed thus far, the substrate surface isnot modified or is modified only slightly by the grow-ing film. This is not the case on less densely packedsurfaces such as the W(111) surface, where facetingoccurs upon deposition of certain metals at high tem-peratures. Pt growth on W(111) is a case study for thisprocess [9.238, 239]. The growth of ferromagnetic lay-ers will be discussed in Sect. 9.6 in connection withSPLEEM.

In many cases, the deposited metal forms a surfacealloy with the substrate. Pd on Mo(100) is an exam-ple that was studied in detail up to several monolay-ers using LEEM [9.240]. Clear alloying was observedup to a monolayer, followed by two-dimensional Pdgrowth without faceting. Alloying of Sn with Cu(111)is strikingly different: at very low coverages, largetwo-dimensional islands form that travel across the sur-face, leaving alloy behind and thereby decreasing insize [9.241]. Pb forms on the Cu(100) surface ini-tially in several two-dimensional structures, followedby the growth of three-dimensional crystals. LEEMclearly shows the correlation between the crystals andthe initial layer [9.242], evidence for the surface alloy-ing in the initial layer at low coverages, followed bydealloying [9.243, 244]. The nature of the disorderingtransitions of these phases was also determined [9.244].One of the most impressive results produced by LEEMto date is the self-assembly of stress domain patternsin the two-dimensional Pb-Cu alloy on Cu(111), whichconsist of domains of a Pb-rich and Pb-poor phase.This process has been studied in great detail, whichhas produced a wealth of information on stress-inducedordering phenomena [9.245–249], and has been well re-viewed [9.250]. Finally, the influence of the interfacebetween Pb droplets and the Cu(111) surface on theshape and melting of the droplets was the subject ofa LEEM study [9.251, 252].

b) c) d)a)Fig. 9.35a–d Dark-fieldimages taken during thereaction of NO with H2

on Rh(110) with LEEDspots characteristic ofthe various phases inthe oscillatory reaction.Reprinted from [9.253],with permission fromElsevier

9.5.8 Reactions on Metal Surfaces

Although LEEM is well suited for the study of reac-tions on surfaces with gases or impurities from the bulk,very little work has been done up to now. Segrega-tion of impurities from the bulk and the formation ofprecipitation products on the surface are routinely ob-served before the crystal has been cleaned completely.Surface carbide formation on W and Mo surfaces isan example. Images of such carbides and of carbidesformed by CO dissociation have been published [9.122,211, 232], but the precipitation process was never stud-ied in detail. As far as oxidation is concerned, onlythree systems have been studied: the initial oxidation ofW(100) [9.254, 255], the low-oxygen-coverage regionon Nb(100) [9.256], and the growth of oxide domainson NiAl(110) [9.257]. All three studies show interest-ing, unexpected phenomena.

More work has been done on surface reactionsin which the reaction products are desorbed, that is,in heterogeneous catalysis. The first studies lookedwith very limited resolution at reactions of CO withO2 [9.258, 259] and with NO [9.260], and at the reac-tion of NO and H2 [9.261], mainly at the propagationof the reaction front and its pinning by defects. OnPt(110), pattern formation during CO oxidation hasbeen studied [9.262, 263], and on Rh(110) the reactionof NO [9.253] and of O2 with H2 [9.264, 265]. Mostof this work has been done with low resolution, butthe possibilities for LEEM in this particular field areevident in some studies [9.253, 264, 265]. The contrastis due to the fact that surface regions with differentreactant composition have different structures, whichproduce characteristic LEED patterns. These can beused for dark-field imaging of these regions [9.253].Such dark-field images are shown in Fig. 9.35 for theNOCH2 reaction on Rh(110). The propagation of a spi-ral wave reaction is imaged with the diffraction spotsof pure N phases (Fig. 9.35a,b), a mixed NCO phase(Fig. 9.35c), and a pure O phase (Fig. 9.35d), which oc-cur during the oscillatory reaction. Because of the highbrightness, LEEM is preferable to PEEM because it al-lows one to follow the kinetics of the reaction wave


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propagation, and it is superior to MEM because of itsbetter resolution.

9.5.9 Oxides and Nitrides

The only LEEM work on oxides published up to nowis that on the TiO2(110) surface [9.266–269]. TiO2 isa particularly good example of the influence of the ex-change of defects between the bulk and the surface onits structure, as its stoichiometry can vary considerably.On the nonstoichiometric surface, a (1� 1)-to-(1� 2)phase transition occurs as a function of temperaturedue to vacancy exchange between the bulk and thesurface, which has been studied thoroughly [9.266–268]. When exposed to oxygen, the nonstoichiometricsurface grows via step flow at high temperatures, butvia two-dimensional nucleation at lower temperatures.With suitable growth conditions, the surface topogra-phy can be changed and the oxygen content of the bulkcan be increased [9.269, 270]. The only nitride studiedto date—silicon nitride and GaN, which were discussedin Sects. 9.5.5 and 9.5.6, respectively, excepted—is the(111) surface of TiN layers. These layers form moundswith spiral steps or stacks of two-dimensional islands.The mass transport at high temperatures can be stud-ied well by following the step motion and the diffusionprocesses, and constants can be derived from it [9.270,271].

The applications discussed in this subsection clearlyshow the many possibilities for LEEM in the study ofsurfaces and thin films of a wide variety of materi-als. Important information on the physical and chemicalproperties of surfaces and thin films has been extractedfrom these studies. These can be found in the referencescited. Diffraction contrast, step contrast, and quantumsize contrast, together with real-time capability, are theessential features that make LEEM so powerful in thesestudies. It should also be emphasized that LEEM isprimarily an imaging method for in situ studies. Sam-ple exchange is more time-consuming than in standardtransmission and scanning electron microscopy becausethe sample has to be transferred into ultrahigh vacuum,usually after cleaning in a preparation chamber, andthen aligned in the microscope. These steps are nec-essary because of the high surface sensitivity of themethod and because the sample is part of the objectivelens, so its surface must be exactly perpendicular to theoptical axis.

An important aspect of LEEM is that it can be eas-ily combined with other, in part complementary, surface

imaging techniques.MEM is useful whenever no strongdiffracted beam is available for imaging, for example,on fine-grained polycrystalline or amorphous samples.In this case, the sample potential is chosen such that theelectrons cannot penetrate the sample. Contrast is thendetermined by surface topography, surface potential,and work function differences. MEM has been used,for example, in the study of chemical reactions [9.258,259]. Ultraviolet light-excited photoemission electronmicroscopy (UVPEEM) is probably the most popularauxiliary imaging method in LEEM instruments. It isused when fields of view larger than those possible inLEEM have to be imaged or in samples in which it pro-duces better contrast than LEEM. A good example isthe study of the growth of pentacene films on oxidizedSi [9.272, 273]. The application range of UVPEEMis the same as that of MEM, but the resolution isgenerally much better. Synchrotron radiation-excitedXPEEM provides chemical information and chemicallyspecific magnetic information. It can be easily com-bined with LEEM, in particular in instruments equippedwith an energy filter. This is the SPELEEM mentionedin Sect. 9.2, whose application will be discussed inSect. 9.7 of this chapter. XPEEM is a subject of anotherchapter in the book.

Other imaging methods possible in LEEM in-struments include thermionic emission electron mi-croscopy (TEEM). TEEM is generally useful onlyfor samples with locally varying work functions andthat can be heated high enough for thermionic emis-sion. Metastable impact electron emission microscopy(MIEEM) is another imaging method with limitedapplication range. In this method [9.274, 275], de-excitation of metastable He� atoms at the surface causeselectron emission up to energies of 15�20 eV. Becauseof the chromatic aberration, resolutions of 100 nm orless can be achieved only with a band-pass energy fil-ter [9.276]. The main application for MIEEM is in thestudy of adsorbates, which consist of regions with dif-ferent de-excitation processes. In LEEM instrumentsthat allow higher energies in the illumination systemthan in the imaging system, SEEM and in particu-lar AEEM are possible. AEEM is useful for chemicalanalysis but inferior to XPEEM because of the largercharacteristic peak width and the larger background. Asin XPEEM, a band-pass energy filter is indispensablefor selection of a narrow energy band at the Auger elec-tron peaks. The energy filter is also useful for SEEM,whose application range is similar to that of UVPEEMand MEM.

PartA|9.6


9.6 Spin-Polarized LEEM (SPLEEM)

SPLEEM is a version of LEEM that requires a separatetreatment because it does not give structural but mag-netic information. It differs from LEEM only in that theillumination system produces a partially spin-polarizedelectron beam. The polarization is achieved by illu-minating a GaAs(100) surface with circular polarizedlight, with the wavelength corresponding to the bandgap of GaAs. The surface is activated by Cs and O2

exposure to negative electron affinity so that electronsthat have been excited to the bottom of the conduc-tion band can escape the surface. Optical selection rulesproduce a spin selection in the excitation process suchthat the spin of the emitted electrons points normal tothe surface, either inward or outward depending uponthe helicity (right or left) of the exciting light. Ordi-nary GaAs cathodes usually have a polarization of onlyabout 20�30%. Details of this kind of cathode canbe found in Pierce [9.277], a more recent analysis ofits properties [9.278]. The theoretical degree of polar-ization of this cathode is limited to 50% by the spindegeneracy of the valence band of GaAs. The degener-acy can be eliminated by strain, resulting in a theoreticaldegree of polarization of 100%. Polarization of 92%has been achieved with a strained GaAs/GaAsP su-perlattice on a GaAs substrate [9.279]. Replacing theGaAs substrate with GaP enables illumination of thecathode from the backside through a close-proximitylens, resulting in a very small emission area and bright-ness of more than 1�107 A cm�2 sr�1 at an extractionvoltage of 20 kV, higher than that of LaB6 electronsources by more than a factor of 10 [9.280]. Witha strain-compensated superlattice, the quantum effi-ciency can be further increased by nearly a factor of10 [9.281].

In the first generation of spin-polarized electronguns, the electron beam is deflected 90ı after extractionfrom the cathode by a combined electrostatic-magneticsector field. In pure electrostatic deflection, the direc-tion of the spin polarization P is unchanged, and in puremagnetic deflection P is deflected 90ı; if both fields areused for 90ı deflection, P can be rotated in any directionin the plane, indicated in Fig. 9.36 [9.282].

In the early SPLEEM studies [9.283–288], onlyelectrostatic deflection was available. Later, the mag-netic rotator lens indicated in the figure was added,which allows P to rotate in any direction inspace [9.282]. Usually, however, only three directionsare selected, one normal to the surface of the crystaland the other two in preferred directions in the surfaceplane (easy and hard magnetic axes). The spin manipu-lator shown in Fig. 9.36 is incorporated in the originalLEEM described in Sect. 9.2 (Fig. 9.9) and in a secondinstrument [9.51, 289]. The more recent SPLEEM in-

struments have a straight optical axis, and the rotationof the spin from along the optical axis to perpendicu-lar to it was initially achieved by a simple Wien filter.The spin rotation around the optical axis was achievedwith the magnetic lenses of the LEEM illumination op-tics [9.290]. Finally, with an eight-pole Wien filter, thespin can be rotated in any direction without using theillumination lenses [9.291].

The magnetic contrast is due to the fact that the180ı backscattering from magnetic materials dependsupon the relative orientation of the spin of the inci-dent electrons and of the electrons in the material, orin other words, upon the relative orientation of the po-larization P and the magnetization M. The intensity inthe image is then given by I D Istr C cP M, where Istris determined by the structure and topography of thesurface, c is a small proportionality constant, and thedot indicates the scalar product. Pure magnetic con-trast is obtained by subtracting two images taken withopposite P directions, pixel by pixel. Maximum con-trast obviously occurs for P k ˙M. For P ? M, thecontrast between magnetic domains with opposite mag-netization vanishes, and only the domain walls producecontrast. Maximum contrast requires imaging at verylow energies, typically a few electronvolts. At theseenergies the exchange splitting of the band structureand the difference between the inelastic mean free

y

z

x

z

x

P

P

P

Fig. 9.36 Schematic of the spin manipulator. Af-ter [9.282]

LEEM, SPLEEM and SPELEEM 9.6 Spin-Polarized LEEM (SPLEEM) 513Part

A|9.6

paths of the spin-up and spin-down electrons, whichwere mentioned in Sect. 9.1, cause the strongest dif-ference between the backscattering for the two spindirections. Because the magnetic signal is only a smallfraction of the total signal, the signal-to-noise ratio inthe difference image is small and frequently limits theresolution [9.292]. The addition of two images with op-posite P directions produces only structural contrast,which makes SPLEEM ideal for the correlation be-tween magnetism and structure. More information onmagnetic contrast formation may be found in severalSPLEEM reviews [9.39, 142, 293–296].

In most SPLEEM studies, the ferromagnetic sam-ples are prepared in situ while observing the magneticstructure together with the crystal structure and to-pography via several contrast mechanisms. However,ex situ-prepared samples can also be studied afterproper surface cleaning, as illustrated in Fig. 9.37 fora Co(0001) surface that had been sputter-cleaned andannealed [9.284]. Ex situ-prepared samples can also bestudied when passivated with a thin layer that is suf-ficiently transparent for slow electrons such as noblemetals [9.297]. In this method, it must be taken intoaccount that such overlayers can change the magnetiza-tion in the film below [9.298, 299]. The in situ studiescover single ferromagnetic layers, ferromagnetic layerscovered with nonmagnetic overlayers, ferromagnetic–nonmagnetic–ferromagnetic sandwiches, and small fer-romagnetic crystals.

Under certain conditions, large regions of a thinFe film can be grown with constant thickness andatomically flat surfaces. These show pronounced quan-tum size effects (Fig. 9.26) [9.87]. The intensity re-flected from regions with different thickness showspronounced spin-dependent quantum size oscillationsas a function of energy (Fig. 9.27) [9.88], from whichthe exchange-split band structure above the vacuumlevel [9.88, 300] and the spin dependence of the inelas-tic mean free path [9.301] can be derived.

The early SPLEEM work concentrated on Co filmson W(110) without [9.283–285] and with [9.286]

Fig. 9.37SPLEEM im-age of the closuredomains ona Co(0001) sur-face. Electronenergy 2 eV.From [9.284],reproduced withpermission

nonmagnetic overlayers, on Co on Au films onW(110) [9.287], and on initial studies of Co=Cu=Cosandwiches [9.288]. Some of the more interesting re-sults were the large difference in the damping of themagnetic signal by Cu and Pd overlayers due to thedifferent inelastic mean free paths in these materials,and the quantum size effect in the magnetic signal withincreasing Cu overlayer thickness [9.286]. This phe-nomenon was later studied in considerable detail in Cufilms on fcc Co(100) on Cu(100) [9.90] and in MgOfilms on Fe(100) on MgO(100) [9.302].

The introduction of the spin manipulator finally en-abled measurement of all three M components. First, itwas found that Co films on W(110) have up to about10 monolayers and interesting wrinkled magnetization,with M tilting with increasing thickness in an apparentoscillatory manner toward in-plane [9.303]. In contrast,the out-of-plane to in-plane spin-reorientation transi-tion (SRT) in Co layers on Au(111) layers on W(110)was found to occur in a completely different man-ner within a small thickness range [9.304]. Subsequentwork revealed that in addition to the effects of the filmand substrate material (e. g., Co on Ru(0001) [9.305]),other parameters such as substrate step structure (e. g.,Ni on Cu(100) [9.306, 307]), noble metal overlayers(e. g., on Co on Ru(0001) [9.308]), hydrogen adsorp-tion (e. g., on Co on Ru(0001) [9.309]), underlayers(Fe on one [9.310] and two [9.311] Cu monolayers onW(110)), and growth conditions (deposition rate, resid-ual gas pressure, e. g., in Fe on Cu(100) [9.312, 313])exerted a strong influence on this transition with in-creasing thickness. For the latter system, the usefulnessof SPLEEM for local hysteresis curve measurementswas also demonstrated [9.314]. A good example ofthe power of SPLEEM is the study of the SRT of Fe-Co alloy layers on Au(111) layers on W(110) [9.315,316]. As shown in Fig. 9.38, the magnetic contrastis already quite strong at 1:22 monolayers, increasesonly slightly with thickness, and then decreases whenthe film approaches the spin reorientation transitionat about 2:7 monolayers. During the approach of thetransition, a pronounced striped phase develops andthe magnetization tilts increasingly, but abruptly con-verts into large, predominantly in-plane magnetizeddomains. The surface has large step bunches pointingtoward and away from the vapor source, which causesthe transition to occur earlier or later. In lateral averag-ing studies, this would smear out the transition so that itwould appear more continuous. Many other details canbe extracted from the SPLEEM images, which indicatea rather complex transition.

Ordered substrate surface alloys have a significantinfluence on the evolution of magnetization with filmthickness, as illustrated for Fe on noble metal surface

PartA|9.6


a)

b)

Fig. 9.38a,b The spin-reorientation transition ina Fe-Co alloy layer onAu(111). (a) Shows theout-of-plane componentof the magnetizationand (b) the in-planecomponent. Electronenergy 2:5 eV anddiameter of field of view10 m. Reprinted withpermission from [9.316].John Wiley & Sons

alloys on W(100) [9.317]. The substrate orientation,however, has a much stronger influence on this evo-lution due to the strong dependence of the magneticstructure on the microstructure of the film, which is de-termined by the misfit and symmetry of the interface.This is evident from a comparison of Co [9.318, 319]and Fe [9.88, 300, 320, 321] films grown on W(110),W(100), and W(111). In contrast to the predominantlylateral growth on W(110) and W(111), Co grows onW(100) on top of a nonmagnetic double layer in three-dimensional needle-shaped crystals bounded by facetsso that no electrons are reflected along the opticalaxis in the energy range in which the magnetic sig-nal is strong [9.318]. Only when the film is continuouscan a weak magnetic signal be seen. Fe grows on allthree surfaces initially two-dimensionally, but the filmexperiences considerable structural changes thereafter,which cause major changes in the magnetic structure.For example, on W(111), magnetization disappears atroom temperature in one-monolayer intervals at the endof the two-dimensional growth [9.320], and on W(100),a complex SRT sets in at this stage, breaking up theoriginal large single domains [9.321].

SPLEEM is also very well suited for the studyof exchange coupling. Coupling through nonferromag-netic metal layers is mediated by quantum size oscil-lations, which cause ferromagnetic, antiferromagnetic,

or biquadratic coupling between the layers, dependingupon interlayer thickness and interface roughness, asdemonstrated for Co=Au=Co [9.322] and Co=Cu=Cotrilayers [9.323]. The strong local coupling at an anti-ferromagnetic interlayer, however, imposes the domainstructure of the interlayer on the ferromagnetic layer, asillustrated by the system Fe=NiO=Fe [9.324].

Perpendicular magnetization in ferromagnetic su-perlattices is of great interest for spin-current devices.To understand its evolution with increasing number nof periods, SPLEEM was used in a study of a pro-totype system, .Ni2Co/n grown on W(110) [9.325–327]. The fast image acquisition—compared with othermagnetic imaging methods—allowed quasicontinuousimaging of in-plane and out-of-plane magnetizationduring growth. Figure 9.39 shows selected images fromsuch a growth sequence. The magnetization is initiallyin-plane, and turns with increasing n, oscillating out-of-plane due to the increasing number of Ni=Co=Ni inter-faces, staying nearly completely out-of-plane after n D4, with some in-plane magnetization left only at stepbunches. Perpendicular magnetization up to large filmthickness has also been obtained by intercalating Co un-der graphene on Ir(111), thanks to high Co=grapheneinterface anisotropy [9.328, 329]. The magnetizationis completely out-of-plane up to 13 monolayers, incontrast to six monolayers without graphene cover.

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a b c d e f g h i

Fig. 9.39 The evolution of the perpendicular magnetization in a Ni2Co multilayer with increasing number of layers.The bottom row show the in-plane images, the center row the out-of-plane images, and the top row enlarged regions ofthe in-plane images, marked by the frames in the bottom row and contrast-enhanced to reveal the domain boundaries.A LEEM image of the surface covered with one Ni2Co pair shows the step (bunches) on the surface. From [9.325]

The transition to in-plane magnetization occurs overa wide thickness range of between 13 and four mono-layers, with a complex, wavy three-dimensional spindistribution. This system shows another interesting ef-fect: a breakdown of the spin asymmetry above a filmthickness-independent energy, causing the loss of mag-netic contrast [9.330].

While most of the SPLEEM work been done onmagnetic domains, domain walls were studied onlybriefly in the early years [9.331], until domain wall de-vices made domain walls an important subject. Systemswith chiral domain walls enable fast wall motion withlow-threshold currents. The Dzyaloshinskii–Moriya in-teraction, which occurs at interfaces with broken in-version symmetry, allows control of the chirality. Inthin films, interfaces can be controlled easily and stud-ied in situ with SPLEEM. Studies of several interfacesystems, Fe=Ni bilayers on Cu(100) [9.332], .Co=Ni/nmultilayers on Pt(111) [9.333] and Ir [9.334], anda Fe/Ni bilayer onW(110) [9.335] have brought consid-erable insight into interface engineering of monochiraldomain walls. An overview of this work, which has ledto the design of a skyrmion phase [9.336], can be foundin [9.337].

The systems discussed thus far were all later-ally unconfined. In confined systems, an additionalanisotropy, the shape anisotropy, plays an increasingrole [9.338–340]. This is illustrated in Fig. 9.40 forFe crystals on W(110). Confinement in one directionleads to either closure domain Fig. 9.40a,b [9.338] or

single domain Fig. 9.40c,d [9.339] formation depend-ing upon the width of the long crystals, and a similarbehaviour is seen in small crystals depending upon lat-eral extension. An example of sub-100nm-sized crys-tals is shown in the images of a Fe film on W(001)Fig. 9.40e,g [9.321].

The crystals have formed clusters during thebreakup of a highly strained Fe film, with a moreor less random magnetization distribution within theclusters. In contrast, the magnetization M is preferen-tially aligned along the h110i directions in the three-monolayer-thick regions shown in Fig. 9.40f, whilein the four-monolayer-thick regions, M is preciselyaligned along the h100i directions. In spatially well-separated Co on Ru(0001) nanocrystals, in addition tothe single-domain states, vortex domains occur, vary-ing with thickness and height, which was presented ina corresponding phase diagram [9.341].

The phase diagram of ultrathin ferromagnetic filmshas been studied extensively with lateral-averagingmethods, which give mean values over regions varyingin magnetic properties, in particular of the critical be-havior. This problem can be overcome with SPLEEM,thanks to its high lateral resolution. An example isthe study of a Fe monolayer on two monolayers ofAu on W(110) with a locally strongly varying stepdensity. Finite size effects caused the ferromagnetic-to-paramagnetic transition in regions with small terracewidth at a significantly lower temperature than on wideterraces. Averaging over all terraces led to an incor-

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a)

b)

c)

e) f) g)

d)

Fig. 9.40a–g Size dependence of the mag-netic domain structure. (a–d) One-dimensionalFe crystals on W(110) forming a closuredomain structure above 1 m width (a,b)(reprinted with permission from [9.338], JohnWiley & Sons), and single, shape anisotropy-determined domains below 1 m width (c,d)(reprinted from [9.339], with permission fromElsevier). The crystals are aligned along theW[001] direction, the polarization along (a,c)and perpendicular to the length of the crystals.(e–f) Fe nanocrystals formed during growth onW(001) at 600K. (e) LEEM image, showingthree- and four-monolayer-thick regions inaddition to nanocrystal clusters on top of thewetting layer; (f,g) show the angular magne-tization distribution in the three-monolayerregions (preferentially along h110i) and inthe nanocrystal clusters (nearly random) inthe color wheel code. Because of the lowenergy (0:5 eV), the clusters are not resolvedin LEEM. In SPLEEM, the magnetic contrastallows partial resolution. (e–g) reprinted withpermission from [9.321]. Copyright 2017 bythe American Physical Society

rect Curie temperature and critical exponents, whileselecting only the largest terrace gave the correct valueexpected for the 2-D Ising model [9.342].

SPLEEM is not limited to the study of ferromag-netic materials, but can be used equally successfullyfor ferrimagnetic materials. Magnetite is a particularlyinteresting ferrimagnetic compound because of its tran-sition from the cubic high-temperature phase to a mon-oclinic low-temperature phase (Verwey transition).A study of the (100) surface with SPLEEM/LEEMfound that the surface develops a roof-like distortion

during the phase transition but maintains its surfacereconstruction [9.343]. Contrary to the easy h111i mag-netization directions in the bulk, the magnetization inthe surface is in-plane along the h011i direction and hasa complicated domain structure [9.344].

Concluding this section, the strengths of SPLEEMshould be emphasized once more: correlation withthe microstructure, fast image acquisition comparedto other magnetic imaging methods, high surface sen-sitivity, and easy access to all three magnetizationcomponents.

9.7 SPELEEM

A LEEM instrument that is equipped with an energyfilter enables real-space and reciprocal-space imagingwith emitted electrons, in particular photoelectrons, andconverts the instrument into a spectroscopic photoemis-sion and low-energy electron microscope (SPELEEM),as mentioned already in Sects. 9.2 and 9.3 [9.15, 45, 46,48]. The increasing availability of SPELEEM instru-ments at synchrotron radiation sources has led to nu-merous studies combining LEEM or LEED with x-rayphotoemission spectroscopy (XPS) and microscopy(XPEEM), and also in particular angle-resolved spec-

troscopy (ARXPS), which is frequently called k-spaceimaging. This section discusses work in which real-and/or reciprocal-space imaging with reflected andenergy-selected emitted electrons is combined. Purephotoemission microscopy and spectroscopy studies,along with their theoretical background, are discussedin Chap. 10.

Figure 9.41 [9.345] shows the three fundamentaloperation modes of a SPELEEM: imaging, diffraction,and spectroscopy—or real-space, reciprocal-space, andenergy-space imaging. While energy filtering in LEEM

http://dx.doi.org/10.1007/978-3-030-00069-1_10

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is necessary only if at the same time strong emission oc-curs, such as thermionic emission at high temperatures,in LEED it is useful for eliminating inelastically scat-tered and secondary electrons from the background. Ofcourse, in the emission modes of operation, energy fil-tering is always needed in order to select the electronswith the desired energy. Before concentrating on thecommonly used photoemission modes, which requirean external excitation source, Auger electron emissionmicroscopy (AEEM) using the LEEM electron gunshould be briefly discussed. This was the only methodfor spectroscopic imaging before bright synchrotron ra-diation light became available, and soon displaced it.The desire for chemical imaging to complement thestructural imaging with LEEM in a laboratory envi-ronment may see a resurrection of this method. Thepossibilities and limitations are illustrated in an earlyexperiment shown in Fig. 9.42.

The short image acquisition time is due to the highelectron impact ionization cross sections: at 3�5 timesthe energy of the inner shell involved in the Auger tran-sition (here M4;5), this cross section is comparable tothat near the maximum of the x-ray photo ionizationcross sections, making AEEM intensities comparable toXPEEM intensities. This is particularly true for shellswith low inner-shell ionization energies. For example,for the Ag M4;5 levels, it is nearly 100%. However,there are two drawbacks: (i) the characteristic peaksare usually much wider because two valence bands (V)are involved, and (ii) the background is much higher.Therefore, XPEEM is preferable whenever available.

r

a)

k

b) c)

Fig. 9.41a–c Schematic of the SPELEEM operation modes. Several lenses, except the objective lens, and the field-limiting andangle-limiting apertures are switched between the different modes. After [9.345]

Fig. 9.42 Auger electron emission image of a thin Agmicrocrystal on a contaminated Si(111) surface. Imageswere taken in 2 eV steps with 1 eV resolution aroundthe M4;5 VV transitions, requiring 15 s/image. The spec-tra were obtained by integration of 1:5 m2 regions.From [9.45]

In the laboratory, in addition to spectroscopic imag-ing, AEEM enables the study of noncrystalline or roughsurfaces, in which LEEM fails. All that is needed toconvert a LEEM instrument into an AEEM instrumentis an auxiliary high-voltage supply so that the illumi-

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nating beam can be put at a sufficiently higher energythan the imaging beam and a split beam separator witha higher deflection field on the illumination site.

In combination with photoemission methods,LEEM and LEED frequently serve only to determinethe quality of the film surface, the film thickness, andthe crystal structure. In other studies, photoemissionmethods play the supporting role, as will be illustratedby a number of studies in the following. Some re-views cover additional examples [9.142, 345–347]. Inone combination of LEEM and photoemission meth-ods, XASPEEM (x-ray absorption PEEM), the energyfilter is not needed, only a monochromatic photonbeam, because contrast stems from energy-dependentabsorption of the photon beam. Its most importantapplication is in x-ray magnetic circular dichroismPEEM (XMCDPEEM). The combination with LEEMhas been very fruitful in enabling a better understand-ing of ferromagnetic systems such as MnAs films onGaAs(100) [9.348, 349], as such understanding, bothof the LEEM images [9.350] and of the XMCDPEEMimages [9.351, 352], has been challenging. Figure 9.43shows XMLDPEEM (x-ray magnetic linear dichroismPEEM) Fig. 9.43a and an XMCDPEEM image 9.43b ofa MnAs film.

In antiferromagnetic samples, magnetic informationcan be obtained without SPLEEM or x-ray magneticdichroism PEEM if they show antiferromagnetism-caused superstructure spots in the LEED pattern. Usingthese spots in LEEM, this was demonstrated con-vincingly for NiO by comparison with XMLDPEEMimages (AFM-LEEM) [9.353]. In general, LEEM andLEED are used in magnetic studies mainly for the de-termination of the crystal structure and the particle sizeand shape. For example, for the correlation of the mag-netization on the surface of three-dimensional Fe crys-tals on Mo(110) with that in the bulk, it was necessary

a) b)

Fig. 9.43a,b XMLDPEEM and XMCDPEEM image ofa 300 nm-thick MnAs film on GaAs(100) at room tempera-ture at which paramagnetic (gray in (b)) and ferromagneticphases (black/white in (b)) coexist with a complicated do-main structure. Reprinted with permission from [9.349].Copyright 2007, American Vacuum Society

to precisely determine the shape of the crystals [9.354].In an XMCDPEEM study of the magnetization distri-bution in three-monolayer-thick triangular Co crystalson Ru(0001) as a function of crystal size, LEEM wasessential for the preparation and structural character-ization of the crystals [9.355]. In the XMCDPEEMstudy of the magnetization pattern and Curie tempera-ture of nanometer-thick magnetite crystals, LEED alsohad to be used for the determination of the crystal struc-ture [9.356].

The combination of LEEM and LEED withXPEEM and XPS is particularly important when struc-tural changes cause chemical changes and vice versa.The changes may be caused by heating, electron or pho-ton irradiation, or chemical reactions. While irradiationeffects are generally disturbing, they can also be usefulfor surface modification, as was done, for example, inthe case of the TiO2(110) surface. Here, MEM, LEED,and XPS were combined in a study of one-dimensional(1-D) Au crystals on the irradiation-modified sur-face [9.357]. Another application of irradiation isAg(111) surface patterning by irradiation-assisted ox-idation [9.358]. Examples of studies of temperature-induced changes include the metal–insulator transi-tion in epitaxial VO2 films on TiO2(110) [9.359, 360],the phase transformations in thin iron oxide films onPt(111) and Ag(111) surfaces [9.361], or the cleaningof ZnO powders, in which reactive growth was studiedwith LEEM, XPEEM, and XPS [9.362].

Reactive growth is a field in which SPELEEM is in-dispensable. A prime example is the growth of Fe onZnS(100) [9.363, 364], in which several reaction prod-ucts are formed. One of these, which in earlier transmis-sion electron microscopy/diffraction studies had beeninterpreted as Fe, required LEEM, LEED, XPSPEEM,and XMCDPEEM for reliable identification as Fe3S4.This was achieved by taking many XPEEM imagesaround the Fe 3p and S 2p photoelectron energies andmeasuring the intensities in windows set on the crys-tals of interest, which produced the spectra shown inFig. 9.44. XMCDPEEMwas used to check themagneticstate and to determine a lower limit of the Curie temper-ature. Quantitative evaluation was limited mainly by theaberrations of the objective lens, which produced con-tributions from the surroundings of very small crystalsto their signal, and by the fact that the Fe3S4 crystalswere faceted and had an unknown angular distributionof the emission. Similar problems must be expected inthe analysis of small or faceted crystals formed in otherreactive growth systems, for example, in the study of thegrowth of Co germanide crystals on Ge(100), in whichthe chemical state of the crystals was determined by se-lected area XASPEEM [9.365]. For larger crystals withflat surfaces and well-known structure, these problems


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c)

a) b)

d)

Fig. 9.44a–d LEEM image takenwith 3 eV electrons (a) and windowsset in the XPSPEEM image (b) fromwhich the XPS spectra (c,d) wereobtained. A is a Fe crystal witha chemisorbed S layer, B is greigite.It requires two components forfitting (surface and bulk) in the S2p spectrum (c) and a completelydifferent majority-to-minority spinratio for fitting the Fe 3p spectrum (d),indicating a completely differentelectronic structure. Reproducedfrom [9.363], with permission fromthe Royal Society of Chemistry

do not occur, for example, in the study of the oxidationpathways of iron oxides on Ru(0001) [9.366]. In anotherSPELEEM study,MEM andLEEDwere combinedwithnear-edge x-ray absorption fine structure (NEXAFS) todetermine the influence of crystal orientation and grainboundaries on the oxidation of polycrystalline Ni. Sur-prisingly, at higher temperature (673K), no oxidationoccurred around the grain boundaries and on the grains.NiO crystals in the 1 m range formed on the otherwiseclean surface [9.367].

SPELEEM has been used in a variety of otherstudies of oxides. MEM and XPSPEEM were com-bined with AFM to investigate the contact poten-tial difference between SrO and TiO2 on the phase-separated SrTiO3(100) surface [9.368]. Another phe-nomenon, which could not have been demonstratedwithout SPELEEM, is the quantum size effect (QSE)in the oxidation of thin-metal films. This was done

with Mg [9.369] and Al [9.370] films on W(110) byusing the QSE contrast in LEEM for local thicknessdetermination and the O 2p contrast in XPSPEEM formeasuring the local oxide coverage. The most exten-sively studied oxide, however, is CeO2, which plays animportant role in catalysis as catalyst support material.It is very radiation-sensitive, and so the emphasis is gen-erally on LEEM and LEED, while XPEEM and XPSare used only briefly to determine the oxidation state.Examples include studies on the growth and structureof (111) [9.371] and (100) [9.372] oriented CeO2 crys-tals on Ru(0001), the reduction process from CeO2 toCo2O3 on Ru(0001) [9.373], and the growth and struc-ture of another oxide, Pr2O3, on Ru(0001) [9.374].

In other work on ceria on metal substrates, both re-flection and emission imaging and spectroscopy wereused, with more emphasis on the latter. The first ofthese studies [9.375] explored the possibilities for imag-

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ing the oxidation state of ceria islands on a Re(0001)substrate by using resonant Ce3C (4f1) and Ce4C(4f0)valence band XPS as well as Re 4f and O 1s, the latterto determine the oxidation state of the substrate. Res-onance XPS makes use of the chemical shift betweenthe two oxidation states. When the photon energy isslightly above the lower binding energy, emission fromonly this state will occur, and if it is above the higherbinding energy, electrons from both states are emitted.Proper spectrum subtraction provides high sensitivity inthe distinction between different binding states, simul-taneously minimizing radiation damage, as illustratedconvincingly in this study. A second study made useof this method in an investigation of the growth, mor-phology, and oxidation state of thin ceria crystals onRh(111) and of Au nanoparticles on these crystals asa model system for the water-gas reaction [9.376]. Inthe most detailed work, the reoxidation of thin CeO2

nanoislands on Rh(111) after reduction by photon irra-diation was investigated [9.377]. Figure 9.45 illustratesthe experimental procedure. The images on top wererecorded during the slow decrease of the oxygen pres-sure starting from nearly completely oxidized CeO2

at the energies characteristic for Ce4C(red) and Ce3C(blue) at 3:5 and 1:5 eV binding energy in the valenceband spectrum, shown at the bottom right. With de-creasing loss of oxygen due to irradiation, conversioninto Ce2O3 occurs, and at the lowest pressure, partialreduction to metallic Ce0, as seen in the intensities plot-ted below the images. At the bottom, enlarged sectionsof the images shown on the top are shown at severalstates of the reduction process, together with a dark-field LEEM image. Reoxidation was found to occur viaspillover from oxygen adsorbed on the Rh surface.

SPELEEM has also been used successfully for elu-cidating surface processes that occur at metal surfacesduring catalysis, albeit at low pressures. The waterformation reaction H2 CO2 in the presence of sub-monolayer adsorbed metals on Rh(110) has been thesubject of several studies. It was found that the reactioninduces a lateral distribution of the metal into metal-rich and metal-poor regions with well-defined reactionfronts [9.265, 378]. With co-adsorbed Au and Pd, well-defined stripe phases were formed at coverages between0:3 and 0:7 monolayers under certain reaction condi-tions (temperature, gas pressures) [9.379, 380]. Similaradsorbate redistribution processes were found with ad-sorbed VO2 on Rh(111) [9.381]. In a study of chemicalwaves and rate oscillations in the same reaction onRh(111) covered with a NiRh alloy, XPEEM and XPSwere also used for local composition analysis [9.382].Another interesting application is the monitoring ofthe local coverage of adsorbates during chemical wavepropagation using the XPS signals of the adsorbates

involved. This was done in the NOCH2 reaction onRh(110) with low potassium coverages. The lateralchemical resolution of XPEEM gave detailed insightinto the propagation process, for example, showing thatK accumulated in front of the N front [9.383]. A finalexample is the location of chemically active oxygen onAg(111). The chemical shift of the O 1s peak betweenthe oxygen-reconstructed terraces and step bunches en-abled imaging of the location of an oxygen specieswith higher binding energy at step bunches, which wereidentified as the chemically active sites [9.384].

Probably the most extensive use of SPELEEM hasbeen in the study of graphene on metals and SiC, par-ticularly the intercalation of metals and gases betweengraphene and the substrate, driven by the many pos-sible applications of graphene. Of the more than 50studies using SPELEEM published up to 2016, onlya few can be cited here as examples of the method.More information can be found in [9.142, Chap. 6.1].Most of the work has been on SiC—the intercalationstudies nearly inclusively—but some interesting studiesalso on metal surfaces. On SiC, graphene is generallyformed by sublimation of Si in situ in UHV or, bet-ter, in inert gases at high pressure, which enables bettercontrol of the sublimation process. On metals, decom-position of a hydrocarbon gas or segregation of C fromC-doped crystals is used. Depending on the goal of thestudies, either all or a few selected SPELEEM meth-ods are used. These include LEEM for determiningthe topography, selected-area LEED for local structureanalysis, I00.V/ for the measurement of the number ofgraphite layers, XPS of the elements involved (sub-strate, graphene, adsorbate, intercalant) for the determi-nation of their bonding states, XPEEM for imaging ofthe lateral distribution of these elements, XPEEM at theK and � points in reciprocal space (where graphene hasa high and zero density of states, respectively), and k-space (kx, ky) imaging at constant energy E and of E.k/in a plane through the K point for the analysis of theDirac cone characteristic for graphene.

Figure 9.46 [9.385] shows an example of some ofthese methods. The graphene flake, which had beengrown by decomposition of ethylene at high tempera-ture, developed upon cooling to room temperature thestriations seen in the LEEM and XPEEM images. TheXPEEM image at the K point shows no intensity inthe stripes, suggesting no graphene. However, there isintensity in the � point image, which is attributed toregions with contact to the substrate, resulting in con-tributions of the substrate density of states. Most ofthe flake, however, is decoupled from the substrate,as the intensity in the K point image and the k-spacedata in the center shows. Thus the graphene layer isrippled and attached to the substrate only along peri-


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p

p

Fig. 9.45 Resonant Ce 4d–4f spectroscopy of the reduction of CeO2(111) islands on Rh(111) by photon irradiation.Reprinted with permission from [9.377]. Copyright 2016 American Chemical Society

odic lines, as shown in the LEED pattern. The periodis below the resolution of the images, which showonly regions with more extended contact with the sub-strate. In other studies, particularly when more thanone graphene monolayer are involved, I00.V/, XPS,and E.k/ cuts play a major role in the determina-tion of the thickness, bonding, and band structure asa function of thickness. This is illustrated, for exam-ple, in [9.386], which summarizes some of the workup to 2011. A study of the electronic band struc-ture of a graphene trilayer on SiC uses LEEM tolocate regions with the desired thickness, identifiedby I00.V/ curves, to image the Dirac cone E.k/ viaARPES [9.387], as a function of thickness. SPELEEM

was also very useful for more device-oriented stud-ies. One example is the study of nanoribbons grownon sidewalls of trenches, which are of interest be-cause of the high electron mobility in them [9.388].Another is the fabrication of semiconducting grapheneribbons on nitrogen-doped SiC [9.389]. In both stud-ies, nearly all methods mentioned above were used forcharacterization.

A fascinating application of SPELEEM is the studyof intercalation between substrate and graphene. As al-ready mentioned in Sect. 9.6, many Co monolayerscan be intercalated between graphene and an Ir sub-strate [9.328, 329]; however, in most cases only one isused. The first was H2, which was used to break the

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a)

b)

c)

d)

e)

f) E

E

k

Fig. 9.46a–f Graphenemonolayer island on anIr(100) surface. (a) LEEMimage taken with thediffraction spots markedin the LEED pattern(b) XPEEM images takenat the (e) K and (f) � pointnear the Fermi energy EF,respectively, (c) k-spacecut near Fermi level,(d) E.k/ in-plane throughK point. ED is the energyof the Dirac point (the tipof the Dirac cone shownin (d), which is a measureof the doping of thegraphene. Reprinted withpermission from [9.385].Copyright 2013 AmericanChemical Society

strong bond between the first C layer on SiC, transform-ing it into a graphene monolayer. Many other atoms andcompounds followed: Li, Na, Cs, Si, Ge, Al, Cu, Au, Pt,Bi, AlBr, NiC. Only a few recent studies, in which ref-erences to older work can be found, can be mentionedhere. Na [9.390], Al [9.391], and Cu [9.392] were in-tercalated between graphene and SiC by deposition andannealing at metal-dependent temperatures. XPS com-bined with LEEM and LEED is essential in this methodfor the distinction between adsorbed and intercalatedmaterial. Intercalation of Na was enhanced by soft x-rayirradiation. Intercalated Al was found to be stable upto high temperatures and to form ordered Al, Si, andAlSi phases as identified by LEED and XPS. Interca-lated Cu also produced a superlattice and resulted ina significant modification of the electronic structure ofgraphene, as seen in the formation of mini-Dirac cones.Another intercalation method is segregation from thesubstrate. This was demonstrated with graphene on C-doped Ni on which annealing produced a Ni carbideintercalation layer [9.393]. Segregation and dissolutionin the substrate are reversible, which allows switchingbetween different coupling strengths between grapheneand the substrate. Still another method for intercalationis by ion implantation, which can be used if other meth-ods fail, as demonstrated for Bi intercalation [9.394].Ion bombardment does not produce a two-dimensionalintercalation layer if the atoms have only weak interac-tions with the substrate, such as noble gas atoms. In thiscase, the intercalated atoms form nanobubbles whichcan be stable under GPa pressures. Ar 2p XPS shows

that their size increases with increasing temperature,from small clusters at room temperature, and becomingvisible in LEEM and XPEEM after annealing to veryhigh temperatures (> 1000 ıC) [9.395]. This exampleillustrates that gases can be kept at high pressures un-der graphene, which makes graphene-covered surfacesideal systems for gas reactions at high pressures. Ex-periments along this line have already been performedin separate LEEM-UVPEEM and XPS systems, but notyet in SPELEEM instruments.

A final example of the importance of combiningLEEM and XPEEM is the study of the phase separationand coexistence in a two-dimensional FeNi alloy mono-layer on a W(110) surface. LEEM shows a complexpattern on the 100 nm scale. Without chemical analysiswith high-resolution XPEEM, it would be impossible tounderstand this phase separation process [9.396].

Concluding this section, the examples discussedhere clearly show the power of SPELEEM in many ap-plications. By proper combination of the various meth-ods available in SPELEEM instruments at synchrotronradiation sources, many problems can be much bettersolved than by studying them in separate instruments.With the continued development of high-brightness lab-oratory x-ray sources and the use of AEEM, SPELEEMstudies should become feasible in the laboratory aswell.

Acknowledgments. The author thanks AnastassiaPavlovska for preparing the figures, references, permis-sions and for the editorial work of this chapter.

LEEM, SPLEEM and SPELEEM References 523Part

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References

9.1 C.J. Davisson, L.H. Germer: Diffraction of electronsby a crystal of nickel, Phys. Rev. 30, 705–740 (1927)

9.2 E. Bauer: Low energy electron reflection mi-croscopy. In: Proc. 5th Int. Congr. Electron Microsc.,Vol. 1, ed. by S.S. Breese (Academic Press, New York1962) pp. 11–12

9.3 W. Telieps, E. Bauer: An analytical reflection andemission UHV surface electron microscope, Ultra-microscopy 17, 57–66 (1985)

9.4 E. Brüche, W. Knecht: Bemerkung über die Errei-chung hoher Auflösungen mit dem elektronenop-tischen Immersionsobjektiv, Z. Phys. 92, 462–466(1934)

9.5 H. Mahl: Über das elektrostatische Elektronen-mikroskop hoher Auflösung, Z. Tech. Phys. 20, 316(1939)

9.6 H.E. Farnsworth: Electron emission and diffractionby a copper crystal, Phys. Rev. 34, 679 (1929)

9.7 H.E. Farnsworth: A simple contamination-freeelectron gun, Rev. Sci. Instrum. 21, 102 (1950)

9.8 R.E. Schlier, H.E. Farnsworth: Low-energy electrondiffraction investigation of chemisorbed gases onthe (100) faces of copper and nickel single crystals,J. Appl. Phys. 25, 1333 (1954)

9.9 E.J. Scheibner, L.H. Germer, C.D. Hartman: Appa-ratus for direct observation of low-energy electrondiffraction patterns, Rev. Sci. Instrum. 31, 112 (1960)

9.10 L.H. Germer, C.D. Hartman: Improved low energyelectron diffraction apparatus, Rev. Sci. Instrum. 31,784 (1960)

9.11 A. Recknagel: Theorie des elektrischen Elektronen-mikroskops für Selbststrahler, Z. Phys. 117, 689–708(1941)

9.12 E. Bauer: Optical properties of the uniform electricfield, J. Appl. Phys. 35, 3079 (1964)

9.13 D.R. Cruise: A numerical method for the deter-mination of an electric field about a complicatedboundary, J. Appl. Phys. 34, 3477–3479 (1964)

9.14 E. Bauer: Surface electron microscopy: The firstthirty years, Surf. Sci. 299/300, 102–115 (1994)

9.15 T. Schmidt, S. Heun, J. Slesak, J. Diaz, K.C. Prince,G. Lilienkamp, E. Bauer: SPELEEM: Combining LEEMand spectroscopic imaging, Surf. Rev. Lett. 5, 1287–1296 (1998)

9.16 E. Bauer, T. Schmidt: Multi-method high resolutionsurface analysis with slow electrons. In: High Res-olution Imaging and Spectroscopy of Materials, ed.by F. Ernst, M. Ruehle (Springer, Berlin, Heidelberg2003) pp. 363–390

9.17 R. Wichtendahl, R. Fink, H. Kuhlenbeck, D. Preik-szas, H. Rose, R. Spehr, P. Hartel, W. En-gel, R. Schlogl, H.J. Freund, A.M. Bradshaw,G. Lilienkamp, T. Schmidt, E. Bauer, G. Ben-ner, E. Umbach: SMART: An aberration-correctedXPEEM/LEEM with energy filter, Surf. Rev. Lett. 5,1249–1256 (1998)

9.18 E. Bauer: Interaction of slow electrons with sur-faces, J. Vac. Sci. Technol. 7, 3–12 (1970)

9.19 E. Bauer: Interaction of slow electrons withrandium-jellium. In: Les interactions des électronsavec la matière condensée. Applications a I’etudedu solide (AVCP, Verbier 1972) pp. 42–96

9.20 T.-Y. Wu, T. Ohmura: Quantum Theory of Scattering(Prentice, Englewood Cliffs 1962)

9.21 H.-J. Herlt: Elastische Rückstreuung sehrlangsamer Elektronen an reinen und an gas-bedeckten Wolfram-Einkristalloberflächen,Ph.D. Thesis (TU Clausthal, Clausthal-Zellerfeld1982)

9.22 J.B. Pendry: Low Energy Electron Diffraction (Aca-demic Press, London 1974)

9.23 M.A. Van Hove, W.H. Weinberg, C.-M. Chen: Low-Energy Electron Diffraction (Springer, Berlin 1986)

9.24 F. Seitz: The Modern Theory of Solids (McGraw-Hill,New York 1940) p. 313

9.25 P. Feibelman, D.E. Eastman: Photoemission spec-troscopy: Correspondence between quantum the-ory and experimental phenomenology, Phys. Rev. B10, 4932–4947 (1974)

9.26 E. Bauer: Low energy electron microscopy and nor-mal incidence VLEED. In: Physics of Solid Surfaces,Studies in Surface Science and Catalysis, Vol. 40, ed.by J. Koukal (Elsevier, Amsterdam 1988) pp. 26–36

9.27 H.-J. Herlt, R. Feder, G. Meister, E. Bauer: Experi-ment and theory of the elastic electron reflectioncoefficient from tungsten, Solid State Commun. 38,973–976 (1981)

9.28 B.I. Lundqvist: Some numerical results on quasi-particle properties in the electron gas, Phys. StatusSolidi (b) 32, 273–280 (1969)

9.29 E. Bauer: Zum gegenwaertigen Stand derAugerelektronenspektroskopie (AES) und Beu-gung langsamer Elektronen (LEED), Z. Metallkd. 63,437–447 (1972)

9.30 E. Bauer: Low energy electron diffraction (LEED) andauger methods. In: Interactions on Metal Surfaces,ed. by R. Gomer (Springer, Berlin 1975) pp. 225–274

9.31 S. Tanuma, C.J. Powell, D.R. Penn: Calculations ofelectorn inelastic mean free paths. II. Data for 27elements over the 50–2000 eV range, Surf. InterfaceAnal. 17, 911–927 (1991)

9.32 Z.-J. Ding, R. Shimitzu: A Monte Carlo modeling ofelectron interaction with solids including cascadesecondary electron production, Scanning 18, 92–113(1996)

9.33 E. Bauer: Inelastic scattering of slow electrons insolids, Z. Phys. 224, 19–44 (1969)

9.34 C.J. Powell, A. Jablonski: Evaluation of calculatedand measured inelastic mean free paths near solidsurfaces, J. Phys. Chem. Ref. Data 28, 19–62 (1999)

9.35 T. Goulet, J.-M. Jung, M. Michaud, J.-P. Jay-Gerin,L. Sanche: Conduction-band density of statesin solid argon revealed by low-energy-electronbackscattering from thin films: Role of the electronmean free path, Phys. Rev. B 50, 5101–5109 (1994)

9.36 E. Keszei, T. Goulet, J.-P. Jay-Gerin: Probabilis-tic description of particle transport. II. Analysis

PartA|9


of low-energy electron transmission through thinsolid Xe and N2 films, Phys. Rev. A 37, 2183 (1988)

9.37 H.C. Siegmann: Surface and 2-D magnetism withspin polarized cascade electrons, Surf. Sci. 307,1076–1086 (1994)

9.38 J. Hong, D.L. Mills: Spin dependence of the inelas-tic electron mean free path in Fe and Ni: Explicitcalculations and implications, Phys. Rev. B 62,5589–5600 (2000)

9.39 E. Bauer: SPLEEM. In:Magnetic Microscopy of Nano-structures, ed. by H. Hopster, H.P. Oepen (Springer,Berlin, Heidelberg 2005) pp. 111–136

9.40 N. Georgiev, A. Pavlovska, E. Bauer: Surface disor-dering without surface roughening, Phys. Rev. B 52,2878–2888 (1995)

9.41 J. Rundgren: Electron inelastic mean free path,electron attenuation length, and low-energy elec-tron-diffraction theory, Phys. Rev. B 59, 5106–5114(1999)

9.42 G. Bartz: Über ein Elektronenmikroskop undein Verfahren zur direkten Sichtbarmachung vonisolierten Oberflächen. In: Proc. 4th Int. Conf. Elec-tron Microsc., ed. by G. Mollenstedt (Springer,Berlin 1960) pp. 201–207

9.43 R.M. Tromp, M.C. Reuter: Design of a new photo-emission/low-energy electron microscope for sur-face studies, Ultramicroscopy 36, 99–106 (1991)

9.44 L.H. Veneklasen: Design of a spectroscopic low-energy electron microscope, Ultramicroscopy 36,76–90 (1991)

9.45 E. Bauer, T. Franz, C. Koziol, G. Lilienkamp,T. Schmidt: Recent advances in LEEM/PEEM forstructural and chemical analysis. In: Proc. NATOAdv. Res. Workshop Chem. Struct. Electron. Anal.Heterog. Surfaces Nanom. Scale, Trieste, ed. byR. Rosei (Kluwer, Dordrecht 1997) pp. 75–91

9.46 E. Bauer, C. Koziol, G. Lilienkamp, T. Schmidt: Spec-tromicroscopy in a low energy electron microscope,J. Electron Spectrosc. Relat. Phenom. 84, 201–209(1997)

9.47 E. Bauer: Low energy electron microscopy, Rep.Prog. Phys. 57, 895–938 (1994)

9.48 E. Bauer: Photoelectron spectromicroscopy:Presentand future, J. Electron Spectrosc. Relat. Phenom.114–116, 975–987 (2001)

9.49 R.M. Tromp, M. Mankos, M.C. Reuter, A.W. Ellis,M. Copel: A new low energy electron microscope,Surf. Rev. Lett. 5, 1189–1197 (1998)

9.50 P. Adamec, E. Bauer, B. Lencova: Compact low-en-ergy electron microscope for surface imaging, Rev.Sci. Instrum. 69, 3583–3787 (1998)

9.51 K. Grzelakowski, E. Bauer: A flange-on type lowenergy electron microscope, Rev. Sci. Instrum. 67,742–747 (1996)

9.52 H. Liebl, B. Senftinger: Low-energy electron micro-scope of novel design, Ultramicroscopy 36, 91–98(1991)

9.53 Y. Sakai, M. Kato, S. Masuda, Y. Harada, T. Ichi-nokawa: Development of a low energy electronmicroscopewith an energy analyzer, Surf. Rev. Lett.5, 1199–1211 (1998)

9.54 K. Grzelakowski: The novel surface science instru-ment: Double reflection electron emission micro-scope, Rev. Sci. Instrum. 70, 3346–3350 (1999)

9.55 H. Rose, D. Preikszas: Outline of a versatile cor-rected LEEM, Optik 92, 31–44 (1992)

9.56 D. Preikszas, P. Hartel, R. Spehr, H. Rose: SMARTelectron optics. In: Proc. 12th Eur. Congr. ElectronMicrosc. EUREM 12, Vol. 2, ed. by L. Frank, F. Či-ampor (Czech. Soc. Electron Microscopy, Brno 2000)pp. 181–184

9.57 R. Fink, M.R. Weiss, E. Umbach, D. Preikszas,H. Rose, R. Spehr, P. Hartel, W. Engel, R. Degen-hardt, R. Wichtendahl, H. Kuhlenbeck, W. Erlebach,K. Ihmann, R. Schlogl, H.J. Freund, A.M. Bradshaw,G. Lilienkamp, T. Schmidt, E. Bauer, G. Benner:SMART: A planned ultrahigh-resolution spectromi-croscope for BESSY II, J. Electron Spectrosc. Relat.Phenom. 84, 231–250 (1997)

9.58 T. Schmidt, U. Groh, R. Fink, E. Umbach, O. Schaff,W. Engel, B. Richter, H. Kuhlenbeck, R. Schlögl,H.-J. Freund, A.M. Bradshaw, D. Preikszas, P. Har-tel, R. Spehr, H. Rose, G. Lilienkamp, E. Bauer,G. Benner: XPEEM with energy-filtering: Advan-tages and first results from the SMART project, Surf.Rev. Lett. 9, 223–232 (2002)

9.59 L.H. Veneklasen: The continuing development oflow-energy electron microscopy for characterizingsurfaces, Rev. Sci. Instrum. 63, 5513–5532 (1992)

9.60 J. Chmelik, L. Veneklasen, G. Marx: Comparingcathode lens configurations for low energy electronmicroscopy, Optik 83, 155–160 (1989)

9.61 G.D. Archard, T. Mulvey: Magnetic deflexion of elec-tron beams without astigmatism, J. Sci. Instrum.35, 279–283 (1958)

9.62 M. Mankos, V. Kolařik, L.H. Veneklasen: Electron-optical properties of multiple magnetic prism sys-tems, Nucl. Instrum. Methods Phys. Res. A 298,189–198 (1990)

9.63 V. Kolařík, M. Mankos, L.H. Veneklasen: Close-packed prism arrays for electron microscopy, Optik87, 1–12 (1991)

9.64 R. Degenhardt: Korrektur von Aberrationen in derTeilchenoptik mit Hilfe von Symmetrien, Ph.D. The-sis (Technische Hochschule Darmstadt, Germany1992)

9.65 H.-C. Kan, D. Auerbach, R.J. Phaneuf: Approach forinvestigating the astigmatism of a magnetic prismin low-energy electron microscopy, Rev. Sci. In-strum. 74, 1008–1015 (2003)

9.66 H.-C. Kan, T. Dürkop, R.J. Phaneuf: Comparison ofstigmatically focusing magnetic prisms of squareversus round symmetries, J. Vac. Sci. Technol. B 20,2519–2525 (2002)

9.67 H. Müller, D. Preikszas, H. Rose: A beam separatorwith small aberrations, J. Electron Microsc. 48, 191–204 (1999)

9.68 R. Spehr, P. Hartel, H. Müller, D. Preikszas, H. Rose:Design and manufacturing of magnetic electronoptical systems with midsection symmetry. In:Proc. 12th Eur. Congr. Electron Microsc., ed. by P. Či-ampor, L. Frank (Czech. Soc. Electron Microscopy,Brno 2000), pp. I85/I86


A|9

9.69 Y.K. Wu, D.S. Robin, E. Forest, R. Schlueter, S. An-ders, J. Feng, H. Padmore, D.H. Wei: Design andanalysis of beam separator magnets for third gen-eration aberration compensated PEEMs, Nucl. In-strum. Methods Phys. Res. A 519, 230–241 (2003)

9.70 D.A. Swenson: Achromatic translation system forcharged particle beams, Rev. Sci. Instrum. 35, 608(1964)

9.71 K. Tsuno: Simulation of a Wien filter as beamseparator in a low energy electron microscope, Ul-tramicroscopy 55, 127–140 (1994)

9.72 V.K. Zworykin, G.A. Morton, E.G. Ramberg, J. Hillier,A.W. Vance: Electron Optics and the Electron Micro-scope (Wiley, New York 1945)

9.73 E.G. Ramberg: Aberration correction with electronmirrors, J. Appl. Phys. 20, 183–186 (1949)

9.74 W.P. Skoczylas, G.F. Rempfer, O.H. Griffith: A pro-posed modular imaging system for photoelectronand electron probe microscopy with aberrationcorrection and for mirror microscopy and low-en-ergy electron microscopy, Ultramicroscopy 36, 252–261 (1991)

9.75 D. Preikszas, H. Rose: Correction properties of elec-tron mirrors, J. Electron Microsc. 46, 1–9 (1997)

9.76 P. Hartel, D. Preikszas, R. Spehr, H. Rose: Per-formance of the mirror corrector for an ultra-high-resolution spectromicroscope. In: Proc. 12thEur. Congr. Electron Microsc., ed. by P. Čiampor,L. Frank (Czech. Soc. Electron Microscopy, Brno2000) pp. I153–I154

9.77 W. Wan, J. Feng, H.A. Padmore, D.S. Robin: Simula-tion of a mirror corrector for PEEM3, Nucl. Instrum.Methods Phys. Res. A 519, 222–229 (2004)

9.78 S. Cherifi, R. Hertel, J. Kirschner, H. Wang,R. Belkhou, A. Locatelli, S. Heun, A. Pavlovska,E. Bauer: Virgin domain structures in mesoscopicCo patterns: Comparison between simulation andexperiment, J. Appl. Phys. 98, 043901 (2005)

9.79 W. Telieps, E. Bauer: The .7�7/ $ .1�1/ phasetransition on Si(111), Surf. Sci. 162, 163–168 (1985)

9.80 W. Telieps: Surface imaging with LEEM, Appl.Phys. A 44, 55–61 (1987)

9.81 W. Świech, E. Bauer, M. Mundschau: A low energyelectron microscopy study of the system Si(111)-Au,Surf. Sci. 253, 283–296 (1991)

9.82 T. Yasue, T. Koshikawa, M. Jalochowski, E. Bauer:LEEM observation of formation of Cu nano-islandson Si(111) surface by hydrogen termination, Surf. Sci.493, 381–388 (2001)

9.83 M.S. Altman, W.F. Chung, C.H. Liu: LEEM phase con-trast, Surf. Rev. Lett. 5, 1129–1141 (1998)

9.84 W.F. Chung, M.S. Altman: Step contrast in low en-ergy electron microscopy, Ultramicroscopy 74, 237–246 (1998)

9.85 T. Müller: Bildentstehung im LEEM, Ph.D. Thesis (TUClausthal, Clausthal-Zellerfeld 1995)

9.86 M. Mundschau, E. Bauer, W. Święch: Initial epi-taxial growth of Cu on Mof011g by low-energyelectron microscopy and photoemission electronmicroscopy, J. Appl. Phys. 65, 581–584 (1989)

9.87 R. Zdyb, A. Pavlovska, A. Locatelli, S. Heun, S. Cher-ifi, R. Belkhou, E. Bauer: Imaging low-dimensional

magnetismwith slow electrons, Appl. Surf. Sci. 249,38–44 (2005)

9.88 R. Zdyb, E. Bauer: Spin-resolved unoccupied elec-tronic band structure from quantum size oscil-lations in the reflectivity of slow electrons fromultrathin ferromagnetic crystals, Phys. Rev. Lett. 88,166403 (2002)

9.89 M.S. Altman: Low energy electron microscopy ofquantum well resonances in Ag films on W(110),J. Phys. Condens. Matter 17, 1305–1310 (2005)

9.90 Y.Z. Wu, A.K. Schmid, M.S. Altman, X.F. Jin,Z.Q. Qiu: Spin-dependent Fabry-Pérot interferencefrom a Cu thin film grown on fcc Co(001), Phys. Rev.Lett. 94, 027201 (2005)

9.91 M.S. Altman, W.F. Chung, Z.Q. He, H.C. Poon,S.Y. Tong: Quantum size effect in low energyelectron diffraction of thin films, Appl. Surf. Sci.169/170, 82–87 (2001)

9.92 W.F. Chung, Y.J. Feng, H.C. Poon, C.T. Chan,S.Y. Tong, M.S. Altman: Layer spacings in coherentlystrained epitaxial metal films, Phys. Rev. Lett. 90,216105 (2003)

9.93 K.L. Man, Z.Q. Qiu, M.S. Altman: Kinetic limitationsin electronic growth of Ag films on Fe(100), Phys.Rev. Lett. 93, 236104 (2004)

9.94 W. Witt: Untersuchungen 2-dimensionalerPhasenübergänge mit einem hochauflösendenLEED-Diffraktometer, Ph.D. Thesis (TU Clausthal,Clausthal-Zellerfeld 1984)

9.95 E. Bauer: Phase transitions on single crystal sur-faces and in chemisorbed layers. In: Structure andDynamics of Surfaces II, Topics in Current Physics,Vol. 43, ed. by W. Schommers, P.V. Blanckenhagen(Springer, Berlin 1987) pp. 115–179

9.96 W. Telieps, E. Bauer: Kinetics of the .7�7/ $ .1�1/transition on Si(111), Ber. Bunsenges. Phys. Chem.90, 197–200 (1986)

9.97 E. Bauer,M. Mundschau, W. Swiech,W. Telieps: Lowenergy electron microscopy of semiconductor sur-faces, J. Vac. Sci. Technol. A 9, 1007–1013 (1991)

9.98 R.J. Phaneuf, N.C. Bartelt, E.D. Williams, W. Swiech,E. Bauer: LEEM investigations of the domain growthof the (7�7) reconstruction on Si(111), Surf. Sci. 268,227–237 (1992)

9.99 J.B. Hannon, H. Hibino, N.C. Bartelt, B.S. Swartzen-truber, T. Ogino, G.L. Kellogg: Dynamics of thesilicon (111) surface phase transition, Nature 405,552–554 (2000)

9.100 H. Hibino, C.W. Hu, T. Ogino, I.S.T. Tsong: Decay ki-netics of two-dimensional islands and holes onSi(111) studied by low-energy electron microscopy,Phys. Rev. B 63, 245402 (2001)

9.101 J.B. Hannon, F.-J. Meyer zu Heringdorf, J. Ter-soff, R.M. Tromp: Phase coexistence during surfacephase transitions, Phys. Rev. Lett. 86, 4871–4874(2001)

9.102 C.W. Hu, H. Hibino, T. Ogino, I.S.T. Tsong: Hysteresisin the .1�1/� .7�7/ first-order phase transitionon the Si(111) surface, Surf. Sci. 487, 191–200 (2001)

9.103 J.B. Hannon, R.M. Tromp: Phase boundary fluctua-tions on Si(111), J. Vac. Sci. Technol. A 19, 2596–2600(2001)

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9.104 H. Hibino, C.-W. Hu, T. Ogino, I.S.T. Tsong: Diffusionbarrier caused by 1�1 and 7�7 on Si(111) duringphase transition, Phys. Rev. B 64, 245401 (2001)

9.105 J.B. Hannon, J. Tersoff, R.M. Tromp: Surface stressand thermodynamic nanoscale size selection, Sci-ence 295, 299–301 (2002)

9.106 J.B. Hannon, J. Tersoff, R.M. Tromp: The stabilityof triangular ‘Droplet’ phases on Si(111), J. Cryst.Growth 237–239, 181–187 (2002)

9.107 J.B. Hannon, J. Tersoff, M.C. Reuter, R.M. Tromp:Influence of supersaturation on surface structure,Phys. Rev. Lett. 89, 266103 (2002)

9.108 H. Hibino, Y. Homma, C.W. Hu, M. Uwaha, T. Ogino,I.S.T. Tsong: Structural and morphological changeson surfaces with multiple phases studied by low-energy electron microscopy, Appl. Surf. Sci. 237, 51–57 (2004)

9.109 R.M. Tromp, J.B. Hannon: Thermodynamics of nu-cleation and growth, Surf. Rev. Lett. 9, 1565–1593(2002)

9.110 J.B. Hannon, R.M. Tromp: Low-energy electron mi-croscopy of surface phase transitions, Annu. Rev.Mater. Res. 33, 263–288 (2003)

9.111 R.J. Phaneuf, W. Święch, N.C. Bartelt, E.D. Williams,E. Bauer: LEEM investigations of orientationalphase separation on vicinal Si(111) surfaces, Phys.Rev. Lett. 67, 2986–2989 (1991)

9.112 E.D. Williams, R.J. Phaneuf, N.C. Bartelt, W. Święch,E. Bauer: The role of surface stress in the facettingof stepped Si(111) surfaces, MRS Proceedings 238,219–227 (1991)

9.113 R.J. Phaneuf, N.C. Bartelt, E.D. Williams, W. Święch,E. Bauer: Crossover from metastable to unstablefacet growth on Si(111), Phys. Rev. Lett. 71, 2284–2287 (1993)

9.114 M.S. Altman,W.F. Chung, T. Franz: LEEM determina-tion of critical terrace widths for Si/Si(111) step flowgrowth, Surf. Rev. Lett. 5, 27–30 (1998)

9.115 W.F. Chung, M.S. Altman: Kinetic length, step per-meability, and kinetic coefficient asymmetry on theSi(111) (7�7) surface, Phys. Rev. B 66, 075338 (2002)

9.116 W.F. Chung, K. Bromann, M.S. Altman: The transi-tion to step flow growth on the clean and surfac-tant covered Si(111) surface studied by in-situ LEEM,Int. J. Mod. Phys. B 16, 4353–4362 (2002)

9.117 H. Hibino, Y. Watanabe: Growth of twinned epi-taxial layers on Si(111)

p3� p

3-B studied by low-energy electron microscopy, Jpn. J. Appl. Phys. 44,358–364 (2005)

9.118 M. Mundschau, E. Bauer, W. Telieps, W. Święch:Atomic steps on Sif100g and step dynamics duringsublimation studied by low-energy electron mi-croscopy, Surf. Sci. 223, 413–423 (1989)

9.119 M. Mundschau, E. Bauer, W. Telieps, W. Święch:Atomic step and defect structure on surfaces ofSif100g and Sif111g observed by low-energy electronmicroscopy, Philos. Mag. A 61, 257–280 (1990)

9.120 E. Bauer,M. Mundschau, W. Swiech, W. Telieps: Lowenergy electron microscopy of surface processes,Vacuum 41, 5–10 (1990)

9.121 W. Święch, E. Bauer: The growth of Si on Si(100):A video-LEEM study, Surf. Sci. 255, 219–228 (1991)

9.122 E. Bauer: Low energy electronmicroscopy of surfaceprocesses, Appl. Surf. Sci. 60/61, 350–358 (1992)

9.123 R.M. Tromp, M.C. Reuter: Wavy steps on Si(001),Phys. Rev. Lett. 68, 820–823 (1992)

9.124 R.M. Tromp, M.C. Reuter: Step morphologies onsmall-miscut Si(001) surfaces, Phys. Rev. B 47,7598–7601 (1993)

9.125 N.C. Bartelt, R.M. Tromp, E.D. Williams: Step cap-illary waves and equilibrium island shapes onSi(001), Phys. Rev. Lett. 73, 1656–1659 (1994)

9.126 W. Theis, N.C. Bartelt, R.M. Tromp: Chemical po-tential maps and spatial correlations in 2-D-islandripening on Si(001), Phys. Rev. Lett. 75, 3328–3331(1995)

9.127 W. Theis, R.M. Tromp: Nucleation in Si(001) ho-moepitaxial growth, Phys. Rev. Lett. 76, 2770–2773(1996)

9.128 S. Tanaka, C.C. Umbach, J.M. Blakely, R.M. Tromp,M. Mankos: Fabrication of arrays of large stepfreeregions on Si(001), Appl. Phys. Lett. 69, 1235–1237(1996)

9.129 N.C. Bartelt, R.M. Tromp: Low-energy electron mi-croscopy study of step mobilities on Si(001), Phys.Rev. B 54, 11731–11740 (1996)

9.130 N.C. Bartelt, W. Theis, R.M. Tromp: Ostwald ripeningof two-dimensional islands on Si(001), Phys. Rev. B54, 11741–11751 (1996)

9.131 S. Tanaka, N.C. Bartelt, C.C. Umbach, R.M. Tromp,J.M. Blakely: Step permeability and the relaxationof biperiodic gratings on Si(001), Phys. Rev. Lett. 78,3342–3345 (1997)

9.132 S. Tanaka, C.C. Umbach, J.M. Blakely, R.M. Tromp,M. Mankos: Atomic step distributions on annealedperiodic Si(001) gratings, J. Vac. Sci. Technol. A 15,1345–1350 (1997)

9.133 R.M. Tromp, M. Mankos: Thermal adatoms onSi(001), Phys. Rev. Lett. 81, 1050–1053 (1998)

9.134 J.M. Blakely, S. Tanaka, R.M. Tromp: Atomic stepdynamics on periodic semiconductor surface struc-tures, J. Electron Microsc. 48, 747–752 (1999)

9.135 J.-F. Nielsen, J.P. Pelz, H. Hibino, C.-W. Hu,I.S.T. Tsong: Enhanced terrace stability for prepara-tion of step-free Si(001)-(2�1) surfaces, Phys. Rev.Lett. 87, 136103 (2001)

9.136 V. Zielasek, F. Liu, Y. Zhao, J.B. Maxson, M.G. La-gally: Surface stress-induced island shape transi-tion in Si(001) homoepitaxy, Phys. Rev. B 64, 201320(2001)

9.137 K. Wurm, R. Kliese, Y. Hong, B. Röttger, Y. Wei,H. Neddermeyer, I.S.T. Tsong: Evolution of surfacemorphology of Si(100)-(2�1) during oxygen ad-sorption at elevated temperatures, Phys. Rev. B 50,1567–1574 (1994)

9.138 J.B. Hannon, M.C. Bartelt, N.C. Bartelt, G.L. Kellogg:Etching of the Si(001) surface with molecular oxy-gen, Phys. Rev. Lett. 81, 4676–4679 (1998)

9.139 J.B. Hannon, G.L. Kellogg, M.C. Bartelt, N.C. Bartelt:Quantitative analysis of the evolution of surfacegrowth morphology in LEEM, Surf. Rev. Lett. 5, 1151(1998)


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9.140 R.M. Tromp, A.W. Denier van der Gon, M.C. Reuter:Surface stress as a driving force for interfacial mix-ing, Phys. Rev. Lett. 68, 2313–2316 (1992)

9.141 D.E. Jones, J.P. Pelz, Y. Hong, E. Bauer, I.S.T. Tsong:Striped phase and temperature dependent stepshape transition on highly B-doped Si(001)-(2�1)surfaces, Phys. Rev. Lett. 77, 330–333 (1996)

9.142 E. Bauer: Surface Microscopy with Low Energy Elec-trons (Springer, New York 2014)

9.143 J.B. Hannon, N.C. Bartelt, B.S. Swartzentruber,J.C. Hamilton, G.L. Kellogg: Step faceting at the(001) surface of boron doped silicon, Phys. Rev. Lett.79, 4226–4229 (1997)

9.144 J.B. Hannon, B.S. Swartzentruber, G.L. Kellogg,N.C. Bartelt: LEEM measurments of step energies atthe (001) surface of heavily boron-doped silicon,Surf. Rev. Lett. 5, 1159–1165 (1998)

9.145 J.-F. Nielsen, J.P. Pelz, H. Hibino, C.-W. Hu,I.S.T. Tsong, J. Kouvetakis: Controlled striped phaseformation on ultraflat Si(001) surfaces during di-borane exposure, Appl. Phys. Lett. 79, 3857–3859(2001)

9.146 P. Sutter, M.G. Lagally: Quantitative determinationof dislocation-induced strain at the surface of (001)silicon-on-insulator, Phys. Rev. Lett. 82, 1490–1493(1999)

9.147 R.M. Tromp, W. Theis, N.C. Bartelt: Real-time mi-croscopy of two-dimensional critical fluctuations:Disordering of the Si(113)-(3�1) reconstruction,Phys. Rev. Lett. 77, 2522–2525 (1996)

9.148 W. Swiech, T. Schwarz-Selinger, D.G. Cahill: Phasecoexistence and morphology at the Si(110) surfacephase transition, Surf. Sci. 519, L599–L603 (2002)

9.149 F. Watanabe, S. Kodambaka, W. Swiech,J.E. Greene, D.G. Cahill: LEEM study of islanddecay on Si(110), Surf. Sci. 572, 425–432 (2004)

9.150 E. van Vroonhoven, H.J.W. Zandvliet, B. Poelsema:.2�1/� .1�1/ phase transition on Ge(001): Dimerbreakup and surface roughening, Phys. Rev. Lett.91, 116102 (2003)

9.151 R.M. Tromp, M.C. Reuter: Local dimer exchange insurfactant-mediated epitaxial growth, Phys. Rev.Lett. 68, 954–957 (1992)

9.152 P. Sutter, E. Mateeva, M.G. Lagally: Si growth onpartially relaxed Ge islands, J. Vac. Sci. Technol. B16, 1560–1563 (1998)

9.153 P. Sutter, M.G. Lagally: Embedding of nanoscale 3-DSiGe islands in a Si matrix, Phys. Rev. Lett. 81, 3471–3474 (1998)

9.154 P. Sutter, E. Mateeva, J.S. Sullivan, M.G. La-gally: Low-energy electron microscopy of nano-scale three-dimensional SiGe islands on Si(100),Thin Solid Films 336, 262–270 (1998)

9.155 F.M. Ross, R.M. Tromp,M.C. Reuter: Transition statesbetween pyramids and domes during Ge/Si islandgrowth, Science 286, 1931–1934 (1999)

9.156 P. Sutter, M.G. Lagally: Nucleationless three-di-mensional island formation in low-misfit het-eroepitaxy, Phys. Rev. Lett. 84, 4637–4640 (2000)

9.157 R.M. Tromp, F.M. Ross, M.C. Reuter: Instability-driven SiGe island growth, Phys. Rev. Lett. 84,4641–4644 (2000)

9.158 J.B. Maxson, D.E. Savage, F. Liu, R.M. Tromp,M.C. Reuter, M.G. Lagally: Thermal roughening ofa thin film: A new type of roughening transition,Phys. Rev. Lett. 85, 2152–2155 (2000)

9.159 P. Sutter, M.G. Lagally: Nucleationless island forma-tion in SiGe/Si(100) heteroepitaxy, Mater. Sci. Eng. B89, 45–48 (2002)

9.160 J.B. Hannon, M. Copel, R. Stumpf, M.C. Reuter,R.M. Tromp: Critical role of surface steps in the al-loying of Ge on Si(001), Phys. Rev. Lett. 92, 216104(2004)

9.161 R.M. Tromp, F.M. Ross: Advances in in situ ultra-high vacuum electron microscopy: Growth of SiGeon Si, Annu. Rev. Mater. Sci. 30, 431–449 (2000)

9.162 A.W. Dernier van der Gon, R.M. Tromp, M.C. Reuter:Low energy electron microscopy studies of Ge andAg growth on Si(111), Thin Solid Films 236, 140–145(1993)

9.163 T. Yasue, T. Koshikawa, E. Bauer: LEEM and LEEDstudy of the growth of Ge on the Si(113) surface,J. Vac. Sci. Technol. B 20, 2496–2499 (2002)

9.164 J. Falta, M.C. Reuter, R.M. Tromp: Growth modes ofGe on GaAs(001), Appl. Phys. Lett. 65, 1680–1682(1994)

9.165 E. Bauer, M. Mundschau, W. Swiech: Low energyelectron microscopy of nanometer scale phenom-ena, J. Vac. Sci. Technol. B 9, 403–406 (1991)

9.166 M. Horn-von Hoegen, F.-J. Meyer zu Heringdorf,D. Kaehler, T. Schmidt, E. Bauer: Adsorption in-duced giant faceting of vicinal Si(100), Thin SolidFilms 336, 16–21 (1998)

9.167 F.-J. Meyer zu Heringdorf, D. Kähler, T. Schmidt,E. Bauer, M. Copel, H. Minoda, M. Horn-von Hoe-gen: Giant faceting of vicinal Si(001) induced by Auadsorption, Surf. Rev. Lett. 5, 1167–1178 (1998)

9.168 M. Horn-von Hoegen, F.-J. Meyer zu Hennigdorf,R. Hild, P. Zahl, T. Schmidt, E. Bauer: Au-inducedgiant faceting of vicinal Si(100), Surf. Sci. 433-435,475–480 (1999)

9.169 F.-J. Meyer zu Heringdorf, R. Hild, P. Zahl,T. Schmidt, B. Ressel, S. Heun, E. Bauer, M. Horn-von Hoegen: Local Au coverage as driving force forAu induced faceting of vicinal Si(001): A LEEM andXPEEM study, Surf. Sci. 480, 103–108 (2001)

9.170 A.W. Dernier van der Gon, R.M. Tromp: Phase for-mation sequence and domain structure: A low-energy-electron-microscopy study of the Si(111)-(p3�p

3) Ag surface, Phys. Rev. Lett. 69, 3519–3522(1992)

9.171 J. Tersoff, A.W. Dernier van der Gon, R.M. Tromp:Shape oscillations in growth of small crystals, Phys.Rev. Lett. 70, 1143–1146 (1993)

9.172 R.M. Tromp, A.W. Denier van der Gon, F.K. LeGoues,M.C. Reuter: Observation of buried interfaces withlow energy electron microscopy, Phys. Rev. Lett. 71,3299–3302 (1993)

9.173 W.X. Tang, K.L. Man, H. Huang, C.H. Woo, M.S. Alt-man: Growth shapes of Ag crystallites on the Si(111)surface, J. Vac. Sci. Technol. B 20, 2492–2495 (2002)

9.174 J. Tersoff, R.M. Tromp: Shape transition in growth ofstrained islands: Spontaneous formation of quan-tum wires, Phys. Rev. Lett. 70, 2782–2785 (1993)

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9.175 T. Michely, M.C. Reuter, M. Copel, R.M. Tromp: Sym-metry, structure, and step induced ordering of theSi(001)-(2�3)Ag surface, Phys. Rev. Lett. 73, 2095–2098 (1994)

9.176 R.M. Tromp, T. Michely: Atomic-layer titration ofsurface reactions, Nature 373, 499–501 (1995)

9.177 B.Q. Li, W. Swiech, J.A. Venables, J.M. Zuo: A LEEMstudy of bamboo-like growth of Ag crystals onSi(001) surfaces, Surf. Sci. 569, 142–148 (2004)

9.178 K.R. Roos, K.L. Roos, M. Horn-von Hoegen,F.-J. Meyer zu Heringdorf: High temperature self-assembly of Ag nanowires on vicinal Si(001), J. Phys.Condens. Matter 17, S1407–S1414 (2005)

9.179 M. Mundschau, E. Bauer, W. Telieps, W. Święch:Initial epitaxial growth of copper silicide on Si(111)studied by low energy electron microscopy andphotoemission electron microscopy, J. Appl. Phys.65, 4747–4752 (1989)

9.180 T. Yasue, T. Koshikawa, M. Jalochowski, E. Bauer:Dynamic LEEM observation of Cu nanostructure for-mation processes on Si(111) with hydrogen, Surf.Rev. Lett. 7, 595–599 (2000)

9.181 T. Yasue, T. Koshikawa, M. Jalochowski, E. Bauer:Dynamic observations of the formation of thin Culayers on clean and hydrogen-terminated Si(111)surfaces, Surf. Sci. 480, 118–127 (2001)

9.182 N. Kuroiwa, Y. Fukushima, P. Rajasekar, H. Ned-dermeyer, M. Jalochowski, E. Bauer, T. Yasue,T. Koshikawa: Copper nanostructure formation andstructure analysis on hydrogen-terminated Si(111)surface, Surf. Interface Anal. 35, 24–28 (2003)

9.183 T. Michely, M.C. Reuter, R.M. Tromp: Al on Si(111):Phase diagram and atomic mechanisms, Phys.Rev. B 53, 4105–4108 (1996)

9.184 T. Schmidt, E. Bauer: Interfactant-mediated quasi-Frank-van der Merwe growth of Pb on Si(111), Phys.Rev. B 62, 15815–15825 (2000)

9.185 T. Schmidt, B. Ressel, S. Heun, K.C. Prince, E. Bauer:Growth of thin metal films studied by spectromi-croscopy. In: Proc. 6th Int. Conf. X-ray Microsc., ed.by W. Meyer-Ilse, T. Altwood (AIP Melville, New York2000) pp. 27–32

9.186 T. Schmidt, E. Bauer: Influence of interfactants onthinmetal film growth, Surf. Sci. 480, 137–144 (2001)

9.187 L. Li, C. Koziol, K. Wurm, Y. Hong, E. Bauer,I.S.T. Tsong: Surface morphology of Pb overlayersgrown on Si(100)-(2�1), Phys. Rev. B 50, 10834–10842 (1994)

9.188 M. Jalochowski, E. Bauer: Self-assembled parallelmesoscopic Pb-wires on Au-modified Si(533) sub-strates, Prog. Surf. Sci. 67, 79–97 (2001)

9.189 M. Jalochowski, E. Bauer: Growth of metallic na-nowires on anisotropic Si substrates: Pb on vicinalSi(001), Si(755), Si(533) and Si(110), Surf. Sci. 480,109–117 (2001)

9.190 A. Pavlovska, E. Bauer, M. Giesen: Low energy elec-tron microscopy study of in on Si(111), J. Vac. Sci.Technol. B 20, 2478–2491 (2002)

9.191 A. Pavlovska, E. Bauer: Surface reconstructions intwo and three dimensions: In on Si(111), Surf. In-terface Anal. 37, 110–114 (2005)

9.192 H.A. McKay, R.M. Feenstra: Low energy electronmi-croscopy of indium on Si(001) surfaces, Surf. Sci.547, 127–138 (2003)

9.193 E. Bauer: Epitaxial growth studies by low energyelectron microscopy, Scanning Microsc. 8(4), 765–772 (1994)

9.194 R.J. Phaneuf, Y. Hong, S. Horch, P.A. Bennett: Twodimensional phase separation for Co adsorbed onSi(111), Phys. Rev. Lett. 78, 4605–4608 (1997)

9.195 R.J. Phaneuf, Y. Hong, S. Horch, P.A. Bennett: Ob-servations of reversible and irreversible structuraltransitions of cobalt on Si(111) with LEEM, Micron 30,13–20 (1999)

9.196 R.J. Phaneuf, P.A. Bennett, M. Marsi, S. Gunther,L. Gregoratti, L. Casalis, M. Kiskinova: Equilibrationof ring-cluster surface phases and silicide islandsfor Co adsorbed on Si(111), Surf. Sci. 431, 232–241(1999)

9.197 P.A. Bennett, M.Y. Lee, S.A. Parikh, K. Wurm,R.J. Phaneuf: Surface phase transformations in theNi/Si(111) system observed in real time using low-energy electron microscopy, J. Vac. Sci. Technol. A13, 1728–1732 (1995)

9.198 P.A. Bennett, B. Ashcroft, Z. He, R.M. Tromp: Growthdynamics of titanium silicide nanowires observedwith low-energy electron microscopy, J. Vac. Sci.Technol. B 20, 2500–2504 (2002)

9.199 R.M. Tromp, M.C. Reuter: Kinetic instability in thegrowth of CaF2 on Si(111), Phys. Rev. Lett. 73, 110–113(1994)

9.200 R.M. Tromp, F.K. LeGoues, M.C. Reuter: Strain reliefduring growth: CaF2 on Si(111), Phys. Rev. Lett. 74,2706–2709 (1995)

9.201 E. Bauer, Y. Wei, T. Müller, A. Pavlovska, I.S.T. Tsong:Reactive crystal growth in two dimensions: Siliconnitride on Si(111), Phys. Rev. B 51, 17891–17901 (1995)

9.202 A. Pavlovska, E. Bauer, D.J. Smith: In situ studies ofthe role of excess Ga on the growth morphology ofthin GaN layers, Surf. Sci. 496, 160–178 (2002)

9.203 J.B. Maxson, N. Perkins, D.E. Savage, A.R. Woll,L. Zhang, T.F. Kuech, M.G. Lagally: Novel dark-fieldimaging of GaNf0001g surfaces with low-energyelectron microscopy, Surf. Sci. 464, 217–222 (2000)

9.204 A. Pavlovska, V.M. Torres, J.L. Edwards, E. Bauer,D.J. Smith, R.B. Doak, I.S.T. Tsong, D.B. Thomson,R.F. Davis: Homoepitaxial GaN layers studied bylow-energy electron microscopy, atomic force mi-croscopy transmission electron microscopy, Phys.Status Solidi (a) 176, 469–473 (1999)

9.205 A. Pavlovska, E. Bauer: Wetting and reactive thinfilm growth, Surf. Sci. 480, 128–136 (2001)

9.206 A. Pavlovska, E. Bauer, V.M. Torres, J.L. Edwards,R.B. Doak, I.S.T. Tsong, V. Ramachadran, F.M. Feen-stra: In situ real-time studies of GaN growth on 6H-SiC(0001) by low energy electron microscopy (LEEM),J. Cryst. Growth 189/190, 310–316 (1998)

9.207 A. Pavlovska, E. Bauer: Low energy electron mi-croscopy studies of the growth of GaN on 6H-SiC(0001), Surf. Rev. Lett. 8, 337–346 (2001)

9.208 C.W. Hu, D.J. Smith, R.B. Doak, I.S.T. Tsong: Mor-phological control of GaN buffer layers grown by


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molecular beam epitaxy on 6H-SiC(0001), Surf. Rev.Lett. 7, 565–570 (2000)

9.209 M. Mundschau, E. Bauer, W. Święch: Defects on thesurface of Mof011g observed by low-energy electronmicroscopy, Philos. Mag. A 59, 217–226 (1989)

9.210 W. Święch, M. Mundschau, C.P. Flynn: Interfacialdefects in thin refractory metal films imaged bylow-energy electron microscopy, Appl. Phys. Lett.74, 2626–2628 (1999)

9.211 W. Święch, M. Mundschau, C.P. Flynn: Characteri-zation of single crystal films of molybdenum (011)grown by molecular beam epitaxy on sapphire(11 N20) and studied by low-energy electron mi-croscopy, Surf. Sci. 437, 61–74 (1999)

9.212 M. Mundschau, W. Święch, C.S. Durfee, C.P. Flynn:Slip propagation in epitaxial Mo (011) studied bylow-energy electron microscopy, Surf. Sci. 440,L831–L834 (1999)

9.213 M. Ondrejcek, W. Swiech, G. Yang, C.P. Flynn: Lowenergy electron microscopy studies of steps on sin-gle crystal thin films of refractory metals, J. Vac. Sci.Technol. B 20, 2473–2477 (2002)

9.214 M. Ondrejcek, W. Swiech, C.S. Durfee, C.P. Flynn:Step fluctuations and step interactions on Mo(011),Surf. Sci. 541, 31–45 (2003)

9.215 M. Ondrejcek, W. Swiech, C.P. Flynn: Studies of stepstiffnesses and relaxation on Pt(111), Pd(111) andMo(011), Surf. Sci. 566, 160–164 (2004)

9.216 C.P. Flynn, W. Święch: Periodic states in the con-strained equilibrium of vicinal Nb(011) miscut along[100], Phys. Rev. Lett. 83, 3482–3485 (1999)

9.217 C.P. Flynn, W. Święch, R.S. Appleton, M. Ondrejcek:Nanofaceting of vicinal Nb(011), Phys. Rev. B 62,2096–2107 (2000)

9.218 R.S. Appleton, W. Swiech, M. Ondrejcek, C.P. Flynn:Studies of threading dislocations in Nb(011) films,Philos. Mag. A 83, 1639–1651 (2003)

9.219 C.P. Flynn, M. Ondrejcek, W. Swiech: Capillary wavesand thermodynamics of multisteps on Pt(111),Chem. Phys. Lett. 378, 161–166 (2003)

9.220 M. Ondrejcek, W. Swiech, G. Yang, C.P. Flynn:Crossover from bulk to surface diffusion in the fluc-tuations of step edges on Pt(111), Philos. Mag. Lett.84, 69–77 (2004)

9.221 B. Poelsema, J.B. Hannon, N.C. Bartelt, G.L. Kel-logg: Bulk-surface vacancy exchange on Pt(111),Appl. Phys. Lett. 84, 2551–2553 (2004)

9.222 M. Ondrejcek, W. Swiech, M. Rajappan, C.P. Flynn:Ripples formed in the sputter erosion of Pd(111),J. Phys. Condens. Matter 15, L735–L742 (2003)

9.223 G.L. Kellogg, N.C. Bartelt: Surface-diffusion-lim-ited island decay on Rh(001), Surf. Sci. 577, 151–157(2005)

9.224 M. Ondrejcek, M. Rajappan, W. Swiech, C.P. Flynn:Step fluctuation spectroscopy of Au(111) by LEEM,Surf. Sci. 574, 111–122 (2005)

9.225 W. Telieps, M. Mundschau, E. Bauer: Dark fieldimaging with LEEM, Optik 77, 93–97 (1987)

9.226 W. Telieps, M. Mundschau, E. Bauer: Surface do-main structure of reconstructed Au(100) observedby dark field low energy electron microscopy, Surf.Sci. 225, 87–96 (1990)

9.227 J. Tersoff, A.W. Denier van der Gon, R.M. Tromp:Critical island size for layer-by-layer growth, Phys.Rev. Lett. 72, 266–269 (1994)

9.228 R. Gerlach, T. Maroutian, L. Douillard, D. Mar-tinotti, H.J. Ernst: A novel method to determinethe Ehrlich-Schwoebel barrier, Surf. Sci. 480, 97–102 (2001)

9.229 M.S. Altman, S. Chiang, P. Statiris, T. Gustafsson,E. Bauer: Stress-induced microfacetted reconstruc-tions of the Pb(110) surface. In: The Structure ofSurfaces, Vol. IV, ed. by X.D. Xi, S.Y. Tong, M.A. vanHove (World Scientific, Singapore 1994) pp. 183–191

9.230 M.S. Altman, E. Bauer: Reconstructions of thePb(110) surface studied by low energy electron mi-croscopy, Surf. Sci. 344, 51–64 (1995)

9.231 K.F. McCarty, J.A. Nobel, N.C. Bartelt: Vacancies insolids and the stability of surface morphology, Na-ture 412, 622–625 (2001)

9.232 M. Mundschau, E. Bauer, W. Święch: Modifica-tion of atomic steps by adsorbates observed bylow energy electronmicroscopy and photoemissionmicroscopy, Catal. Lett. 1, 405–412 (1988)

9.233 E. Bauer: Ultrathin metal films: From one to threedimensions, Ber. Bunsenges. Phys. Chem. 95, 1315–1325 (1991)

9.234 E.Z. Luo, Q. Cai, W.F. Chung, B.G. Orr, M.S. Altman:Competing desorption pathways during epitaxialgrowth: LEEM investigation of Cu/W(110) heteroepi-taxy, Phys. Rev. B 54, 14673 (1996)

9.235 I.K.H. Man, M.S. Altman: Low-energy electron mi-croscopy of layer spacings and quantum electronicstructure of ultrathin films, Surf. Interface Anal. 37,235–238 (2005)

9.236 L. Aballe, A. Barinov, A. Locatelli, S. Heun, S. Cher-ifi, M. Kiskinova: Spectromicroscopy of ultrathin Pdfilms on W(110): Interplay of morphology and elec-tronic structure, Appl. Surf. Sci. 238, 138–142 (2004)

9.237 W.L. Ling, T. Giessel, K. Thürmer, R.Q. Hwang,N.C. Bartelt, K.F. McCarty: Crucial role of substratesteps in de-wetting of crystalline thin films, Surf.Sci. 570, L297–L303 (2004)

9.238 K. Pelhos, J.B. Hannon, G.L. Kellogg, T.E. Madey:LEEM investigation of the faceting of the Pt coveredW(111) surface, Surf. Sci. 432, 115–124 (1999)

9.239 K. Pelhos, J.B. Hannon, G.L. Kellogg, T.E. Madey:Nucleation and growth of the platinum-coveredW(111) surface, Surf. Rev. Lett. 6, 767–774 (1999)

9.240 D. Wu, W.K. Lau, Z.Q. He, Y.J. Feng, M.S. Altman,C.T. Chan: Ordered alloying of Pd with the Mo(100)surface, Phys. Rev. B 62, 8366–8375 (2000)

9.241 A.K. Schmid, N.C. Bartelt, R.Q. Hwang: Alloying atsurfaces by the migration of reactive two-dimen-sional islands, Science 290, 1561–1564 (2000)

9.242 G.L. Kellogg, R. Plass: The relationship between thegrowth shape of three-dimensional Pb islands onCu(100) and the domain orientation of the under-lying C(5

pf2g �pf2g)R45 structure, Surf. Sci. 465,L777–L782 (2000)

9.243 R. Plass, G.L. Kellogg: Surface morphology changesduring Pb deposition on Cu(100): Evidence for sur-face alloyed Cu(100)-c(2�2) Pb, Surf. Sci. 470, 106–120 (2000)

PartA|9


9.244 G.L. Kellogg, R.A. Plass: Mesoscopic scale observa-tions of surface alloying, surface phase transitions,domain coarsening, and 3-D island growth: Pb onCu(100), Surf. Rev. Lett. 7, 649–655 (2000)

9.245 R. Plass, J.A. Last, N.C. Bartelt, G.L. Kellogg: Nano-structures: Self-assembled domain patterns, Na-ture 412, 875 (2001)

9.246 R. Plass, N.C. Bartelt, G.L. Kellogg: Dynamic obser-vations of nanoscale self-assembly on solid sur-faces, J. Phys. Condens. Matter 14, 4227–4240 (2002)

9.247 R. van Gastel, R. Plass, N.C. Bartelt, G.L. Kellogg:Thermal motion and energetics of self-assembleddomain structures: Pb on Cu(111), Phys. Rev. Lett. 91,055503 (2003)

9.248 R. van Gastel, N.C. Bartelt, P.J. Feibelman,F. Léonard, G.L. Kellogg: Relationship betweendomain-boundary free energy and the tempera-ture dependence of stress-domain patterns of Pbon Cu(111), Phys. Rev. B 70, 245413 (2004)

9.249 F. Léonard, N.C. Bartelt, G.L. Kellogg: Effects of elas-tic anisotropy on the periodicity and orientationof striped stress domain patterns at solid surfaces,Phys. Rev. B 71, 045416 (2005)

9.250 G.L. Kellogg: Surface alloying and de-alloying ofPb on single-crystal Cu surfaces. In: The ChemicalPhysics of Solid Surfaces, ed. by D.P. Woodruff (El-sevier, Amsterdam 2002) pp. 152–183

9.251 M.S. Altman, Q. Cai, W.F. Chung, E.Z. Luo,H. Pinkvos, E. Bauer: Role of surface steps in thinfilm growth and properties studied by LEEM, MRSProceedings 355, 235 (1994)

9.252 E.Z. Luo, Q. Cai, W.F. Chung, M.S. Altman: Interfaceeffects in melting of Pb clusters on the Cu(111) sur-face, Appl. Surf. Sci. 92, 331–334 (1996)

9.253 T. Schmidt, A. Schaak, S.G. Guenther, B. Ressel,E. Bauer, R. Imbihl: In situ imaging of structuralchanges in a chemical wave with low energy elec-tron microscopy: The system Rh(110)/NOCH2, Chem.Phys. Lett. 318, 549–554 (2000)

9.254 M.S. Altman, E. Bauer: The reaction of oxygen withthe hot W(001) surface studied by low-energy elec-tron microscopy, J. Vac. Sci. Technol. A 9, 659–660(1991)

9.255 M.S. Altman, E. Bauer: LEEM/LEED investigation ofreconstruction and initial oxidation of the W(001),Surf. Sci. 347, 265–279 (1996)

9.256 M. Ondrejcek, R.S. Appleton, W. Swiech,V.L. Petrova, C.P. Flynn: Thermally activatedstripe reconstruction induced by O on Nb(011),Phys. Rev. Lett. 87, 116102 (2001)

9.257 K.F. McCarty: Imaging the crystallization andgrowth of oxide domains on the NiAl(110) surface,Surf. Sci. 474, L165–L172 (2001)

9.258 B. Rausenberger, W. Świech, W. Engel, A.M. Brad-shaw, E. Zeitler: LEEM and selected-area LEED stud-ies of reaction front propagation, Surf. Sci. 287/288,235–240 (1993)

9.259 B. Rausenberger, W. Swiech, C.S. Rastomjee,M. Mundschau, W. Engel, E. Zeitler, A.M. Bradshaw:Imaging reaction-diffusion fronts with low-energyelectron microscopy, Chem. Phys. Lett. 215, 109–113(1993)

9.260 A.K. Schmid, W. Świech, C.S. Rastomjee, B. Rausen-berger, W. Engel, E. Zeitler, A.M. Bradshaw: Thechemistry of reaction-diffusion fronts investigatedby microscopic LEED I–V fingerprinting, Surf. Sci.331–333, 225–230 (1995)

9.261 B. Rausenberger, W. Swiech, A.K. Schmid, C.S. Ras-tomjee, W. Engel, A.M. Bradshaw: Investigation ofthe NO+H2 reaction on Pt(100) with low-energyelectron microscopy, J. Chem. Soc. Faraday Trans.94, 963–970 (1998)

9.262 K.C. Rose, B. Berton, R. Imbihl, W. Engel, A.M. Brad-shaw: Pattern formation in an oscillatory mediumwith memory effects: Reversible roughening ina surface reaction, Phys. Rev. Lett. 79, 3427 (1997)

9.263 H. Wei, G. Lilienkamp, R. Imbihl: Surface topo-graphical changes and chemical wave patterns incatalytic CO oxidation on Pt(110), Chem. Phys. Lett.389, 284–288 (2004)

9.264 H. Marbach, G. Lilienkamp, H. Wei, S. Günther,Y. Suchorski, R. Imbihl: Ordered phases in alkaliredistribution during a catalytic surface reaction,Phys. Chem. Chem. Phys. 5, 2730–2735 (2003)

9.265 A. Locatelli, S. Heun, M. Kiskinova: Direct obser-vation of reaction-induced lateral redistribution ofsub-monolayers of Au deposited on a Rh(110) sur-face, Surf. Sci. 566-568, 1130–1136 (2004)

9.266 K.F. McCarty, N.C. Bartelt: Role of bulk thermal de-fects in the reconstruction dynamics of the TiO2(110)surface, Phys. Rev. Lett. 90, 046104 (2003)

9.267 K.F. McCarty, N.C. Bartelt: The 1�1=1�2 phasetransition of the TiO2(110) surface—Variation of tran-sition temperature with crystal composition, Surf.Sci. 527, L203–L212 (2003)

9.268 K.F. McCarty, N.C. Bartelt: Spatially resolved dy-namics of the TiO2(110) surface reconstruction, Surf.Sci. 540, 157–171 (2003)

9.269 K.F. McCarty: Growth regimes of the oxygen-defi-cient TiO2(110) surface exposed to oxygen, Surf. Sci.543, 185–206 (2003)

9.270 S. Kodambaka, S.V. Khare, W. Święch, K. Ohmori,I. Petrov, J.E. Greene: Dislocation-driven surfacedynamics on solids, Nature 429, 49–52 (2004)

9.271 S. Kodambaka, N. Israeli, J. Bareño, W. Święch,K. Ohmori, I. Petrov, J.E. Greene: Low-energy elec-tron microscopy studies of interlayer mass trans-port kinetics on TiN(111), Surf. Sci. 560, 53–62 (2004)

9.272 F.-J. Meyer zu Heringdorf, M.C. Reuter, R.M. Tromp:Growth dynamics of pentacene thin films, Nature412, 517–520 (2001)

9.273 F.-J. Meyer zu Heringdorf, M.C. Reuter, R.M. Tromp:The nucleation of pentacene thin films, Appl.Phys. A 78, 787–791 (2004)

9.274 Y. Harada, S. Yamamoto, M. Aoki, S. Masuda, T. Ichi-nokawa, M. Kato, Y. Sakai: Surface spectroscopywith high spatial resolution using metastableatoms, Nature 372, 657–659 (1994)

9.275 S. Yamamoto, S. Masuda, H. Yasufuku, N. Ueno,Y. Harada, T. Ichinokawa, M. Kato, Y. Sakai: Study ofsolid surfaces by metastable electron emission mi-croscopy: Energy-filtered images and local electronspectra at the outermost surface layer of silicon ox-ide on Si(100), J. Appl. Phys. 82, 2954–2960 (1997)


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9.276 G. Lilienkamp, H. Wei, W. Maus-Friedrichs,V. Kempter, H. Marbach, S. Günther, Y. Suchorski:Metastable impact electron emission microscopyof the catalytic H2 oxidation on Rh(110), Surf. Sci.532–535, 132–136 (2003)

9.277 D.T. Pierce: Spin-polarized electron sources. In:Atomic, Molecular and Optical Physics: ChargedParticles, Methods in Experimental Physics,Vol. 29A, ed. by F.B. Dunning, R.G. Hulet (AcademicPress, San Diego 1995) pp. 1–38

9.278 X. Jin, A.A.C. Cotta, G. Chen, A.T. N’Diaye,A.K. Schmid, N. Yamamoto: Low energy electronmicroscopy and auger electron spectroscopy stud-ies of Cs-O activation layer on p-type GaAs photo-cathode, J. Appl. Phys. 116, 174509 (2014)

9.279 T. Nishitani, T. Nakanishi, M. Yamamoto, S. Okumi,F. Furuta, M. Miyamoto, M. Kuwahara, N. Ya-mamoto, K. Naniwa, O. Watanabe, Y. Takeda,H. Kobayakawa, Y. Takashima, H. Horinaka, T. Mat-suyama, K. Togawa, T. Saka, M. Tawada, T. Omori,Y. Kurihara, M. Yoshioka, K. Kato, T. Baba: Highlypolarized electrons from GaAs-GaAsP and InGaAs-AlGaAs strained-layer superlattice photocathodes,J. Appl. Phys. 97, 094907 (2005)

9.280 X. Jin, N. Yamamoto, Y. Nakagawa, A. Mano, T. Kato,M. Tanioku, T. Ujihara, Y. Takeda, S. Okumi, M. Ya-mamoto, T. Nakanishi, T. Saka, H. Horinaka, T. Kato,T. Yasue, T. Koshikawa: Super-high brightness andhigh-spin-polarization photocathode, Appl. Phys.Express 1, 0450023 (2008)

9.281 X. Jin, B. Ozdol, M. Yamamoto, A. Mano, N. Ya-mamoto, Y. Takeda: Effect of crystal quality on per-formance of spin-polarized photocathode, Appl.Phys. Lett. 105, 203509 (2014)

9.282 T. Duden, E. Bauer: A compact electron spin polar-ization manipulator, Rev. Sci. Instrum. 66, 2861–2864 (1995)

9.283 H. Pinkvos, H. Poppa, E. Bauer, G.-M. Kim: A time-resolved SPLEEM study of magnetic microstructurein ultrathin Co films on W(110). In: Magnetism andStructure in Systems of Reduced Dimensions, ed.by R.F.C. Farrow, B. Dieny, M. Donath, A. Fert,B.D. Hermsmeier (Plenum, New York 1993) pp. 25–31

9.284 M.S. Altman, I. Hurst, G. Marx, H. Pinkvos,H. Poppa, E. Bauer: Spin polarized low energy elec-tron microscopy of surface magnetic structure, MRSProceedings 232, 125 (1991)

9.285 H. Pinkvos, H. Poppa, E. Bauer, J. Hurst: A spin-polarized low energy electron microscopy study ofthe magnetic microstructure of ultrathin epitaxialcobalt films on W(110), Ultramicroscopy 47, 339–345(1992)

9.286 H. Poppa, H. Pinkvos, K. Wurm, E. Bauer: Spin po-larized low energy electron microscopy (SPLEEM) ofsingle and combined layers of Co, Cu, and Pd onW(110), MRS Proceedings 313, 219 (1993)

9.287 M.S. Altman, H. Pinkvos, E. Bauer: Spin polarizedlow energy electron microscopy for thin film mag-netism and microstructure, J. Magn. Soc. Jpn. 19,129–134 (1995)

9.288 E. Bauer, T. Duden, H. Pinkvos, H. Poppa, K. Wurm:LEEM studies of the microstructure and magneticdomain structure of ultrathin films, J. Magn. Magn.Mater. 156, 1–6 (1996)

9.289 K. Grzelakowski, T. Duden, E. Bauer, H. Poppa,S. Chiang: A new surface microscope for magneticimaging, IEEE Trans. Magn. 30, 4500–4502 (1994)

9.290 M. Suzuki, M. Hashimoto, T. Yasue, T. Koshikawa,Y. Nakagawa, T. Konomi, A. Mano, N. Yamamoto,M. Kuwahara, M. Yamamoto, S. Okumi, T. Nakan-ishi, X. Jin, T. Ujihara, Y. Takeda, T. Kohashi,T. Ohshima, T. Saka, T. Kato, H. Horinaka: Realtime magnetic imaging by spin-polarized low en-ergy electron microscopy with highly spin-polar-ized and high brightness electron gun, Appl. Phys.Express 3, 026601 (2010)

9.291 T. Yasue, M. Suzuki, K. Tsuno, S. Goto, Y. Arai,T. Koshikawa: Novel multipole Wien filter as three-dimensional spin manipulator, Rev. Sci. Instrum.85, 043701 (2014)

9.292 T. Duden, E. Bauer: Spin-polarized low energyelectron microscopy, Surf. Rev. Lett. 5, 1213–1220(1998)

9.293 E. Bauer: Spin-polarized low energy electron mi-croscopy (SPLEEM). In: Novel Techniques for Charac-terizing Magnetic Materials, ed. by Y. Zhu (KluwerAcademic, Boston 2005) pp. 361–379

9.294 E. Bauer: Spin-polarized low energy electron mi-croscopy. In: The Handbook of Magnetism and Ad-vanced Magnetic Materials, Vol. 3, ed. by H. Kron-müller, S. Parkin (Wiley, Chichester 2007) pp. 1470–1487

9.295 N. Rougemaille, A.K. Schmid: Magnetic imag-ing with spin-polarized low-energy electron mi-croscopy, Eur. Phys. J. Appl. Phys. 50, 20101 (2010)

9.296 E. Bauer: Spin-polarized low-energy electron mi-croscopy. In: Handbook of Nanoscopy, ed. by G. vanTandeloo, D. van Dyck, S.J. Pennycook (Wiley, Wein-heim 2012) pp. 697–707

9.297 E.D. Tober, G. Witte, H. Poppa: Variable temperatureand ex-situ spin-polarized low-energy electronmicroscope, J. Vac. Sci. Technol. A 18, 1845 (2000)

9.298 T. Duden, E. Bauer: Spin-polarized low en-ergy electron microscopy of ferromagnetic layers,J. Electron Microsc. 47, 379–385 (1998)

9.299 T. Duden, E. Bauer: Influence of Au and Cu over lay-ers on the magnetic structure of Co films on W(110),Phys. Rev. B 59, 468–473 (1999)

9.300 R. Zdyb, E. Bauer: Spin dependent quantum sizeeffects in the electron reflectivity of ultrathin fer-romagnetic crystals, Surf. Rev. Lett. 9, 1485–1491(2002)

9.301 R. Zdyb, E. Bauer: Spin-resolved inelastic mean freepath of slow electrons in Fe, J. Phys. Condens. Mat-ter 25, 272201 (2013)

9.302 Y.Z. Wu, A.K. Schmid, Z.Q. Qiu: Spin-dependentquantum interference from epitaxial MgO thin filmson Fe(001), Phys. Rev. Lett. 97, 217205 (2006)

9.303 T. Duden, E. Bauer: Magnetization wrinkle in thinferromagnetic films, Phys. Rev. Lett. 77, 2308–2311(1996)

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9.304 T. Duden, E. Bauer: Magnetic domain structureand spin reorientation transition in the systemCo/Au/W(110), MRS Proceedings 475, 283 (1997)

9.305 F. El Gabaly, S. Gallego, C. Muñoz, L. Szunyogh,P. Weinberger, C. Klein, A.K. Schmid, K.F. McCarty,J. de la Figuera: Imaging spin-reorientation tran-sitions in consecutive atomic Co layers on Ru(0001),Phys. Rev. Lett. 96, 147202 (2006)

9.306 C. Klein, R. Ramchal, M. Farle, A.K. Schmid: Di-rect imaging of spin-reorientation transitions inultrathin Ni films by spin-polarized low-energyelectron microscopy, Surf. Interface Anal. 38, 1550–1553 (2006)

9.307 C. Klein, R. Ramchal, A.K. Schmid, M. Farle: Control-ling the kinetic order of spin-reorientation tran-sitions in Ni/Cu(100) films by tuning the substratestep structure, Phys. Rev. B 75, 193405 (2007)

9.308 F. El Gabaly, K.F. McCarty, A.K. Schmid, J. de laFiguera, M.C. Muñoz, L. Szunyogh, P. Weinberger,S. Gallego: Noble metal capping effects on thespin-reorientation transitions of Co/Ru(0001), NewJ. Phys. 10, 073024 (2008)

9.309 B. Santos, S. Galego, A. Mascaraque, K.F. McCarty,A. Quesada, A.T. N’Diaye, A.K. Schmid, J. de laFiguera: Hydrogen-induced reversible spin-reori-entation transition and magnetic stripe domainphase in bilayer Co on Ru(0001), Phys. Rev. B 85,134409 (2012)

9.310 Q. Wu, M.S. Altman: Spin polarized low energyelectron microscopy of quantum well resonancesin Fe films on the Cu-covered W(110) surface, Ultra-microscopy 130, 109–114 (2013)

9.311 Q. Wu, M.S. Altman: Probing buried magnetic in-terface structure with the quantum size effectin spin-dependent electron reflectivity, Ultrami-croscopy 159, 530–535 (2015)

9.312 K.L. Man, M.S. Altman, H. Poppa: Spin polarizedlow energy electron microscopy investigations ofmagnetic transitions in Fe/Cu(100), Surf. Sci. 480,163–172 (2001)

9.313 K.L. Man, W.L. Ling, S.Y. Paik, H. Poppa, M.S. Alt-man, Z.Q. Qiu: Modification of initial growth andmagnetism in Fe/Cu(100), Phys. Rev. B 65, 024409(2001)

9.314 H. Poppa, E.D. Tober, A.K. Schmid: In situ obser-vation of magnetic domain pattern evolution inapplied fields by spin-polarized low energy elec-tronmicroscopy, J. Appl. Phys. 91, 6932–6934 (2002)

9.315 R. Zdyb, E. Bauer: Magnetic domain structure andspin reorientation transition in ultrathin Fe-Co al-loy films, Phys. Rev. B 67, 134420 (2003)

9.316 R. Zdyb, A. Locatelli, S. Heun, S. Cherifi, R. Belkhou,E. Bauer: Nanomagnetism studies with spin-po-larized low energy electron microscopy and x-raymagnetic circular dichroism photo emission elec-tron microscopy, Surf. Interface Anal. 27, 239–243(2005)

9.317 C. Ji, Z. Wang, Q. Wu, L. Huang, M.S. Altman: Con-trolling magnetic interfaces using ordered surfacealloys, Phys. Rev. B 94, 134425 (2016)

9.318 T. Duden, R. Zdyb, M. Altman, E. Bauer: Micromag-netic and microcrystalline structure of ultrathin co

layers on W single crystal surfaces, Surf. Sci. 480,145–152 (2001)

9.319 K.L. Man, R. Zdyb, Y.I. Feng, T. Chan, M.S. Altman,E. Bauer: Growth morphology, structure and mag-netism of ultrathin co films on W(111), Phys. Rev. B67, 184402 (2003)

9.320 Q. Wu, R. Zdyb, E. Bauer, M.S. Altman: Growth,magnetism and ferromagnetic thickness gap in Fefilms on the W(111) surface, Phys. Rev. B 87, 104410(2013)

9.321 Y.R. Niu, K.L. Man, A. Pavlovska, E. Bauer, M.S. Alt-man: Fe onW(001): From continuous films to nano-particles: Growth and magnetic domain structure,Phys. Rev. B 95, 064404 (2017)

9.322 T. Duden, E. Bauer: Biquadratic exchange in fer-romagnetic/nonferromagnetic sandwiches: A spin-polarized low energy electron study, Phys. Rev. B59, 474–479 (1999)

9.323 T. Duden, E. Bauer: Exchange coupling in Co/Cu/Cosandwiches studied by spin-polarized low energyelectron microscopy, J. Magn. Magn. Mater. 191,301–312 (1999)

9.324 N. Rougemaille, M. Portalupi, A. Brambilla, P. Bia-gioni, A. Lanzara, M. Finazzi, A.K. Schmid, L. Duò:Exchange-induced frustration in Fe/NiO multilay-ers, Phys. Rev. B 76(6), 214425–214421 (2007)

9.325 M. Suzuki, T. Yasue, T. Koshikawa, E. Bauer: Mag-netic structure of Co/Ni thin film magnetic structureof Co/Ni thin films onW(110) studied by high bright-ness and highly spin-polarized LEEM. In: Proc. 8thInt. Symp. At. Level Charact. New Mater. DevicesALC’11, Seoul (2011) pp. 437–440

9.326 M. Suzuki, K. Kudo, K. Kojima, T. Yasue, N. Akutsu,W.A. Diño, H. Kasai, E. Bauer, T. Koshikawa: Mag-netic domain patterns on strong perpendicularmagnetization of Co/Ni multilayers as spintronicsmaterials: I. Dynamic observations, J. Phys. Con-dens. Matter 25, 406001 (2013)

9.327 K. Kudo, M. Suzuki, K. Kojima, T. Yasue, N. Akutsu,W.A. Diño, H. Kasai, E. Bauer, T. Koshikawa: Mag-netic domain patterns on strong perpendicularmagnetization of Co/Ni multilayers as spintronicsmaterials: II. Numerical simulations, J. Phys. Con-dens. Matter 25, 395005 (2013)

9.328 N. Rougemaille, A.T. N’Diaye, J. Coraux, C. Vo-Van, O. Fruchart, A.K. Schmid: Perpendicular mag-netic anisotropy of cobalt films intercalated undergraphene, Appl. Phys. Lett. 101, 142403 (2012)

9.329 H. Yang, A.D. Vu, A. Hallal, N. Rougemaille,J. Coraux, G. Chen, A.K. Schmid, M. Chshiev:Anatomy and giant enhancement of the perpen-dicular magnetic anisotropy of cobalt-grapheneheterostructures, Nano Lett. 16, 145–151 (2016)

9.330 A.D. Vu, J. Coraux, G. Chen, A.T. N’Diaye,A.K. Schmid, N. Rougemaille: Unconventionalmagnetisation texture in graphene/cobalt hybrids,Sci. Rep. 6, 24783 (2016)

9.331 R. Ramchal, A.K. Schmid, M. Farle, H. Poppa:Magnetic domains and domain-wall structure inNi/Cu(001) films imaged by spin-polarized low-en-ergy electron microscopy, Phys. Rev. B 68, 054418(2003)


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9.332 G. Chen, J. Zhu, A. Quesada, J. Li, A.T. N’Diaye,Y. Huo, T.P. Ma, Y. Chen, H.Y. Kwon, C. Won, Z.Q. Qiu,A.K. Schmid, Y.Z. Wu: Novel chiral magnetic do-main wall structure in Feo/Ni/Cu(001) films, Phys.Rev. Lett. 110, 177204 (2013)

9.333 G. Chen, T. Ma, A.T. N’Diaye, H. Kwon, C. Won, Y. Wu,A.K. Schmid: Tailoring the chirality of magnetic do-mainwalls by interface engineering, Nat. Commun.4, 2671 (2013)

9.334 G. Chen, A.T. N’Diaye, Y. Wu, A.K. Schmid: Ternarysuperlattice boosting interface-stabilized mag-netic chirality, Appl. Phys. Lett. 106, 062402 (2015)

9.335 G. Chen, A.T. N’Diaye, S.P. Kang, H.Y. Kwon, C. Won,Y. Wu, Z.Q. Qiu, A.K. Schmid: Unlocking Bloch-type chirality in ultrathin magnets through uniaxialstrain, Nat. Commun. 6, 6598 (2015)

9.336 G. Chen, A. Mascaraque, A.T. N’Diaye, A.K. Schmid:Room temperature skyrmion ground state stabi-lized through interlayer exchange coupling, Appl.Phys. Lett. 106, 242404 (2015)

9.337 G. Chen, A.K. Schmid: Imaging and tailoring thechirality of domain walls in magnetic films, Adv.Mater. 27, 5738–5743 (2015)

9.338 E. Bauer, R. Belkhou, S. Cherifi, R. Hertel, S. Heun,A. Locatelli, A. Pavlovska, R. Zdyb, N. Agarwal,H. Wang: Microscopy of mesoscopic ferromagneticsystems with slow electrons, Surf. Interface Anal.38, 1622–1627 (2006)

9.339 R. Zdyb, A. Pavlovska, M. Jałochowski, E. Bauer:Self-organized Fe nanostructures on W(110), Surf.Sci. 600, 1586–1591 (2006)

9.340 N. Rougemaille, A.K. Schmid: Self-organizationand magnetic domain microstructure of Fe nano-wire arrays, J. Appl. Phys. 99, 08S502 (2006)

9.341 H.F. Ding, A.K. Schmid, D. Li, K.Y. Guslienko,S.D. Bader: Magnetic bistability of Co nanodots,Phys. Rev. Lett. 94, 157202 (2005)

9.342 R. Zdyb, E. Bauer: Coexistence of ferromagnetismand paramagnetism in a ferromagnetic monolayer,Phys. Rev. Lett. 100, 155704 (2008)

9.343 J. de la Figuera, Z. Novotny, M. Setvin, T. Liu, Z. Mao,G. Chen, A.T. N’Diaye, M. Schmid, U. Diebold,A.K. Schmid, G.S. Parkinson: Real-space imaging ofthe Verwey transition at the (100) surface of mag-netite, Phys. Rev. B 88, 161410(R) (2013)

9.344 J. de la Figuera, L. Vergara, A.T. N’Diaye, A. Quesada,A.K. Schmid: Micromagnetism in (001) magnetiteby spin-polarized low-energy electron microscopy,Ultramicroscopy 130, 77–81 (2013)

9.345 T.O. Menteş, A. Locatelli: Angle-resolved x-rayphotoemission electron microscopy, J. ElectronSpectrosc. Relat. Phenom. 185, 323–329 (2012)

9.346 A. Locatelli, L. Aballe, T.O. Menteş, M. Kiskinova,E. Bauer: Photoemission electron microscopy withchemical sensitivity: SPELEEM methods and appli-cations, Surf. Interface Anal. 38, 1554–1557 (2006)

9.347 T.O. Menteş, M.A. Niño, A. Locatelli: Spectromi-croscopy with low-energy electrons: LEEM andXPEEM studies at the nanoscale, e-J. Surf. Sci. Na-notechnol. 9, 72–79 (2011)

9.348 E. Bauer, S. Cherifi, L. Daeweritz, M. Kaestner,S. Heun, A. Locatelli: Low-energy electron mi-

croscopy/x-ray magnetic circular dichroism pho-toemission electron microscopy study of epitaxialMnAs on GaAs, J. Vac. Sci. Technol. B 20, 2539–2542(2002)

9.349 E. Bauer, R. Belkhou, S. Cherifi, A. Locatelli,A. Pavlovska, N. Rougemaille: Magnetostructure ofMnAs on GaAs revisited, J. Vac. Sci. Technol. B 25,1470–1475 (2007)

9.350 A.B. Pang, A. Pavlovska, L. Däweritz, A. Locatelli,E. Bauer, M.S. Altman: LEEM image phase contrastof MnAs stripes, Ultramicroscopy 130, 7–12 (2013)

9.351 R. Engel-Herbert, D.M. Schaadt, S. Cherifi, E. Bauer,R. Belkhou, A. Locatelli, S. Heun, A. Pavlovska,J. Mohanty, K.H. Ploog, T. Hesjedal: The natureof charged zig-zag domains in MnAs thin films,J. Magn. Magn. Mater. 305, 457–463 (2006)

9.352 R. Engel-Herbert, T. Hesjedal: Micromagnetic anal-ysis of unusual, V-shaped domain transitions inMnAs nanowires, J. Magn. Magn. Mater. 323, 1840–1845 (2011)

9.353 K.S.R. Menon, S. Mandal, J. Das, T.O. Menteş,M.A. Niño, A. Locatelli, R. Belkhou: Surface anti-ferromagnetic domain imaging using low-energyunpolarized electrons, Phys. Rev. B 84, 132402 (2011)

9.354 R. Hertel, O. Fruchart, S. Cherifi, P.-O. Jubert,S. Heun, A. Locatelli, J. Kirschner: Three-dimen-sional magnetic-flux-closure patterns in meso-scopic Fe islands, Phys. Rev. B 72(11), 214409–214401(2005)

9.355 A. Mascaraque, L. Aballe, J.F. Marco, T.O. Menteş,F. El Gabaly, C. Klein, A.K. Schmid, K.F. McCarty,A. Locatelli, J. de la Figuera: Measuring the mag-netization of three monolayer thick Co islands andfilms by x-ray dichroism, Phys. Rev. B 80, 172401(2009)

9.356 M. Monti, B. Santos, A. Mascaraque, O.R. dela Fuente, M.A. Niño, T.O. Menteş, A. Locatelli,K.F. McCarty, J.F. Marco, J. de la Figuera: Magnetismin nanometer-thick magnetite, Phys. Rev. B 85,020404 (2012)

9.357 T.O. Menteş, A. Locatelli, L. Aballe, A. Pavlovska,E. Bauer, T. Pabisiak, A. Kiejna: Surface modifica-tion of oxides by electron-stimulated desorptionfor growth-mode control of metal films: Exper-iment and density-functional calculations, Phys.Rev. B 76, 155413 (2007)

9.358 S. Günther, S. Bocklein, R. Reichelt, J. Wintterlin,A. Barinov, T.O. Menteş, M.A. Niño, A. Locatelli: Sur-face patterning of silver using an electron- or pho-ton-assisted oxidation reaction, ChemPhysChem 11,1525–1532 (2010)

9.359 J. Laverock, S. Kittiwatanakul, A.A. Zakharov,Y.R. Niu, B. Chen, S.A. Wolf, J.W. Lu, K.E. Smith: Di-rect observation of decoupled structural and elec-tronic transitions and an ambient pressure mono-cliniclike metallic phase of VO2, Phys. Rev. Lett. 113,216402 (2014)

9.360 J. Laverock, S. Kittiwatanakul, A.A. Zakharov,Y.R. Niu, B. Chen, J. Kuyyalil, S.A. Wolf, J.W. Lu,K.E. Smith: Simultaneous spectroscopic, diffrac-tion and microscopic study of the metal-insulator

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transition of VO2, MRS Proceedings 1730, Mrsf14-1730-n05-04 (2015)

9.361 F. Genuzio, A. Sala, T. Schmidt, D. Menzel,H.-J. Freund: Phase transformations in thin ironoxide films: Spectromicroscopic study of velocityand shape of the reaction fronts, Surf. Sci. 648, 177–187 (2016)

9.362 L.K.E. Ericsson, K.O. Magnusson, A.A. Zakharov: ZnOnanocrystals on SiO2/Si surfaces thermally cleanedin ultrahigh vacuum and characterized using spec-troscopic photoemission and low energy electronmicroscopy, J. Vac. Sci. Technol. A 28, 438–442(2010)

9.363 E. Bauer, K.L. Man, A. Pavlovska, A. Locatelli,T.O. Menteş, M.A. Niño, M.S. Altman: Fe3S4 (greig-ite) formation by vapor-solid reaction, J. Mater.Chem. A 2, 1903–1913 (2014)

9.364 K.L. Man, A. Pavlovska, E. Bauer, A. Locatelli,T.O. Menteş, M.A. Niño, G.K.L. Wong, I.K. Sou,M.S. Altman: Growth, reaction and nanowire for-mation of Fe on the ZnS(100) surface, J. Phys.Condens. Matter 26, 315006 (2014)

9.365 M. Ewert, T. Schmidt, J.I. Flege, I. Heidmann,T. Grzela, W.M. Klesse, M. Foerster, L. Aballe,T. Schroeder, J. Falta: Morphology and chemi-cal composition of cobalt germanide islands onGe(001), Nanotechnology 27, 325705 (2016)

9.366 M. Monti, B. Santos, A. Mascaraque, O.R. dela Fuente, M.A. Niño, T.O. Menteş, A. Locatelli,K.F. McCarty, J.F. Marco, J. de la Figuera: Oxida-tion pathways in bicomponent ultrathin iron oxidefilms, J. Phys. Chem. C 116, 11539–11547 (2012)

9.367 A. Cornish, T. Eralp, A. Shavorskiy, R.A. Bennett,G. Held, S.A. Cavill, A. Potenza, H. Marchetto,S.S. Dhesi: Oxidation of polycrystalline Ni studiedby spectromicroscopy:Phase separation in the earlystages of crystallite growth, Phys. Rev. B 81, 085403(2010)

9.368 L. Aballe, S. Matencio, M. Foerster, E. Barrena,F. Sánchez, J. Fontcuberta, C. Ocal: Instability andsurface potential modulation of self-patterned(001)SrTiO3 surfaces, Chem. Mater. 27, 6198–6204(2015)

9.369 L. Aballe, A. Barinov, A. Locatelli, S. Heun, M. Kiski-nova: Tuning surface reactivity via electron quan-tum confinement, Phys. Rev. Lett. 93, 196103 (2004)

9.370 L. Aballe, A. Barinov, N. Stojic, N. Binggeli,T.O. Menteş, A. Locatelli, M. Kiskinova: The electrondensity decay length effect on surface reactivity,J. Phys. Condens. Matter 22, 015001 (2010)

9.371 B. Kaemena, S.D. Senanayake, A. Meyer, J.T. Sad-owski, J. Falta, J.I. Flege: Growth and morphologyof ceria on ruthenium (0001), J. Phys. Chem. C 117,221–232 (2013)

9.372 J.I. Flege, J. Höcker, B. Kaemena, T.O. Menteş,A. Sala, A. Locatelli, S. Gangopadhyay, J.T. Sad-owski, S.D. Senanayake, J. Falta: Growth and char-acterization of epitaxially stabilized ceria(001) na-nostructures on Ru(0001), Nanoscale 8, 10849–10856 (2016)

9.373 J. Höcker, T.O. Menteş, A. Sala, A. Locatelli,T. Schmidt, J. Falta, S.D. Senanayake, J.I. Flege: Un-

raveling the dynamic nanoscale reducibility (Ce4+

! Ce3+) of CeOx-Ru in hydrogen activation, Adv.Mater. Interfaces 2, 1500314 (2015)

9.374 J. Höcker, J.-O. Krisponeit, J. Cambeis, A. Zakharov,Y. Niu, G. Wei, L. Colombi Ciacchi, J. Falta, A. Schae-fer, J.I. Flege: Growth and structure of ultrathinpraseodymium oxide layers on ruthenium(0001),Phys. Chem. Chem. Phys. 19, 3480–3485 (2017)

9.375 D.C. Grinter, C.-M. Yim, C.L. Pang, B. Santos,T.O. Menteş, A. Locatelli, G. Thornton: Oxidationstate imaging of ceria island growth on Re(0001),J. Phys. Chem. C 117, 16509–16514 (2013)

9.376 D.C. Grinter, C. Muryn, B. Santos, B.-J. Shaw,T.O. Menteş, A. Locatelli, G. Thornton: Spectromi-croscopy of a model water–gas shift catalyst: Goldnanoparticles supported on ceria, J. Phys. Chem. C118, 19194–19204 (2014)

9.377 D.C. Grinter, C. Muryn, A. Sala, C.-M. Yim, C.L. Pang,T.O. Menteş, A. Locatelli, G. Thornton: Spillover re-oxidation of ceria nanoparticles, J. Phys. Chem. C120, 11037–11044 (2016)

9.378 A. Locatelli, C. Sbraccia, S. Heun, S. Baroni,M. Kiskinova: Energetically driven reorganization ofa modified catalytic surface under reaction condi-tions, J. Am. Chem. Soc. 127, 2351–2357 (2005)

9.379 A. Locatelli, T.O. Menteş, L. Aballe, A. Mikhailov,M. Kiskinova: Formation of regular surface-sup-ported mesostructures with periodicity controlledby chemical reaction rate, J. Phys. Chem. B 110,19108–19111 (2006)

9.380 A. Locatelli, L. Aballe, T.O. Menteş, F.Z. Guo,M. Kiskinova: A spectro-microscopic study of thereactive phase separation of AuCPd and O onRh(110), Surf. Sci. 601, 4663–4668 (2007)

9.381 F. Lovis, M. Hesse, A. Locatelli, T.O. Menteş,M.A. Nino, G. Lilienkamp, B. Borkenhagen, R. Im-bihl: Self-organization of ultrathin vanadium ox-ide layers on a Rh(111) surface during a catalyticreaction. Part II: A LEEM and spectromicroscopystudy, J. Phys. Chem. C 115, 19149–19157 (2011)

9.382 F. Lovis, T. Smolinsky, A. Locatelli, M.A. Niño, R. Im-bihl: Chemical waves and rate oscillations in theH2C O2 reaction on a bimetallic Rh(111)/Ni catalyst,J. Phys. Chem. C 116, 4083–4090 (2012)

9.383 S. Günther, H. Liu, T.O. Menteş, A. Locatelli, R. Im-bihl: Spectromicroscopy of pulses transporting al-kali metal in a surface reaction, Phys. Chem. Chem.Phys. 15, 8752–8764 (2013)

9.384 S. Günther, S. Böcklein, J. Wintterlin, M.Á. Niño,T.O. Menteş, A. Locatelli: Locating catalytically ac-tive oxygen on Ag(111)—A spectromicroscopy study,ChemCatChem 5, 3342–3350 (2013)

9.385 A. Locatelli, C. Wang, C. Africh, N. Stojić, T.O. Menteş,G. Comelli, N. Binggeli: Temperature-driven re-versible rippling and bonding of a graphene su-perlattice, ACS Nano 7, 6955–6963 (2013)

9.386 L.I. Johansson, S. Watcharinyanon, A.A. Zakharov,T. Iakimov, R. Yakimova, C. Virojanadara: Stackingof adjacent graphene layers grown on C-face SiC,Phys. Rev. B 84, 125405 (2011)

9.387 C. Coletti, S. Forti, A. Principi, K.V. Emtsev, A.A. Za-kharov, K.M. Daniels, B.K. Daas, M.V.S. Chan-


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drashekhar, T. Ouisse, D. Chaussende, A.H. Mac-Donald, M. Polini, U. Starke: Revealing the elec-tronic band structure of trilayer graphene on SiC: Anangle-resolved photoemission study, Phys. Rev. B88, 155439 (2013)

9.388 M.S. Nevius, F. Wang, C. Mathieu, N. Barrett, A. Sala,T.O. Menteş, A. Locatelli, E.H. Conrad: The bottom-up growth of edge specific graphene nanoribbons,Nano Lett. 14, 6080–6086 (2014)

9.389 F. Wang, G. Liu, S. Rothwell, M.S. Nevius, C. Math-ieu, N. Barrett, A. Sala, T.O. Menteş, A. Locatelli,P.I. Cohen, L.C. Feldman, E.H. Conrad: Pattern in-duced ordering of semiconducting graphene rib-bons grown from nitrogen-seeded SiC, Carbon 82,360–367 (2015)

9.390 S. Watcharinyanon, C. Xia, Y. Niu, A.A. Zakharov,L.I. Johansson, R. Yakimova, C. Virojanadara:Soft x-ray exposure promotes Na intercalation ingraphene grown on Si-face SiC, Materials 8, 4768–4777 (2015)

9.391 C. Xia, L.I. Johansson, A.A. Zakharov, L. Hult-man, C. Virojanadara: Effects of Al on epitaxial

graphene grown on 6H-SiC(0001), Mater. Res. Ex-press 1, 015606 (2014)

9.392 S. Forti, A. Stöhr, A.A. Zakharov, C. Coletti, K.V. Emt-sev, U. Starke: Mini-Dirac cones in the band struc-ture of a copper intercalated epitaxial graphenesuperlattice, 2D Materials 3, 035003 (2016)

9.393 C. Africh, C. Cepek, L.L. Patera, G. Zamborlini,P. Genoni, T.O. Menteş, A. Sala, A. Locatelli,G. Comelli: Switchable graphene-substrate cou-pling through formation/dissolution of an interca-lated Ni-carbide layer, Sci. Rep. 6, 19734 (2016)

9.394 A. Stöhr, S. Forti, S. Link, A.A. Zakharov, K. Kern,U. Starke, H.M. Benia: Intercalation of grapheneon SiC(0001) via ion-implantation, Phys. Rev. B 94,085431 (2016)

9.395 G. Zamborlini, M. Imam, L.L. Patera, T.O. Menteş,N. Stojić, C. Africh, A. Sala, N. Binggeli, G. Comelli,A. Locatelli: Nanobubbles at GPa pressure undergraphene, Nano Lett. 15, 6162–6169 (2015)

9.396 T.O. Menteş, A. Sala, A. Locatelli, E. Vescovo,J.M. Ablett, M.A. Niño: Phase coexistence in two-dimensional Fe0.70Ni0.30 films on W(110), e-J. Surf.Sci. Nanotechnol. 13, 256–260 (2015)

Ernst BauerDept. of PhysicsArizona State UniversityTempe, AZ, [email protected]

Ernst Bauer is a German-American physicist who received his PhD from the Universityof Munich in 1955. He is a pioneer in surface and thin-film physics, and is the inventorof low-energy electron microscopy (LEEM). He is currently involved in a number ofinternational research collaborations investigating magnetic materials and catalysisusing a multi-technique approach.

LEEM,SPLEEM 9.LEEM,SPLEEMandSPELEEMincluding: low-energy electron microscopy (LEEM), its extension...

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