Electron-hole recombination on ZnO(0001) single-crystal surface studied by time-resolved soft X-ray photoelectron spectroscopyR. Yukawa, S. Yamamoto, K. Ozawa, M. Emori, M. Ogawa, Sh. Yamamoto, K. Fujikawa, R. Hobara, S. Kitagawa, H. Daimon, H. Sakama, and I. Matsuda Citation: Applied Physics Letters 105, 151602 (2014); doi: 10.1063/1.4897934 View online: http://dx.doi.org/10.1063/1.4897934 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Time-resolved x-ray photoelectron spectroscopy techniques for real-time studies of interfacial charge transferdynamics AIP Conf. Proc. 1525, 475 (2013); 10.1063/1.4802374 Carrier recombination losses in inverted polymer: Fullerene solar cells with ZnO hole-blocking layer fromtransient photovoltage and impedance spectroscopy techniques J. Appl. Phys. 109, 074514 (2011); 10.1063/1.3561437 Experimental observation of bulk band dispersions in the oxide semiconductor ZnO using soft x-ray angle-resolved photoemission spectroscopy J. Appl. Phys. 105, 122403 (2009); 10.1063/1.3116223 Donor-acceptor pair luminescence of nitrogen-implanted ZnO single crystal J. Appl. Phys. 97, 043528 (2005); 10.1063/1.1854208 Surface chemistry of II–VI semiconductor ZnSe studied by time of flight secondary ion mass spectrometry and x-ray photoelectron spectroscopy J. Vac. Sci. Technol. B 16, 3048 (1998); 10.1116/1.590340

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Electron-hole recombination on ZnO(0001) single-crystal surface studiedby time-resolved soft X-ray photoelectron spectroscopy

R. Yukawa,1 S. Yamamoto,1 K. Ozawa,2 M. Emori,3 M. Ogawa,1 Sh. Yamamoto,1

K. Fujikawa,1 R. Hobara,1 S. Kitagawa,4 H. Daimon,4 H. Sakama,3 and I. Matsuda1,a)

1Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan2Department of Chemistry and Materials Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551,Japan3Department of Physics, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan4Nara Institute of Science and Technology (NAIST), Ikoma, Nara 630-0192, Japan

(Received 16 July 2014; accepted 27 September 2014; published online 13 October 2014)

Time-resolved soft X-ray photoelectron spectroscopy (PES) experiments were performed with time

scales from picoseconds to nanoseconds to trace relaxation of surface photovoltage on the

ZnO(0001) single crystal surface in real time. The band diagram of the surface has been obtained

numerically using PES data, showing a depletion layer which extends to 1 lm. Temporal evolution

of the photovoltage effect is well explained by a recombination process of a thermionic model, giv-

ing the photoexcited carrier lifetime of about 1 ps at the surface under the flat band condition. This

lifetime agrees with a temporal range reported by the previous time-resolved optical experiments.VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4897934]

Zinc oxide (ZnO) has attracted much attention for its

optoelectronic applications, e.g., photovoltaics, light-emitting

diode, and photocatalyst.1–4 One of the important factors in

the photo-induced function is dynamics of photoexcited car-

riers that govern optical and electronic phenomena, and their

lifetime is directly related to efficiency of the optoelectronic

functionalities. Time-resolved measurements of reflection,

transmission, and photoluminescence spectroscopies have

been carried out by a pump-probe method on various ZnO

samples, i.e., nanocrystals and single crystals.5–10 These

authors have reported that the recombination processes of the

photo-generated electron-hole (e-h) pairs or excitons are

enhanced by the trapping of the carriers at impurity levels in

the bulk band gap. Recently, a time-resolved photoelectron

spectroscopy (PES) experiments were carried out on a

ZnOð10�10Þ surface to trace the relaxation process of the sur-

face photovoltage (SPV) effect to clarify the photo-generated

carrier dynamics as one of the important elementary proc-

esses.11 Surface-sensitive results are obtained by PES as com-

pared to optical methods.5–10 The temporal variation of the

relaxation process was described in terms of surface e-hrecombination after the carrier transfer from the internal bulk

to the surface over the surface potential. Time-resolved PES

measures the temporal variations of the surface potential. On

the other hand, time-resolved optical reflectivity and photolu-

minescence experiments monitor the variations of the reflec-

tivity by the change in the carrier density and of

photoluminescence by electron-hole recombination, respec-

tively. Each experimental technique measures different physi-

cal quantities in the relaxation processes of photo-excitation.

Therefore, it is essential to understand the systematic relation-

ship between the results obtained by different experimental

probes for developing optoelectronic devices.

Here, we report the detailed investigation of the time-

resolved soft X-ray PES experiments on a ZnO(0001)

surface to clarify the surface recombination process. A ZnO

crystal has wurtzite structure and the (0001) surface is ter-

minated with zinc atoms (Fig. 1(a)). The e-h pairs are gen-

erated by the irradiation of the femtosecond-pulse laser, and

the following charge separation leads to the SPV genera-

tion. The relaxation of the SPV was found to take nanosec-

onds. The relaxation process is fairly described by a

thermionic model, in which the photoexcited carriers over-

come the surface potential, Vs, to be transported to the sur-

face and then recombine with the counterparts accumulated

at the surface. The potential, generated by the bulk band-

bending effect near the surface and the Vs value, was eval-

uated by valence band PES measurements of the sample

surface. By tracing the relaxation of the SPV effect, we

determined a photoexcited carrier lifetime at the surface

under the flat band condition on the ZnO(0001) surface to

be 1.7 ps. The obtained carrier lifetime at the surface is

close to the values reported by the previous time-resolved

optical experiments.6–8 The agreements draw an overall

temporal picture during the relaxation of the photovoltage

effect at the ZnO surface.

Time-resolved PES experiments were conducted with a

pump-probe method at the soft X-ray beamline BL07LSU of

SPring-8.13–15 The photon energies h� of the pumping laser

and the probing synchrotron radiation were 3.1 eV (a pulse

width of <50 fs) and 253 eV (about 50 ps), respectively. A

detailed experimental set-up is described elsewhere.13 All

the measurements were performed at room temperature. A

hydrothermally grown ZnO(0001) wafer was commercially

purchased from Goodwill, Russia. Figure 1(b) shows an

absorption spectrum of the ZnO wafer taken at a photon

energy range of 1.2–3.7 eV. The absorption edge is located

at h�¼ 3.16 eV, which is smaller than the bulk band gap of

ZnO (EZnOg ¼ 3:37 eV).16 The spectral features within the

band gap are likely assigned to the defect or impurity states

that would be a recombination center of the photoexcited

carriers.17 The surface of the ZnO wafer was cleaned by Arþa)Electronic mail: imatsuda@issp.u-tokyo.ac.jp

0003-6951/2014/105(15)/151602/4/$30.00 VC 2014 AIP Publishing LLC105, 151602-1

APPLIED PHYSICS LETTERS 105, 151602 (2014)

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sputtering for 10 min and by successive annealing at

920–1000 K for 10 min in an oxygen atmosphere of

2� 10�4 Pa. The quality of the crystal surface was ascer-

tained by a 1� 1 pattern of low-energy electron diffraction.

Figure 1(c) is a valence band PES spectrum that shows

a spectral edge at the binding energy of EB¼ 2.66 eV. Since

the escape depth of photoelectrons is about 1 nm,18 the

energy position corresponds to the valence band maximum

(VBM) of the bulk band near the surface. Using the effec-

tive mass of 0.3 me (Refs. 19 and 20) (me is the free electron

mass) and a bulk carrier density, the energy position of the

conduction band minimum (CBM) in the bulk was calcu-

lated to be located 0.26 eV above the Fermi level. The bulk

carrier density was obtained by Hall measurements to be

2� 1014 cm�3, which is of the same order of magnitude

with the carrier densities of ZnO crystals grown by the

hydrothermal method.21 The amount of the surface poten-

tial Vs was determined to be Vs¼ 0.45 eV from EZnOg , the

VBM at the surface, and the CBM in the bulk. Using a rela-

tive dielectric constant of �¼ 7.88,22 the band diagram at

the ZnO(0001) surface was calculated self-consistently by

solving Poisson’s equation with the Fermi-Dirac distribu-

tion function so that Vs and the CBM position in the bulk

agree with the experimentally obtained results (Fig. 1(d)).23

In the calculation, the distribution of the bulk carriers

within the band bending region was solved under the

assumption that the bulk donors, whose density ND was the

same as the bulk carrier density, were distributed uniformly

in the sample. The space-charge layer is a depletion-type

and extends to 1 lm from the surface. The depth of the

depletion layer LD is roughly estimated by the equation23

LD ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2Vs��0=ðe2NDÞ

p, where �0 is the vacuum dielectric

constant. LD¼ 1400 nm is obtained for our sample, support-

ing the calculated band structure in Fig. 1(d). With laser

irradiation on the surface, the photoexcited electrons and

holes are transferred to the bulk and surface sides, respec-

tively, by the driving force of the potential gradient near the

surface. As a result, reduction of the bulk band bending and

the potential variation by the surface photovoltage effect

VSPV are induced.24–26

The energy variation of the bulk bands of a material is

directly traced by following the PES peak positions.24–26

Figure 2(a) shows the Zn 3d peaks with and without pump-

ing laser irradiation. The shift of the Zn 3d peak to the higher

binding energy side is clearly observed. Since the bulk band

gap of ZnO is 3.37 eV, the direct transition of electrons from

the VBM to the CBM in a ZnO crystal requires the photon

energies larger than 3.37 eV. Thus, the optical pumping asso-

ciates with a multiphoton-absorption process or an optical

transition mediated by the in-gap states as detected in

Fig. 1(b).

The laser power dependence of the SPV shifts is shown

in Fig. 2(b). The VSPV increases with the laser intensity (I),and the data are fitted by the following equation:26

VSPV ¼ g0kT lnð1þ cIÞ; (1)

where k, T, and c denote the Boltzmann constant, tempera-

ture, and a proportional factor, respectively. g0 is an ideal-

ity factor, which is obtained from the curve fitting to be

FIG. 1. (a) Atomic structure of an ideal

ZnO crystal.12 The formation of facet

structures is neglected in the figure. (b)

Absorption spectrum of the ZnO crys-

tal with an arrow indicating the bulk

band gap position, EZnOg ¼ 3:37 eV. (c)

A valence band PES spectrum taken at

the normal emission with a photon

energy of h�¼ 253 eV. The spectral

edge corresponds to the bulk valence

band maximum (VBM) at the surface.

(d) Calculated bulk band bending near

the ZnO(0001) surface.

151602-2 Yukawa et al. Appl. Phys. Lett. 105, 151602 (2014)

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2.1. The proportional factor, c, relates the number of pho-

toexcited carriers against the laser intensity I. In the pres-

ent case, we obtained c ¼ 1:9� 10�4 cm2lJ�1 for the ZnO

crystal irradiated by the laser light of h�¼ 3.1 eV. This cvalue is much smaller than c ¼ 10� 20 cm2lJ�1, which is

reported on Si crystal surfaces pumped by photons of

h�¼ 1.55 eV.25 The difference can be understood by the

photoexcitation process. In the latter case, the photon

energy exceeds the Si bulk band gap (ESig ¼ 1:1 eV), and

the photoexcitation proceeds with a single step. The former

case may require the multiphoton-absorption process for

photoexcitation, which likely results in the smaller c value

for the present ZnO experiment.

Relaxation of the SPV effect proceeds through recombi-

nation between the holes accumulated at the surface and the

electrons transferred to the surface (Fig. 3, inset).11,25–27

Figure 3(a) shows the temporal variation of VSPV that shows

completion of measurable relaxation after 20 ns. The time

dependence is well-fitted with the following equation of the

thermionic relaxation model11,25–27 as shown in the inset in

Fig. 3(a)

VSPV tð Þ¼�gkT ln 1� 1�exp �VSPV 0ð ÞgkT

!( )e�t=ss

" #; (2)

where VSPV (0) is VSPV at t¼ 0, and ss is a relaxation time in

the absence of the SPV (a dark carrier lifetime). The data

were well-fitted as displayed in Fig. 3. The ideality factor

was determined to be g¼ 2.0, which is almost the same as g0

obtained from the power dependence in Fig. 2(b). The ideal-

ity factors of the Schottky contacts on various metal/ZnO

interfaces28 lie in the range between 1 and 5, and the present

g (g0) values are consistent with these results. In the thermi-

onic relaxation model, the electron-hole recombination time

depends on the amount of bulk band bending in a depletion

layer, and thus, it varies with the delay time. The fitted relax-

ation time of ss¼ 13.6 ns corresponds to the value evaluated

for the initial band bending state before the optical pumping

(the dark condition with a barrier height of Vs¼ 0.45 eV).

In the case of the flat band condition, or absence of the

band bending effect, the carrier lifetime s0 can be expressed

as a following equation:11,25–27

s0 ¼ ss exp � Vs

gkT

� �: (3)

By using the band parameter of Vs¼ 0.45 eV, as depicted in

Fig. 1, one obtains s0¼ 1.7 ps for the present ZnO(0001)

sample surface. The surface recombination time of 1.7 ps for

the photoexcited carriers at the ZnO(0001) surface is in good

agreement with values of the surface carrier decay time (�1

ps) reported in the literatures of time-resolved reflectivity

and transmissivity experiments.5–8 These agreements indi-

cate that the relaxation of the SPV effect on the ZnO(0001)

crystal proceeds with thermal diffusion of the electrons over

the surface potential from the internal bulk, followed by e-hrecombination at the same recombination centers reported in

the literatures.5–8 A proper value of the recombination time

allows one to evaluate the diffusion length of the photocar-

riers as l ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis0lkT=e

p, where l is a carrier mobility. Taking

the value (l ¼ 131� 205 cm2 V�1 s�1) for a bulk ZnO crys-

tal from the reference,28 the diffusion length is estimated as

l� 30 nm at room temperature. Previous time-resolved

reflectivity research estimated the thickness of the surface

layer, which is responsible for e-h recombination, to be

30–100 nm.6 It was discussed that the density of singly ion-

ized oxygen vacancy traps (which can be the recombination

centers) is much higher in such a surface recombination layer

than in the interior of the crystal.6,8,17 Therefore, the

FIG. 2. (a) PES spectra (h�¼ 253 eV)

of the Zn 3d states with (orange

circles) or without (black squares) the

pumping laser. The smoothed curves

are shown as solid lines. The pumped

spectrum was taken at the delay time

of t¼ 1 ns with the laser intensity (I) of

998 mJ/cm2/pulse. (b) VSPV taken at

delay time of t¼ 1 ns with different I.The solid curve is obtained by fitting

the SPV shifts using Eq. (1).

FIG. 3. Time-dependences of (a) VSPV taken with the laser intensity of

113 mJ/cm2/pulse, and (b) carrier lifetime (st) during the relaxation of the

SPV effect. The inset shows the schematic drawing of electron-hole recom-

bination at or near the surface. In (a), the data points are curve-fitted with

Eq. (2) of the thermionic model. Variation of st is obtained with

st ¼ s0 exp fðVs � VSPVÞ=ðgkTÞg, where g¼ 2.0 and s0¼ 1.7 ps.

151602-3 Yukawa et al. Appl. Phys. Lett. 105, 151602 (2014)

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163.221.235.114 On: Tue, 21 Oct 2014 07:15:05

photocarriers located between the surface and 30 nm from

the surface recombination layer play an important role for

the ultrafast (�1 ps) recombination process in the flat-band

ZnO crystals. On the other hand, it has been known in sur-

face science that the electronic properties of the topmost sur-

face electronic structure are completely different from those

in the bulk,29 and electronic states localized in the topmost

surface (surface states) can also become the recombination

centers in the bulk band gap. We infer that e-h recombination

takes place at the trapping sites at the topmost surface (the

surface state) and in the subsurface layer (the impurity states)

with the different carrier-capturing cross-sections.

The bulk band-bending effect, dealt in the present work,

is inevitable in studying carrier dynamics on semiconductor

surfaces. The amount of band bending depends sensitively

on the surface and its treatment.23,30 The flat-band carrier

lifetime, s0, allows evaluation of the relaxation time of vari-

ous depletion layers depending on the surface and its treat-

ment. The carrier lifetime increases exponentially with the

amount of band bending at the surface (Vs). The relation can

be used to design ZnO-based photocatalysts or photovoltaics

elements.1–4

In summary, the time-resolved soft X-ray PES experi-

ment was carried out on the ZnO(0001) surface to trace

relaxation of the SPV effect. With the simultaneous evalua-

tion of the bulk band bending effect, the flat-band carrier

lifetime at the surface was determined to be 1.7 ps, which

was the same as those reported by the previous time-

resolved optical experiments. The consistency allows one to

draw the overall temporal evolution of photoexcited carriers

at the ZnO surface that leads to both developments of a new

optoelectronically functional materials and improvements of

the efficiency.

The authors would like to thank N. Sarukura, T.

Shimizu, Y. Minami, and R. Arita for fruitful discussions

and technical assistance in reproducing the experiments of

the absorption spectrum. We are grateful to T. Sakurai for

help with the Hall measurements. This work was partly

supported by JSPS (Grant Nos. KAKENHI 23560020 and

25870193). This work was performed using facilities of the

Synchrotron Radiation Research Organization, The

University of Tokyo (Proposal No. 7401 for 2009-2013).

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