Chemical and Structural Characterization of Epitaxial Compound Semiconductor Layers using X=Ray Photoelectron Diffraction

Stephen Evans and Michael D. Scott Edward Davies Chemical Laboratories, University College of Wales, Aberystwyth, Dyfed, SY23 lNE, UK

The application of angle-resolved X-ray photoelectron spectroscopy (X-ray photoelectron diffraction) to characterize thin layers of compound semiconductors grown on single crystal substrates is described. in studies of ZnSe grown on (100) GaAs, confirmation of the epitaxial character of the growth, together with a quantitative estimate (f5%) of the stoichiometry of the ZnSe layer, were achieved by means of a single experiment. The techniques described are equally applicable to very thin layers (<lo nm) which cannot easily be characterized by established methods.

INTRODUCTION

There is currently considerable interest in developing techniques for the epitaxial growth of thin layers of binary, ternary and quaternary compound semiconduc- tors (of Groups 111-V, IV-VI and 11-VI) on suitably chosen high quality single crystal substrates. This inter- est has arisen largely because the reliable and predict- able growth of such layers would open up a wide range of possibilities for the development of technologically and commercially important homo- and heterostructure optoelectronic devices.’ Possible devices range from solar cells to the light emitting diodes, photodetectors and lasers currently of great importance fo; the develop- ment of optical communications systems.

The quality of thin epitaxially grown layers is critically dependent on the substrate ~ s e d . ~ , ~ Lattice parameters, crystallographic orientations, defect concentrations and coefficients of thermal expansion are amongst the numerous factors which must be considered when choosing an appropriate substrate. The most commonly adopted materials are consequently the extensively researched 111-V binary semiconductors, e.g. GaAs (employed in this study) and InP, which are now readily available as large well-characterized wafers with very low defect concentrations. The quality of a grown layer is also critically dependent on its rate of growth, and in fact the rates employed in most device fabrication tech- niques have to be maintained at a very low level to ensure good layer crystallinity. Relatively thick layers are thus not always readily obtainable, and indeed may not be required in device manufacture, which often involves the production of only very thin films.

There thus exists an urgent need for characteriza- tion techniques which are applicable to very thin films (<<1 Fm) and which can establish unambiguously (a) whether a grown layer is monocrystalline or polycrystal- line, and (in some cases) whether its crystal structure is the same as that of the substrate; (b) whether it is epitaxial with the substrate; and (c) to what extent a grown layer is non-stoichiometric, both in the bulk and in the exposed surface.

For relatively thick layers, X-ray crystallography can be used to answer the first two questions, and electron microprobe analysis the last, in part, but the necessary procedures become increasingly less straightforward as the layer thickness decreases. In this paper we describe the characterization of thin layers of ZnSe (grown on GaAs) by angle-resolved X-ray photoelectron spectro- scopy (XPS), demonstrating the resoIution of ail three issues more rapidly than would have been possible using other techniques.

The crystallographic aspects are investigated by monitoring the diffraction of the initially isotropically ejected photoelectrons by the single crystal solid sur- rounding the core-ionized atoms. The existence of such effects has been known for a number of years,5 but it is only very recently that their potential for revealing structural information has become recognized.6 The required analytical (chemical) information in the pres- ent application is derived from the mean (angle- integrated) XPS peak intensity ratios between the relevant core-level peaks: and again it is only recently that models have been developed and evaluated which are sufficiently accurate and reliable to be of value in such a context.’

EXPERIMENTAL

The electron spectroscopic measurements were made using an AEI (now Kratos) ES200A electron spec- trometer with a Mg Ka X-ray source. Results from two samples are reported here; more extensive studies are in progress. The present specimens were an ‘as received’ GaAs (100) single cryst$ supstrate (MCP Ltd, Wembley; n-type, 6.7 x 10 cm- ) and a comparable crystal on which a ZnSe layer had been deposited by H2 transport of elemental vapours in an open tube apparatus as described elsewhere.’ Both samples were approximately 15 x 6 x 0.2 mm in size, and were moun- ted with a (110) direction parallel to the axis of rotation of the spectrometer probe. Much smaller samples have,

@ Heyden & Son Ltd, 1981

CCC-0142-2421/81/0003-0269 $01 S O SURFACE AND INTERFACE ANALYSIS, VOL. 3, NO. 6, 1981 269

S. EVANS AND M. D. SCOTT

however, subsequently been studied successfully, using suitably increased data acquisition times.

Initially the GaAs crystal was found to have a poly- crystalline surface oxide layer (i.e. one showing no oscillatory variations in peak intensity with the electron take-off angle) and only a very weak photoelectron diffraction pattern (range * 7 % - c f . Fig. 1) could be detected from the underlying GaAs. Although a sub- stantial chemical shift (3.3 eV) was only observed on the As core photoelectron peaks, those of Ga being merely broadened, the existence of oxidized Ga in addition to the oxidized As at the surface was clearly evident from the angular variation in the relative intensities of the two well-resolved components (maxima separated by 4 eV) of the principal Ga LMM Auger peaks. The Ga(oxide)/Ga(GaAs) Auger peak height ratio varied from about 2 : 1 at a take-off angle of 75" down to about 0.7 at 15", roughly paralleling the As 3d(oxide)/As 3d(GaAs) ratio, which declined from 2 to 0.4 over the same angular range. (This observation incidentally reinforces a comment by Ludeke' on the studies by Spicer el a1." of the initial stages of oxidation of GaAs: the absence of a large XPS chemical shift following exposure to 0 2 does not necessarily imply lack of involvement of the element in question in the oxida- tion process.)

Before attempting to grow a ZnSe layer on such a substrate, any such oxide layer would normally be removed by passing HCI vapour over the crystal at elevated temperatures: the crystal was therefore removed from the spectrometer, washed with concen- trated HCl for about 60 s, rinsed well in deionized water and reinserted. This gave a surface almost free from 0, but showing traces of C1, which, however, became negli- gibly small after heating in a vacuum to about 150 "C.

The ZnSe surface, examined within a few days of growth, needed no such pretreatment. The precleaning of such samples, when made necessary by oxidation or contamination subsequent to growth, will be described elsewhere:" HCI is not satisfactory for cleaning ZnSe.

Both samples showed the C 1 s contaminant peaks typical of air prepared surfaces;12 but thin contaminant layers such as these, only a few Angstroms in thickness, do not have any seriously deleterious effect on the results. Elastic scattering in thicker polycrystalline over- layers (>lo ,&) does, however, progressively attenuate the photoelectron diffraction pattern from the underly- ing single crystal.

Preliminary examinations of normal analogue X-ray photoelectron spectra established precise electron kinetic energies (KE) for the Ga, As, Zn and Se 3p and 3d peak maxima in GaAs and ZnSe: the intensities (net heights in counts sec-') of these peaks were then deter- mined digitally as a function of electron take-off angle, using the timer-scaler modules in the ES200 instrumentation. Count rates were measured at the maximum and at the background to higher KE for each peak; the net peak heights were then obtained by sub- traction and the appropriate peak height ratios, after normalization by division by the mean ratio in each case, plotted as a function of the take-off angle to yield the x-ray photoelectron diffraction (XPD) patterns reproduced below.

It was, however, necessary also to determine the mean peak area ratio in each case, for the determination of

the surface stoichiometry by the method previously described. '*13 As the XPS peak profiles were essentially independent of the take-off angle, conversion factors linking the mean height of each signal to its mean area were obtained by 'square-counting' typical analogue scans for each peak.

RESULTS AND DISCUSSION ~

The XPD patterns for the GaAs substrate and the ZnSe layer are shown in Fig. 1. The positions of the diffraction maxima and minima on the angular scale are identical for the two crystals, an observation which immediately confirms (a) that the ZnSe has the same (zinc blende) structure as the substrate (and not the wurtzite alterna- tive, which can grow under some experimental condi- tions) and (b) that the growth was, as had been hoped, epitaxial. The overall ranges of variation (peak-peak diffraction height) in the two XPD patterns are closely similar, suggesting that the crystal quality in the over- layer is comparable with that of the substrate; although it should be acknowledged that it is not yet known in any quantitative sense just how sensitive the XPD pat- terns are to such factors.

The wavelengths of the photoelectrons concerned are almost identical, because the photoelectrons are close in KE, and the agreement between the patterns obtained using the 3p and 3d peaks is therefore, as expected, excellent in both cases. The differences in the intensities of the various diffraction maxima between the two patterns must consequently be real; they prob- ably reflect differences in the electronic environment surrounding the emitting atoms in the two solids, which

0 Ga 3p/As 3 p 0 Go 3d/As 3 d

1.3

0 0 Zn 3p/Se 3 p 0 Zn 3d/Se 3 d

.- c o 1.3 - LL

~

0.7 - I I I I I I

15 45 75 Toke-o f f angle (degrees)

Figure 1. XPD patterns for (a) (100) GaAs wafer and (b) ZnSe layer grown on an identical GaAs substrate by Hz transport. The axis of rotation lay parallel to (110), and the take-off angle is given with respect to the surface normal (i.e. 90" corresponds to glancing electron exit).

270 SURFACE AND INTERFACE ANALYSIS, VOL. 3, NO. 6. 1981 @ Heyden & Son Ltd, 1981

X-RAY PHOTOELECTRON DIFFRACTION OF SEMICONDUCTORS

have the same structure and virtually identical lattice parameters.

The remarkably large magnitude (range of variation) of these XPD patterns, from a solid in which all the atoms are tetrahedrally co-ordinated with a constant bond length, arises simply because the two mutually perpendicular (1 10) directions on the (100) surface are not equivalent as rotation axes in an XPD experiment: the Ga sites in one orientation transform into As sites in the other, and vice versa. Rotation of the crystal azimuthally through 90” followed by retaking of the XPD pattern thus results in an inversion of the patterns. ’’

Turning to the question of stoichiometry, in Table 1 we show the results obtained by applying the model for quantitative XPS analysis previously discussed and e ~ a l u a t e d ” ’ ~ to the mean peak area ratios obtained as described in the previous Section.

As previously discussed: we consider experimentally based cross-sections, when available, to be more reliable in analytical contexts than the calculated values. In confirmation of this, the nearest approach to the known 1 : 1 stoichiometry of the GaAs substrate was indeed achieved using experimentally based values for the 3p cross-sections. Moreover, as all four of the experi- mentally based cross-sections used here were originally obtained’ from the same extensive set of measurements, the very satisfactory result for GaAs lends added authority to the observed (and initially unexpected) zinc selenide stoichiometry calculated in a precisely analogous manner. The qualitative result, that the layer is considerably deficient in Se for at least 50-100 A below its surface, is further confirmed by the 3d results also included in Table 1. Subsequent measurements on other specimens gave similar results, and it was con- cluded that Se was being volatilized from the surfaces after their removal (while still hot) from the growth zone, so that the surface and bulk compositions differed very substantially.*’

An overall accuracy of better than a few percent cannot reasonably be claimed for these XPS measure- ments, but (as demonstrated here) such an accuracy is proving very valuable in optimizing growth and post- growth conditions for these semiconductors. The final characterization of more closely stoichiometric speci- mens inevitably requires more accurate techniques, but this XPS method can ensure that only

Table 1. Stoichiometries of GaAs substrate and ZnSe layer from XPS

Peak areas Stoichiometry obtained using Comments calcd x-snsb used exptl x-sns*

Ga 3p/As 3p Gal,&sl.oa Gal.&sl.os All within 8% Ga 3d/As 3d Not available Gal.oAs,93 of known 1 : 1

stoichiometry Mean within

1%

Zn 3p/Se 3p Znl.oSeo.,e Znr.oSeo84 All within

Znl.oSeo.80 Zn 3d/Se 3d Not available Znl.oSeo.80 5% of

a See Ref. 7. See Ref. 14.

near-stoichiometric samples are selected for further investigation.

CONCLUSIONS

The electron diffraction phenomena apparent in angle- resolved x-ray photoelectron spectra from single crys- tals are of considerable value in studies of the growth of thin compound semiconductor layers. They can pro- vide invaluable information on the crystal structure, orientation and order of a grown layer too thin to characterize by other techniques, and in addition a quantitative elemental analysis can easily be extracted from the same data. Moreover, unlike low energy elec- tron diffraction (LEED), which can also, in principle, be used to characterize thin overlayers, XPD does not require an atomically clean surface. Specimens can thus normally be exposed to air before characterization, a major advantage in the present context. Any technique for obtaining the epitaxial layer should thus be equally compatible with the characterization techniques described.

Acknowledgements

We thank the SRC and The Plessey Co. Ltd for financial support, Drs J. 0. Williams and J. M. Adams for many valuable discussions and Miss E. Raftery for experimental assistance.

REFERENCES

1. T. B. Tang, J. Electron. Mafer. 4, 1229 (1975). 2. A. Descombes and W. Guggenbiihl, IEEE Trans. Electron

3. K. Aggarwal, J. S. Baijal and R. G. Mendiratta, J. Phys. Chern.

4. W. Stutius, Appl Phys. Lett. 33,656 (1978). 5. K. Siegbahn, U. Gelius. H. Siegbahn and E. Olson, Phys. Len.

Devices ED-28, 395 (1981).

Solids 41, 1271 (1980).

A 32,121 (1970). 6. S. Evans. J. M. Adams and J. M. Thomas. Philos. Trans. R.

SOC. London Ser. A 292,563 (1979).

Spectrosc. Relar. Phenorn. 14,341 (1978).

Growth 51,267 (1981).

7. S. Evans, R. G. Pritchard and J. M . Thomas, J. Elecrron

8. M . D. Scott, J. 0. Williams and R. C. Goodfellow, J. Cryst.

9. R. Ludeke, Phys. Rev. B 16,5598 (1977). 10. P. Pianetta, I. Lindau, C. Garner and W. E. Spicer, Phys. Rev.

l’r. E. S. Crawford, S. Evans and M. D. Scott, unpublished. 12. D. Briggs, in Handbook of X-ray and UV Phoroelectron

Spectroscopy, edited by D. Briggs, p. 157. Heyden, London (1977).

13. S. Evans, E. Raftery and J. M . Thomas, Surf. Sci. 89,64 (1979). 14. J. H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8. 129

Lett 35, 1356 (1975); Phys. Rev. Lett. 37, 1166 (1976).

(1976).

Received 7 July 1981; accepted 13 August 1981

@ Heyden & Son Ltd, 1981

0 Heyden & Son Ltd, 1981 SURFACE AND INTERFACE ANALYSIS, VOL. 3, NO. 6, 1981 271