Oscillatory evolution of an ion bombarded InSb(100) surface

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Surface Science 318 (1994) 281-288

EISEVIER

Oscillatory evolution of an ion bombarded InSb( 100) surface I.N. Evdokimov

a~1,R. Valizadeh a, D.G. Armour a~*,N.V. Richardson b, CF. McConville ’

‘DepartmentofElectronic and Electrical Engineering, University of Saijord, Saljord, M5 4wT, UK ’ Sudace Science Research Centre, University ofliverpool, Liverpool, L69 3BX, UK ‘Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

Received 12 April 1994; accepted for publication 12 July 1994

Abstract Low energy ion-scattering and ion-recoil (LEIS and LEIR) spectroscopies have been used to monitor the structure and composition of an InSb( 100) surface during continuous irradiation with argon ions (4.8 keV) directed at grazing incidence. Oscillations in both the scattered and recoil ion intensities have been observed during all bombardment sequences (for doses > 2 X 1016 ions/cm’). The periods of these oscillations show some dependence on the experimental geometry, and are close to those predicted for sputter-removal of one or two surface layers. Interestingly, the surface shows no sign of becoming amorphous even following excess ion bombardment. Details of the variations in surface composition are assigned, not only to layer-bylayer depth profiling, but also by rapid diffusion of Sb to the surface from the Sb depleted lattice. These data indicate that LEIS and LEIR can be used to monitor composition profiles with monolayer depth resolution such that different states of the same surface can be achieved simply and controllably by terminating ion bombardment following a pre-determined irradiation dose.

1. Introduction In recent years, there has been an increasing number of studies on III-V semiconductor surfaces. Although the majority of this work has been performed on gallium arsenide ( GaAs),indium antimonide ( InSb) has also attracted considerable attention due to its narrow band-gap, low effective mass for electrons and its application as an infrared sensitive material (e.g. see Ref. [ 11, and references therein). As with all studies of IIIV materials the method of surface preparation, particularly for the (100) surface, is crucial in order to produce clean, ordered and stoichiometric surfaces. In contrast to the ( 110) surface, the (100) surface cannot * Corresponding author. ’ Permanent address: Physics Department, State Academy for Oil and Gas, Lenin&i prospekt 65, Moscow 11796, Russia. 0039-6028/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSD10039-6028(94)00433-l

be prepared simply by cleaving the bulk material in situ. In addition, conventional methods of surface oxide removal, using thermal desorption, are not applicable in view of the low melting point of this material ( N 800 K) . Although there has been some recent speculation regarding hydrogen plasma surface cleaning of InSb, based on the success of cleaning GaAs( 100) in this way [ 21, the generally adopted method for preparing InSb(100) substrates is by several cycles of lowenergy ion bombardment and subsequent annealing to remove the majority of the radiation-induced damage. The surface structures observed on InSb( 100) substrates by low energy electron diffraction (LEED), following ion bombardment and annealing, demonstrate increased surface ordering at the higher anneal temperatures. The sequence of LEED structures indicates a (1X1) +(4X1)/(4X2) +c(8X2) series

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with the c( 8 X 2) forming at 450-700 K [ 3,4]. This is in contrast to evidence suggesting that clean Si( 100) and Ge( 100) surfaces produced by means of ion bombardment and annealing are independent of the precise annealing procedure [ 51. Moreover, relatively little is known about the structural properties of InSb( 100) subjected merely to room temperature bombardment (i.e. without further modification induced by annealing). Whilst LEED shows a basic (1 X 1) structure, HREELS data indicates that a disordered near surface layer is formed [ 31, similar to GaAs, where ion bombardment is known to produce an amorphous surface layer [ 61. Widely quoted structural models of In- and Sb-terminated surfaces (e.g. see Refs. [7,8]) have been introduced mainly by an analogy with previous GaAs studies. The surface chemical composition of clean InSb( loo), prepared by bombardment and annealing has, however, only been studied by ARS and XPS. From AES data ]4], a bombarded and unannealed sample was found to have an Sb/In Auger peak-to-peak height ratio of 0.75, which remained unch~ged after annealing at 300”C, indicting persistent In enrichment at the surface. Core-level photoemission data, however, indicates that ion bombardment followed by annealing produces a surface terminated by only 3/4 monolayer In [ 71. This latter model is supported by recent STM studies of the In-terminated InSb( 100) surface [ 81. Generally, it is the details of both the bombardment and an annealing stage that are crucial in determining the resulting surface. Unfortunately, in the majority of published work, only the annealing procedures are described in any detail with only cursory mention of the ion bomb~dment procedure. (For example, the angle and plane of ion incidence with respect to the surface orientation, the energy spread and the total ion dose within the primary beam are parameters that rarely receive a mention.) Variation of any of these parameters may result in considerable changes to the surface composition and morphology, radiation damage profiles etc. [ 9,101. There is also no reason, a priori, to expect that the damage induced in the material will be completely eliminated by any subsequent annealing procedure [ 111. Low-energy ion scattering and ion-recoil (LEIS and LEIR) spectroscopies have for many years been used to monitor variations in the surface structure and composition of materials by measuring the energy and

Fig. 1. The experimental geometry of the incident and scattered ions is shown in (a), and examples of the basic scattering and recoilproducing events at the bombarded surface (b) single scattering (SS), (c) single recoil (SR), (d) double scattering (DS), (e) double recoil and ( f) above surface scattering ( CHS) .

angular spread of inert gas ions scattered from well ordered surfaces (e.g. see Refs. [ 13,14 7). These techniques not only provide a high degree of surface sensitivity, but also allow in situ measurements during continuous ion bombardment. At low energy and large angles, both ion scattering and ion recoil spectroscopies involve binary atomic collisions. These single scattered, primary ions (SS) and recoil ions (SR), shown in Figs. lb and lc, contribute to comparatively narrow peaks in the measured energy spectra and relate to the presence and concentration of particular atomic species on the surface. The comparatively low secondary ion intensities observed, due to the strong angular dependence of the scattering cross-section, are enhanced by using grazing incidence, where improved surface sensitivity is also achieved. The measured energy spectra also contain peaks resuiting from correlated binary collision sequences with several neighbouring surface atoms and are therefore sensitive to a degree of shortrange order in the outer-most surface layer [ 141. Another specific low-angle effect is above surface channelling (CHS). This involves scattering of primary ions by a large number of surface atoms and is therefore sensitive to the surface roughness. Attributing these non-binary events to particular peaks in the energy spectra is somewhat complicated and usually involves model ~c~ations. Several ion-surface events, relevant to the discussion of results presented here, are illustrated in Figs. lb-lf. In the specular scat-

I.N. Evdokimov et al. I Surface Science 318 (1994) 281-288

tering geometry used, the correlated collisions with two neighbouring surface atoms, double scattered (DS) and deflected recoil (DR) events, may be regarded simply as successions of two binary collisions [ 131. In this paper, we report LEIS and LEIR spectroscopy results of a detailed investigation of a continuously bombarded InSb( 100) surface. These data indicate that the angle of ion incidence is an extremely important parameter in the ion-surface interaction when sputtering this III-V semiconductor material. Also, the surface did not appear to become amorphous even at ion doses in excess of those required to produce the same effect on other semiconductor surfaces. Changes in the surface composition, as a function of ion incidence, were also detected indicating an oscillatory dependence of the surface termination on the total bombardment dose.

2. Experimental Measurements were carried out on an InSb( 100) wafer ca. 10 mm2 and 500 pm thick. The Te doped (4 X 1017 cmw3) InSb crystals (MCP Wafer Technology, UK) were cut, polished and solvent cleaned by the manufacturer. Prior to insertion in the vacuum, the sample was chemically etched to remove sub-surface damage and to produce a stable oxide layer [ 3,4]. The sample was then mounted onto a circular molybdenum holder and held firmly in place by three tungsten wire springs. In vacuum, each sample was outgassed for one hour at 525 K and sputter-cleaned at room temperature by low energy argon ion bombardment (500 eV, 2 fl/ cm’) at normal incidence and then annealed for one hour at 625 K. Previous work indicates [ 3,4] that three such cycles of surface cleaning are sufficient to remove the oxygen and carbon contaminants from the InSb surface. Indeed, no indication of these impurities was detected in the ion scattering spectra. After the initial cleaning cycles the sample was cooled to room temperature and transferred (under III-IV conditions) into the low-energy ion scattering spectrometer. A detailed description of the LEIS spectrometer is given elsewhere [ 151. However, the main features of this UHV instrument are a highly collimated Ar+ ion beam (divergence < 0.2”) directed on to a precise sample orientation system (A8<0.1”). This enabled the scattering geometry, with respect to the incident ion direction and the position of the energy analyser, to be

283

accurately controlled and makes this spectrometer uniquely suited for the study of specific effects observed at grazing ion incidence. In most LEIS experiments, small-angle effects tend to be smeared out due to poor angular resolution, however, in this system the energy spectra of all scattered ions were collected in a spherical electrostatic analyser (501 channels per scan with a single channel width of 10 eV) . The experimental geometry is shown in the upper part of Fig. 1. The plane of ion incidence was parallel to [ 1 lo] directions of the surface and grazing, specular scattering conditions were used ( 0, = 0, = 10” relative to the surface) for all the experimental work presented here. The InSb( 100) sample was subjected to bombarded at room temperature with Ar+ primary ions with an energy of 4.8 keV. The beam had a circular crosssection, 3 mm in diameter, and the beam current never exceeded 70 nA. The output signal was recorded in pulse-counting mode allowing the use of a low-current primary ion beam, thus facilitating quasi-static, ion scattering studies. As a result of these measurements it was possible to monitor, with a high degree of accuracy, the dose-dependent variations of surface properties during the course of continuous ion bombardment.

3. Results A typical energy spectrum of scattered and recoil ions, taken under conditions of grazing ion bombardment, is shown in Fig. 2. The positions of the SS and SR peaks, calculated in the approximation of purely elastic binary collisions, are also shown. The intense high-energy peak is a result of SS events at surface In and Sb atoms. The poor mass resolution observed in the scattered ion peaks is a common feature of smallangle scattering spectroscopy (e.g. see Ref. [ 141) and a consequence of the proximity in masses of the In and Sb isotopes. In fact, all the calculated SS peak positions fall inside a 2 eV interval, i.e. less than the channel width of the energy analyzer. A low-intensity shoulder at the high-energy side of the SS peak is the result of DS-type events involving both In and Sb surface atoms. The broad peak in the energy spectrum, to the left of the SS peak multiple, is attributed to CHS events. In view of the existing models [ 161, the double-peak structure of this feature in Fig. 2 is attributed to the presence of some sub-monolayer coverage on top of

I.N. Evdokimov et al. I Surface Science 318 (1994) 281-288

I

Recoil

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Energy (keV) Fig. 2. A typical energy spectrum of scattered and recoil ions from InSb( 100) surface bombarded with a 4.8 keV Ar+ ion beam under conditions of grazing incidence. The vertical lines represent the single scattering (SS) and recoil (SR) peak positions calculated assuming purely binary collisions. From left to right, SR peaks for the ‘%b, lzlSb, l151n isotopes and finally the SS peak for both Sb and In.

the expected outer-most layer of the surface. Finally, the peaks in the low-energy part of the spectrum in Fig. 2 correspond to indium and antimony positively ionized recoils, removed from the bombarded surface by direct collisions with primary ions. The vertical lines also shown in Fig. 2 represent the calculated SS and SR peak positions, assuming purely binary collisions, for the lz3Sb, ‘*’Sb and “‘In isotopes. These higher energy recoils originate in different DR events involving In and Sb atoms at the surface (cf. Fig. 4) and the background signal, at low energy, may be attributed to the production of recoils in multiple and cascade-type collisions. Dose dependent variations in the terminated state of the bombarded InSb( 100) surface were monitored by observing the changes in peak shape and intensity of features in the energy spectra. Two modes of spectrometer operation were employed to achieve this. In the first, the secondary ion analyser was fixed at a certain energy channel corresponding to the maximum intensity of a particular peak in the energy spectrum. It was, therefore, possible to reduce the data collection time and to follow the process of surface evolution continuously. This mode of operation can, however, lead to

artifacts in the detected peak positions because of small energy shifts. Hence the complete energy spectrum was measured in each case and the energy intervals for particular scattered and recoil ion production events determined subsequently. The probability of these events was calculated by integrating the total number of detected particles within the time interval. These integration intervals for scattered ions, CHS effects and for all recoils (both In and Sb) are indicated in the upper part of Fig. 2. Using this procedure significantly increased the time intervals for data collection and greatly improved the reproducibility of the results. Experimental data sets were obtained for two orientations of the sample in both cases with the ion beam directed along the [ 1101 azimuth. The first data set (solid triangles in Fig. 3) was collected and then the sample rotated 180” about the surface normal so that the plane of ion incidence was again parallel to the [ 1101 surface directions. The second data set (open circles in Fig. 3) was measured at this new sample orientation after a 10 h interval with the sample held at room temperature. A comparison of the data in Fig. 3 indicates that, even after previous bombardment with doses as high as 2 X 1018 ions/cm*, the InSb( 100) surface is evidently restored even without any high-temperature annealing and the previously recorded stages of surface evolution are repeated during the course of each bombardment sequence. It can be also seen that for bombardment doses lower than 5 X 1015 ions/cm* the observed intensity oscillations always pass through

I 0’ azimuth o 180” azimuth 1.0 h 0

, 1

2

3

4

5

6

7

I 8

Ion Dose (xl015 ions/cmz) Fig. 3. Dependenceof the integral intensity of above surfacescattering (CHS) on the bombardment ion dose. The oscillatory dependence of intensity as a function of ion dose, discussed in the text, can clearly be seen.

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doses up to 101’ ions/cm’. In addition to a considerable intensity increase observed, the simple double-peak structure is now smeared, due to the appearance of some new poorly resolved recoil peaks. These are attributed to the emergence of In atoms at the bombarded surface. This conclusion is further supported by deconvolution of the composite energy spectrum (Fig. 4b) and subtracting the Sb+ ~nt~bution (Fig. 4a), which results in the observation of a double-peak structure in agreement with the binary collision calculations for SR and DR indium. Finally, Fig. 5 shows the results of the detailed study of InSb( 100) surface evolution at the initial stages of Ar + bombardment, for doses up to 5 X 101’ ions/cm’. The effects of increased ion dose are illustrated for: (a) intensity of SS; (b) intensity of scattering in CHS events; (c) intensity of SR and DR, (a, b, c were obtained by inte~ating the energy spectra over the energy intervals indicated in Fig. 2); (d) ratio of In/ Sb SR intensities (obtained by deconvolution of com-

-Lr!!L C

2.5

3.0

3.5

Energy tkeV) Fig. 4. Effect of the changes in surface composition on the shape of recoil ion energy spectra: (a) initial Sb-covered surface, (b) surface after a bombardment dose of lOr* ions/cm, and (c) the deconvoluted contribution of In+ recoils to the spectrum shown in (b) .

an evolutionary stage, where the period and amplitude vary. However, at the higher ion doses, the amplitudes remain approximately constant and indicate a period of N 1.4 X 1015 ions/cm2. Operating the analyser in a fixed energy channel mode, a spectrum was collected for the life-time of the ion source filament in an attempt to detect damping effects in the intensity oscillations. Under these conditions more than twenty consecutive periods of undamped oscillation were recorded without any noticeable decrease in amplitude. Recording complete spectra also allowed changes in intensities of scattered and recoil ions along with variations of individu~ peak shapes and energy positions, to be detected directly. This is illustratedin Fig. 4 which shows the In and Sb recoils produced in both SR and DR collision events. The plot in Fig. 4a results from the standard small-angle geometry, immediately after a previous high-dose, large-angle bombardment (10’s ions/cm’). By comparing this result with the predictions of the binary collision approximation, the low and high energy peaks may be attributed to the production of Sb+ recoils in SR and DR events, respectively. The energy spectrum shown in Fig. 4b was measured after prolonged grazing incidence bombardment with ion

Ion Dose (xl015 ions/cm21 Fig. 5. A combination plot of the dose dependencies of normalised, scattered and recoil ion intensities and peak positions; (a) I,,,,, (b) I&_*, fc) Z-a, (d) In : Sb ratio of Z_,a, and (e) gMWtir,a The individual errors associated with each of these plots are indicated by the error bars located on the right hand side.

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posite recoil peaks - cf. Fig. 4); (e) energy position of the narrow SS peak in the energy spectra (cf. Fig. 2). From these results the oscillatory dependence as a function of ion dose for several experimental parameters can clearly be determined.

4. Discussion. This is the first application of ISS and IRS to the study of an InSb( 100) surface and the first obse~ation of continuous oscillations in scattered and recoil ion intensities interpreted in terms of the changes of surface structure and composition. The most obvious effects are variations of IRS peak intensities as a direct consequence of changes in surface concentration of one particular atomic species. Hence the sharp maxima observed in Fig. 5d at ion does of 0.7 and 3.5 X lo*’ ions/cm2, may be attributed to temporary enhancement of the In inundation at the InSb( 100) surface during ~on~nuous ion i~adiation. However, Sb is always found to be the predominant surface component, as indicated by specific features of the integral recoil energy spectra (cf. Fig. 4). One of the more unusual observations made during this study was, however, that the InSb( 100) surface did not appear to become amorphous even at ion doses ( > 4 X 1018 ions/cm’), well in excess of those required to produce the same effect on GaAs and Si surfaces [ 10,111. This unusual effect was investigated in some detail and is the subject of a separate publication [ 121. Comparison with other dose dependencies shown in Fig. 5 allows the position of In atoms to be determined by considering the shadowing and blocking effects, where maxima in the overall recoil intensity may be attributed to the development of an atomically rougher surface [ 171. However, at such a surface there is a smaller probability of CHS, resulting in the anti-phase features clearly seen in Figs. 5b and 5~. The general anti-phase relationship between shadowing effects and recoil intensities over several recorded periods of oscillation supports the conclusion that observed surface “roughening” is a process involving a temporary build-up of a sub-monolayer of indium on top of an antimony-terminated InSb( 100) surface. During sputtering, this In coverage is quickly removed and changes in the atomic ordering of the underlying Sb layer are induced. This is indicated by the continuing

variations observed in Figs. 5b and 5c following bombardment dose in the range (l-3) X 101’ ions/cm2. The most ordered surface is evidently achieved following an ion dose of -2.5 X lOr5 ions/cm2 where the maximum of CHS and minimum of In recoil intensities are observed. The observed dose-dependent variations in the scattering peak intensity and energy (Figs. 5a and 5e) may also be explained in terms of the above model of InSb surface evolution. Since the majority of primary ions scatter at small angles by correlated sequences of collisions with ordered arrays of atoms at the surface layer, the appearance of a new atomic species on top of this layer may be expected to effectively interrupt such collision sequences, thus decreasing the intensity of this type of small-angle ions scattering. Hence, if the surface is Sb terminated, the appearance of In atoms at the surface should readily be detectable. A corresponding values (Fig. Se) may then be attribincrease of E scaaered uted to smaller inelastic energy losses along shorter ion trajectories in the near surface region. On the basis of these results, it is suggested that the state of the InSb( 100) surface during the course of continuous grazing ion bombardment develops sequentially. Following a grazing incidence bombardment cycle, a reproducible surface state is restored, even without high-temperature annealing, merely by holding the sample at room temperature in UHV for N 10 h. This initial state of the surface was determined purely from ion scattering data as no diffraction techniques were available to confirm the long range order of the surface. During any cycle of continuous ion bombardment, the state of the initial surface is quickly modified (at doses < 2 X 1Or4ions/cm’) and following an initial adjustment stage, a sub-monolayer of In coverage is briefly detected on top of the Sb-terminated surface. The lifetime of this In layer is very short (corresponding to a dose interval < 7 X 1014ions/cm2) and, once removed, an Sb terminated surface persists for a relatively long time (dose interval N 2 X 1015ions/cm2), with varying structure and composition, and only traces of In present in the recoil ion spectra. The highest Sb surface concentration, and most ordered surfaces are achieved in about the middle of the Sb-dominated interval (after a bombardment dose of w 1.4X 1015 ions/ cm2 with respect to the middle of the In-terminated stage). At the beginning and near the end of each Sbterminated interval, the surface layer goes through rel-

I.N. Evdokimov et al. /Sulfate

atively disordered structural stages before the In-terminated stage is again repeated, allowing the period of oscillation for the surface chemical composition to be defined as 2.8 X 1015 ions/cm*. A second, smaller, period may be also defined if the surface is considered in terms of its structural perfection. During the course of further continuous bombardment, a state of saturation is achieved when the distinction between above consecutive states is gradually smeared. At saturation, the more reliably detected structural oscillations persist without any signs of damping, up to ion doses > 2 X 1016 ions/cm*. At present it is not possible to draw any detinite conclusions about the crystallographic structure of the surface layer during the course of grazing-incidence bombardment. Such structural analysis would require measurements of azimuthal dependencies in the LEIS and LEIR spectra at each stage of the surface evolution. However, in the absence of other published ion scattering data on binary compound semiconductors, such as InSb( lOO), a comparison has been made with the experimental data from the sputtering of binary metal alloys [ 181. This revealed persistent contradictions between the surface concentrations measured by AES and ion scattering spectroscopies for conditions, as with InSb sputtering, when the topmost atomic layer is enriched in one component (Sb) , but underlying layers are depleted in the same species. Under these conditions ISS measurements will always show enrichment and AES measurements will show depletion of the same component due to the different sampling depths of the two techniques. It is tempting to explain the observed surface evolution solely as a process of layer-by-layer sputtering of a composite InSb crystal, primarily on account of the measured periods of oscillation. The largest period, corresponding to a dose of 2.8 X 10” ions/cm*, could therefore be related to the sputtering of one In layer and of an underlying Sb layer, and the smallest period of 1.4 X 10” ions/cm’, determined by variation in the surface roughness, could then be ascribed to the sputtering of a single surface atomic layer irrespective of its chemical composition. A reasonable test for these assumptions can be obtained by calculating the corresponding sputtering yields ( Y) . Assuming the atomic density of one monolayer of InSb( 100) is 4.7 X 1014 atoms/cm* [ 191, a value of Y = 3 is obtained. Although little is still known about the sputtering yields of multi-

Science 318 (1994) 281-288

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component materials, the determined values of Y seem to support the above model, being in reasonable agreement with known yields for Ar+ sputtering of various elemental solids [ 201. Within the limits of such a model, it is also possible to argue in favour of a macroscopic in-phase surface development by taking into account the specific grazing-incidence conditions. There is both experimental and theoretical evidence (e.g. see Ref. [ 211) to suggest that, under these conditions, the cascade sputtering processes are effectively suppressed and target atoms are only sputtered from the outer most layer as a result of direct impact by primary ions. Consequently, irregular surface morphologies do not tend to evolve and the surface becomes atomically polished over large areas. An analysis of these results, however, shows that a number of the observed regularities cannot be explained by this simple layer-by-layer sputtering model. The most obvious problem is the persistently high surface concentration of Sb. There is also experimental evidence that Sb is preferentially removed from all InSb surfaces by sputtering [20] resulting in subsurface Sb depletion extending deep into the bulk [ 41. Hence, indium enrichment at the surface might be expected in the case of simple depth profiling. These results, therefore, imply that there is a constant and effective transport of Sb atoms to the surface from the underlying layers, evidently by diffusion, which determines the rate and the manner of surface evolution. One other consequence of the synergic effect of sputtering and diffusion is that the speed of surface recession will necessarily be less than that calculated on the basis of sputtering yields as some sputtered material is constantly being replenished by diffusion. A number of models for surface composition changes, induced by ion bombardment, has been developed for two-component metallic alloys (e.g. see Ref. [ 181) . These models take into account not only target erosion, including preferential sputtering effects, but also thermal and radiation enhanced diffusion of components. Contrary to the above conclusions, most of these models agree in one important respect, that for high-dose, steady-state conditions the final surface composition is governed only by the component sputtering yields, which do not include any diffusion. However, compared to the case of III-V semiconductors, the diffusion processes, considered in these models are extremely slow. For example, Cu-Ni alloys under con-

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I.N. Evdokimov et al. I Surface Science 318 (1994) 281-288

ditions of effective enhancement by ion bombardment, display diffusivity values approximately an order of magnitude smaller than those reported for III-V materials [ 11,221. With InSb, self-diffusion is mainly substitutional and both components diffuse independently in their respective sub-lattices [ 91. Hence sub-surface Sb depletion, induced by the ion bombardment, should further aid thetransport processes and the effect on the final state of the bombarded InSb( 100) surface.

5. Conclusions Low-energy ion-scattering and ion-recoil spectroscopies have been used to study the InSb( 100) surface and, in particular, in situ observation of the surface evolution under continuous ion bombardment. The most notable results of these studies are that the surface does not become amorphous following ion bombardment and that both the structure and composition of the surface evolve in an oscillatory manner. The observed, self-sustaining oscillations may be attributed to layerby-layer sputtering through an Sb-depleted crystal lattice and radiation-assisted, segregation or transport of Sb to the surface. As a consequences of such surface development, different specific states of the same surface may be achieved by terminating ion bombardment at pre-determined doses. Additionally, these experimental results indicate that in materials with comparatively low diffusion rates, grazing-incidence LEIS and LEIR spectroscopies techniques may serve as a methods of profiling composition in the near-surface region with better than monolayer depth resolution unattainable by conventional SIMS techniques.

References [l] S.R. Kurtz, G.C. Osbourn, R.M. Bielfeld, L.R. Dawson and H.J. Stem,Appl. Phys. Lett. 52 (1988) 831; R.G. van Welzenis and B.K. Ridley, Solid State Electron. 27 (1984) 113, and references therein. [2] CM. Rouleau and R.M. Park, J. Appl. Phys. 73 (1993) 4610. [3] T.S. Jones, M.Q. Ding, N.V. Richardson and C.F. McConville, Surf. Sci. 247 (1991) 1. [4] R.G. Jones, N.K. Singh and C.F. McConville, (1989) L34.

Surf. Sci. 208

[5] A. Many, Y. Goldstein and N.B. Gorver, Semiconductor Surfaces (North-Holland, Amsterdam, 1981). [6] L.H. Dubois and G.P. Schwartz, J. Vat. Sci. Technol. B 2 (1984) 101. [7] P. John, I. Miller and T.-C. Chiang, Phys. Rev. B 39 (1989) 1730. [8] M.O. Schweitzer, F.M. Lcibsle, T.S. Jones, C.F. McConville and N.V Richardson, Surf. Sci. 280 (1993) 63.

[ 91 R. Behrisch, Ed., Sputtering by Particle Bombardment,

Vols. l/2 (Springer, Berlin, 1981/ 1983). [ 101 J.M. Mayer, E. Eriksson and J.A. Davies, Ion Implantation in Semiconductors (Academic Press, New York, 1970). [ 111 0. Madelung, Physics of III-V Compounds (Wiley, New York, 1974).

[ 121 I.N. Evdokimov, R. Valizadeh, D.G. Armour, N.V. Richardson and C.F. McConville,

to be published.

[ 131 D.G. Armour, in: Methods of Surface Analysis, Ed. J.M. Walls (Cambridge University Press, Cambridge, 1992). [14] E.S. Mashkova and V.A. Molchanov, Medium-Energy Ion Reflection from Solids (North-Holland, Amsterdam, 1985).

[ 151 J.A. Van den Berg and D.G. Armour, Inst. Phys. Conf. Ser. 38 (1978) 298.

[ 161 I.N. Evdokimov, Phys. Chem. Mech. Surf. 7 (1991) 49.5. [ 171 Y. Nakagawa, A. Karen, M. Hatada, K. Okuno, F. Soeda and A. Ishitani, Proceedings of the 8th International Conference on SIMS, Eds. A. Bemringhoven, K.T.F. Janssen, J. Tumpner and H.W. Werner (Wiley, New York, 1992) p. 335. [18] G. Betz and Bombardment, 1983) p. 37.

G.K. Wehner, in: Sputtering by Vol. 2, Ed. R. Behrisch (Springer,

Particle Berlin,

Acknowledgements

[19] H.H. Andersen and H.L. Bay, in: Sputtering by Particle Bombardment, Vol. 2, Ed. R. Behrisch (Springer, Berlin, 1983) p. 145.

The authors wish to thank Dr. S.E. Donnelly for valuable critical discussions. One of the authors (I.N.E.) is grateful to the IRC in Surface Science at the University of Liverpool for financial support.

[20] D.M. Hill, F. Xu, Z. Lin and J.H. Seaver, Phys. Rev. B 38 (1988) 1893. [21] B. Baretzky and E. Taglauer, Surf. Sci. 162 (1985) 996. [ 221 P.S. Ho, J.E. Lewis, H.S. Wildman and J.K. Howard, Surf. Sci. 57 (1976) 393.