Surface characterisation of indium phosphide

Surface Science 51 (1975) 14-28 0 North-Holland Publishing Company

SURFACE CHARACTERISATION

OF INDIUM PHOSPHIDE

R.H. WILLIAMS and LT. MCGOVERN School of Physical Sciences,

The New University of Ulster, Coleraine, N. Ireland, U.K.

Received 4 March 1975; revised manuscript received 20 March 1975

LEED, characteristic loss spectroscopy, Auger electron spectroscopy, photoemission and light modulated contact potential methods have been employed to study indium phosphide surfaces. Cleaved (110) faces have a surface unit mesh identical to that of the bulk and surface states lead to band bending in the surface region. Reaction with oxygen leads to loss of phosphorus and the formation of an oxide layer near the surface. Features in the characteristic loss spectrum are discussed in terms of interband transitions and plasmon losses and the Auger spectra of oxidized surfaces are thought to contain components due to cross transitions. The surface composition, band bending and photothresholds of etched (100) surfaces of epitaxial InP layers have been established. Surfaces cannot be cleaned by heat treatment alone since decomposition occurs at elevated temperature. The influence of evaporated silver on the sign of the surface photovoltage has also been investigated.

1. Introduction The potential applications of indium phosphide in photoemitting devices [ 1,2] and in microwave oscillators [3] has led to a surge of interest in the properties of this solid. The efficiency and performance of InP based microwave oscillators depends very critically on the electrical contacts made to the solid and can be improved substantially by appropriate fabrication of the electron injecting contact [3]. Such contacts are normally deposited on contaminated InP surfaces and an understanding of the exact mechanisms involved has hitherto not been possible due largely to the lack of adequate information regarding the nature of the InP surface. There is a need, therefore, to establish the geometrical, chemical and electronic properties of clean surfaces, to study contamination on these faces, and to understand the electronic nature of such contaminated surfaces, in particular with regard to the establishment of electrical contacts to them. In view of the potential application of InP it is surprising that their surface properties have not been studied more widely. Fischer [4] briefly investigated the photoemitting behaviour of atomically clean cleaved (110) surfaces as well as cesium coated faces. Others [5,6] have investigated cesiated surfaces either from the aspect of practical photocathodes or with a view to exploring the lower conduction bands of the

R.H. Williams, I. T. McGovern/Surface

characterisation of InP

15

solid. Recently Bayliss and Kirk [7] attempted to clean (100) faces by argon bombardment and annealing and although the surfaces were never free of oxygen a number of low energy electron diffraction patterns were observed depending on the thermal treatment undergone by the surface. In general, however, the surface parameters and properties of InP are very poorly understood, in comparison to other III-V compound semiconductors such as GaAs or GaP. In these studies we have examined atomically clean and contaminated (110) single crystal faces prepared by cleavage. Surface geometry, photothresholds, bending of the energy bands, and surface chemical compositions have been established. In addition etched (100) faces of thin epitaxial layers, of the type used in microwave devices, have been examined. Preliminary investigations of the effect of thermal treatment on surface properties of these etched faces and of the influence of sub monolayer amounts of evaporated silver are also discussed.

2. Experimental The ultra high vacuum experimental chamber in which most the workwas carried out is shown in fig. 1. A carrousel supports numerous samples simultaneously, and each sample can be rotated in turn to face any one of several ports or stations. The LEED port employs three grid electron optics which may also be used for energy analysis of backscattered electrons. The Auger spectra described, however, were obtained using a Vacuum Generators Ltd. high resolution cylindrical mirror analyser on another port. An electromagnetically driven Kelvin probe allows a determination of work function and surface photovoltages can be established by noting the change in the work function as the sample surface is illuminated by white light or by radiaLEE0

GUN

AND F-l

OPTICS

CLEAVING

AUGER SPECTROMETER

KELVIN

PROBE

u.v. RADIATION VIEWING

PORT LASER

RAOIATI~N

Fig. 1. The experimental u.h.v. chamber

16

R.H. Willkmx,I. T. McGovern/Surface characterisationof InP

tion from an argon ion laser of 1 W maximum output. Another station allows the photoemissive properties of the samples to be established. Ultra-violet radiation of energy ranging from 2 to 11 eV is incident on the sample via a lithium fluoride window, and photoelectrons are collected using a hemi-spherical collector. Either yield or energy distribution of emitted electrons may be established. Surface photovoltages may also be measured at this port by noting the shift of the U.V. stimulated energy distribution curves as a result of irradiation by laser light. Cleavage was accomplished at another port by means of a bellows operated vice. Facilities are also available for argon ion etching, for electron beam heating and for deposition of metals on the sample surfaces by evaporation. The vacuum system is ion pumped and has a pressure capability well below lo-lo torr. Other experimental details of the system are included elsewhere [8]. Both single crystal and epitaxially deposited samples were n-type with a carrier concentration of around 1016 cme3. Crystals were cut into bars about 1 cm X 0.7 cm X 0.7 cm before mounting for cleavage. The epitaxial films were 10 pm thick and grown on high conductivity n-type InP single crystal substrates.

3. Results and discussion 3. I. Cleaved single crystal surfaces 3.1.1. Surface structure and characteristic energy loss Indium phosphide surfaces cleavea in the atmosphere and subjected to no further cleaning other than the vacuum bakeout showed no low energy electron diffraction patterns. As a result of cleavage in u.h.v., however, diffraction features of high quality were observed. These are drawn in fig. 2a. (We do not present photographs since the carrousel and associated components shielded a large fraction of the pattern from the camera.) The observed pattern is a simple (1 X 1) as generally observed from (110) surfaces of III-V compound semiconductors with zinc-blende structure. The repeat distance of the surface unit mesh is therefore identical to that of the parallel substrate unit mesh, and it may be concluded that the cleaved (110) face of InP is highly stable structurally. The characteristic energy loss spectrum of clean InP is shown in fig. 2b. For the case shown the electron beam energy was 80 eV, the beam was incident normal to the (110) face, and energy analysis was performed using the three grid system of the LEED optics. A very prominent peak is observed at 11.2 eV with a smaller subsidiary peak, possibly split, at 21 eV. Small shoulders are observed at approximately 14.5,7.5 and 5 eV but due to the fact that they are situated on a large rising background an accurate evaluation of their positions is difficult. These features were reproducible for a series of incident beam energies ranging from 50 to 120 eV, but with the peak at 11.2 eV being more prominent at the higher energy. Characteristic loss phenomena in general may be described in terms of interband

R.H. Williams,I. T. McGovern/Surface characterisationof InP

I

0

10

20 30 ENERGY

17

LO k+‘)

Fig. 2. (a) LEED pattern for u.h.v. cleaved InP (110) surface; beam energy is 128 eV. (b) Electron energy loss spectrum for u.h.v. cleaved InP (110) surface; incident electron beam energy is 80 eV.

transitions and volume or surface plasma oscillations. In order to establish the origin of the various features in fig. 2b, it is therefore necessary to examine optical absorption and reflectivity data, together with the imaginary part of the dielectric constant as determined from characteristic losses in high energy electron transmission experiments. Unfortunately no previous characteristic energy loss experiments appear to have been reported for InP. Optical data, in the energy range up to 9.5 eV [9] and in the range 15 to 40 eV [lo] have been reported but to date the range 9.5 to 15 eV does not seem to have been covered. For the loss structure of fig. 2b the features at 5 and 7.5 eV are consistent with interband transitions. Peaks are observed in the reflectivity spectrum at similar energies [9] and attributed to the direct transitions X,, --f (X3,, Xl,) for the former and L,, + Llc for the latter. The peak at 14.5 eV is almost certainly due to a volume plasma oscillation. Assuming four oscillating electrons per atom and a free electron model, then a value of 14.9 eV may be calculated at zero scattering angle. Thus, in agreement with observations on other III-V compounds [ 111 the location of the plasma oscillation is little affected by single electron transitions. The peak at 11.2 eV may be due to an interband transition or to a surface plasma oscillation. A surface plasmon might be expected at 10.5 eV and in view of the fact that the 11.2 eV peak in fig. 2b appears strong at a beam energy of 120 eV, where the electron escape depth is shorter than, say, at 50 eV, this interpretation is tentatively favoured. In the absence of optical data, however, a band to band transition cannot be ruled out. The feature at 21 eV remains to be explained. This is almost certainly due to transitions originating in the metal species’ outermost d-level (see fig. 7). Absorption due to transitions from this level to conduction band states has been observed at just this energy by Gudat et al. [lo] using synchrotron radiation.

18

R.H. Williams, I. T. McGovern/Surface

characterisation of InP

8- X 1O-2

PHOTON

ENERGY

IeV)

Fig. 3. Plot of cube root of yield, in electrons per incident photon, against photon energy. Open circles for u.h.v. cleaved (110) InP surfaces. Full circles for cleaved ( 110) face exposed to the atmosphere.

3.1.2. Photothreshold and surface photovoltage The photoemission near threshold was investigated by recording the yield as a function of photon energy immediately after cleavage. In common with a whole range of semiconductors a plot of the cube root of the yield near threshold, as a function of photon energy, shows linear behaviour (fig. 3). This plot extrapolates to zero yield for a photon energy of 5.6 eV. Inspection of the widths of a series of energy distribution curves, for various photon energies, and allowing for instrumental resolution, leads to the conclusion that this value of 5.6 eV is close to the separation of the vacuum level and valence band edge at the surface. For these clean surfaces irradiation by the argon ion laser leads to a decrease of 15 mV in the work function. In principle this may arise due to a reduction of the band bending as a result of interband transitions within a diffusion length of the space charge layer or as a result of excitation of carriers from surface states. Alternatively, it could come about due to the Dember effect as a result of the different mobilities of electrons and holes. However, calculation of the Dember voltage, along the lines described by Galbraith and Fischer [ 131, leads to the conclusion that its magnitude is less than 1 mV, i.e. below the sensitivity of our instrumentation. The surface photovoltage is therefore due to a flattening of the energy bands near the surface and the band bending results from intrinsic surface states. More studies are required on crystals of various doping levels in order to characterise these states further and such studies are in progress. The surface parameters for InP are shown in fig. 4.

R.H. Williams, I. T. McGovern/Surface

VACUUM

1.1 5.6eV i

characterisation of InP

19

LEVEL

i

Fig. 4. Schematic illustration of the photothreshold a u.h.v. cleaved (110) InP face.

and band bending

in the surface region for

Exposure of a clean surface to the atmosphere leads to a distinct reduction of the photothreshold and a large increase in yield. It is seen in the next section that the main contaminants present are carbon and oxygen. After such contamination the surface photovoltage increases to 140 mV, corresponding to a depletion of electrons from the bulk and an increased negative surface charge. In addition there is a marked increase in the time taken to reach equilibrium following illumination. The LEED patterns of a clean surface are obliterated by exposure to atmospheric contaminants suggesting a disordered surface layer. 3.1.3. Surface chemical composition The Auger electron spectrum (AES) for a freshly cleaves InP (110) surface is shown in fig. 5. All the observed features may be attributed to indium and phosphorus, together with characteristic losses, and are labelled accordingly. It will be noticed that the dominant feature of the distribution is due to phosphorus. Also shown in fig. 5 is an Auger spectrum after exposing the cleaved surface to the atmosphere for 15 min. Carbon and oxygen are now present in copious amounts and, in addition, the peak to peak height of the phosphorus line at 120 eV is drastically reduced relative to the indium. For the contaminated case shown the In : P peak ratio is 3 : 1, whereas for an atomically clean surface the same ratio has a value of 0.3. The surface impurities are tightly bound and cannot be removed easily by heating (see later). If a surface which has been exposed to the atmosphere is then subjected to the vacuum bakeout there is a further large increase in the amplitude of the carbon and oxygen Auger peaks and the phosphorus practically disappears. It is not possible from these results alone to decide whether a monolayer of contamination has been adsorbed or whether a thicker oxide layer has been formed. The very large decrease in the amplitude of the phosphorus peak relative to the indium suggests that the process is not simple adsorption. In order to test this hypothesis, oxygen was allowed into the u.h.v. system in controlled doses and the various peak amplitudes monitored as a function of exposure. Fig. 6 shows the resulting variation of the P : In peak ratio, as well as the oxygen peak height. The curves

20

100

200

300

400 ENERGY

500 ieV1

Fig. 5. Auger electron spectra for InP: {a) cleaved in ubv,; @) after exposure to the atmosphere. Beam current < 1 rA. Modulation amplitude < 1 V p-p. Note that the main phosphorus peak in (a} is divided by 3.

shown tend towards ~turation after exposures of about 3 X 10W3torr sec. If this were due simply to monolayer formation then an oxygen sticking coefficient of about 3 X ICY4 is predicted, i.e. the (1.10) InP surface is not very reactive. However, fig. 6 does not present the whole story. For pressures approaching an atmosphere there are further slow changes with the P : In peak ratio decreasing by a factor of 100%. The behaviour is much more typical of a slow oxidation process in which the oxygen species penetrate the bulk, rather than a simple adsorption. As-

0

12

Fig. 6. Plots of oxygen Auger peak height and P : In peak height ratios as a function of exposure to oxygen.

R.H. Williams,I. T. McGovern/Surface characterisationof InP

21

suming that the Auger transition probabilities do not change significantly and allowing for the differing escape depths of 120 and 400 eV electrons, it would appear that phosphorus is being removed from the surface region, probably as an oxide, leaving behind an easily oxidized indium rich layer. Such a process is consistent with all experimental observations. Furthermore, the oxides of phosphorus are volatile products and would be easily removed under the experimental conditions of these studies. Further mass spectroscopic studies in progress may enable the reaction products to be identifled. The in depth sensitivity of electron spectroscopic experiments depends on the escape depth of excited electrons, which in turn is energy dependent. At the energies considered here the sampling depth of AES is 4 to 8 A. It can safely be concluded therefore, that after oxidation, there is little phosphorus remaining in the top monolayer or two of InP. In order to probe a deeper surface layer, crystals were examined by X-ray photoelectron spectroscopy (XPS). Using Al Kcr exciting radiation (energy 1486 eV) the emitted electron energy and therefore escape depth may be substantially higher. The XPS spectra are shown in fig. 7. For a clean vacuum cleaved surface indium and phosphorous core levels are clearly visible (with a small amount of C and 0 due to irradiation of the sample holder). Interestingly, short exposures to the atmosphere leads to little change in this spectrum. However for a surface exposed to the atmosphere for several days no phosphorous is observed and the carbon and oxygen peaks dominate the XPS spectrum. Since the escape depth at the energies involved in XPS is of the order of 30 A [ 141 it is concluded that after fairly short exposure to the atmosphere the surface impurity layer on InP is only a few monolayers thick, whereas after prolonged exposure it may be somewhat thicker.

1000 KINETIC

1500 ENERGY

leV)

Fig. 7. X-ray (Al Ka 1486 eV) induced photoelectron spectra for: (a) u.h.v. cleaved face;(b) cleaved (110) InP face exposed to the atmosphere for several days.

22

R.H. Williams, 1. T. McGovernjSurface

characterisation

of InP

3.1.4. Details of the Auger spectm

During the course of these studies it was observed that the nature of the Auger spectra in the vicinity of phosphorus L2,, W transition, at 120 eV, varied in an interesting manner. For an atomically clean cleaved face the resolved peaks are due to the L2,3W transition and its characteristic energy loss (109 eV). After expsoure to high oxygen pressures these peaks are severely reduced but generally no new well resolved features are observed in the vicinity. However, for shorter oxygen exposures, typically 10m3 torr set, new peaks appear with minima at 111 and 102 eV, and with subsidiary shoulders at 93 and 86 eV. Those are shown in fig. 8. These extra peaks have only been observed when oxygen has been adsorbed on the surface and are undoubtedly associated with the phosphorus species. They are not apparent for clean indium phosphide, or for slightly oxidized metallic indium, and are not consistent with transitions involving indium species. The amplitudes of the peaks, in addition to being dependent on the oxygen pressure, also showed a dependence on the time of exposure to the electron beam and on the beam current density, For relatively low levels of oxidation, high beam currents led to a reduction of the peak heights, presumably due to desorption of adsorbates. For high levels of oxidation on the other hand, large beam current densities led to the appearance of small peaks at 111 and 102 eV, where none had previously been clearly resolved. One possible explanation for the extra features is that excited secondary electrons suffer elastic scattering on the way out of the crystal and are scattered into or out of the escape angle [ 151. The dependence on beam current, however, makes this interpretation very unlikely. By far the most likely explanation is that the peaks are due

I

I

100

200

300

200

ENERGY

500 IeVi

Fig. 8. Auger electron spectrum of u.h.v. cleaved InP under oxidation. Note appearance of new peaks below 120 eV. Incident beam current < 1 &A. Modulation amplitude < 1 V p-p.

R.H. Williams,I. T. McGovern/Surface characterisationof InP

23

to cross transitions involving phosphorus and oxygen atoms. The existence of such transitions has been used to explain similar phenomena in metals [ 171 and in the related semiconducting material gallium phosphide [ 161. For these transitions to be possible it is necessary for the oxygen and phosphorus species to be in close proximity. This situation exists at the interface between the InP and the adsorbed monolayer or oxide layer. If the transitions do take place predominantly at this interface the associated Auger spectra would be observable for coverages of oxygen of the order of a monolayer but would be much reduced if the oxide layer were two or three layers thick, due to the small escape depth of electrons at these energies. This is entirely consistent with out experimental findings. Furthermore, high exciting beam current densities presumably cause some desorption of oxygen from highly oxidized faces, thus increasing the contribution from the interface region. Possible interpretations in terms of adsorbed oxides of phosphorus on the surface have also been considered but cannot be reconciled with these electron beam effects. Tentatively, therefore, the observed extra features are identified in table 1 in terms of possible cross transitions. However, further detailed and well controlled studies of the influence of contaminants and electron beams on the indium phosphide Auger spectra are desirable and such studies are in progress. 3.2. Epitaxial layer surfaces Microwave devices based on InP are fabricated from n-type epitaxial layers grown on a highly conducting InP substrate. The surfaces to which electrical contact is made are (100) and they are normally etched using (1) 2% bromine in methanol or (b) a solution of 2HC1,2H,O and 1HN03 (Aqua Regia), prior to the deposition of the injecting contact. In this section we describe studies of the surface properties of these “real” (100) faces and compare them with those of cleaved (110) single crystal already determined.

Table 1 Observed and calculated peak positions for the Auger spectrum of oxidised indium phosphide. The oxygen Lz,~ level has been taken as 10 eV below the vacuum level [ 17) and the InP valence band density of states maximum as 6 eV below Transition

PL2,3

vv

pL2,3°L2,30L2,3

pL2,3°L2,30Li PL2,3%

oJ4

Observed peak position (eV)

Calculated peak position (eV)

120 111

120 112

102

102

93 84

98 84

24

R.H. Williams, I.T, McGovern/Swf~ce

characterisatkm

of InP

Fig. 9. Auger electron spectra of etched (100) InP epitaxial layers: (a) etched using bromine in

methanol; (b) etched using 2HCl-2HzO-lHN03. 12 VP-p.

Beam current 1 PA. Modulation amplitude

Auger electron spectra for etched hrP slices are shown in fig. 9. All surfaces were subjected to the bakeout necessary to achieve u.h.v. (- 200 “C for 12 hr typically). For all surfaces there is an abundance of oxygen and carbon and the curves in general are remarkably similar to those of contaminated cleaved (110) faces. In particular, the small amplitude of the phosphorus peak is very obvious, su sting that the surface region of an etched face has been depleted of phosphorus in a similar way to that of a contaminated cleaved (1 IO) face. No bromine was observed on surfaces etched in the methanol solution but some chlorine was always visible on the corresponding cases etched in aqua regia. For the unetched surfaces investigated there was usually more oxygen present compared with the etched faces. 3.2.2. Phot~t~r~~hold and surface photo~oIt~~~ Fig. 10 shows the cube root of photoelectric yield as a function of photon energy for etched InP epitaxial layers, The curves have not been corrected for reflectivity

R.H. Willlams,I. T. McGovern/Surface characterisationof InP

25

Fig. 10. Plot of cube root of yield, not corrected for spectral dependence of reflectivity, as a function of photon energy. Full circules for samples etched using 2HCl-2HzO-lHN03. Open circles for samples etched using bromine in methanol.

variation with wavelength. Both plots yield good linear relationships and extrapolate to zero yield for a photon energy of - 5.1 eV. This is 0.5 eV below that for a clean cleaved (110) surface, but in view of the agreement with the yield of a contaminated (110) face, the difference may be attributed to the surface impurities rather than to a crystallographic effect. Similar studies on other solids [8] have shown that considerable emission may originate in these surface impurities. Illumination of the surfaces by radiation from the argon ion laser leads to two noticeable effects. Firstly, the yield close to threshold increases by about 4%, though it was established that the laser light itself did not give rise to emission directly. However, the effect may be understood in terms of the energy bands near the surface being flattened as a result of laser illumination. If the bands were initially bent as shown in fig, 4 then decreasing the band bending increases the number of electrons which may be excited at a given depth below the surface such that they are capable of escaping over the surface barrier, Thus strong evidence is provided for the existence of negative surface charge from this observation. This is confirmed by the second effect, namely a decrease in work function as a result of laser illumination. A plot of the work function variation, or surface photovoltage, as a function of the logarithm of the light intensity, is shown in fig. 11, for surfaces prepared using different etchants. It may be seen that an approach to saturation is reached in each case, and since the Dember effect may be ignored, the saturation values of the photo-voltages yield the magnitudes of the band bending. For sur-

26

R.H. Williams,i.T, McGovernjSurface characterisationof InP

1

?I

IO LIbHT

IN:ENSI;Y

IREt.

01T41

Fig. 11. Surface photovoltage variation with relative light intensity. Full circles for samples etched using bromine in methanol. Open circles for samples etched using 2HCl-2H,O-1HNOs.

faces etched in aqua regia the energy bands are bent by about 87 mV, while the bending is about 12 mV smaller for faces etched in the bromine solution. A brief study was aIso made of the influence of deposited silver on the measured photovoltage. Silver was chosen since it has a comparatively low work function and thus, according to traditional models of the metal semiconductor contact, it might be expected to yield an ohmic contact to these InP surfaces. The silver film would thus donate electrons to the semi~onducting solid and the sign of the band bending near the interface would be of the opposite sign to that measured for a free surface. In these experiments silver was deposited by evaporation off a tungsten filament, in u.h.v. The sign of the surface photovoltage was monitored as silver was slowly deposited. For thick continuous fdms the photovoltage was screened but for small coverages, as measured by Auger spectroscopy, photovoltages were observable under laser ihumination. At no time, however, did we observe a change in the sign of the photovoltage from that of a nonsilvered surface. This suggests that a Schottky barrier is formed at the interface when silver is deposited by evaporation onto these faces. This is in agreement with similar measurements using capacitance and photoresponse techniques [ 191. It is of relevance to note that silver has previously been investigated with respect to its performance as a cathode material in high efficiency microwave devices [3]. In those studies it was also concluded that Schottky barriers existed at the contact.

R.H. Williams, 1. T. McGovern/Surface

characterisation of InP

21

3.2.3. The effect of heat treatment The studies described in the previous sections on samples grown by vapour epitaxy all refer to contaminated surfaces. Since cleavage is not possible along (100) it is not feasible to obtain clean surfaces by this method. It is however, desirable to obtain clean surfaces and for this reason the influence of heating was briefly investigated. Slices were heated for various periods at various temperatures in ultra high vacuum. Initially there was a small decrease in the carbon peak height and at higher temperatures, above 300 ‘C, a small reduction in the size of the phosphorus peak, possibly due to evaporation from the surface region. Carbon and oxygen, however, were present in substantial amounts at all temperatures until decomposition of the surface occurred. For temperatures well in excess of 500 “C the oxygen peak suddenly almost disappeared and the carbon peak was much reduced. Inspection of the sample, however, revealed that the hotter region contained pure indium on the surface, i.e. decomposition had taken place. It was concluded therefore that these etched (100) surfaces could not be cleaned by heat treatment alone since decomposition occurs first. Our observations confirm the result of Bayliss and Kirk [7], namely that out diffusion of phosphorus occurs at temperatures much in excess of 320 “C. These workers also conclude that atomically clean stoichiometric (100) surface cannot be obtained by argon ion bombardment and annealing, although oxygen stabilised faces, showing (1 X 1 LEED structure were obtained if the annealing temperature did not exceed 300 ‘C.

4. Conclusions (1) Cleaved single crystal InP, exposing the (110) face, possesses a surface two dimensional unit mesh indentical to that of the bulk. For a clean surface the photothreshold is 5.6 eV and for the n-type crystals studied surface states leads to a depletion layer near the surface. (2) Reaction with oxygen, at room temperature, leads to loss of phosphorus from the surface and the formation of an oxide layer. For a cleaved surface exposed to the atmosphere there is also an abundance of carbon. (3) (100) faces of vapour grown epitaxial layers have a composition similar to that of contaminated (110) crystal surface. For surfaces etched in aqua regia some chlorine is also present. For etched surfaces of n-type material depletion layers exist near the surface. Barrier heights are of the order of 0.1 V, being slightly larger for surfaces etched using aqua regia than for those etched suing bromine in methanol. (4) The deposition of silver, by evaporation, onto contalninated n-type (100) epitaxial layer surfaces, leads to a Schottky barrier in the contact region. (5) Indium phosphide surfaces cannot be cleaned by heat treatment alone since decomposition occurs before carbon and oxygen impurities can be removed. Decomposition of the surfaces appears quite severe for temperatures well above 300 “C.

28

R.H. Wiliiams,I. T. McGovern/Surface characterisationof InP

Acknowledgements

The authors wish to thank Professor G.G. Roberts for valuable discussions and support. We also wish to acknowledge valuable discussions with K.W. Gray and J.E. Pattinson and thank them and their colleagues at R.R.E., Malvern, for supply ing the indium phosp~de crystals,

References [ 11 R-L. Bell and J,J. Uebbing, Appl. Phys. Letters 12 (1968) 76. [Z] R.L. Bell, Negative Efectron Affinity Devices (Oxford Univ. Press, 1973). [ 31 H.D. Rees, Metal Semiconductor Contacts, Conf. Series No. 22, Inst. of Phys. (1974) 105. [4] T.E. Fischer, Phys Rev. 142 (1966) 519. [S] L.W. James, J.P. van Dyke, F. Herman and D.M. Chang, Phys Rev. Bl (1970) 3998. [6] R.L. Bell, L.W. James and R.L. Moon, Appl. Phys. Letters 25 (1974) 645. [ 71 CR. Bayllss and D.L. Kirk, to be published, [ 81 LT. McGovern, R.H. Was and C.H.B. Mee, Surface Scl. 46 (1974) 427. [Q] M. Cardona, in: Proc. 7th Intern. Conf. on Physics of Semiconductors, Paris, 1964, p.181. [IO] W. Cudat, E.E. Koch, P.Y. Yu, M. Cardona and CM. Penchina, Phys. Status Solidi fb) 52 (1972) 505. [ 111 C.V. Festenberg, Z. Physik 227 (1969) 453. [12] R.H. Ritchie, Phys. Rev. 106 (1957) 874. [13] L.K. Galbraith and T.E. Fischer, Surface Sci. 30 (1972) 185. [ 141 J.C. Riviere, Contemp. Phys. 14 (1973) 513. [ 151 L. McDonnell, B.D. PowelI and D.P. Woodruff, Surface Sci. 40 (1973) 669. [ 161 A.E. Morgan and W.J.M. Van Velzen, Surface Sci. 40 (1973) 360. [ 171 A.P. Janssen, R.C. Schoonmaker, A. Chambers and M. Prutton, Surface Sci. 45 (1974) 45. [ 181 A. Many, B. Goldstein and B. Clover, Semiconductor Surfaces (North-Holland, Amsterdam, 1965). [ 191 CA. Mead and W.G. Spitzer, Phys. Rev. 134 (1964) A713.