XPS stoichiometry measurements on surfaces of III–V crystalline compounds

Journal of Electron Spectroscopy and Related Phenomena, 49 (1989) 159-173 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

XPS STOICHIOMETRY MEASUREMENTS III-V CRYSTALLINE COMPOUNDS

ON SURFACES

159

OF

P. ALNOT and J. OLIVIER LCR THOMSON-CSF Domaine de Corbeville 91401 Orsay (France) C.S. FADLEY Department of Chemistry, University of Hawaii at Monoa, Honolulu, Hawaii 96822 (U.S.A.) (First received 28 July 1988; in final form 21 November 1988)

ABSTRACT The important and often-disregarded problem of stoichiometry measurements of single-crystal III-V compounds is studied. Several experimental procedures utilizing XPS are examined in order to obtain the actual stoichiometry or to follow its qualitative evolution, e.g. after successive chemical treatments of the sample. In particular, a correct allowance for the effects of photoelectron diffraction is found to be necessary.

INTRODUCTION

In surface analysis, one desires the elemental composition or stoichiometry of the outermost atom layers of a solid, and if possible, a detailed knowledge of the chemical bonding states. X-ray electron spectroscopy (XPS) is a wellestablished technique that in principle can provide such information. However, the measurement of core electron intensity or Auger electron intensity from a single crystal surface as a function of the electron take-off angle gives rise to plots in which pronounced fine structure is superimposed on the instrumental response function [ 11. Because of this dramatic anisotropy of the XPS signal from single crystals due to photoelectron diffraction in the atomic network, great care should be taken in quantitative interpretation [ 21. The single scattering cluster (SSC) description of such X-ray photoelectron diffraction (XPD ) effects as first discussed by Kono et al. [ 3 ] has been found to describe very well the observed XPD features for GaAs and InP singlecrystal substrates in the energy range around 300-1500 eV [ 2,4]. This model utilizes a kinematical approach, where the Auger or photoelectron intensity for an electron wave vector k is given by the superposition of the primary wave and waves elastically scattered once from all other atoms in the vicinity of the emitter. Much of the simplicity and utility of this model derives from the dom-

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0 1989 Elsevier Science Publishers B.V.

160

inance of forward-peaked electron scattering at higher energies [ 31. There is currently considerable interest in knowing the surface stoichiometry and chemical composition after different chemical treatments of III-V binary compounds with formula AB. In XPS analysis of such surfaces, measurements are usually performed on the A and B core levels. The stoichiometry is then obtained through ratios I( A) /I( B ) of the intensities of these core level peaks. In previous papers ]2,4 ] , we have obtained good agreement concerning both the shape and angular peak positions, between experimental and theoretical ratios I(As3d 1/I( Ga3d) and I( P2p ) /I(In4d) as a function of the electron takeoff angle, provided that substrate XPD effects are taken into account. As a result of these effects, there is a very large variation in these ratios (factors of 2 to 3) depending upon the polar angle. These results thus indicate the care which has to be taken in order to measure correctly the stoichiometry of any mono-crystalline substrate: accurate crystal orientation within the spectrometer must be achieved in order to probe emission along well defined crystal axes. only in that case can the variation of surface stoichiometry due for example to different surface treatments be meaning~lly determined. In this paper, we will explore three ways of obtaining accurate stoichiometry values of GaAs and InP (001) surfaces. (a) The first way consists in finding pairs of azimuthal and polar angles for which the diffraction effects on both the anion and cation photoelectron intensities will be zero or at least negligible. (b) The second method explored is to take advantage of the dependence of the XPD patterns on the crystallographic site (but not necessarily the species of emitter atoms in the crystal) and thus also of the zinc blende symmetry of the III-V compounds, in order to determine polar scans for which intensity distributions from anion and cation will be similar. (c 1 The last procedure explored is to average over oscillations due to atomic order by enlarging the solid acceptance angle of the electron analyser. INSTRUMENTAL

The instrument and the procedures used for the measu~ments are described in detail elsewhere [2], and only a summary and certain new features of the experimental set-up are presented here. Photoelectrons are excited by Al KCY radiation. The conical acceptance angle of the hemispheric analyser of the VG Mark II system is +-6’. But the effective angular resolution is somewhat better than this value, because of the combined effects of retardation and aberration in the electron lens [5 1, and is estimated to be + 3’. Angular scans were performed with a specimen manipulator driven by stepping motors and there is overall a precision of angle setting of 0.1’. The polar angle B of photoelectron take-off relative to the (001) surface, is scanned by sample rotation about an axis lying in the sample surface as shown in Fig. 1 (a). With our sample mount-

161

(b)

Fig. 1. (a)

Anglereferences and Miller indices of the planes and axes used in this study. (b) GaAs crystal structure. Black and white spheres represent galliumand arsenic atoms.

ing geometry, two azimuthal crystal positions could be chosen with sufficient accuracy by checking the positions of cleavage faces, the (110) and (li0) planes. We will call the two different polar scans corresponding to these two azimuthal positions, the [liO] scan (axis of rotation being [liO] for which @=90” in Fig. 1) and the [ 1101 scan (axis of rotation being [ 1101, for which @= 180” ). As the [ ii01 scan (@=O” ) will be symmetry-equivalent for all sites to the [110] scan, we will for simplicity designate [llO] as @=O”. The InP (001) surfaces were obtained by performing a chemical etch in acidic solution (HF : H,O ). The GaAs (001) surfaces also were etched in acidic solution (5H,SO, : Hz02 : Hz0 ). Finally, the two types of crystals were rinsed with deionized water for 15 minutes. These etching procedures are carried out inside a glove box maintained under a nitrogen atmosphere in order to avoid surface oxidation. The crystals were finally transferred from the glove box into the spectrometer via a hermetically-sealed vessel. As verified by XPS, these procedures are found to leave both GaAs and InP surfaces free of any oxides. A second type of (001) surface has been obtained by the MBE growth of a GaAs epitaxial layer. The sample is protected from contamination during its transfer from the MBE growth system to the XPS apparatus by condensing a protective layer of elemental As onto the sample surface. After transfer to the XPS vacuum system, the sample is heated to around 400” C to evaporate the As overlayer. EXPERIMENTAL

RESULTS

Polar-angle distribution curves (ADC’s) of surfaces cleaned by the previously described processes are shown in Figs. 2-7, for three azimuthal positions ( @= 0 ‘, 45 O, 90’ ) and for several spectral lines (Ga3d, As3d, In4d and P2p), all of which have high kinetic energies in the narrow range from 1357 eV to 1468 eV (see Table 1). No correction has been made for the o-dependent in-

162

t 110.0

30.0

70.0

POLAR

ANGLE

.

.

.

.

110.0

(8)

.

.

.

70.0

POLAR

..i 30.0

ANGLE (13)

Fig. 2. Comparison between experimental (squares) and theoretical ADCs for a Gdd and an As3d photopeak in a [ 1 lo] scan (@= 0 o ). The theoretical ADCs were obtained with the SSC-DE model (cf. ref. 2). They have been convoluted by an angular broadening function of 3” and multiplied by a e-dependent instrument response function. Fig. 3. Comparison between experimental (squares) and theoretical ADCs for a Ga3d and an As3d photopeak in a [ liO] scan (@=90’). The theoretical ADCs were obtained with the SSC-DE model. They have been convoluted by an angular broadening function of 3” and multiplied by a B-dependent instrument response function.

I

30.0

70.0

110.0

POLAR

ANGLE

70.0

110.0

POLAR

(8)

Fig. 4. Experimental scans.

ADCs corresponding

to a Ga3d and an As3d photopeak

Fig. 5. Experimental (100) (@=45”).

ADCs corresponding

to a Ga3d and As3d photopeak

ANGLE

30.0

(@)

for [ 1101 and [ liO] for a scan in the plane

163

In4d

2

110.0 POLAR

70.0 ANGLE

110.0

30.0 (8)

POLAR

70.0 ANGLE

(G)

30.0

Fig. 6. Comparison between experimental (squares) and theoretical ADCs for a In4d and a PZp photopeak in a [ 1101 scan (@=a’ ). The theoretical AJXs were obtained with the SSC-DE model. They have been convoluted by an angular broadening of 3” and multiplied by a e-dependent inst~ment response function. Fig. 7. Comparison between experimental (squares) and theoretical ADCs for a In4d and a P2p photopeak in a [ IriO] scan (@=90”). The theoretical AD& were obtained with the SSC-DE model. They have been convoluted by an angular broadening function of 3 ’ and multiplied by a e-de~ndent inst~ment response function. TABLE 1 Kinetic energy E, inelastic mean free path a and wave vector k of the pho~lect~n core levels of gallium, arsenic, indium and phosphorus Level E(_eV) @1 k(A-‘)

Ca3d 1467 29.90 19.62

As3d 1446 29.50 19.48

In4d 1468 31.50 19.63

excited from

P2P 1357 29.70 18.87

strument response function. Due to this response function, XPS intensities generally decrease as the polar orientation moves towards grazing angles of emission [ 11. If we group an anion ADC for a @= 0” (0= 90” ) scan with a cation ADC for a CD=90” (@=O” ) scan, it is obvious that trends and peak positions are the same for both pairs of curves. Due to the symmetry of the zinc blende crystal, the positions of the anions in the lattice as viewed by the electron analyzer during the @=O” scan, are equivalent to those of the cations during the @=90” scan (Fig. l), as discussed in detail in ref. 2. In addition, it is clear that the polar scans taken in the azimuthal ( 100 ) plane ( @= 45’ ), give

164 I’(P2P)

I'(AS3d) /I'(GaBdi

/I

iI”4d)

I-“,“..‘~

110.0

70.0 POLAR ANGLE

30.0 (6)

iio.0

70.0 POLAR ANGLE

30.0 (e)

Fig. 8. Comparison between experimental and theoretical ratios of the photoelectron intensities Z’ (Adcl)/Z’ (Ga3d) for the two scans [llO] and [liOJ: (a) MBE grown and annealed GaAs substrate; (b) Chemically cleaned GaAs surface; (c) theoretical curves. In (a) the ratio for the third azimuth (plane (MO), @=45”) is presented (crosses). Fig. 9. Comparison between experimental and theoretical ratios of the photoebctron intensities I’ (PZp)/Z’(In4d) for the two scans [X10] and [liO]: (a) chemically cleaned InP surface; (bf theoretical curves.

intensity distributions which for both ions follow essentially the same variation pattern (Fig. 5)) again for reasons of crystalline symmetry, as shown below. Therefore, as previously noted [ 21, an important result drawn from these curves if that ADC’s are very sensitive to the positions of anions and cations within the crystal lattice. The acidic chemical cleaning (5H,SO, : Hz02 : H,O) of the GaAs (001) surface is different from that of the preceding paper [2] which involved an additional basic chemical etching ( 5NH40H : H2G2: 200 H,G). Besides the implication of the results for surface stoichiometry that will be examined later, we can also draw some qualitative conclusions from the measurement of the degree of anisotropy (I,,, - fmin)/I~,=&/&,,, of the XPD patterns [Table 21, which is directly related to the change of surface structure and stoichiometry, or in particular the degree of near surface crystalline order. The systematically lower values of the degree of anisotropy in the case of the single-step acidic etching is probably due to the very first layers of GaAs being slightly more disordered than in the case of the two-step etching. The MBEgrown and annealed GaAs layer shows a greater degree of anisotropy that reaches, at @=90”, the values obtained in the case of the two-step chemical cleaning which seems at this stage of the study the softer etching process. Figures 8 (a), 8 (b ) and 9 (a) show the polar angle dependence for different

165

azimuthal planes of experimental ratios I’ (As3d) /I’ (Ga3d) and I’ (P2p) / I’ (In4d) between the intensities of the photopeaks but with a normalization by Scofield’s theoretical differential cross sections [6]. That is, for a given peak, we define I’ = I/ (da/U) where I is the observed intensity and (da/U) is the differential photoelectric cross section of the level involved. Depending upon the polar and azimuthal angles, there is a very large variation in these ratios, through which one must now attempt to obtain the stoichiometry. As noted in our previous paper [ 21, stoichiometry values differing by as much as a factor of 2 could result for GaAs, with this factor being even higher at 2.5 for InP. DISCUSSION

XPS Quantitative analysis Chemical cleaning and ion beam techniques such as plasma etching are very useful techniques in electronic processing. But they induce stoichiometry variation and damage on the surface of the processed materials. Quantitative analysis by XPS aims to determine the various concentrations in the elements existing at the “surface” of the sample. We understand here that “surface” means the few outermost atom layers illuminated by the X-ray beam. The photoelectron created has a probability of l/e of travelling a distance characterized by the inelastic mean free path in the matrix, II, before being inelastically scattered and no longer appearing in the spectral line. Thus the flux of photoelectron decays as exp ( -d/L), where d is the distance from the point of origin. The energy-dependent characteristic vertical depth from which electrons can be emitted, often called the escape depth, is thus ;1 sin 8, where 8 is the angle of emission with respect to the surface. Its knowledge allows the contribution to the signal from atoms in each atom layer to be determined. In the following we will use the A values obtained from the equations established by Szajman et al. [ 71, which well fit experimental values obtained for narrow-band-gap semiconductors near 1 keV kinetic energy [ 7,8], [Table

11.

If an amorphous or polycrystalline binary compound AB is homogeneous over a depth several times A, the intensity of a photopeak is given by I(A,k) =1x xJ2(&)

xA(Ed

XD(&c)

XPAX

(da/dQ)

XA(&)

where Ix = the flux of X-rays, Ek = the kinetic energy of the photoelectron excited from the core level lzof the atom A, da,/d,& the differential photoelectric cross section of the level k, PA is the density of atoms A and the analyzer is assumed to be described in terms of an effective solid angle of 52acting over an effective source area of A, and to have a detection efficiency D. For photoelectron peaks very close in energy, as in the case here (cf. Table 1) , the SZ,A,D

166

and il factors will cancel in such an intensity ratio. Therefore, the ratio I’ (A)/ I’ (B ) between the intensities of the photopeaks normalized by the Scofield’s theoretical differential cross sections will give a good estimate of the stoichiometry pA/pB of the compound. Also, in a homogeneous semi-infinite sample, this ratio will be independent of the take-off angle [ 91. However, in a more common experimental situation of a homogeneous specimen for which alteration of the surface composition is present, stoichiometry measurement will be expressed in terms of the composition averaged over the technique’s effective sampling depth Izsin 13.Thus two common ways to probe the surface stoichiometry are to enhance the surface sensitivity by (i) measuring the ratio I’ (A)/f’ (B) at more and more grazing angles of emission, and/or, (ii) working with photoelectrons of low kinetic energy. As a matter of fact, the energy dependence of A is such that one gets a flat minimum near 5 A between 40 and 100 eV [lo]. In all of the above procedures, any effects due to atomic order are thus implicitly assumed to be fully averaged over.

Allowance for XPD

effects

However, for c~stalline substrates, our experimen~l results (Figs. 2-7) show clearly that performing such ratio calculations without considering substrate diffraction effects may result in considerable error concerning stoichiometry. This may explain the wide variation observed in the experimental values of the ratio I’ (As)/l’ (Ga) for the several well defined surface reconstructions of GaAs (001) [ 11-161, since these studies make use of analyzers that angle-integrate the diffraction structures in different ways and to different degrees. If we desire now to theoretically estimate the diffraction component of intensity variations for low kinetic energies, the calculations may involve multiple-scattering theory, which is rather cumbersome. Thus, in the following, we will be concerned only with photoelectron peaks of energy l-l.5 keV [Table l]. For such energies, a variety of experimental results have been correctly predicted by the single-scattering cluster model [l-5 ] , particularly if it is empirically corrected for the combined effects of spherical wave scattering and for defocussing effects due to multiple scattering along low-index rows of atoms (SSC-DE), as discussed in ref. 2. Briefly, the XPS signal can be calculated as the square of the photoelectron wave amplitude whose expression is given by the superposition of the primary wave originating from a given source (a certain atom in the lattice) and those waves scattered singly by surrounding atoms; If a0 (?$) is the photoelectron wave at ; as emitted directly into direction k and C~~(?~<-tk) is the wave resulting from initial at, emission toward a scattererj at rj and $hen subsequent scattering so as to emerge from the surface in the direction of k, the photoelectron intensity in the same direction is given by [ 1]

167

With some simplifications that make use of the strongly forward peaked nature of electron atom scattering at these energies, as discussed in ref. 2, this equation can be written as I(i)

oc [da(cx)/dQ]

Jo + CJ$“, J

where Jo and Jj are defined as Jo =sin cvkexp (-L/211) Jj=sin

Olj( ]f,(8,)I/rj)exp(-L;/2n)

exp{i[krJ(1-cos8i)+y/,(8J)]}

where 0, is the scatter$g angle, o!ykis the angle between the direction of radiation propagation and k, Ocjis the angle between the radiation propagation direction and i!j, L and Lj are the path lengths below the sample surface, h( ej> =vj( 6j)I exp [i Wj( Oj) ] is the complex electron scattering factor, and do( a) / +2 is the differential photoionization cross-section, as evaluated in direction k. Idi==Ji is the non-diffracting or direct part of the intensities given by the anions and cations, as normalized by the differential photoionization crosssection. From the expressions of I(k) and Idirywe can extract the diffracting part, Idiff = IJ0 + C JjI” - Jg . This diffracting part normalized by the direct part (a quantity ver; closely related to the x functions used in EXAFS and SEXAFS) is displayed in Figs. 10 and 11 as a function of polar angle for two scans ( @= 0 o and @= 90’ ) in the case of GaAs (001) and InP (001) . In all these figures, a noteworthy result is that Idifflldir exhibits a wide minimum of value near zero that is situated between about 1940” and tV= 50”. In addition, for the two azimuthal angles @=O” ([lio]) and @=90” ([llo]), the Idifr/ldir curves for the anions and cations are equal near the polar angles f&=44’ for GaAs (Fig. lo), and 0,=48O for InP (Fig. 11). Thus, for III-V compounds like GaAs and InP (001) , an important result is that pairs of azimuthal and polar angles exist where we can measure an effective surface stoichiometry that should be little affected by diffraction effects. From the theoretical ADC’s of Figs. 2,3,6 and 7 which take into account the apparatus response including an angular resolution of about 3” and the electron transmission within the analyzer, we can calculate theoretical ratios I’ (A) / I’ (B ) that clearly agree with a stoichiometry of one, as noted at the polar angles t9, (Figs. 8 (c ) and 9 (b ) ) . Concerning the experimental ratios (Figs. 8(a), 8 (b) and 9 (a) ), we can notice (i) a good agreement with the theoretical results, and especially with the shape of the curves; (ii) a weaker anisotropy than that of the theoretical

Idlff/Idlr

Idiff/Idir

70.0

110.0

POLAR ANGLE

30.0

ilO.

(G)

30.0

70.0

POLAR ANGLE

(G)

Fig. 10. Theoretical ratios of the diffracting and non-diffracting parts of the photoelectron sities I’ (GaSd) and Z’ (As3d) for the two scans [ liO] and [ 1101.

inten-

Fig. 11. Theoretical ratios of the diffracting and non-diffracting parts of the photoelectron sities I’ (In4d) and I’ (P2p) for the two scans [ liO] and [ 1101.

inten-

TABLE 2 Degree of anisotropy AZ/Z,,. at the two azimuths 0~0” and @=90” and value of the stoichiometry Z’ (As) /I’ (Ga) as the polar angle 0, = 44 ‘, as determined theoretically for an ideal GaAs (001) crystal and experimentally for different treatments of the GaAs surface GaAs surface treatments

Degree of anisotropy AZ/Z,,,,, (between f&60” and 0=90’)

Stoichiometry (8,=44”)

As3d

Ga3d

Two-step etching [ 2 ]

0.42 (@=O”) 0.46 (@=90’)

0.37 (@=90”) 0.48 (@=O”)

0.9

One step acidic chemical cleaning

0.32 (@=O”) 0.43 (@=90°)

0.33 (@=90°) 0.42 (@=O’)

0.87

MBE-grown and annealed

0.35 (@CO”) 0.46 (0=90”)

0.34 (@=90”) 0.48 (@=O”)

0.80

Theory

0.51 (&O” ) 0.59 (0=90”)

0.53 (@=90”) 0.67 (@=O”)

1

curves; (iii) an offset of the over-all experimental curves towards sub-stoichiometry, i.e. an anion depletion. At 0, = 44 O,the measured surface stoichiometries are about 0.80 for the MBEgrown and annealed GaAs layer and 0.90 for the chemically cleaned GaAs sur-

169

face (Figs. 8(a) and 8(b)). From the InP curves (Fig. 9(a)) at 1!$=48’, we estimate a stoichiometry value of 0.70 (cf. Table 2). Due to the chemical etching of the samples, we might expect that the very first layers will be disordered. Therefore they give little diffraction and act like an attenuating amorphous layer for the photoelectrons coming from the underlying crystalline substrate (acting to explain observation (ii) ). In addition, if the composition of the attenuating layer is different from that of the bulk, and for example, consists of a surface depleted in anion, one will observe behavior such as (iii). Application to specific cases Let us examine the particular case of polar scans in azimuthal ( 100) planes (@= 45’ ) which give anion and cation ADC’s of similar shape like those of the MBE-grown and annealed GaAs layer shown in Fig. 5. Because (100) planes contain successive and alternating centred faces of the two crystalline sublattices of the zinc blende structure (Fig. 1) , intensity distributions from cation and anion along the (100) plane are expected to be very similar, assuming that the difference in their scattering properties is weak. The anionic and ca$ionic photoelectron intensities of III-V compounds AB in the direction of k, normalized by their differential photoionization cross-section, are given by I*(Z) CCIJO+ &&I2

with J,, and Jj defined as above. Here the atoms surrounding the anion and cation occupy equivalent sites, so CJAj and CJ,,. differ only in the scattering fac$r f inv$ved. If, as in the case of GaAs, fA and fB are nearly equal [ 171 IA (lz) = In (Iz). Thus the ratio I’ (As3d) /I’ (Ga3d) is about constant (crosses in Fig. 8 (a) ) , supporting the value of 0.80 given by the 2 other scans ( @= 0’ and @=90’) at &=44”. In the general case of III-V (001) surfaces where fA is different from fB, the diffraction effects are different for the anion and the cation. The ratio of their XPS intensities thus cannot simply give the stoichiometry, except for polar angles where the two diffraction contributions are negligible with regard to the primary wave. However, we can follow qualitatively the evolution of stoichiometry provided that we maintain the same azimuthal and polar sample position. An analogous, but more general situation arises from the zinc blende symmetry which is such that, at the azimuth 0, the anion (cation) occupies identical sites with the cation (anion) at cP-+7r/2. The anionic and cationic photoelectron intensities are now given by

170

L,(&WdJo

+

CJA, + CJd2 n j

I,(~,~+a/2)cclJ,+CJB,+CJA,12 n i Comparing GaAs (fA=fB) and the other III-V compounds (fA # fn), we can draw the same previous conclusions as for @=45”. For example, in the case of GaAs the ratio I_.,’(@=O’ )/IB’ (0=90’ ) (not shownon the figures),isnearly constant with only slight fluctuations around a horizontal line ( t 7% ). A leastsquares fit of the experimental data gives a stoichiometry value of 0.82 for MBE-grown and annealed layer, and of 0.87 for the chemically etched surface in the experimental angular range (30” 5 85 100” ). In the case of InP, this ratio suffers important oscillations with varying 19. Effect of analyser angular averaging We now consider the effects of analyzer acceptance angle enlargement on the stoichiometry measurement. In general, in the absence of inelastic scattering effects, single- or even multiple-elastic scattering cannot create or destroy photoelectron flux. Thus, integrating such scattered photoelectron intensities over the full 47cSr solid angle around a given emitter would give atotal intensity proportional to the photoelectric cross section cr.Adding in the effects of inelastic scattering as well as considering the variety of analyzers which angle average to differing degrees complicates this problem, however. We begin by considering GaAs, since it is a compound for which Ga and As scattering factors are very similar [ 171, and so the diffraction effects have the same order of magnitude (Fig. 10) and the ratio of peak intensities I’ (As)/ I’ (Ga) that are very fully angular averaged will give a reasonable value of the stoichiometry. This would be the case for example with a hemispherical retarding field analyzer [lo]. InP is a less favourable compound, for which values of anion and cation scattering factors are considerably different [ 171. Thus, the diffraction effects are also very different (Fig. 11) and the ratio I’ (P2p)/I’ (In4d), even when measured in an angle-integrated mode may not give an accurate estimate of PPIPin-

However, even for such situations we can follow the qualitative stoichiometry evolution, e.g., after successive chemical treatments. However, if the diffraction effects are only partially averaged, it is important to consistently use the same azimuthal and polar conditions of sample measurement. Most of the experimental set-ups used to measure stoichiometry ratios of well defined reconstructed surfaces of GaAs (001) have involved the cylindrical mirror analyzer (CMA) [ 12,13,15] of apex angle (x=42”, CMA apertures accept a spread of angles about cyof about Aa = t- 3’. Under these conditions the

171

electron paths through the analyzer consist of annuli corresponding to about 0.5 Sr solid angle. The importance of diffraction effects was demonstrated some years ago by Chang [ 181 who measured the Auger signal from a Si (111) substrate covered with about 10 A of oxide in a CMA with a coaxial gun. He compared the spectra taken before and after small changes in crystal orientation (5’ ) and noted that Auger peaks from the amorphous surface oxide did not change amplitude significantly while peaks from the crystalline Si changed by large amounts. Our preliminary results concerning GaAs (001) surfaces show that Auger currents measured by CMA should be dependent on the angle between the sample normal and the CMA axis. Though they are not as large as for Si, the amplitude variations of the Ga LMM (E= 1066 eV) and of the As LMM (E= 1227 eV) Auger peaks are in the range 10% to 20%. Moreover, the shape of the variations is different for the two elements and, for an electron primary energy of 6 KeV, has a higher anisotropy for arsenic upon rotation from 0' (near normal incidence of the primary electron beam) to about 15’ [ 191. One contribution to that effect is from the anisotropic emission of the Auger electrons themselves, but, in contrast to XPS, a second contribution might come from changes in the surface ionization as a result of diffraction of the incident electron beam. However, Chambers et al. have found Auger electron diffraction features to be rather independent of the incident beam direction [20] and largely governed by forward scattering of the outgoing electrones [ 211. Their conclusion is that the incoming beam scattering plays a relatively minor role. Perhaps this is because of the high probability of primary ionization by diffusely backscattering incident electrons. Once again, one can only carry out measurements of qualitative stoichiometry evolution if the CMA axis is perpendicular to the sample surface. Otherwise, 8 is no longer constant along the CMA entrance annulus, and we have to orient the sample in the same way from one experiment to an other in order to obtain confident qualitative stoichiometry evolution. Some photoemission studies were made with a two-dimensional display-type spectrometer [ 141, which has a collection angle of about 1 Sr solid angle in the angle-integrated mode. From experimental two-dimensional patterns obtained from a GaAs (001) surface by Owari et al. [ 221, we have graphically integrated the As3d and Ga3d currents over 1 Sr. Their ratio very close to 1 is equal to the value obtained by graphically integrating the currents along @= 45’) a procedure which gives the actual stoichiometry, as previously shown in the paragraph, “Application to specific cases”. At least for GaAs, 1 Sr solid angle provides sufficient integration. Before summarizing our work, we make the following remark: the determination of the spectrometer intensity response function in both kinetic energy and 8 is of critical importance, and successive round-robins involving relativeintensity measurements on high-purity samples have refined the reference

172

procedures [ 23,241. Among them, the choice of reference material is critical. Taking into account the preceding discussion, the metallic polycrystalline foils used must be free of preferential crystallite orientation in order to avoid the anisotropic angular intensity effects. If Cu, Ag and Au rolled sheets are used [ 241, the principal orientation texture for these face-centered cubic metals is one in which the (110) plane is parallel to the surface of the sheet and the [ i12] direction is parallel to the rolling direction [25]. Obviously, subsequent annealing or ion bombardment cleaning can modify or remove the initial texture. Thus one always ought to check the purely random crystallite orientation of reference standards used in inter-laboratory comparison e.g., by X-ray diffraction. Powell et al. [23] have not commented concerning this potential problem. On the other hand, Seah et al. [ 24 ] observed that the Cu2p,,,/Cu3p intensity ratio was independant of the sample orientation for angles of emission between 30’ and 60’ from the surface normal, which seems to prove that the Cu foil was a good standard, CONCLUSIONS

The impo~ant problem of XPS stoichiometry measurement of mono-crystalline III-V compounds is studied and several solutions are proposed. (i) From the empirically-adjusted single-scattering cluster (SSC ) model, pairs of azimuthal angles and polar angles are calculated for which the diffraction effects are negligible. Thus the actual stoichiometry can be measured for all III-V compounds. (ii) Taking advantage of the zinc blende crystal symmetry, the ratio I’ (anion, @)/I’ (cation, @& n/2) gives the actual stoichiometry for any 0, but only for GaAs for which the Ga and As scattering factors are nearly equal. For all other III-V compounds, this ratio cannot give the stoichiometry except for certain polar angles where diffraction contributions are negligible with regard to the primary wave. However, we can follow quali~tive stoi~hiomet~ evolutions, provided that, for each measurement the same azimuthal and polar sample positions are used. (iii) Concerning the influence of the analyser acceptance angle, we conclude that: for GaAs, since the Ga and As scattering coefficients are nearly equal, we can expect to obtain the actual stoichiometry by the ratio of fully or widelyaveraged peak intensities I’ (As) /I’ (Ga). By contrast, most other binary compounds AB represent very different scattering coefficients for the anion and for the cation, and thus in general, the ratio I’ (A)/I’ (B ) measured in an angle-integrated mode, may not give the actual stoichiometry. However, one can in any case follow qualitative stoichiometry evolution, again noting that, if partial angle inte~ation is performed, fully reproducible angular measurement conditions of the sample must be used.

173 ACKNOWLEDGEMENTS

We thank J.P. Hirtz and M.N. Charasse for providing the MBE samples. We would like to thank the Direction des Recherches et Techniques (DRET) for financial support (contract no. 8534496). One of us (C.S.F. ) also gratefully acknowledges the support of the National Science Foundation (Grant CUEES20200), the Office of Naval Research (contract no. 00014-87-lc-0512), and L.U.R.E., C.N.R.S., Orsay. REFERENCES 1

2 3 4 5 6 7

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

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C.S. Fadley, Prog. Surf. Sci., 16 (1984) 275; Phys. Ser., T17 (1987) 39. P. Alnot, J. Olivier, F. Wyczisk and C.S. Fadley, J. Electron Spectrosc. Relat. Phenom., 43 (1987) 263. S. Kono, SM. Goldberg, N.F.T. Hall and C.S. Fadley, Phys. Rev. Lett., 41 (1978) 1831; S. Kono, SM. Goldberg, N.F.T. Hall and C.S. Fadley, Phys. Rev. B, 22 (1980) 6085. P. Alnot, J. Olivier and F. Wyczisk, Mater. Res. Sot. Symp. Proc., 94 (1987) 231. M. Owari, M. Kudo and Y. Nihei, J. Electron Spectrosc. Relat. Phenom., 34 (1984) 215; R.C. White, C.S. Fadley and R. Trehan, J. Electron Spectrosc. Relat. Phenom., 41 (1986) 95. J.H. Scofield, J. Electron Spectrosc. Relat. Phenom., 8 (1976) 129. J. Szajman and R.C.G. Leckey, J. Electron Spectrosc. Relat. Phenom., 24 (1981) 83; J. Szajman, J. Liesegang, J.G. Jenkin and R.C.G. Leckey, J. Electron Spectrosc. Relat. Phenom., 23 (1981) 97. H. Gant and W. Month, Surf. Sci., 105 (1981) 217. B.L. Henke, Phys. Rev. A, 6 (1972 ) 94. M.P. Seah and W.A. Dench, Surf. Interface Anal., 1 (1979) 2. A.J. Van Bommel J.L. Crombeen and T.G.J. Van Oirschot, Surf. Sci., 72 (1978) 95. P. Drathen, W. Ranke and K. Jakobi, Surf. Sci., 77 (1978) L162. R.Z. Bachrach, R.S. Bauer, P. Chiarada and G.V. Hansson, J. Vat. Sci. Technol., 19 (1981) 335. T.C. Chiang, R. Ludeke, M. Aono, G. Landgren, F.J. Himpsel and D.E. Eastman, Phys. Rev. B, 27 (1983) 4770. J.F. van der Veen, P.K. Larsen, J.H. Neave and B.A. Joyce, Solid State Commun., 49 (1984) 659. A.D. Katnani, H.W. Sang, Jr., P. Chiaradia and R.S. Bauer, J. Vat. Sci. Technol, B, 3 (1985) 608. M. Fink and A.C. Yates, At. Data, 1 (1970) 285; M. Fink and J. Ingram, At. Data 4 (1972) 129; G. Gregory and M. Fink, At. Nucl. Data Tables, 14 (1974) 39. C.C. Chang, Appl. Phys. Lett., 31 (1977) 305. J. Olivier, unpublished results. S.A. Chambers, H.W. Chen, S.B. Anderson and J.H. Weaver, Phys. Rev. B, 34 (1986) 3055. S.A. Chambers, I.M. Vitomirov and J.H. Weaver, Phys. Rev. B, 36 (1987) 3007. M. Owari, M. Kudo, Y. Nihei and H. Kamada, Jpn. J. Appl. Phys., 24 (1985) L394. C.J. Powell, N.E. Erickson and T.E. Madey, J. Electron Spectrosc. Relat. Phenom., 17 (1979) 361; J. Electron Spectrosc. Relat. Phenom., 25 (1982) 87. M.T. Anthony and M.P. Seah, Surf. Interface Anal., 6 (1984) 95; Surf. Interface Anal., 6 (1984) 107; M.P. Seah and M.T. Anthony, Surf. Interface Anal., 6 (1984) 230; M.P. Seah, M.E. Jones and M.T. Anthony, Surf. Interface Anal., 6 (1984) 242. H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures, Wiley, New York, 1962, p. 557.