The effects of microstructure on interface characterization

Surface Science 168 (1986) 290-300 North-Holland, Amsterdam

THE EFFECTS OF MICROSTRUCTURE ON INTERFACE CHARACTERIZATION R LUDEKE I B M Thoma9 J Watson Re~earcl~ ('enter, P 0 YorktoBn Hetghts, New YorX 10598, U~A

B o t 218,

Recewed 10 June 1085, accepted for pubhc,mon 4 September 1985

The growth morphologyot metal layers on semiconductors is discussed m terms ot its influence on photoelectronspectra Examplesare givenfor nucleated growthof non-interacting metal semiconductorsystems, for whichthe stabdlzaUonot the surface potenual may be delayedto coverages exceeding many monola}eers For strongly interacting systems the need for caretul llneshape analysis ISshown to be essenual for the correct chemical and electromc charactenzaUon ot the developing interlace

1. Introduction The chemical and electronic characterizations ot sohd surfaces and shallow interfaces is one of the principal applications of X-ray and synchrotron radiation excited photoelectron spectroscopy Whereas the bulk of past work dealing with the chemical characterizaUon of surfaces has been predominantly qualitative, the advent of improved spectroscopic facllmes and computeraided data analysis permits, in principle, improved quantitative characterization The avallablhty of synchrotron radiation facilities and high-resolution electron spectrometers (~<0 2 eV) [1] allow, for instance, the probing of predominantly surface or bulk features, by merely varying the photon energy [2] To fully utlhze the added information obtained requires a more thorough understanding of the photoemisslon process and how it is affected by the environment The photoemlssion process is sufficiently well understood that for well-defined systems the experimental hneshapes of core and valence spectra can be predicted [3-7] This success has been achieved on homogeneous metals and clean semiconductors Once such systems become disturbed, a meaningful analysis becomes more difficult As a rule disturbances on semiconductors not only affect the surface morphology and chemistry, which may be quite complex for binary systems, but induce profound changes in the electronic properties of the surface and near-surface region The latter include mhomo0039-6028/86/$03 50 © Elsevier Science Publishers B V (North-Holland Physics Pubhshlng Division)

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geneltles in the surface potentml and doping effects which affect resolution, these effects and their possible prevention must be realized for meaningful interpretation of the data In this work we will be mostly concerned with the effects of metal adsorbates and films on semiconductor surfaces The morphology of the resulting nascent interface has a profound effect on the interpretation of the electron spectra Aspects of the morphology of semiconductor interfaces and the formarion of epltaxlal interfaces has been published elsewhere [8], and need not be detailed here However, in simple terms the interface may be abrupt or reacted (including lnterdlffused) In addition, the formation process itself is of importance and the growth mode relevant to the interpretation The latter point IS all too often either neglected or assumed without substantiation [8] From an analysis point of view the simplest and most desirable growth mode is the laminar or layer-by-layer mode In spite of numerous assertions this m o d e is seldom encountered for heteroepltaxlal growth on semiconductors [9] The most frequently observed m o d e is clustered growth, which may sometimes be preceded by a few monolayers of laminar growth (Stranskl-Krastanov mode) We will discuss the effects on spectroscopic data of the clustered growth mode In section 2, and begin with the simplest case, the abrupt and unreacted interface We will subsequently discuss reacted interfaces and the importance of detailed knowledge of the spectral composition of the core levels of the clean semiconductor In the last section we will stress the importance of hneshape analysis for a strongly lnterdlffused interface in the multl-monolayer coverage limit

2. The effect of clustered growth The most obvious effect of clustered growth is the non-exponential decay of the strength of the core level emission peaks with metal coverage Such behavior could be wrongly interpreted as resulting from lnterdlffUSlOn, unless substantiated by other observations (such as different decay curves of the core spectra of the constituent atoms of the semiconductor taken at the same kinetic energy) Depending on the shape of the clusters, their growth rate and distribution, as well as on the electron mean free path, diverse shapes in the decay curves can be encountered [8,10] A frequently used method of determining changes in band bending with surface treatment is to follow the changes in kinetic energy (KE) of the photoemitted core electrons For n-type (p-type) material an increase in the band bending (or equivalently the Fermi level at the semiconductor surface moves deeper into the band gap) results in an increase (decrease) in the kinetic energy of the electrons The results of such a study are shown in fig 1 for Ag deposited at room t e m p e r a t u r e on n- and p-type G a A s ( l l 0 ) [10] The solid

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Fig 1 T h e position as a function ot A g t h i c k n e s s of the surtace F e r m i level l o r n- a n d p-t,~pe G a A s ( 1 1 0 ) The s~mbols r e p r e s e n t the e x p e r i m e n t a l results (ref [10]), the d a s h e d cur,m is a corr e c n o n for d o p i n g effects and fimte p r o b i n g d e p t h s (see text)

curves represent the measured position of the Fermi level relative to the band edges It should be noted that the Fermi level for n- and p-type do not meet. but stay separated by about 0 2 eV Similar observations were made for numerous other metal and adsorbate systems [11]. and interpreted as resultlng from an lntrmslc property of the semmonductor, a norton whmh led to the formulation of the universal defect model for Schottky barners [11] An alternative explanation for the lack of convergence of the Fermi level m the two types of doping can be based on the work by Tang and Freeouf [12]. who determined the effect of distributed pinning sates on the measured surface potential For commonly used semiconductors typmally doped to 10 )~ cm -~ and flmte electron escape depths (5-20 ~,). these authors determined that average or measured Fermi-level positions may deviate by several tenths of an eV from the actual pinning position The reason for the dopant dependence hes m the effectiveness of screening and the associated short depletion width (~100 .~) in the heavily doped material It was estimated that the average separation of the Ag clusters in the data of fig 1 was 60 A [12] For the highly doped material used. th~s separation produces considerable variation (-~15(1 meV) m the surface potential Fermi-level convergence is achieved when the experimental data are corrected for this effect, as shown by the dotted lines in fig 1 Freeouf and Tang suggested that low doped (<101.' cm ~) materml be used to minimize these effects [12] The slow approach to the saturation poslnon of the Fermi level (the pruning position) with coverage in fig 1. whmh has also been observed for Au [13]. can also be attributed to clustering The noble and other weakly interact-

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ing metals generally have high surface mobihtles which leads to discrete nucleation and growth sites If their separation is of the order of the semiconductor screening length or greater and if pmnlng occurs only at the nucleation sites, then considerable variations in the band bending will occur between clusters The net effect of this is a slow approach to a uniform surface potential, which ms not achieved until the surface is nearly covered with metal or when the average separation between ~slands is much smaller than the screening length This delayed stabilization ~s not only observed m photoemisslon but in any spectroscopy sensitive to the surface potential A corresponding delay ~s expected if one measures the surface potentml with a Kelwn probe, for example There is, however, a major difference, m that an external surface potential measurement is sensmve not only to the potential of the bare semiconductor, but to the newly developing metal surface layer as well Thus the potential follows mltmlly the band bending, as for photoem~sslon, but subsequently becomes dominated by the metal work function potential Thus for n-type matermls the mmal increase in the contact potentml due to band bendmg will continue to increase or reverse depending on whether the metal work function ~s greater or less than that of the semiconductor, the reverse occurs for p-type material These effects have been observed, but were ascribed to the formation of a surface dipole [14]

3. Core level analysis of the reacted interface 3 1

Low coverage

In order to detect incipient chemical changes in the surface due to adsorbate-surface interactions and to accurately determine their strength and chemical shifts a precise knowledge of the hneshape of the clean surface is necessary Most semiconductor core level spectra contain a contribution from the surface atoms, the strength of which depends on the kinetic energy For energies of 30-100 eV typically used in many experiments using synchrotron radmtion, the contribution of the surface component is about 30% For binary semiconductors, such as GaAs, this surface contribution is shifted to higher kinetic energies for the As anion and to lower energies for the cation because of charge transfer Although the surface contribution may be resolvable in high-resolution spectra [15], the lack of its detectability under conditions of inferior resolution does not preclude its absence This fact is generally Ignored in the analysis of core level data and can lead to erroneous interpretations We will next discuss an example to illustrate these points A problem will arise in the early detection of certain chemical reactions on binary semiconductor surfaces, such as oxidation reactions of the cation or reduction reactions of the anion Since the surface cation is already partially oxidized rela-

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tlve to the bulk atoms (through an electron transfer to the anion) further surface oxidation will have little apparent effect on the total hneshape of the cation An example of this is shown in fig 2 for the real oxidation of G a A s [2] Fig 2a shows a surface-sensitive spectrum for an oxygen exposure of 108 L The surface c o m p o n e n t of the clean surface (shown in fig 3a) has largely been replaced by two components attributed to different oxidation states To a good approximation the hneshape has not changed appreciable, particularly under condition of poorer resolution This lack of detection of a shifted component has led to the erroneous conclusion that Ga ~s not initially involved in the oxidation process [16] The oxidation of the As on the other hand, causes the appearance of additional emission on the low K E side of the bulk peak and the removal of the surface contribution on the high K E side, which is more readily detected Fig 2b shows the spectrum taken under bulk sensitive conditions (KE ~ 6 eV for which the electron escape depth is ~ 2 0 A), and demonstrates that the oxidation of G a A s , even for coverages much less than a monolayer, involves considerable bulk or subsurface oxidation [2] This use of the tuneabllity of synchrotron radiation to distinguish bulk and surface features in the XPS spectra has not yet been applied widely Similar examples, but involving the additional reduction reactions of As are encountered In the metalhzation of G a A s with d-electron metals [17]

3 2 High coverage As the metal coverage increases for the strongly interacting system the contrlbutlon of the bulk signal progressively weakens relative to that of the reacted components, which often either diffuse into the growing metal film or segregate to the surface This is frequently observed [17,20], and poses problems with the decomposition of the spectra into individual contributions An example of a series of spectra for increasing metal coverage is shown for the Pd/GaAs system In fig 3 The prominent bulk signal observed at low coverage rapidly weakens and becomes difficult to extract beyond a few monolayers of coverage The reason for this rapid attenuation is due to the combined effect of the Pd and the G a and As reaction products, which effectively reduce the escape depth [17] As stated before, a precise knowledge of the hneshape of the components is needed in order to determine the energetic position and intensity of the weaker constituents A casual, but by no means simple approach to the lineshape analysis is to assume similar overal hneshapes for all spectral components The results of such decompositions are shown in fig 4, again for the Pd/ G a A s ( l l 0 ) system The figure depicts the changes In kinetic energy with Pd coverage of the various G a 3d and As 3d spectral components, both bulk (1 e G a A s ) and chemically shifted constituents The data analysis consxsted first of a subtraction of a background, which was simulated by a sphne fit to the wings

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Fig 3 Decompositions of Ga 3d spectra for indicated coverages ot Pd deposited on GaAs(110) at room temperature The hneshape of (a) is essentially that of the clean surface, except for band bending shifts The high K E spectral components in (b)-(d) were fitted with asymmetric hneshapes (Domach-~unjl6) The larger dots represent the raw data after a linear background subtraction, the solid line is the fit to the data and is the sum of the various spectral components (dotted curves) (Ref [17] )

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of the raw data, followed by spectral synthesis using a least-square fitting routine [2] The latter assumed typical semiconductor hneshapes consisting of spin-orbit split pairs of convoluted Gausslans and Lorentzlans [21, 22] The Lorentzlan was kept constant for all components, the Gausslans were allowed to vary to simulate variations in the bonding environment and other lnhomogeneous broadening mechanisms The results in fig 4 were baffling for coverages ~>2 ,& and p r o m p t e d a re-evaluation of the data analysis The previously used background subtraction method was suspect for the higher coverages, as stronger deviations from the hnear background of the clean and low-coverage limit were needed to produce zero background on the low K E side of the spectra However, It could be ascertained experimentally by decreasing the photon energy by ~ 5 eV that the Inelastic background was essentmlly hnear [23], and that the resulting background-corrected spectra exhibited considerable emission in the low KE tails, as evident from fig 3d This inelastic tall suggested many-body contributions, which have previously been shown to be important in the hneshape analysis of metals [4] A useful hneshape was developed by Doniach and gUnll~, and consists of convolution o! an inverse power term in energy with a Lorentzian We have used this function in our F

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hneshape analysm tor all chemically shifted peaks of the As and Ga 3d core levels The suitability of this hneshape for these peaks is based on the presence ot the low KE tails, as well as the observaUon that the reaction products are dispersed m a metal host and should thus be analyzed m terms ot metal hneshapes [4] Whereas prewously the spectrum of fig 3d, and others like it, could only be fitted properly with two components, the use of the D o n l a c h ~unjl(: hneshape reduced the fitting to a single c o m p o n e n t tor the Ga 3d spectra at coverages of Pd or Ti of >~10 A The fitted spectrum in fig 3d is an example of this The asymmetry parameter of the Domach-gunjlO function obtained m our data analys~s was 0 1+__0 01, m excellent agreement with that ot other simple metals [4] l h e consequences of considering the D o m a c h - g u n j l e hneshape are several (1) reduction of the intensity of structure m the low KE tails, including elimlnanon of otherwise spurious structure, (2) shifts to lower KE of this 04

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structure, and (3) an apparent decrease in the K E of the peak position in the Donlach-~unlld hneshape relative to the true Lorentzlan peak [4] The second consequence allows for a more precise monitoring of the attenuating bulk signal and hence a better m e a s u r e m e n t of the band bending The results of the analysis just described for the Pd spectra are shown in fig 5 The differences with the former treatment of fig 4 are substantial The seemingly erratIC trends for large exposures in fig 4 are transformed into systematic trends in fig 5 The bulk Ga 3d shifts for both bulk- and surface-sensitive conditions are more nearly equal, but extractable from the raw spectra only for Pd coverages of ~<10 A It should be noted in fig 5 that the surface-sensitive shift at 5 A of the Ga 3d for G a A s (open circle) is dewating from the stablhzed position corresponding to the pinning of the Fermi level This shift is unphyslcal and is attributed to difficulties in extracting the bulk c o m p o n e n t from the stronger chemically shifted component, shown at the bottom of the figure The latter continues to increase in K E with coverage, which has been attributed to final-state effects as the Ga is diluted m the Pd host [17] In view of these difficulties it seems fortmtous that band bending could be ascertained from surface-sensitive spectra for coverages ~>10 A as previously reported [19] The use of bulk-sensitive conditions alleviates the problem somewhat, however, the electron escape depths in the metal become comparable for KE of ~ 6 eV and ~ 5 0 eV, in contrast to those for the clean or non-metalhzed surface [2] The observed chemical shifts for the As 3d core level components have been attributed to the formation of elemental As [17,19], and an A s - P d compound, possibly involving Ga, as indicated m fig 5 [17]

4. Conclusions We have given several examples of how the mlcrostructure of the nascent interface can affect the spectroscopic characterization These examples show the importance of the morphological characterization of metal growth For the quantitative characterization of the chemistry and band bending, particularly for interacting metal-semiconductor systems, detailed analysis of the photoemlsslon core level spectra is needed This analysis must include the proper handling of the backgrounds [7], as well as hneshape considerations consistent with the atomic environment

Acknowledgements The author acknowledges fruitful discussions with Professor John H Weaver, which led to the Donlach-gunlld hneshape analysis This research was partially supported by the US A r m y Research Office under contract #DAAG-29-83-C-0026

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References [1] D E Eastman, J Donelon, N C Hlen and F J Hlmpsel, Nucl lnstr Methods 172 (198(I) 327 [2] G Landgren, R Ludeke Y Jugnet, J F Morar and F J HJmpsel, J V a c u u m Scl Technol B2 (1984) 351 [3] M Cardona and L Le}, Eds , Photoemlsslon m Sohds, Vols 1 & 2 (Springer Berlin 1977) [4] G K Werthelm and P H Cgtrm, m Photoemmsslon m Sohds, Vol 1, Eds M Cardona and L LeT (Springer, Berhn, 1977) ch 5 [5] S Hufner, m Photoemlsslon m Sohds, Vol 2, Eds M Cardona and L LeT (~prmger Berhn, 1977) ch 3 [6] M C a m p a g n a , G K Werthelm and Y Baer, in Photoemmssmon m Sohds Vol 2, Eds M Cardona and L LeT (Springer, Berhn, 19771 ch 4 [71 P Sterner, H Hochst and S Hutner, m Photoemls~mon m Sohds, Vol 2 Eds M Cardona and L LeT (Springer, Berhn, 19771 ch 7 [8] R Ludeke, Surface Scl 132 (1983) 143, J Vacuum Sct Technol B2 (19841 4011 [9] R Ludcke, R M King and E H C Parker, m The Technology and Physics of Molecular Beam Epltaxy, Eds E H C Parker and M G Dowsett (Plenum, New York, 19851 ch 16 [1(I] R Ludeke, T - C C h g a n g a n d T Mdler, J V a c u u m Scl Technol BI (1983)581 [11] P Skeath [ Lmdau, P W Chye C Y Su and W E Splcer, J V a c u u m Scl Technol 16 (19791 1143 [12] J ~ -F Tang and J L Freeout J V a c u u m Scl Technol B2 (19841 459 [13] P Skeath, C Y Su, I Hlno, I L m d a u a n d W E Splcer Appl Phys Letters 39(19811 ~,49 [14] L J Brlllson J Vacuum Scl Technol 16(1979)11~,7 115] D E Eastman, T -C Chlang P Helm mn and F J Hlmpsel, Ph)s Re~ Letters 4S (198(11 675 [16] P PJanetta I Lmdau, C Garner and W E Splcer Phys Re~ Letters 37 (1976) 1166 C Y Su I L m d a u P W Chve P R S k e a t h a n d W E Spmecr P h y s R e ~ B25(1982) 404S [17] R Ludeke and G Landgren, Phks Rev B, to be published [181 L J Bnllson and C F Brucker J Vacuum Sol Technol 21 (1982)564 [191 T Kendele~lcz W G Petro S H Pan M D Wflhams 1 Lmdau ,md W E Sptcer Appl Phys Letters 44 (19841 11 ~, [20] J H Wea~er, M G n o m , J J Joyce and M del Gmdace, Phvs Re~ B31 (19851 5290~ J H Weaver M Grk)m and J Jo',ce, Phys Re~ B'~I (19851 S348 [21] R Ludeke, I -C C h l a n g a n d D E Eastman Ph'~slca 117/118B (1988) 819 [22] J F van der Veen L Smlt, P K Larsen and J H Neave, Phvslca 117/118B (1983)822 [23] The linear background approximation is cxtenslvel'¢ used m XPS anal)s~s ot metals, see rot [7] [241 S Domach and M ~un/lc J Ph)s C'~ (1970)285