Photoemission studies of semiconductor interfaces: electronic structure and barrier heights

Photoemission studies of semiconductor interfaces: electronic structure and barrier heights

Surface Science 269/270 (1992) 938-952 North-Holland "surface science Photoemission studies of semiconductor interfaces: electronic structure and ba...

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Surface Science 269/270 (1992) 938-952 North-Holland

"surface science

Photoemission studies of semiconductor interfaces: electronic structure and barrier heights K. Horn Fntz-Haber-lnstaut der Max-Planck-Gesellschaft, D-IO00 Berlin 33, Germany Received 9 September 1991; accepted for publication 12 September 1991

Semiconductor interfaces are an interesting class of systems from a fundamental as well as apphcations-oriented point ot wew. Several recent examples wdl be given to demonstrate how photoelectron spectroscopy permits a detailed investlgatton t~f the electronic structure of semiconductor surfaces and interfaces, in parttcular of features which determine the transport properties, e.g. the valence band offset AE v in heterojunctions and the Schottky barrier height ~ b m metal-III-V-semiconductor junctions. For m SltU studies, these properties can be examined at a microscopic level, i.e. as the interface is bruit from the lowest coverages onwards. The influence of chemical reactions at the interface, which in turn may have an influence on the electronic properties, is considered. Attention wdl also be paid to non-equilibrium processes such as the surface photovoltage, which may have a severe influence on the interpretation of Ferm~ level pmnmg data from photoem~ssmn. A combination of electromc structure information with recent advances m the determination of the geometric structure and morphology of overlayers on semtconductors provides new mslghts into th~s class of interfaces.

I. Introduction

A knowledge of the electronic structure of semiconductor interfaces is crucial for an understanding of the role such interfaces play in transport processes in modern semiconductor devices; it is also important in those instances where such devices are used to study electron confinement phenomena. The gap between applications-oriented and fundamental research is particularly small in this field since devices are usually of good crystalline quality, and cleanliness requirements are similar to those typically found in the kind of ultra-high vacuum experiments familiar to the surface scientisl. Ever since the first evidence for surface states on silicon was obtained, photoelectron spectroscopy has been of prime importance for the study of clean semiconductor surfaces, and there is now a large body of valence band spectra available by means of which the electric structure of many common semiconductor surfaces has been determined [1]. However, photoelectron spectroscopy has also found impor-

tant applications in other fields of semiconductor interface research. Core level spectra have yielded an enormous amount of information about the interaction of adsorbates with semiconductor surfaces, including such processes as oxidation and passivation. The surface and bulk band structure of semiconductors has been examined in detail [2,3]. The combination of valence and core level spectroscopy, by which band bending phenomena can be examined [4], has been widely used for the study of metal-semiconductor interfaces (Scbottky barriers) [5] and semiconductor-semiconductor interfaces (heterojunctions) [6]. Since photoemission experiments are routinely carried out under ultra-high vacuum conditions, clean and well-ordered surfaces are prepared and examined, such that the experimental results may be directly compared with modern electronic structure calculations. The investigation of semiconductor interfaces also often proceeds through an in situ preparation of pure and well-defined metal or semiconductor overlayers, providing a testing ground for theoretical descriptions of interfaces.

0039-0028/92/$05 00 ~_) 1992 - Elsevier Science Pubhshers B.V All rights reserved

K. Horn / Electromc structure and barrier hetghts of semtcondmtor tnterface~

fJl

I* =ho~- R

R

L

Ad~

-'1

energy analyser core level

~

eAVs l

Fig. 1 Schematic drawmg of the photoemlsslon process as apphed to two semiconductor surfaces m whtch different band bending prevads The quantities eAV,, Aq~, and l * which are directly accessible m photoemlsslon, are also indicated.

The photoemission technique itself has received ample attention in recent years, and a detailed outline of its advantages for surface and interface research can be found in the literature [7-9]. There are a few aspects which merit attention, however. In particular, the question of the photoemission reference level and its importance for band bending studies will have to be discussed. Typically, in a photoemission experiment involving semiconductor surfaces, the valence band region is measured along with the region of accessible core levels. Schematically, a diagram of the electronic structure of a semiconductor is shown in fig. 1. In the lower part, valence band and core level binding energies are shown as a function of distance from the surface, indicating the effect of band bending eAV,. Band bending

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can be caused by a variety of processes, one of them being deposition of foreign material on the surface of the semiconductor, causing a depletion or accumulation layer in the surface [10,11]. Now recall that the reference level for the determination of electron kinetic energies is the Fermi level E~ of the electron energy analyser (shown on the right-hand side), such that changes in the position of E F in the semiconductor substrate due to band bending will cause a shift of the whole photoelectron spectrum relative to this reference level. This is indicated by the two spectra shown for different magnitude of band bending in fig. 1. The band bending region usually covers several hundred to several thousand ~Ingstr/Sms. Since the photoelectron attenuation length is normally on the order of 5 to 15 ,~ [12], the photoemission experiment probes the immediate surface region and thus the maximum extent of the band bending; at high doping concentrations, when the band bending region is smallest, some smearing out of features due to the finite sampling depth of the photoemission technique may occur, but it usually amounts to only a few meV. The change in band bending can thus be measured by the shift of the valence band maximum emission (top part). Since this usually involves a more cumbersome experiment, it is common practice to determine the shift of a (normally sharp and intense) core level emission line. Note, however, that this method also has its dangers, since core level lines frequently change their shape due to occurrence of chemically reacted species, disappearance of surface core level shifts, etc. Only the shift of the bulk core level contribution to the peak (coming from about 2 to 10 ,~ below the surface region) actually reflects the band bending. Reports by some groups that different values for the band bending were obtained from different core level lines are due to this neglect of changes in the line shape, which can be overcome by determining the bulk contribution to the peak by careful numerical line shape analysis ("curve fitting") as outlined below. Fig. 1 also shows that, apart from measuring valence and core level binding energies, we can determine the ionization energy, i.e. the binding energy of the highest occupied valence levels with respect to

940

K Horn /Electrontc structure and barrier heights ~f semiconductor ~nterfaces

the vacuum level, through a know~edge of the precise phc energ~ used for ionization. By using tabula., ' ~,alues for the magmtude of the fundamental band gap, the electron affinity of the semiconductor surface can also be measured. These quantities are important it, the study ef the properties of such surfaces, and ,~,ill be used in the interpretation of heterojunction band offset data below. Finally, a shift in the secondary electron cutoff reflects the change in semic~onductor work function q~¢. For a semiconductor, the work function consists of the electron affinity ,~, which is a material-dependent quantity, and the distance of the Fermi level from the bottom of the conduction band (Ec-EF), which is affecl~ed by band bending, ~c =X + ( E c - E F ) .

(1)

By determining changes in band bending eAV~ from the core level shifts, we can thus distinguish between its contribution to the work function change, and can determine char~ges in the electron affinity, brought about by the formation of dipole layers. This separation cannot be achieved by other methods of work function change measurement such as the Kelvin probe, for example. These possibilities will be most important in the study of semiconductor junctions. The combination of extreme surface sensitivity and the access to barr'er heights makes photoelectron spectroscopy unique for such studies; this capability can be enhanced by utilizing synchrotron radiation for photoexcitation, since the surface sensitivity can be varied, and one normally has superior resolation for the study of relevant core levels which can be used for determining cAVe. This paper intends to give an account of the current status of semiconductor interfaces derived from ohotoelectron spectroscopy data, to identify achi~wcments and issucs currently under debate, and to recognize limitations imposed on the interpretation of photoelectron speet,a. I,vestigations of surface and bulk band structures have been reviewed in several recent papers [1315] and compilations [1], so x~e will restrict ourselves to an invest,~gatie~ of electronic structure in relation to hete~ojunction band offsets and Schottky barriers.

2. Inuestigation of semiconductor heterojunctions The unique capabilities of photoemission in the study of surfaces and interfaces find a particularly important application in semiconductor heterojunctions [16], which are of current technological interest and hold promise for application in novel devices. Properties of such junctions can be determined with high sensitivity, reaching down to the monolayer scale, by photoelectron spectroscopy. This also applies to the electrical barriers which govern electron transport across the interface; They can be measured with an accuracy presently unmatched by any other technique, including transport (C-V and I - V ) experiments and optical techniques [2]. These have serious drawbacks when applied to heterojunctions. In the important case of GaAs/AIAs for example, widely used in applications, the original 15-85% rule for the distribution of A Ev and AE c among the difference in band gaps of GaAs and AlAs derived from Dingle's optical absorption measurements [18], and used for well over 10 years, recently had to be revised on the basis of photoemission results by Katnani and Bauer, who found that AE v = 0.5 AEg [19]. It is obvious that the magnitude of the band discontinuity has a profound influence on the energy and level spacing of the quantum well states, and a precise knowledge of AE,, is therefore important. Photoemission determinations of the valence band offset E,. have also been relevant for a critical assessment of current theories concerning valen~ceband lineup in heterojunctions. In the above GaAs-AlAs example, for instance, the previously accepted value had been in agreement with the theoxretical approaches of the time, while th,e new value gave credence to tile midgap level ap~ roach revived by Tersoff [20], and has thus helped to promo'~e the midgap or ,.,,,~t~;~ ,,,~u,,,~a~ models which have gained widespread acceptance. These are based ,on the notion, originally proposed by Heine [21], of evanescent states in the semiconductor, which act as states that provide a level which the bands align to, on both sides of the junction [22]. This so-called charge neutrality level (CNL) is derived through different approaches, and comparison between ttds kind of theory and

K. Horn / Electromc structure and barrier heights of semtconductor interfaces

experiment seems to work well [23]. The models based on midgap states belong to the group of so-called linear theories, since they are solely based on bulk properties of the two semiconductors. Thus the band offsets as described by these theories fulfill the requirements of commutativity, i.e. that the band offset among two materials is independent of growth sequence (AEv(A-B)-AEv(B-A)), and transitivity, which means that the band offsets between three semiconductors should add up to zero (AEv(A-C)= AE,(A-B) + AEv(B-C) [16]. The other, more elaborate approach extends the self-consistent calculations of the electronic structure of a slab consisting of several layers of a crystalline solid, by combining several layers of each semiconductor into a single slab [24,25]. These calculations also include the influence of the interface dipole on the band offset, and have recently also been modified to take into account the effect of strain, which is induced in one of the constituents in a latticemismatched heterojunction, on the magnitude of the band offset [26]. These are probably the most precise schemes fol the prediction of band offsets. Among the wide variety of heterojunctions, the lattice-matched ones may serve as model systems

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since the influence of lattice imperfections, which invariable occur at thicknesses beyond a few layers in mismatched junctions, can be neglected in the thin ( ~ 30 to 100 ,~) layers used in many studies. The case of AIAs/GaAs was briefly mentioned above. Others include ZnS/Si, GaP/Si, GaSb/ZnTe. As an example for a lattice-matched heterojunction between a I I I - V and a II-VI semiconductor, consider the case of cadmium sulphide on indium phosphide (110). This system has some interesting properties which will be presented in more detail below. Consider the valence band spectra of the CdS/InP(ll0) heterojunction under different conditions of overlayer thickness shown in fig. 2a [28]. The bottom spectrum exhibits the characteristic feature~ of emission from a clean InP(110) s,-rface, recorded in normal electron emission at a photon energy of 70 eV. Upon deposition of CdS, the sharp peaks of the clean surface are broadened, and at higher depositions, new peaks emerge, which for the highest thickness shown are those representative of bulk CdS. In the intermediate region, the valence band emission consists of a mixture of features from the substrate and overlayer, depending on its thickness (center spectrum). The top part shows the spectrum from a thick

CdS/InP (110)

l~u=?0eV

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~.

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Energy below E F(evl Fig. 2. (a) Valence level spectra of CdS on lnP(110) recorded m normal eh;ctron emission at a photon energy of 70 eV The clear changes in the spectra indicate the gradual emergence o: the CdS valence band Note the shift in the valence band maximum as the CdS laver forms. (b) Schematic d,agram of the band lineup at the CdS/InP(110) interface (see text)

K Horn / Electromc structure and barrier hetghts of semiconductor interfaces

942

CdS overlayer, in which the contribution from the substrate is almost obscured; here the valence band. features of the overlayer dominate. Comparing the bottom and top spectra, there is an obvious shift in the valence band edge (VBM), and an additional shift due to band bending as read off from the core levels. The total valence band offset is then simply AE v = VBM A

-

VBMa - enV~

(1)

and may thus be directly be derived from the experimental data. The symmetry and spot positions of the LEED pattern does not change when CdS is deposited onto I n P ( l l 0 ) [28]. Now the stable modification of CdS at room temperature is the wurtzite structure; hence the LEED results indicate that CdS grows in the metastable zincblende structure on InP(ll0). Such substrate-stabilized metastable growth has been observed in a number of cases, in metals as well as semiconductors. There are only a few reports of zlncblende CdS; its existence in thin films was first deduced from reflectivity data of CdS on GaAs [29]. These layers have recently been investigated by Raman spectroscopy and double-crystal X-ray diffractometry [30], spectroscopic ellipsometry [31], and photoluminescence as well as Rutherford backscattering [32]. These data have convincingly demonstrated the presence of well-o, dered zincblende-CdS overlayers with small half-widths in X-ray diffraction, sharp Raman lines, and remarkable differences in the electronic structure between zineblende and wurtzite CdS as reflected in the optical constants. The photoemission data cannot yet be compared with spectra from bulk zincblende CdS since only data for the stable wurtzite CdS are available [331. From the set of valence band spectra showr~ in fig. 2, it is obvious that a shift AVBM in the energy of the valence band maximum can be observed when an overlayer of CdS is grown on the substrate; the band offset A E v may then be derived according to the method outlined above. Since the magnitude of the fundamental gap is known (although with precision only for wurtzite CdS), the conduction band offset AE c is known. Moreover, from the ionization energy I * the

vacuum level may be determined, and thus the change in work function as the overlayer is grown. However, the change in work function • does not directly give access to the interface dipole, a quantity that is of great interest. • consists of two contributions: the so-called internal work function, i.e. the chemical potential, plus the surface dipole contributiorL If one were to deposit a thin layer acting as a dipole, and another thick layer of the same material as that of the substrate on top, the change in work function would under otherwise ideal circumstances (crystalline perfection, absence of defects and foreign atoms, etc.) in fact give access to the interface dipole contribution. Such dipole layers in what is essentially a homojunction have indeed been prepared [34], and an offset was found, but a comparison with the work function change was not reported. In the present case of C d S / I n P ( l l 0 ) , the change in work function will become important in the study of different interlayers, used in order to modify the band offset, as reported below. For C d S / I n P ( l l 0 ) , the band offsets at the interface are summarized in the diagram of fig. 2b. A valence band discontinuity of - 0 . 7 7 eV is found for CdS/InP, such that the heterojunction is of the "straddling" type, i.e. the discontinuities for valence and conduction bands go into opposite directions. There have been many attempts to provide a basis for the prediction of heterojunction band offsets. The semi-empirical scheme of Katnani and Margaritondo [35] has been widely used for such predictions; it is based on the concept of transitivity, a feature common to all linear theories, and involves the deposition of Si and Ge overlayers on a wide variety of semiconductor surfaces; however, effects of strain at the interface are neglected (see below). Various linear theories have been put forward [16], these have been replaced by the so-called midgap theories. These are based on the analogy between Schottk3' barrier formation and heterojunctions pointed out by Tersoff [20] who, starting from earlier work by Heine [21], postulated the existence of a charge neutrality level (CNL)which serves to adjust the valence levels on either side of the heterojunction; this concept is discussed in more detail

K. Horn / Electromc stnwture and barrier heights of semiconductor interfaces

below. There have been many attempts to predict band offsets from theoretical concepts, which have been reviewed by Kroemer [16]. Most present approaches can in fact be divided into two groups [23]: those based on the notion of a midgap energy (CNL), which is characteristic of each semiconductor, and fully self-consistent calculations of bands in a heterojunction based on a supercell geometry. The influence of the CNL may be compared with that of the Fermi level in a metal-metal junction. In such a contact, charge will flow, resulting in a dipole which cancels the difference in work function of the two metals, such that the interface will be in equilibrium. The situation in a contact between two insulators will be totally different; here, no charge flow occurs, such that the discontinuity will occur between the two CNLs, and the vacuum levels will be aligned; there is thus no dipole. The situation in a semiconductor will be somewhere in between these two limiting cases. While some charge flow will occur, causing a dipole to be formed which will compensate the difference in the vacuum levels, it is also reasonable to assume that the CNLs will not be perfectly adjusted. Since we refer the valence band maxima to the CNL, this has a direct influence on A E~. Thus the factors affecting the valence band offset divide up into a dipole term between the vacuum levels, and an energy difference between the neutrality levels. The question is then how close the case of two semiconductors is to either limiting cases. For I I I - V semiconductors, Tejedor and Flores [22] have estimated that the situation is closer to the case of two metals, with energy differences between the two CNLs of up to about 0.2 eV. Cardona and Christensen [23] have treated many different heterojunction systems using their "dielectric midgap energy" ( D M E ) model, in which the charge neutrality level is estimated from average properties of the band structure, and reach good agreement with a large number of experimental data. Finally, the most reliable theoretical values for band offsets are probably derived from ab-initio calculations of band offsets using a supercell geometry, in which a layer of semiconductor A of a given thickness (usually three to nine unit cells) is joined to a similar layer of semicon-

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ductor B, and this arrangement is used, via periodic boundary conditions, as a basis for the selfconsistent calculation of the band structures of these materials across the interface. Such calculations have been carried out by van der Walle and Martin [24] and Christensen [25]. A comparison between experimental values and theoretical predictions yields a mixed picture. While agreement is often good, with deviations below 0.2 eV, including the present case of C d S / I n P [36], there are other cases where large deviations are found. Unfortunately, the picture varies from system to system. Large deviations have been related to extensive interface reaction which is observed in some system, creating what is in effect a different species in between the heterojunction partners, similarly to the "Effective Work Function" model of Freeouf and Woodall [37] for Sehottky barriers, which may lead to a dipole that affects the band lineup. The first example presented here has dealt with a lattice-matched junction. Interfaces in which there is a large mismatch in crystal lattice parameters are also attractive, since there are important ones for applications among them, and also since the influence of strain can lead to interesting physical effects. Consider a junction between silicon and germanium, with its associated lattice mismatch of 4.18%. The conditions under which strained epitaxial layers can be grown are well known, and calculations which explicitly include strain have been performed. The biaxial strain in Ge and Si when deposited on its counterpart in this heterojunction causes a splitting of the topmost valence bands and a strain-dependent shift in the core level to valence band binding energies. In an experimental determination of AE v such shifts have to be taken into account in order to obtain meaningful from the photoemission data. Yu et al. [38] have carried out extensive X-ray photoemission experiments in order to determine the effect of strain on the separation between the core levels and the valence band maximum, and have determined the valence band offset for a layers of Si on Ge(100) and Ge on Si(100). They found a large dependence of AEv on the sequence of heterojunction growth (0.83 eV for Si on Ge(100) and 0.22 eV for Ge on

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K. Horn / Electromc structure and barrter haghts of semtconductor interfaces

Si(100)), which is caused by strain effects, induced in the overlayer through the large lattice mismatch. That strain is responsible for this large deviation from commutativity was demonstrated by the fact that the offset between the weighted averages of the valence band was similar for the t~o growth sequences. These important observations cast some doubt on the semiempirical band offset values by Katnani and Margaritondo [35], which were obtained by depositing Si and Ge on a large variety of substrates in which the strain at the interface was largely unknown. Finally, another cause of deviation from the "ideal" band offset should be considered: interface reaction products of intentionally deposited species which in effect cause a dipole layer at the interface. The influence of such layers has been considered in a thought experiment by Tersoff [20]. He argues that embedding a sheet of dipoles ~x in a semiconductor leads to a net screened dipole 6 = 8x/e, where e is the static (long wavelength) dielectric constant. Since the screening charge in a semiconductor is confined to a region of a few ,~ around the charge being screened, the additional valence band discontinuity by the sheet of dipoles corresponds to the net screened dipole. This extra dipole will thus change the balance between the energy difference of the CNLs and the vacuum level "offset" mentioned above. While in the case of heterojunctions where a strong interface occurs, such as in the junctions containing II-VI semiconductors (ZnTe/GaSb(110) [39], CdS/ZnTe(110) [40], A1Sb/ZnTe(110) [38]), the action of such "chemical" interface dipoles has been invoked in a hand-waving way in order to explain the large differences between experimental and calculated AEv. However, a more stringent test of the influence of such interface dipoles has been performed in a number of experimental studies where interracial layers have been intentionally deposited. Interlayers in heterojunctions have been reported by Perfetti et al. [41]. The junction used by these authors was an extreme one (Si/SiO 2) and the interlayers had an undefined structure. Recently, interlayers have been used in a more controlled way, by using layers which have been well characterized using surface sensitive methods. Maierhofer et al. [42] have

CdStlnP{1101

In/,d

hw--60eV

interlayer

no

interlayer

I

~

-,'0

clean surfQce

B,ndmg energy(eV,EF=OI

Fig. 3. In4d core level spectra from a clean lnP(ll0) surface (bottom), an InP(ll0) surface covered with 8 monolayers ot CdS (center), and an InP/CdS junction with a monolayer of Sb deposited between the two semiconductors, where the CdS layer has the same thickness as that in the center spectrum (top).

studied C d S / I n P ( l l 0 ) heterojunctions with monolayers of Sb and Bi (unreactive) and AI (reactive) using core and valence level photoemission. These metals were chosen since their interaction with InP had been extensively studied before [41,44,43]. Munoz et al. [46] have investigated the limit of such dipoles in atomically thin layers, i.e. introducing a double layer of atoms with different chemical valence at an isovalent interface, which would then act as a microscopic capacitox. Other authors [47] suggest that the changes of the valence band offset are due to modifications in the microscopic interface dipoles due to charge redistribution, being caused by new interface chemical bonds and from migration of atomic species across the junction. It is hard to tell, however, whether such a distinction between dipole effects and charge redistribution is meaningful and can be tested in practice. This is because interfacial layers, by their very. presence, also influence interracial reactivity. Consider the spectral of the In4d level from an InP(ll0)surface (bottom), the same surface after deposition of a CdS layer (discussed in relation to the band offset above) (center), and the influence of a monolayer of antimony located between the InP

BL Horn / Electronic structure and barrier heights of semwonductor interfaces

substrate and the CdS overlayer (top) in fig. 3. The clean surface shows the asymmetry due to the presence of lines from the surface atoms, shifted by a certain amount [48]. Deposition of the CdS layer gives rise to a broader In 4d line; by means of line shape analysis, a new line shifted to higher binding energy is identified, which has been interpreted as due to an In2S 3 reacted phase [28], an assignment verified by Raman spectroscopy [30]. Such interface reactions are also found in other heterojunctions [38,39]. The top spectrum shows the ln4d level when a Sb interlayer has been deposited prior to CdS growth. The interface reaction evident from the center spectrum is totally suppressed by the presence of the Sb interlayer; in the top spectrum, only one doublet is clearly sufficient to describe the core level spectrum. Sb monolayers are known to passivate silicon sdrfaees, and to induce important changes in the growth mode of epitaxial layers [51]. Other interlayers show a similar suppression (Bi) or enhancement (AI) of the interface reaction [42]. The introduction of such interlayers has a marked influence on the magnitude of the valence band offset. For the C d S / I n P ( l l 0 ) j u n c tion, AE v = 0.77 eV was found. For junctions with a Bi or Sb interlayer, AEv dropped to 0.53 eV, while an AI interlayer increases the offset of 0.87 e ~/. While it may be argued that the suppression of interface reactions will lead to a more ideal junction, the presence of the interlayer itself will have an influence also, such that a clear assignment of the change in AEv cannot be performed in the present case. The experiments of McKinle~ et al. [34] using interlayers in homojunctions have obviously demonstrated the influence of dipole layers according to the simple model outlfned above. In heterojunctions, the influence ol the interface dipole was di, ectly confirmed in a very elegant recent experiment by Sorba et al. [47]. These authors prepared AIAs/GaAs interfaces with Si interlayers using MBE techniques; no sign of interface reaction was found. Half a monolayer of Si was found to induce a change in A E~ of 0.4 eV. When the growth sequence of the junction was reversed, the change was found to be opposite to that previ-

945

ously observed, which is a clear verification that the interface dipole itself was responsible for the change in band offset, and that growth sequence-dependent interface reactions, if at all present, did net have an influence on AEv. These results provide a striking example for the possibility of band offset engineering in a technolog'~cally important system.

3. Metal-semiconductor interfaces Metal-semiconductor interfaces play a crucial role in all applications of semiconductor devices. Thus the geometric and electronic properties of such interfaces, their morphology and growth under different preparation conditions, and chemical reactions between the metal and the constituent atoms of the semiconductor substrate, have received considerable interest in recent years. While steady progress has been made in the characterization of metal overlayer growth modes, the prediction of the transport properties of a particular metal-semiconductor junction has remained a controversial issue in spite of decades of intense experimental and theoretical effort [52]. The interpretation of Schottky barrier (SB) heights at metal-semiconductor interfaces has been at the center of interest [53-55]. In the surface science community, the interaction of metals with III-V semiconductor surfaces, and here with the (110) cleavage planes in particular, has received a great deal of attention. This is probably due to the fact that these (110) surfaces have no surface states in the fundamental band gap (as do the technologically important Si(100) and (111) surfaces), such that model studies of band bending may be performed. While most SB height data for applications-oriented metal-semiconductor interfaces are obtained from I - V and C - V experiments, information about the electronic structure of the interface, as well as the evolution of the SB and its final height, Is usually obtained through photoemission experiments. In th; ~ section, the interpretation of band bending data from photoemission experiments in terms Schottky barrier formation models will be critically reviewed. The initial model by Schottky [54] and Mott [55] explains the rectifying properties of such

946

K Horn / Electromc stn~cture and barrier hetghts of semwonductor interfaces

junctions on the basis of the band structures of the two solids. It assumes that a depletion layer is formed, which causes surface band bending, with a characteristic decay length into the bulk, depending on the dopant concentration. This situation is depicted on the left-hand side of fig. 5. The barrier between the conduction band and the metal Fermi level induced by the band bending gives rise to the rectifying properties of the junction. As shown in fig. 1, core level photoemission provides a precise means of measuring changes in the band bending, and by measuring the energy separation between the core level and the valence band maximum, and following the band bending as the overlayer reaches the metallic stage, the Schottky barrier ~b may be determined. This process is quite similar to that for heterojunctions in section 2, with the exception that the Fermi energy of the metal coincides with the electron spectrometer reference level, if the whole system is in equilibrium, i.e. if there is no charge imbalance in the depletion region. The model of Schottky and Mott, which assumed that the barrier ~bn (on n-type material) was equal to the difference between the metal work function and the electron affinity of the semiconductor, is almost never observed. Rather, a value of ~ h independent of metal work function is commonly encountered for semiconductors with gaps smaller than about 3 eV [5]. In a modification of Bardeen's [56] original model. Spicer et al. have proposed that defects created at the interface by the deposition of the metal are responsible for Fermi level pinning [57]. The model which, following Heine's original work [21], and recently revived by Tersoff [58], has emerged as a successful description of electronic processes at the interface, is the so-called virtual gap state (VIG), metal-induced gap state (MIGS) or charge-neutrality (CNL) level model already discussed in section 2. Except for a small correction due to the fact that the dielectric constant of the semiconductor is fiuite, the CNL will line up with the Fermi level of the metal because the metal electrons occupy the acceptor-like state in the gap. While these models for the Schottky barrier height are important for a conceptual description

of the electronic structure at the interface, they need to be transformed into numbers which describe specific systems, i.e. in calculations, in order to demonstrate their predictive power. Here the Schottky-Mott rule fails in the experimental test as mentioned above. The midgap state model has demonstrated fair agreement with experiment [20]. Its shortcoming is the same as in the case of semiconductor heterojunctions, in that only bulk properties of the semiconductor are used, such that the features of a particular junction such as crystallographic orientation, strain, etc. are not included. These are in fact included in the most precise attempt at present, for the description of these junctions through fully self-consistent slab calculations using empirical pseudopotentials [59] or, more recently, the local-density functional approximation (LDA) method [60]. H e r e the gap states are obtained in a natural way through a density of states which can be resolved in the different semiconductor layers. Experimentally, for the most extensively studied I I I - V semiconductor substrates, the following general observations were found [61]. At room temperature, there is a gradual shift of the surface Fermi level from the conduction band minimum (CBM) for n-type substrates or the VBM (for p-type) towards a final common position for both doping types at higher metal coverages. In principle this position should correspond to the barrier height as measured by other techniques, and the pinning position should be the same for n- and p-type substrates. The metal dose at the final position depends on the type of deposited metal, being on the order of several tens of ~ngstriSms of the noble metals. At low temperature, the behaviour is markedly different: for submonolayer coverages the Fermi level remains near the CBM on n-type material, while strong band bending with even a pronounced maximum (the so-called 3vershoot") was found for p-tTpe. After about o . mon,31ayer deposition, the Fermi level on n-type subst~'ates exhibits a step-like decrease, while on p-type a gradual movement to the final position is observed. The step-like decrease was thought to be due to the fact that the ovcrlayer acquired metallic character, such that the gap states filled by the metal were now re-

K. Horn / Electromc stnwture and barrter helghts of semtconductor mterfaces

sponsible for the determination of the position of EF. The different behaviour of n- and p-type substrates at low temperature was assigned to adatom-related surface states of donor character, which affect E F for p-type substrates only (see ref. [61] and references therein). The different behaviour at room and low temperatures was explained by the supposedly lower reactivity at LT, decreasing the influence of reaction-induced defects. While this interpretation provided a fairly consistent picture of the movement of E F at low metal coverages, combining in fact both the defect and the midgap states model, some recent observations did not fit into the model. Thus the Fermi level movement was found to exhibit a considerable dependence on doping level and substrate temperature. It became increasingly clear that other mechanisms were also at work, which were related to the photoemission process itself. These apparent inconsistency were resolved when new experimental data [63] and a thorough analysis [64] of existing data [65,66] demonstrated that many of the photoemission observations could be explained through photovoltaic effects induced by the UV light beam used for photoexcitation. In the vast majority of photoemission studies it had been implicitly assumed that the band bending evaluated from photoelectron spectra was representative of the equilibrium band arrangement, and that photon-induced charge transport processes could be neglected. A careful investigation of the core and valence level spectrum of metal-semiconductor junctions actually reveals that this assumption often is not justified. Consider the core level spectrum of a 15 A layer of indium on G a A s ( l l 0 ) under different conditions [62] shown in fig. 4. From the relative intensity of the Ga3d and the In4d core level peaks we concluded that, when In deposition is performed at low temperature, the surface is largely covered with an iridium layer that grows in a quasi laminar mode, since the Ga 3d emission is laminar mode, since the Ga 3d emission ~s largely suppressed. When this layer is warmed up to room temperature, the core levels show a relative increase of the Ga3d peak, which demonstrates that strong clustering occurs upon warming, which

947

n-GoAs(110) highly doped ÷15AIndium htJ:61.2eV Go 3d

In/*d

.,t

°

cooled d o w n to -

120K t

I

warmed

! t_J "-/" v

up to

300K

-T i

I 120K I .... 1.... I .... I .... 36 39

I .... I .... ! .... I /,0 /-2 Kmehc

..

I,,,,I,,,,I,,,,1~ 55

56

**lt,|t 57

e n e r g y (eV)

Fig. 4. left panel, core level spectra of a 15 layer of In on high!y-doped n-GaAs(ll0) deposited at 120 K, and subsequently warmed up to room temperature The relatwe mcrease c ~ the Ga3d core level Is caused by the strong clustermg of the indmm upon heating, leading to the formation of patches of bare surface Subsequently cooling only leads to marginal changes m the relatwe mtensltses, probably due to diffusion effects (elapsed trine). Right panel corresponding spectra for the Fermi level region of thss layers, exhlblmg the increase of the SPV shift m the center spectlum due to clustering, and due to a decrease of recombination upon recoohng From ref. [62]

causes large parts of the surface to become uncovered (left panel). Clustering can be readdy detected in this way. Apart from the intensity variations with temperature, the core levels also exhibit a considerable shift. Such shifts would normally be (and have been in the past) interpreted as due to band bending changes. However, the overlayer is already metallic under these conditions, and the corresponding spectrum of the region near the Fermi level, shown in the right panel of fig. 4, demonstrates that there is a potential applied te the surface region, which shifts the entire spectrum with respect to the reference Fermi level. The process leading to these shifts is schematically drawn in fig. 5. Under equdibrium conditions, the Fermi level of the semiconductor and that of the metal hne up with that of the reference E~, i.e. of the electron energy analyzer. If the surface region is illuminated with photons of an energy greater than that of the fundamental

K. Horn / Electromc structurt and bamer haghts of semtconductor interfaces

948

equlltbnurn

~.

non - equdtbnurn[SPV)

cb J

cb ~

"hu

~, ~'1 ...... ff,~l EF ...... "~r,

core

"" t level

~_b- An t

Fig. 5, Schematic drawing of the eqmhbrium level ahgnment (left), and the influence of the surface photovoltage on the energies of the substrate and ovedayer peaks (right).

band gap (right panel), electron-hole pairs are generated, and because of the potential difference AV~ induced by the band bending (in the case of n-type material), electrons are driven into the bulk, while the holes accumulate at the surface, compensating the space charge. This causes a rigid shift of all semiconductor and metal levels. This situation can be described by a quasi-Fermi level, shown by the dashed curve. If core level shifts are interpreted as band bending changes, a significant error can be made when such a nonequilibrium situation prevails. In fig. 4 the effect has been demonstrated for n-type material; comparisons for n- and p-type substrates prove that non-equilibrium processes are indeed the cause of the shifts, since they occur ia different directions for n- and p-type semiconductors. The deviation from equilibrium is called the surface photovoltage effect (SPV), and its magnitude can be measured by the shift ot the overlayer Fermi edge with respect to the reference E F. SPV is a selflimiting process since the surface band bending is needed to separate or trap the mobile carriers. This phenomenon had actually been observed before [67,68]. For clean silicon surfaces, Demuth et al. [67] were able to completely compensate the band bending solely by irradiation with the UV source used for photoionization at low temperature. In hindsight, the assumption that the surface region of a metal-semiconductor junction is in equilibrium in a photoemission experiment seems odd, since there is a whole set of literature on the technique of surface photovoltage spoctroscopy [66,67], which utilizes the effect of photon-generated electrov.-hole pair generation and their sep-

aration by the potential in the depletion region, in order to study electron states in the semiconductor band gap. It was not realized that even the feeble photon flux present in a photoemission experiment can lead to a surface photovoltage. The weak dependence of SPV on photon flux, and the cascade processes, which cause a highenergy photon to create a multitude of electronhole pairs were neglected. Following the initial discovery of the significance of this effect [63,64], several groups have in fact demonstrated the importance of SPV effects in photoemmision from metal-semiconductor interfaces and even clean, pinned semiconductor surfaces [43,71,72], and, by means of the Kelvin probe technique, have extended these studies [73] to the low-coverage region where the magnitude of SPV cannot be derived from photoelectron spectra, for lack of a clear E v. Obviously, the study of SPV processes is an interesting field in itself for applications of metal-serniconductor interfaces as solar cells. The influence of SPV has immediate consequences for studies of metal-induced band bending derived from photoemission. Obviously, large corrections have to be applied to many previous data [73], in particular those where e A V b was measured at low temperature; this problem is reviewed in ref. [74]. Consider the data for Pt on GaP(ll0) in fig. 6, recorded at room temperature. Here band bending is plotted as a function of nominal metal coverage for both n- and p-type subs~trates [75]. The shape of the band bending curve has the usual step-like feature for n-type material previously associated with metallization of the overlayer. From the studies c.~ SPV using photoemission [63] and the Kelvin probe it is now clear that this step is caused by a different process, i.e. the breakdown of the surface photovoltage when the metal overlayer becomes contiguous, and establishes contact with the sides of the sample, t,uch that a leakage current can flow [71,73]. It is now well recognized that metallization is not the prime cause for the step-like feature in the band bending curves [73]. SPV may also be the reason behind other puzzling observations of band bending behaviour. Thus group Ib and other less reactive metals were found to induce a band bending that was only complete

K Horn / Electromc structure and barrter hesghts of semiconductor mterface~

Pt/GoP(110): Fermi level at 300K CBM 29-

row deta

15o

I=

correctedfor SPY • n- type • p- type

0 n - type 0 P- type

,o--o_.9... °

o-

i i

,.= 10-

I i

8 ')5-

D VB[

~i~

i

0

II~

i,~,.|

.

0.1

I

"

'''''l

'

1

'

l i ' ' ' ' l

10

Nomlnoi Pt ¢overoge (~1

Fzg. 6. Band bending induced in n- and p-type G a P ( l l 0 ) by depositing platinum onto room-temperature samples The open symbols indicate the uncorrected band bendmgs (affected by the surface photovoltage). The SPV correction is indicated by the arrows The data show that, from the lowest coverage accessible by our analysis, there is a common pruning position for n- and p-type substrates

(i.e. arrived at the final pinning position) after abcut 10 to 30 ,~ of deposited material; similarly, a difference in pinning position for a particular metal on n- and p-type substrates was found [61]. Future studies, in which SPV effects will be taken into account, will prove whether this and other observations are in fact due ~o non-equilibrium processes. It will also be most important to investigate whether the results by Map et al. [73], that SPV-corrected band bending data for A g / GaAs(110) showed a pinning position close to the final value for the lowest coverages onwards, is also true for other metal-semiconductor interfaces. Such data are of importance for the models concerning the emergence of q~b with metal coverage, a debate which at present appears stalled because of the SPV issue. It is anticipated that future investigations will also remove the discrepancies between some Schottky barrier heights derived from photoemission data and those measured by transport techniques [41,80,81]. The quantitative analysis of surface photovoltage effects [64,76,78] indicates that its magnitude can be fairly precisely derived from standard

949

diode theory, in which the main parameters are the barrier height, temperature and substrate doping. Hecht's original calculations for Ti/GaAs (~b = 0.65 V) predict a negligible SPV for high doping over the whole 100 to 300 K temperature range. These calculations should not be interpreted as indicating that it is enough to work with highly doped material in order to avoid SPV. As shown in the top spectra of fig. 4, a large SPV persists at low temperature for this highly doped substrate, and even at room temperature a small SPV is left [62]. The reason for this finding is easily extracted from similar calculations [79]: indium has a much larger barrier than other metals, so the SPV occurs in spite of the high doping level and temperature. This brings us back to the initial question of the origin of specific Schottky barrier heights on III-V surfaces. A recent experiment by Laubschat et al. [77] has suggested that the metallic character of the overlayer is important for pinning. These authors changed the character of a Cs overlayer on GaAs(110) between metallic and insulating by oxidizing the Cs overlayer, and found that the Fermi level reverted from midgap to a position to near the band edges upon loss of metallicity. The importance of this observation lies in the fact that any defects created in the adsorption process will most certainly not be removed when the Cs overlayer is oxidized (in fact rather the opposite would be expected) such that defects cannot be made responsible for the initial midgap movement of E F. Upon subsequent reevaporation of Cs onto the oxidized layer, the band bending reverted to a position near mid-gap. It is unlikely that the full metallic character of the overlayer is recovered upon evaporating Cs onto ar oxidized Cs layer. Thus these results demonstrate that oxidized Cs, in which strongly polar bands are created, does not cause Fermi level pinning, while an only partly oxidized layer does so, at least to some extent. Thus pinning will also be caused by a metal overlayer which does not yet have metallic character, but in which the adsorbed atoms have a sufficiently high density of states in the gap. The importance of the gap density of states has recently been subject of self-consistent slab

950

K. Horn / Electromc structure and bamer hetghts of semwonductor interfaces

calculations of a variety of metals on GaAs(ll0) by van Schilfgaarde and Newman [60]; they found a dependence of barrier height on the nature of the metal, which gives rise to variations in ~t, of up to 0.73 eV. Such a large variation is not found experimentally [41]; in fact, an analysis of Vb's for many different metals on GaAs(ll0), InP(ll0), and GaP(ll0) suggests that there is very little systematic dependence on any of the parameters previously considered, such as electronegativity [41], heat of reaction [82], etcetera. Van Schilfgaarde and Newman concluded that the models which invoke intrinsic interface states in order to explain Fermi level pinning, are not consistent with experimental observations, and that the non-ideal nature of the interfaces used in experiments plays an important role in determining Schottky barrier heights. This approach is similar to the original "effective work function" model by Freeouf and Woodall [37]. This model has found experimental verification through the systematic influence of Ga clusters concentration on ~h and the barrier height of Ai/GaAs(11) [83]. The significant influence of interface structure on Schottky barrier heights was demonstrated by Heslinga et al. [84] who found a difference of about 0.2 eV for Pb junt.tions with Si(lll) when different interface geometries were prepared. The somewhat disappointing aspect of this explanation is the absence of a single, clear-cut mechanism which enables one to actually predict barrier heights. It also resembles our attempts to relate the discrepancies between experimental heterojunction band offsets and slab calculations to an influence of interface reaction species, and the extent of interface reaction in section 2.

4. Conclusions The previous examples, which are by no means meant to give an exhaustive review of currently interesting topics, have shown that photoemission experiments have contributed to the progress in our understanding of the processes which govern barrier height development at semiconductor interfaces. For semiconductor heterojunctions, photoemission data have given a reliable data

base for testing current model calculations, and have identified the role of interlayers which can modify A E~. The large body of photoemission data has also been, and continues to be, most important in judging the merits of different model approaches for the understanding of Schottky barrier heights. An analysis of surface photovoltage effects which affect the potential in the depletion region, has succeeded in explaining some puzzling photoemission data as due to artefacts of the measurement process, and has paved the way to a deeper understanding of photon-ir~duced non-equilibrium processes at such interfaces.

Acknowledgements It is a pleasure to acknowledge the collaboration with Mario Alonso, Roberto Cimino, Andrew Evans, Werner Wilke, Christiane Maierhofer, Thomas Chass6 and Walter Braun, discussions with H. Liith, M. Prietsch and M. Scheffler, and the technical support by Henrik Haak. This work was supported by Bundesministerium for Forschung und Technologie under grant No. 490 FX B as well as the Deutsche Forschungsgemeinschafl through SFB 6 project A 05.

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951

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K Horn / Electromc structure and barrier hetghts of semtconductor mterfaces

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