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Photoemission studies of barrier heights in metal–semiconductor interfaces and heterojunctions
Applied Surface Science 166 Ž2000. 1–11 www.elsevier.nlrlocaterapsusc
Photoemission studies of barrier heights in metal–semiconductor interfaces and heterojunctions K. Horn ) Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany
Abstract The technique of photoelectron spectroscopy has contributed tremendously to our knowledge on the properties of semiconductor interfaces, in aspects such as the electronic structure at the interface, relating to band bending and the evolution of transport barriers such as the Schottky barrier and the heterojunction band offset. This paper describes recent progress in this field, concentrating on metal contacts to wide band gap semiconductors, and the question of band offset engineering through intralayers. Some of the pitfalls of the technique are pointed out, such as in cases where the assumption of an equilibrium situation andror the presence of a flat band condition in overlayers is not fulfilled. This is particularly important with reference to the Ainterface dipoleB interpretation of results from intralayers in GaAsrAlAs junctions, which are discussed in the light of recent experiments. q 2000 Published by Elsevier Science B.V. Keywords: Photoelectron spectroscopy; Metal–semiconductor junctions; Semiconductor heterojunctions; Band ending; Interface dipole
1. Introduction An important contribution to our present knowledge of the electronic structure of semiconductor interfaces has been made through photoelectron spectroscopic studies under well-defined conditions. This technique is well known for its merits in the study of the atomic micropotential, the bulk and surface electronic structure, but its specific advantage for analyzing metal–semiconductor interfaces and semiconductor heterojunctions relates to the fact that also the macropotential, i.e. the band bending at the surface, and its variation under conditions of metal or semiconductor deposition, adsorption, structural changes, and defect formation can be studied )
Tel.: q49-30-84135640; fax: q49-30-84135603. E-mail address: [email protected] ŽK. Horn..
w1x. In this respect, photoemission, in its combination of the investigation of the core and valence level region, provides quite unique possibilities. The method has been used extensively for the study of Schottky barriers under clean and well-defined conditions, leading to a detailed understanding of the factors that affect the barrier height of different metals on elemental and compound semiconductors w2,3x. Similarly, band offsets in heterojunctions have been determined for a large set of combinations, from lattice-matched common anion junctions such as the well known GaAs–AlAs system to junctions with a large strain w4–6x. It should also be mentioned that, as a by-product of core level photoemission, the abruptness of the interface and the occurrence of chemical interactions at the interface w7x can also be investigated from the shape of substrate and overlayer core level lines and the attenuation of substrate
0169-4332r00r$ - see front matter q 2000 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 4 3 5 - 9
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lines. The possibility to measure the band bending from the lowest metal of semiconductor depositions Žfractions of a monolayer. onwards has been particularly useful, and has opened the way to investigate the influence of so-called intralayers, i.e. monolayers of atoms that are foreign to both substrate and overlayer, on the transport barrier that are formed in such interfaces. This so-called band offset engineering w6x has attracted considerable interest since the properties of devices based on such semiconductor interfaces can be significantly altered by even small changes in transport barrier heights. The method of measuring a macropotential variation w8x, i.e. band bending changes, by means of photoelectron spectroscopy is readily explained on the basis of the schematic diagram in Fig. 1, which shows the electronic structure representation of a generic semiconductor Žbottom left.. Electrons excited by photons, and emitted from the surface, give rise to the photoelectron spectrum with the core and valence level region shown above, for a particular position of the Fermi level at the surface. This is important, since the reference level for the kinetic energy of the photoelectrons is the Fermi level of the electron energy analyzer, which is Žor should be. in ohmic electrical contact with the sample. This alignment is shown on the right-hand side. If the position of the Fermi level at the surface changes for some reason Žcenter., this will shift the entire spectrum by a corresponding amount, and the amount of band bending can be precisely determined from the shift of a sharp feature such as a core level line. Since the electron mean free path in solids for typical kinetic energies in a photoemission experiment is small w9x, only the outermost part of the band bending region, which under conditions of moderate doping extends for several hundreds to thousands of angstroms, is measured; this is also indicated in the figure. Thus, the total band bending at the surface is measured Žunder conditions of extremely high doping, the band bending region may be narrow, and photoemission may sample a major part of that region, such that some line broadening may occur.. Even the absolute position of the Fermi level at the surface can be determined, by measuring the magnitude of the separation between the valence band maximum ŽVBM. and the Fermi level. The electron affinity level can also be determined from the total width of the spec-
Fig. 1. Schematic diagram of band bending measurements through core and valence level photoemission. The left-hand side shows a schematic band diagram of an n-type semiconductor under flat band conditions, and following a movement of the surface Fermi level to midgap by Žintrinsic or extrinsic. surface states Žcenter.. The upper part shows the resulting photoelectron spectrum, and the reference Fermi level of the electron energy analyzer on the right-hand side.
trum as shown in Fig. 1. One weakness of the absolute determination of the Fermi level position is the method of VBM evaluation. The commonly used method of extrapolating the leading edge of the valence level spectrum to zero intensity has recently come under criticism, particularly in angle-resolved measurements, where the band character of states giving rise to photoemission peaks is important. It has been argued, also in contributions to this conference w10,11x, that the leading peak, measured at the G point of the bulk Brillouin zone, must be taken as
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a reference. Its influence on the determination of heterojunction band offsets is discussed in Section 3 below; for Schottky barriers the error would be large, but a comparison can be carried out with transport measurement Ži.e., IrV and CrV . data, which, so far, have often yielded good agreement in spite of the fact that for most part, the simple extrapolation method was used for the photoemission data. The important point regarding the determination of the VBM has been discussed by Leckey and Riley w12x, but a comprehensive experimental comparison regarding this important question is most desirable but has not been carried out to date to the author’s knowledge. This paper is intended to address the current topics of semiconductor interface studies using photoemission, such as Schottky barriers on wide band gap materials, the so-called transitivity rule in heterojunction band offsets, and the influence of intralayers on the band offsets at semiconductor heterojunctions. These topics will be discussed to illustrate the merits and shortcomings of the photoemission technique in its application to semiconductor interface studies and will address complications in the interpretation of photoemission data in order to arrive at reliable conclusions.
2. Schottky barrier determinations on large band gap semiconductors A large number of Schottky barrier height measurements by means of photoemission have been carried out on III–V compound semiconductor surfaces, and the Ž110. surface in particular, since this is easily prepared in a stoichiometric and atomically clean manner by cleaving in ultrahigh vacuum. These surfaces also have the Fermi level at the surface at the bulk position governed by the doping for good cleaves, such that it is not ApinnedB, i.e. moved to some energy in the fundamental band gap by intrinsic or extrinsic surface states w3x. Thus, changes brought about by the first stages of adsorption can be readily followed, and a large body of data has been collected, mostly for the III–V semiconductors GaAs, InP and GaP w2x. The universally accepted interpretation of Schottky barrier heights in terms of virtual gap states ŽVIGS. or metal-induced gap states
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ŽMIGS. w13,14x, with an additional description of charge transfer across the interface in terms of the electronegativity of the materials involved w15,16x, has been largely successful in describing these data. In a Žhypothetical. nonpolar bonding situation at the interface, the Fermi level of the metal would adjust at the branch point of the MIGS, and a Schottky barrier height would result, which is equal to energy separation of the branch point of the MIGS and the conduction band minimum, for n-type material. The influence of charge transfer causes the Schottky barrier to be smaller Žlarger. if the electronegativity difference between metal and semiconductor is negative Žpositive.. An important factor is also the density of states of the MIGS near the branch point, which determines the relative influence of MIGS vs. charge transfer effects. From the body of data for GaAs, InP, and even GaP with its larger gap, it appears that the interface density of MIGS in these is so high that the Schottky barrier height is not strongly influenced by the metal electronegativity Žor work function. as suggested from the original Schottky–Mott model w3x, which states that the barrier height scales with the work function of the metal. This topic has attracted considerable interest over the years, and in particular in the early period of Schottky barrier height investigations w17x, where a transition from a more or less constant barrier height to a more Schottky–Mott-like behavior was postulated for semiconductors with a larger band gap or ionicity, for example. Data for materials with larger gaps are thus of interest to examine whether a more ASchottky –MottB-like behavior might be found in these. Wolfframm et al. w18x have recently examined metal overlayers on ZnSŽ110.. The choice of substrate arose from the fact that for ZnS self-consistent calculations, and systematic comparisons with Si and GaAs, have been performed w19x, and the size of the gap Ž3.3 eV. is substantially larger than that of substrates previously studied under clean and stoichiometric conditions. The ZnS surfaces were grown by molecular beam epitaxy on cleaved GaPŽ110. surfaces, since bulk ZnS material is likely to charge in the photoemission experiments, leading to an undefined reference level. This is avoided by using a ˚ thick on medium-doped ZnS layer about 100–200 A GaP, and the choice of substrate is made because there exists only a small lattice mismatch Ž0.5%.
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between the two compound semiconductors. The spectra in Fig. 2 show the features of a well-defined ZnSŽ110. surface Žthe bulk and surface band structure of which had been determined in an earlier experiment w20x., and the evolving emission from the Ag overlayer, ultimately showing a clear Fermi edge. It is obvious that this Fermi edge initially occurs well below the energy of the reference Fermi level. This is due to the occurrence of a surface photovoltage ŽSPV., one of the effects that has to be kept in mind when performing photoemission from semiconductor interfaces, in particular from wide gap materials. Illumination of the surface with photons of energy higher than the band gap leads to a non-equilibrium at the surface w21,22x; under conditions of upward band bending such as here, electrons excited from the valence band top diffuse into the bulk because of the built-in field, while the holes remain at the surface for the same reason. Thus, they compensate the band bending and attempt to drive the system, in a dynamic equilibrium under illumination, towards flat band conditions. If this effect is neglected, and merely the energy of the substrate core level is taken as a measure of band bending, serious
errors in band bending result w22x. This can be avoided under those circumstances where a Fermi level of the evolving overlayer is already apparent, such as in Fig. 2. The apparent band bending can then be corrected for the SPV, as in the inset, and the true position of the Fermi level can be determined, as ˚ metal shown for Ag and Au in Fig. 2. Above 10-A thickness the SPV breaks down due to short-circuiting at the edges of the film, and the position of the Fermi level can be unambiguously determined. The resulting data for Ag and Au Schottky barriers on ZnS provide an interesting view on the dependence of Schottky barrier height on metal work function, and a quantitative test of the predictive powers of current schemes w16x. ZnS is supposed to be a case where charge transfer is expected to be more important than in the III–V compound semiconductors since the density of interface states, and thus their ability to dominate the alignment of the branch point with the Fermi level against the influence of charge transfer between metal and semiconductor, is much lower. The Schottky barriers evaluated from the data are F b ŽAu. s 2.19 " 0.10 eV and F b ŽAg. s 1.81 " 0.10 eV. These values are slightly
Fig. 2. Left: Spectra from silver overlayers of varying thickness on a clean ZnSŽ110. surface, as indicated by the Ag-induced features in the ˚ thickness Žfrom Ref. w18x.. Right: Band bending induced by Ag spectrum; notice the emergence of a Fermi edge in the spectra beyond 3-A and Au depositions onto ZnSŽ110., measured from the movement of the Zn 3d core level Žuncorrected for surface photovoltage, open symbols., and after correction for surface photovoltage Žfilled symbols.. The inset gives the measured Schottky barrier heights.
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larger than those from transport measurements in the literature w23x. These differences may be related to the different methods of measurement, and different surface preparations prior to growth; also, an influence of barrier inhomogeneity w24x cannot be ruled out. The relative influence of the branch point F bp and the electronegativity difference on Schottky barrier height can be written as
F Bn s F bp q S X Ž Xm y Xs .
Ž 1.
where F Bn is the Schottky barrier height for an n-type semiconductor, F bp the energy of the MIGS branch point, and Xm and Xs are the metal and semiconductor electronegativities. S X is the so-called slope parameter that describes the dependence of the barrier height on metal electronegativity and thus on work function. For our present case, we derive the slope parameter S X s dF Bn rd X m s 0.76, based on Pauling’s electronegativities. The present data w18x cover an appreciable range of work functions Ž0.7 eV., and for two AunreactiveB metals, rendering an influence of interface reaction on the magnitude of the barrier height unlikely. Taking the branch point energy of the MIGS from empirical tight binding calculations as F bp s 2.05 eV w25x, and using Monch’s relation between the dielectric constant and ¨ S X , we obtain a predicted slope parameter of 0.64, i.e. close to our value above, and clearly in a range where the work function or electronegativity differences of the metal have a sizeable influence on the Schottky barrier height. Barrier heights calculated from these predicted numbers then yield F B ŽAu. s 2.22 and F B ŽAg. s 1.93, again quite close to the experimental values w18x. This is a nice example where the predictive powers of the AMIGS plus electronegativityB w26x model are well supported by experiments with a wide band gap semiconductor. It is fortunate that self-consistent pseudopotential calculations of metal interaction with the ZnSŽ110. surface have been carried out by Louie et al. w19x. Their results offer an intuitive explanation for the differences in Fermi level pinning observed in semiconductors with different electronic structure and magnitudes of the fundamental band gap. These authors studied the interface between aluminum Žmodeled by jellium. and the Ž110. surfaces of GaAs,
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ZnSe and ZnS. In their calculations of the interface density of states in the band gap region, they find that the density of these MIGS is quite low for ZnS, being much higher for Si, GaAs and ZnSe. Moreover, the penetration of these states into the semicon˚ for Si and 1.9 A˚ for ZnSe to ductor falls from 3.0 A ˚ for ZnS. The authors used these results to only 0.9 A calculate the Schottky barrier heights. They also used a simple model to relate the MIGS surface density of states and the penetration depth to the dipole potential at the surface. For ZnS they find an S parameter of S ; 0.7, quite close to our data from photoemission measurements above. While the calculations of Louie et al. w19x are for AjelliumB metal, they lend themselves to a comparison with our data of the noble metals, with their purely s–p density of states at E F . We thus conclude that our photoemission data from metal deposition onto clean, well-defined ZnSŽ110. surfaces performed under UHV conditions strongly support the notion that the low density of MIGS in ZnS lead to a strong dependence of Schottky barrier heights on metal work function, in agreement with the general concepts based on electronegativity considerations, and the theoretical description of the metal–ZnS bond.
3. Semiconductor heterojunctions
3.1. Test of heterojunction band offset transitiÕity In this section, we first analyze heterojunction band offset measurements between III–V and II–VI compound semiconductors, not as a distinctive class of systems, but in order to discuss a specific aspect of heterojunction band offset properties, i.e. band offset commutativity and transitivity w5x. Briefly, this addresses the question whether band offsets are transferable, i.e. whether a band offset between materials A and B, and the offset involving materials B and C, i.e. D Ev Ž B y C . is equal to an offset at the interface between materials A and C, D Ev Ž A y C . Žtransitivity.; a related question relates to the valence band offset D Ev at an interface between material A and material B being independent of growth se-
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quence Žcommutativity.. Consider the example presented in the left part of Fig. 3, where the measured band offset of a CdSerGaSb heterojunction was determined using photoemission w27x. The band offset is determined from the change of the onset of photoemission at the valence band edge, where the change in band bending that may occur upon interface formation is accounted for by recording the shift in energy of a substrate core level; this procedure is explained in many papers Žsee Ref. w28x, for example.. Aside from these, we noticed that the commonly used practice of using the linear extrapolation of emission intensity to determine the location of the VBM has recently been questioned. Eich et al. w10x and Kreis et al. w11x have argued that a proper VBM determination from angle-resolved photoemission should be performed, such that the highest dispersing peak is assigned to the topmost valence band. This method is based on the assumption that the topmost band at the G point gives rise to a clearly discernible peak, and that the shape of the valence band emission in normal emission is not affected by density of states considerations. It has been shown that the width of the leading peak is affected by surface
perfection, such that a rough overlayer may give rise to erroneous results. While the work of Eich et al. w10x and Kreis et al. w11x touches on a valid point, the error is probably not large if the VBM of both materials is determined using the same procedure, as in the case of CdSe on GaSbŽ110.. The band offset between CdSe and GaSb, in a Ž110. interface orientation, is determined to be 1.09 eV w27x from these data, a value that is close to that determined from a scheme based on charge neutrality considerations in the dielectric midgap energy scheme Ž1.27 eV w29x.. The existence of other band offset determinations within the group of III–VrII–VI interfaces then permits an analysis of transitivity in heterojunction band offsets. Consider a set of heterojunctions between the semiconductors GaSbrZnTe w7 x, ZnTerCdSe w30x, and CdSerGaSb w27x. For each of these, the band offset was measured by photoemission, and the respective values are plotted together on the right-hand side of Fig. 3. If the heterojunction band offset were a bulk property, a round trip from one side of a hypothetical device that encompasses all of these interfaces to the other side, in the above sequence would then lead to a net offset of zero w31x.
Fig. 3. Left: Spectral region near the valence band maximum for clean GaSbŽ110., and after deposition of a thick layer of cubic CdSe, together with the Fermi level reference from a metal plate in ohmic contact with the sample Žfrom Ref. w27x.. Right: Comparison of band offsets determined for GaSbrZnTe w7x, ZnTerCdSe w30x and the data for CdSerGaSb w27x, showing that the deviation form transitivity is about 0.11 eV, i.e. within the experimental accuracy.
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The situation is somewhat more complicated than one might think, since the ZnTerCdSe junction is not lattice matched Žin contrast to the other two., and its offset therefore could be affected by strain effects, which need to be accounted for in theoretical predictions of the band offset. As indicated by the dashed line, the sum of all valence band offsets is 0.11 eV, i.e. within the error margin of the photoemission experiment. This is somewhat surprising since in all of the heterojunctions involved, an interface reaction occurs, which might affect the magnitude of D Ev . It appears unlikely that the small value for the total band offset is fortuitous, since the absolute magnitude of the offsets involved is quite large. We thus conclude that even for those interfaces where an interface reaction occurs, the band offsets do not seem to be strongly modified from the value that they would assume from the bulk properties of the semiconductors involved. This conclusion is drawn here for Ž110.-oriented, i.e. nonpolar interfaces, where dipole effects due to charge exchange across the interface Žsee below. are expected to be small in any case. 3.2. Intentional modification of band offsets by intralayers The magnitude of the band offset has important implications for the use of heterojunctions, affecting carrier injection, carrier confinement, and ionization thresholds in semiconductor devices. Thus, it would be most desirable to control the magnitude of the offset, a process that has been termed heterojunction band offset engineering w6x. Briefly, the idea is to introduce a local dipole layer at the interface, which will act to change the potential step between the two semiconductors; this influence can be modeled by a parallel plate capacitor. This is different from the doping interface dipole, where two thin sheets of charge are introduced on either side of the interface. In fact, large changes in the apparent valence band offset of GaAsrAlAs upon introducing a silicon intralayer have been reported in photoemission experiments w6,32x. These have been interpreted in terms of a variation of the band offset through the introduction of an interfacial dipole layer w33,34x, an interpretation that appears plausible since the magnitude of band offset change showed a variation with
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Si concentration. However, other authors w35x have argued that photoemission, as a surface probe, is sensitive to band bending and Fermi level pinning at the surface, which might mimic a band offset in the data even if only the band bending exhibits change. Since the band offset is measured from the energy of the VBM andror a shallow core level at the surface of the oÕerlayer, a band bending in the overlayer can well be interpreted as an indication of a band offset change. The interface dipole model makes several predictions on the band offset change, which are amenable to experimental verification. First, since the silicon atoms are supposed to assume the role of the anion or cation depending on which site they are located on it suggests that group IV atoms should act as dipoles in polar interfaces, but not in nonpolar ones, since the charge of neighboring intralayer atoms would cancel in the latter. This is schematically indicated in Fig. 4a for interfaces oriented along the Ž100. and Ž110. directions. It is obvious that the silicon atoms which take up the position in a Ž100.oriented interface act as a dipole sheet, while the charges arranged in a Ž110. interface compensate, giving no net effect on the band offset. This is also true for a homojunction ŽGaAsrGaAs. where a Aband offsetB might be introduced by a dipole layer, but only in a polar growth direction. Secondly, intralayers from other than group IV elements should exhibit a weaker influence, since they cannot act as AamphotericB substitutional atoms when replacing anion or cation substrate or overlayer atoms. Moreno et al. w36x have carried out several experiments to tests whether the above predictions agree with experimental observations. They examined the influence of Si intralayers on the band offset in GaAsrAlAs junctions. The samples were grown in a dedicated molecular beam chamber, and were transferred to the photoemission chamber in a small transport ultrahigh vacuum chamber. Their sample geometry and layer arrangement is shown in Fig. 4b. Moreno et al. measured not only the As 3d, Ga 3d and Al 2p core levels, but also the energy of the VBM from the emission near the VBM, and the location of the Fermi level measured from a gold foil in ohmic contact with the samples. In the case of a GaAsrAlAs junction with Si intralayer large apparent band offsets occur as can be
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Fig. 4. Ža. Schematic diagram of the layer arrangement of samples in the experiments of Moreno et al., showing the location of the Si Žand Be. intralayer. The lower diagram shows the action of the ionized donors and the electrons trapped in surface states to create an electric field across the overlayer. Žb. Schematic diagram indicating the action of group IV atoms at a polar interface to create an interface dipole. Žc. The effect of silicon intralayers in various GaAsrAlAs interfaces on the apparent valence band offset measured by photoemission, from several groups as indicated. ŽFrom Moreno et al. w36x..
seen from the compilation of results from several authors in Fig. 4c. The apparent band offset also depends on the amount of intralayer atoms inserted, which finds an easy interpretation in the interface dipole model. It is of opposite sign for the two
growth directions, and even larger than in previous experiments, probably due to an improved incorporation of the Si layer. However, what is also obvious is that for a nonpolar junction Ži.e., the Ž110. direction. apparent band offsets of similar magnitude occur.
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This cannot be understood in terms of the model shown in Fig. 4a, since the neighboring dipoles, in an ordered array, would cancel, resulting in zero net change of band offset. Moreno et al. have provided a different interpretation of the observed effects in terms of a band bending in the overlayer, in line with an earlier report by Hashimoto et al. w35x. This starts from the assumption that the silicon atoms at the interface are ionized, and that the electrons, which they donate, fill the surface states; the charge on the positive ionized Si atoms and the negative charge in the surface states then leads to a strong electric field that builds up across the GaAs overlayer, which terminates the heterostructure. This model of charge sheets is shown in the lower part of Fig. 4b. This interpretation was put to the test in another experiment by Moreno et al., where the action of silicon and beryllium intralayers was compared. The logic behind this choice of intralayer atoms lies in the well-known action of silicon as an n-type and beryllium as a p-type dopant. The interface dipole model makes no obvious predictions about the action of Be atoms in an intralayer; however, they are unlikely to assume a role similar to that of silicon which does exhibit amphoteric behavior in GaAs. Thus, one would not expect the insertion of a Be layer in a GaAsrAlAs interface to cause a significant change in the band offset. The photoemission experiments by Moreno et al. w37x instead showed a marked effect of the Be intralayer, reducing the apparent valence band offset by almost 0.2 eV. This is shown in Fig. 5 where the Al 2p and Ga 3d spectra from a GaAsrAlAs junction with an Si and Be intralayer are compared, referenced to the VBM. The separation between these core levels is often used to determine the magnitude of the valence band photoemission, based on the known binding energy difference between these levels and the VBM in the respective compounds. Thus, the data on the left-hand side of Fig. 5 might be taken to provide a graphic representation of an increase in D Ev upon Si insertion, and a decrease upon Be insertion as stated above. However, the situation is more complex as shown on the right-hand side of Fig. 5. Here, the VBMs for the two different intralayers have been aligned, and the emission from a metallic film in good electrical contact with the samples is compared. It is obvious that the surface Fermi level differs by about 400
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meV between the two samples. This observation cannot be explained in the interface dipole model, but in fact provides strong support for the doping role of the intralayer atoms which form the basis of the interpretation of the photoemission results by Moreno et al. w37x. The Bohr radius of charge carriers liberated from the intralayer atoms in a hydrogen ˚ The GaAs overlayer model amounts to about 100 A. in these heterojunctions must not be too thick to totally attenuate the signal from the interface, i.e. no ˚ depending on photoelectron more than about 50 A, ˚ in the experiments of kinetic energy Žit was 20 A Moreno et al... Thus, the electrons Žfor Si intralayers. or holes Žfor Be. can readily be trapped in the surface states of the top GaAsŽ100. layer. The two sheets of localized charges, ionized atoms in the intralayer and charges in the surface states, build up an electric field across the overlayer, as shown in the schematic diagram in the lower portion of Fig. 4a. The important point to remember is that photoemission, as a surface sensitive technique, derives most of its signal from the immediate surface region, according to an exponential attenuation law for the photoemitted electrons. Thus, if the assumption of flat bands in the overlayer, implicit in almost all photoemission determinations of band offsets, is not fulfilled, the signal from the buried material ŽAlAs. will be derived from the immediate interface, but that from the GaAs side of the interface will have its strongest contributions from the surface. This differs from that at the interface by an amount equal to the magnitude of band bending Žthe overlayer peak will be additionally broadened by an addition of contributions of the overlayer core level with different band bending energies, according to the band bending profile.. The data for Si intralayers inŽ100.-oriented interfaces, and the effect of Si and Be intralayers w37x provide convincing evidence against an interpretation of core level photoemission results from such interfaces in terms of a real band offset modification. Moreno et al. were able to model the potential variation across the overlayer by means of a solution to the one-dimensional Poisson equation, and arrived at an excellent agreement with their observations, without having to introduce variations in the magnitude of D Ev compared to that in GaAsrAlAs. These data do not provide evidence against the interface
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Fig. 5. Top: Spectra from a GaAsrAlAs heterojunction with an Si and Be intralayer, respectively, showing the opposite shift of the Al 2p line in these cases. Bottom: Shift of the reference Fermi level if the spectra are aligned at the valence band maximum, demonstrating the difference in band bending in the overlayers upon Si and Be insertion. ŽFrom Moreno et al. w37x..
dipole model as such, but against the support from photoemission experiments which was thought to demonstrate its validity.
Photoemission has made many important contributions to our understanding of the electronic structure of semiconductor interfaces, and, as amply
K. Horn r Applied Surface Science 166 (2000) 1–11
demonstrated by many papers at this conference, and some of the aspects discussed in this paper, continues to be of great value in elucidating barrier height and band bending phenomena. Applications of this technique necessitate taking into account the importance of the surface aspect of this technique, and the possibility of non-equilibrium effects for a proper interpretation of the data.
Acknowledgements This work was supported by the Bundesministerium fur ¨ Bildung, Forschung und Technologie under grant 05 622 OLA 3. I gratefully acknowledge the long collaboration and discussions with M. Alonso, S.R. Barman, Th. Chasse, ´ D.A. Evans, K.O. Magnusson, M. Moreno, and G. Neuhold.
References w1x H. Luth, ¨ Surfaces and Interfaces of Solids, Springer-Verlag, Berlin, 1993. w2x E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, Clarendon Press, Oxford, 1988. w3x W. Monch, Semiconductor Surfaces and Interfaces, ¨ Springer-Verlag, Berlin, 1993. w4x H. Kroemer, J. Vac. Sci. Technol., B 2 Ž1984. 433. w5x A.D. Katnani, G. Margaritondo, Phys. Rev. B 28 Ž1983. 1944. w6x A. Franciosi, C.G. van de Walle, Surf. Sci. Rep. 25 Ž1996. 1. w7x W.G. Wilke, K. Horn, J. Vac. Sci. Technol., B 6 Ž1988. 1211. w8x E. Spenke, Elektronische Halbleiter, Springer-Verlag, Berlin, 1965. w9x M.P. Seah, W.A. Dench, Surf. Interface Anal. 1 Ž1979. 2. w10x D. Eich, D. Hubner, M. Grabner, K. Ortner, C.R. Becker, C. ¨ ¨ Heske, R. Fink, G. Landwehr, E. Umbach, to be published Ž1999.. w11x C. Kreis, M. Traving, R. Adelung, L. Kipp, M. Skibowski, to be published Ž1999..
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w12x R.C.G. Leckey, J.D. Riley, Crit. Rev. Solid State Mater. Sci. 17 Ž1992. 307. w13x V. Heine, Phys. Rev. 138 Ž1965. A1689. w14x S.G. Louie, M.L. Cohen, Phys. Rev. B 13 Ž1976. 2461. w15x J. Tersoff, Phys. Rev. Lett. 52 Ž1984. 465. w16x W. Monch, in: e.B. Kramer ŽEd.., Advances in Solid State ¨ ., Vieweg, Braunschweig, 1999. Physics ŽFestkorperprobleme ¨ w17x S. Kurtin, T.C. McGill, C.A. Read, Phys. Rev. Lett. 22 Ž1969. 1433. w18x D. Wolfframm, D.A. Evans, G. Neuhold, K. Horn, J. Appl. Phys. Ž1999. in press. w19x S.G. Louie, J.R. Chelikowsky, M.L. Cohen, Phys. Rev. B 15 Ž1977. 2154. w20x S.R. Barman, S.-A. Ding, G. Neuhold, K. Horn, D. Wolfframm, D.A. Evans, Phys. Rev. B 58 Ž1998. 7053. w21x J.E. Demuth, W.J. Thompson, N.J. DiNardo, R. Imbihl, Phys. Rev. Lett. 56 Ž1986. 1408. w22x M. Alonso, R. Cimino, K. Horn, J. Vac. Sci. Technol., A 9 Ž1991. 891. w23x C.A. Mead, Solid State Electron. 9 Ž1966. 1023. w24x R. Cimino, A. Giarante, K. Horn, M. Pedio, Europhys. Lett. 32 Ž1995. 601. w25x W. Monch, J. Appl. Phys. 80 Ž1996. 5076. ¨ w26x W. Monch, Rep. Prog. Phys. 53 Ž1990. 221. ¨ w27x G. Neuhold, K. Horn, K.O. Magnusson, D.A. Evans, J. Vac. Sci. Technol., A 13 Ž1995. 666. w28x W.G. Wilke, R. Seedorf, K. Horn, J. Cryst. Growth 101 Ž1990. 620. w29x N.E. Christensen, I. Gorczyca, O.B. Christensen, U. Schmid, M. Cardona, J. Cryst. Growth 101 Ž1990. 318. w30x E.T. Yu, M.C. Phillips, J.O. McCaldin, T.C. McGill, J. Vac. Sci. Technol., B 9 Ž1991. 2233. w31x H. Kroemer, in: L.L.C.a.K. Ploog ŽEd.., Molecular Beam Epitaxy and Heterostructures, M. Nijhof, Dordrecht, 1985. w32x L. Sorba, G. Bratina, G. Ceccone, A. Antonini, J.F. Walker, M. Micovic, A. Franciosi, Phys. Rev. B 43 Ž1991. 2450. w33x W.A. Harrison, E. Kraut, J.R. Waldrop, R.W. Grant, Phys. Rev. B 18 Ž1978. 4402. w34x S. Baroni, R. Resta, A. Baldereschi ŽEds.., Institute of Physics, Polish Academy of Sciences, Warsaw, 1988. w35x Y.G. Hashimoto, G. Tanaka, T. Ikoma, J. Vac. Sci. Technol., B 12 Ž1994. 125. w36x M. Moreno, H. Yang, M. Horicke, J.-A.M.-G.M. Alonso, R. ¨ Hey, K. Horn, J.L. Sacedon, K. Ploog, Phys. Rev. B 57 Ž1998. 12314. w37x M. Moreno, J.L. Sacedon, M. Alonso, M. Horicke, R. Hey, J. ¨ Avila, M.C. Asensio, K. Horn, K. Ploog, Phys. Rev. B 58 Ž1998..
Photoemission studies of barrier heights in metal–semiconductor interfaces and heterojunctions K. Horn ) Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany
Abstract The technique of photoelectron spectroscopy has contributed tremendously to our knowledge on the properties of semiconductor interfaces, in aspects such as the electronic structure at the interface, relating to band bending and the evolution of transport barriers such as the Schottky barrier and the heterojunction band offset. This paper describes recent progress in this field, concentrating on metal contacts to wide band gap semiconductors, and the question of band offset engineering through intralayers. Some of the pitfalls of the technique are pointed out, such as in cases where the assumption of an equilibrium situation andror the presence of a flat band condition in overlayers is not fulfilled. This is particularly important with reference to the Ainterface dipoleB interpretation of results from intralayers in GaAsrAlAs junctions, which are discussed in the light of recent experiments. q 2000 Published by Elsevier Science B.V. Keywords: Photoelectron spectroscopy; Metal–semiconductor junctions; Semiconductor heterojunctions; Band ending; Interface dipole
1. Introduction An important contribution to our present knowledge of the electronic structure of semiconductor interfaces has been made through photoelectron spectroscopic studies under well-defined conditions. This technique is well known for its merits in the study of the atomic micropotential, the bulk and surface electronic structure, but its specific advantage for analyzing metal–semiconductor interfaces and semiconductor heterojunctions relates to the fact that also the macropotential, i.e. the band bending at the surface, and its variation under conditions of metal or semiconductor deposition, adsorption, structural changes, and defect formation can be studied )
Tel.: q49-30-84135640; fax: q49-30-84135603. E-mail address: [email protected] ŽK. Horn..
w1x. In this respect, photoemission, in its combination of the investigation of the core and valence level region, provides quite unique possibilities. The method has been used extensively for the study of Schottky barriers under clean and well-defined conditions, leading to a detailed understanding of the factors that affect the barrier height of different metals on elemental and compound semiconductors w2,3x. Similarly, band offsets in heterojunctions have been determined for a large set of combinations, from lattice-matched common anion junctions such as the well known GaAs–AlAs system to junctions with a large strain w4–6x. It should also be mentioned that, as a by-product of core level photoemission, the abruptness of the interface and the occurrence of chemical interactions at the interface w7x can also be investigated from the shape of substrate and overlayer core level lines and the attenuation of substrate
0169-4332r00r$ - see front matter q 2000 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 4 3 5 - 9
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lines. The possibility to measure the band bending from the lowest metal of semiconductor depositions Žfractions of a monolayer. onwards has been particularly useful, and has opened the way to investigate the influence of so-called intralayers, i.e. monolayers of atoms that are foreign to both substrate and overlayer, on the transport barrier that are formed in such interfaces. This so-called band offset engineering w6x has attracted considerable interest since the properties of devices based on such semiconductor interfaces can be significantly altered by even small changes in transport barrier heights. The method of measuring a macropotential variation w8x, i.e. band bending changes, by means of photoelectron spectroscopy is readily explained on the basis of the schematic diagram in Fig. 1, which shows the electronic structure representation of a generic semiconductor Žbottom left.. Electrons excited by photons, and emitted from the surface, give rise to the photoelectron spectrum with the core and valence level region shown above, for a particular position of the Fermi level at the surface. This is important, since the reference level for the kinetic energy of the photoelectrons is the Fermi level of the electron energy analyzer, which is Žor should be. in ohmic electrical contact with the sample. This alignment is shown on the right-hand side. If the position of the Fermi level at the surface changes for some reason Žcenter., this will shift the entire spectrum by a corresponding amount, and the amount of band bending can be precisely determined from the shift of a sharp feature such as a core level line. Since the electron mean free path in solids for typical kinetic energies in a photoemission experiment is small w9x, only the outermost part of the band bending region, which under conditions of moderate doping extends for several hundreds to thousands of angstroms, is measured; this is also indicated in the figure. Thus, the total band bending at the surface is measured Žunder conditions of extremely high doping, the band bending region may be narrow, and photoemission may sample a major part of that region, such that some line broadening may occur.. Even the absolute position of the Fermi level at the surface can be determined, by measuring the magnitude of the separation between the valence band maximum ŽVBM. and the Fermi level. The electron affinity level can also be determined from the total width of the spec-
Fig. 1. Schematic diagram of band bending measurements through core and valence level photoemission. The left-hand side shows a schematic band diagram of an n-type semiconductor under flat band conditions, and following a movement of the surface Fermi level to midgap by Žintrinsic or extrinsic. surface states Žcenter.. The upper part shows the resulting photoelectron spectrum, and the reference Fermi level of the electron energy analyzer on the right-hand side.
trum as shown in Fig. 1. One weakness of the absolute determination of the Fermi level position is the method of VBM evaluation. The commonly used method of extrapolating the leading edge of the valence level spectrum to zero intensity has recently come under criticism, particularly in angle-resolved measurements, where the band character of states giving rise to photoemission peaks is important. It has been argued, also in contributions to this conference w10,11x, that the leading peak, measured at the G point of the bulk Brillouin zone, must be taken as
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a reference. Its influence on the determination of heterojunction band offsets is discussed in Section 3 below; for Schottky barriers the error would be large, but a comparison can be carried out with transport measurement Ži.e., IrV and CrV . data, which, so far, have often yielded good agreement in spite of the fact that for most part, the simple extrapolation method was used for the photoemission data. The important point regarding the determination of the VBM has been discussed by Leckey and Riley w12x, but a comprehensive experimental comparison regarding this important question is most desirable but has not been carried out to date to the author’s knowledge. This paper is intended to address the current topics of semiconductor interface studies using photoemission, such as Schottky barriers on wide band gap materials, the so-called transitivity rule in heterojunction band offsets, and the influence of intralayers on the band offsets at semiconductor heterojunctions. These topics will be discussed to illustrate the merits and shortcomings of the photoemission technique in its application to semiconductor interface studies and will address complications in the interpretation of photoemission data in order to arrive at reliable conclusions.
2. Schottky barrier determinations on large band gap semiconductors A large number of Schottky barrier height measurements by means of photoemission have been carried out on III–V compound semiconductor surfaces, and the Ž110. surface in particular, since this is easily prepared in a stoichiometric and atomically clean manner by cleaving in ultrahigh vacuum. These surfaces also have the Fermi level at the surface at the bulk position governed by the doping for good cleaves, such that it is not ApinnedB, i.e. moved to some energy in the fundamental band gap by intrinsic or extrinsic surface states w3x. Thus, changes brought about by the first stages of adsorption can be readily followed, and a large body of data has been collected, mostly for the III–V semiconductors GaAs, InP and GaP w2x. The universally accepted interpretation of Schottky barrier heights in terms of virtual gap states ŽVIGS. or metal-induced gap states
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ŽMIGS. w13,14x, with an additional description of charge transfer across the interface in terms of the electronegativity of the materials involved w15,16x, has been largely successful in describing these data. In a Žhypothetical. nonpolar bonding situation at the interface, the Fermi level of the metal would adjust at the branch point of the MIGS, and a Schottky barrier height would result, which is equal to energy separation of the branch point of the MIGS and the conduction band minimum, for n-type material. The influence of charge transfer causes the Schottky barrier to be smaller Žlarger. if the electronegativity difference between metal and semiconductor is negative Žpositive.. An important factor is also the density of states of the MIGS near the branch point, which determines the relative influence of MIGS vs. charge transfer effects. From the body of data for GaAs, InP, and even GaP with its larger gap, it appears that the interface density of MIGS in these is so high that the Schottky barrier height is not strongly influenced by the metal electronegativity Žor work function. as suggested from the original Schottky–Mott model w3x, which states that the barrier height scales with the work function of the metal. This topic has attracted considerable interest over the years, and in particular in the early period of Schottky barrier height investigations w17x, where a transition from a more or less constant barrier height to a more Schottky–Mott-like behavior was postulated for semiconductors with a larger band gap or ionicity, for example. Data for materials with larger gaps are thus of interest to examine whether a more ASchottky –MottB-like behavior might be found in these. Wolfframm et al. w18x have recently examined metal overlayers on ZnSŽ110.. The choice of substrate arose from the fact that for ZnS self-consistent calculations, and systematic comparisons with Si and GaAs, have been performed w19x, and the size of the gap Ž3.3 eV. is substantially larger than that of substrates previously studied under clean and stoichiometric conditions. The ZnS surfaces were grown by molecular beam epitaxy on cleaved GaPŽ110. surfaces, since bulk ZnS material is likely to charge in the photoemission experiments, leading to an undefined reference level. This is avoided by using a ˚ thick on medium-doped ZnS layer about 100–200 A GaP, and the choice of substrate is made because there exists only a small lattice mismatch Ž0.5%.
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between the two compound semiconductors. The spectra in Fig. 2 show the features of a well-defined ZnSŽ110. surface Žthe bulk and surface band structure of which had been determined in an earlier experiment w20x., and the evolving emission from the Ag overlayer, ultimately showing a clear Fermi edge. It is obvious that this Fermi edge initially occurs well below the energy of the reference Fermi level. This is due to the occurrence of a surface photovoltage ŽSPV., one of the effects that has to be kept in mind when performing photoemission from semiconductor interfaces, in particular from wide gap materials. Illumination of the surface with photons of energy higher than the band gap leads to a non-equilibrium at the surface w21,22x; under conditions of upward band bending such as here, electrons excited from the valence band top diffuse into the bulk because of the built-in field, while the holes remain at the surface for the same reason. Thus, they compensate the band bending and attempt to drive the system, in a dynamic equilibrium under illumination, towards flat band conditions. If this effect is neglected, and merely the energy of the substrate core level is taken as a measure of band bending, serious
errors in band bending result w22x. This can be avoided under those circumstances where a Fermi level of the evolving overlayer is already apparent, such as in Fig. 2. The apparent band bending can then be corrected for the SPV, as in the inset, and the true position of the Fermi level can be determined, as ˚ metal shown for Ag and Au in Fig. 2. Above 10-A thickness the SPV breaks down due to short-circuiting at the edges of the film, and the position of the Fermi level can be unambiguously determined. The resulting data for Ag and Au Schottky barriers on ZnS provide an interesting view on the dependence of Schottky barrier height on metal work function, and a quantitative test of the predictive powers of current schemes w16x. ZnS is supposed to be a case where charge transfer is expected to be more important than in the III–V compound semiconductors since the density of interface states, and thus their ability to dominate the alignment of the branch point with the Fermi level against the influence of charge transfer between metal and semiconductor, is much lower. The Schottky barriers evaluated from the data are F b ŽAu. s 2.19 " 0.10 eV and F b ŽAg. s 1.81 " 0.10 eV. These values are slightly
Fig. 2. Left: Spectra from silver overlayers of varying thickness on a clean ZnSŽ110. surface, as indicated by the Ag-induced features in the ˚ thickness Žfrom Ref. w18x.. Right: Band bending induced by Ag spectrum; notice the emergence of a Fermi edge in the spectra beyond 3-A and Au depositions onto ZnSŽ110., measured from the movement of the Zn 3d core level Žuncorrected for surface photovoltage, open symbols., and after correction for surface photovoltage Žfilled symbols.. The inset gives the measured Schottky barrier heights.
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larger than those from transport measurements in the literature w23x. These differences may be related to the different methods of measurement, and different surface preparations prior to growth; also, an influence of barrier inhomogeneity w24x cannot be ruled out. The relative influence of the branch point F bp and the electronegativity difference on Schottky barrier height can be written as
F Bn s F bp q S X Ž Xm y Xs .
Ž 1.
where F Bn is the Schottky barrier height for an n-type semiconductor, F bp the energy of the MIGS branch point, and Xm and Xs are the metal and semiconductor electronegativities. S X is the so-called slope parameter that describes the dependence of the barrier height on metal electronegativity and thus on work function. For our present case, we derive the slope parameter S X s dF Bn rd X m s 0.76, based on Pauling’s electronegativities. The present data w18x cover an appreciable range of work functions Ž0.7 eV., and for two AunreactiveB metals, rendering an influence of interface reaction on the magnitude of the barrier height unlikely. Taking the branch point energy of the MIGS from empirical tight binding calculations as F bp s 2.05 eV w25x, and using Monch’s relation between the dielectric constant and ¨ S X , we obtain a predicted slope parameter of 0.64, i.e. close to our value above, and clearly in a range where the work function or electronegativity differences of the metal have a sizeable influence on the Schottky barrier height. Barrier heights calculated from these predicted numbers then yield F B ŽAu. s 2.22 and F B ŽAg. s 1.93, again quite close to the experimental values w18x. This is a nice example where the predictive powers of the AMIGS plus electronegativityB w26x model are well supported by experiments with a wide band gap semiconductor. It is fortunate that self-consistent pseudopotential calculations of metal interaction with the ZnSŽ110. surface have been carried out by Louie et al. w19x. Their results offer an intuitive explanation for the differences in Fermi level pinning observed in semiconductors with different electronic structure and magnitudes of the fundamental band gap. These authors studied the interface between aluminum Žmodeled by jellium. and the Ž110. surfaces of GaAs,
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ZnSe and ZnS. In their calculations of the interface density of states in the band gap region, they find that the density of these MIGS is quite low for ZnS, being much higher for Si, GaAs and ZnSe. Moreover, the penetration of these states into the semicon˚ for Si and 1.9 A˚ for ZnSe to ductor falls from 3.0 A ˚ for ZnS. The authors used these results to only 0.9 A calculate the Schottky barrier heights. They also used a simple model to relate the MIGS surface density of states and the penetration depth to the dipole potential at the surface. For ZnS they find an S parameter of S ; 0.7, quite close to our data from photoemission measurements above. While the calculations of Louie et al. w19x are for AjelliumB metal, they lend themselves to a comparison with our data of the noble metals, with their purely s–p density of states at E F . We thus conclude that our photoemission data from metal deposition onto clean, well-defined ZnSŽ110. surfaces performed under UHV conditions strongly support the notion that the low density of MIGS in ZnS lead to a strong dependence of Schottky barrier heights on metal work function, in agreement with the general concepts based on electronegativity considerations, and the theoretical description of the metal–ZnS bond.
3. Semiconductor heterojunctions
3.1. Test of heterojunction band offset transitiÕity In this section, we first analyze heterojunction band offset measurements between III–V and II–VI compound semiconductors, not as a distinctive class of systems, but in order to discuss a specific aspect of heterojunction band offset properties, i.e. band offset commutativity and transitivity w5x. Briefly, this addresses the question whether band offsets are transferable, i.e. whether a band offset between materials A and B, and the offset involving materials B and C, i.e. D Ev Ž B y C . is equal to an offset at the interface between materials A and C, D Ev Ž A y C . Žtransitivity.; a related question relates to the valence band offset D Ev at an interface between material A and material B being independent of growth se-
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quence Žcommutativity.. Consider the example presented in the left part of Fig. 3, where the measured band offset of a CdSerGaSb heterojunction was determined using photoemission w27x. The band offset is determined from the change of the onset of photoemission at the valence band edge, where the change in band bending that may occur upon interface formation is accounted for by recording the shift in energy of a substrate core level; this procedure is explained in many papers Žsee Ref. w28x, for example.. Aside from these, we noticed that the commonly used practice of using the linear extrapolation of emission intensity to determine the location of the VBM has recently been questioned. Eich et al. w10x and Kreis et al. w11x have argued that a proper VBM determination from angle-resolved photoemission should be performed, such that the highest dispersing peak is assigned to the topmost valence band. This method is based on the assumption that the topmost band at the G point gives rise to a clearly discernible peak, and that the shape of the valence band emission in normal emission is not affected by density of states considerations. It has been shown that the width of the leading peak is affected by surface
perfection, such that a rough overlayer may give rise to erroneous results. While the work of Eich et al. w10x and Kreis et al. w11x touches on a valid point, the error is probably not large if the VBM of both materials is determined using the same procedure, as in the case of CdSe on GaSbŽ110.. The band offset between CdSe and GaSb, in a Ž110. interface orientation, is determined to be 1.09 eV w27x from these data, a value that is close to that determined from a scheme based on charge neutrality considerations in the dielectric midgap energy scheme Ž1.27 eV w29x.. The existence of other band offset determinations within the group of III–VrII–VI interfaces then permits an analysis of transitivity in heterojunction band offsets. Consider a set of heterojunctions between the semiconductors GaSbrZnTe w7 x, ZnTerCdSe w30x, and CdSerGaSb w27x. For each of these, the band offset was measured by photoemission, and the respective values are plotted together on the right-hand side of Fig. 3. If the heterojunction band offset were a bulk property, a round trip from one side of a hypothetical device that encompasses all of these interfaces to the other side, in the above sequence would then lead to a net offset of zero w31x.
Fig. 3. Left: Spectral region near the valence band maximum for clean GaSbŽ110., and after deposition of a thick layer of cubic CdSe, together with the Fermi level reference from a metal plate in ohmic contact with the sample Žfrom Ref. w27x.. Right: Comparison of band offsets determined for GaSbrZnTe w7x, ZnTerCdSe w30x and the data for CdSerGaSb w27x, showing that the deviation form transitivity is about 0.11 eV, i.e. within the experimental accuracy.
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The situation is somewhat more complicated than one might think, since the ZnTerCdSe junction is not lattice matched Žin contrast to the other two., and its offset therefore could be affected by strain effects, which need to be accounted for in theoretical predictions of the band offset. As indicated by the dashed line, the sum of all valence band offsets is 0.11 eV, i.e. within the error margin of the photoemission experiment. This is somewhat surprising since in all of the heterojunctions involved, an interface reaction occurs, which might affect the magnitude of D Ev . It appears unlikely that the small value for the total band offset is fortuitous, since the absolute magnitude of the offsets involved is quite large. We thus conclude that even for those interfaces where an interface reaction occurs, the band offsets do not seem to be strongly modified from the value that they would assume from the bulk properties of the semiconductors involved. This conclusion is drawn here for Ž110.-oriented, i.e. nonpolar interfaces, where dipole effects due to charge exchange across the interface Žsee below. are expected to be small in any case. 3.2. Intentional modification of band offsets by intralayers The magnitude of the band offset has important implications for the use of heterojunctions, affecting carrier injection, carrier confinement, and ionization thresholds in semiconductor devices. Thus, it would be most desirable to control the magnitude of the offset, a process that has been termed heterojunction band offset engineering w6x. Briefly, the idea is to introduce a local dipole layer at the interface, which will act to change the potential step between the two semiconductors; this influence can be modeled by a parallel plate capacitor. This is different from the doping interface dipole, where two thin sheets of charge are introduced on either side of the interface. In fact, large changes in the apparent valence band offset of GaAsrAlAs upon introducing a silicon intralayer have been reported in photoemission experiments w6,32x. These have been interpreted in terms of a variation of the band offset through the introduction of an interfacial dipole layer w33,34x, an interpretation that appears plausible since the magnitude of band offset change showed a variation with
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Si concentration. However, other authors w35x have argued that photoemission, as a surface probe, is sensitive to band bending and Fermi level pinning at the surface, which might mimic a band offset in the data even if only the band bending exhibits change. Since the band offset is measured from the energy of the VBM andror a shallow core level at the surface of the oÕerlayer, a band bending in the overlayer can well be interpreted as an indication of a band offset change. The interface dipole model makes several predictions on the band offset change, which are amenable to experimental verification. First, since the silicon atoms are supposed to assume the role of the anion or cation depending on which site they are located on it suggests that group IV atoms should act as dipoles in polar interfaces, but not in nonpolar ones, since the charge of neighboring intralayer atoms would cancel in the latter. This is schematically indicated in Fig. 4a for interfaces oriented along the Ž100. and Ž110. directions. It is obvious that the silicon atoms which take up the position in a Ž100.oriented interface act as a dipole sheet, while the charges arranged in a Ž110. interface compensate, giving no net effect on the band offset. This is also true for a homojunction ŽGaAsrGaAs. where a Aband offsetB might be introduced by a dipole layer, but only in a polar growth direction. Secondly, intralayers from other than group IV elements should exhibit a weaker influence, since they cannot act as AamphotericB substitutional atoms when replacing anion or cation substrate or overlayer atoms. Moreno et al. w36x have carried out several experiments to tests whether the above predictions agree with experimental observations. They examined the influence of Si intralayers on the band offset in GaAsrAlAs junctions. The samples were grown in a dedicated molecular beam chamber, and were transferred to the photoemission chamber in a small transport ultrahigh vacuum chamber. Their sample geometry and layer arrangement is shown in Fig. 4b. Moreno et al. measured not only the As 3d, Ga 3d and Al 2p core levels, but also the energy of the VBM from the emission near the VBM, and the location of the Fermi level measured from a gold foil in ohmic contact with the samples. In the case of a GaAsrAlAs junction with Si intralayer large apparent band offsets occur as can be
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Fig. 4. Ža. Schematic diagram of the layer arrangement of samples in the experiments of Moreno et al., showing the location of the Si Žand Be. intralayer. The lower diagram shows the action of the ionized donors and the electrons trapped in surface states to create an electric field across the overlayer. Žb. Schematic diagram indicating the action of group IV atoms at a polar interface to create an interface dipole. Žc. The effect of silicon intralayers in various GaAsrAlAs interfaces on the apparent valence band offset measured by photoemission, from several groups as indicated. ŽFrom Moreno et al. w36x..
seen from the compilation of results from several authors in Fig. 4c. The apparent band offset also depends on the amount of intralayer atoms inserted, which finds an easy interpretation in the interface dipole model. It is of opposite sign for the two
growth directions, and even larger than in previous experiments, probably due to an improved incorporation of the Si layer. However, what is also obvious is that for a nonpolar junction Ži.e., the Ž110. direction. apparent band offsets of similar magnitude occur.
K. Horn r Applied Surface Science 166 (2000) 1–11
This cannot be understood in terms of the model shown in Fig. 4a, since the neighboring dipoles, in an ordered array, would cancel, resulting in zero net change of band offset. Moreno et al. have provided a different interpretation of the observed effects in terms of a band bending in the overlayer, in line with an earlier report by Hashimoto et al. w35x. This starts from the assumption that the silicon atoms at the interface are ionized, and that the electrons, which they donate, fill the surface states; the charge on the positive ionized Si atoms and the negative charge in the surface states then leads to a strong electric field that builds up across the GaAs overlayer, which terminates the heterostructure. This model of charge sheets is shown in the lower part of Fig. 4b. This interpretation was put to the test in another experiment by Moreno et al., where the action of silicon and beryllium intralayers was compared. The logic behind this choice of intralayer atoms lies in the well-known action of silicon as an n-type and beryllium as a p-type dopant. The interface dipole model makes no obvious predictions about the action of Be atoms in an intralayer; however, they are unlikely to assume a role similar to that of silicon which does exhibit amphoteric behavior in GaAs. Thus, one would not expect the insertion of a Be layer in a GaAsrAlAs interface to cause a significant change in the band offset. The photoemission experiments by Moreno et al. w37x instead showed a marked effect of the Be intralayer, reducing the apparent valence band offset by almost 0.2 eV. This is shown in Fig. 5 where the Al 2p and Ga 3d spectra from a GaAsrAlAs junction with an Si and Be intralayer are compared, referenced to the VBM. The separation between these core levels is often used to determine the magnitude of the valence band photoemission, based on the known binding energy difference between these levels and the VBM in the respective compounds. Thus, the data on the left-hand side of Fig. 5 might be taken to provide a graphic representation of an increase in D Ev upon Si insertion, and a decrease upon Be insertion as stated above. However, the situation is more complex as shown on the right-hand side of Fig. 5. Here, the VBMs for the two different intralayers have been aligned, and the emission from a metallic film in good electrical contact with the samples is compared. It is obvious that the surface Fermi level differs by about 400
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meV between the two samples. This observation cannot be explained in the interface dipole model, but in fact provides strong support for the doping role of the intralayer atoms which form the basis of the interpretation of the photoemission results by Moreno et al. w37x. The Bohr radius of charge carriers liberated from the intralayer atoms in a hydrogen ˚ The GaAs overlayer model amounts to about 100 A. in these heterojunctions must not be too thick to totally attenuate the signal from the interface, i.e. no ˚ depending on photoelectron more than about 50 A, ˚ in the experiments of kinetic energy Žit was 20 A Moreno et al... Thus, the electrons Žfor Si intralayers. or holes Žfor Be. can readily be trapped in the surface states of the top GaAsŽ100. layer. The two sheets of localized charges, ionized atoms in the intralayer and charges in the surface states, build up an electric field across the overlayer, as shown in the schematic diagram in the lower portion of Fig. 4a. The important point to remember is that photoemission, as a surface sensitive technique, derives most of its signal from the immediate surface region, according to an exponential attenuation law for the photoemitted electrons. Thus, if the assumption of flat bands in the overlayer, implicit in almost all photoemission determinations of band offsets, is not fulfilled, the signal from the buried material ŽAlAs. will be derived from the immediate interface, but that from the GaAs side of the interface will have its strongest contributions from the surface. This differs from that at the interface by an amount equal to the magnitude of band bending Žthe overlayer peak will be additionally broadened by an addition of contributions of the overlayer core level with different band bending energies, according to the band bending profile.. The data for Si intralayers inŽ100.-oriented interfaces, and the effect of Si and Be intralayers w37x provide convincing evidence against an interpretation of core level photoemission results from such interfaces in terms of a real band offset modification. Moreno et al. were able to model the potential variation across the overlayer by means of a solution to the one-dimensional Poisson equation, and arrived at an excellent agreement with their observations, without having to introduce variations in the magnitude of D Ev compared to that in GaAsrAlAs. These data do not provide evidence against the interface
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K. Horn r Applied Surface Science 166 (2000) 1–11
Fig. 5. Top: Spectra from a GaAsrAlAs heterojunction with an Si and Be intralayer, respectively, showing the opposite shift of the Al 2p line in these cases. Bottom: Shift of the reference Fermi level if the spectra are aligned at the valence band maximum, demonstrating the difference in band bending in the overlayers upon Si and Be insertion. ŽFrom Moreno et al. w37x..
dipole model as such, but against the support from photoemission experiments which was thought to demonstrate its validity.
Photoemission has made many important contributions to our understanding of the electronic structure of semiconductor interfaces, and, as amply
K. Horn r Applied Surface Science 166 (2000) 1–11
demonstrated by many papers at this conference, and some of the aspects discussed in this paper, continues to be of great value in elucidating barrier height and band bending phenomena. Applications of this technique necessitate taking into account the importance of the surface aspect of this technique, and the possibility of non-equilibrium effects for a proper interpretation of the data.
Acknowledgements This work was supported by the Bundesministerium fur ¨ Bildung, Forschung und Technologie under grant 05 622 OLA 3. I gratefully acknowledge the long collaboration and discussions with M. Alonso, S.R. Barman, Th. Chasse, ´ D.A. Evans, K.O. Magnusson, M. Moreno, and G. Neuhold.
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