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compound semiconductor interfaces: valence band studies
Vacuum/volume 43/numbers Printed in Great Britain
5-7fpages
587 to 590/l
992
0042-207x/92$5.00+.00 Pergamon Press Ltd
Synchrotron radiation photoemission studies the palladium/compound semiconductor interfaces : valence band studies Nan Li and Zhangda Beijing
100080,
Lin, laboratory
for Surface Physics, Institute
of Physics, Chinese Academy
on
of Sciences,
P R China
and
Yongli
Gao and J H Weaver,
Minnesota,
Minneapolis,
Department MN 55455, USA
of Chemical
Engineering
and Material
Science,
University
of
A synchrotron radiation (SR) photoemission study on the valence band of the PdlGaAs(1 IO), PdflnP(7 lo), PdllnSb (110) and Pdf PbS( 100) systems has been presented. It was observed in the study that at Pd coverages ranging from 0.2 to 40 A, all of the valence bands of the interface systems are composed of a main structure characterizing the Pd 4d band, and a shoulder near the Fermi level representing the antibonding states coming from the interaction of Pd with the substrates. With increasing Pd coverages, the main peak in the valence bands developed gradually into the bulk Pd 4d band, and the shoulder appeared to merge into the main band. Two interface processes, namely surface segregation and chemical interaction, have been investigated for the interface systems with the help of peak decomposition and curve fitting for the Pd 4d band, which revealed the evolution of the band shape parameters at various coverages. It is found that for the semiconductor substrate with more reactive anions, chemical interaction will dominate the interface processes at the first stage of interface formation.
1. Introduction The formation and properties of metal/semiconductor (M/S) interfaces have been studied a great deal in past decades for their scientific and technological importance. Abundant experimental results have been reported on many M/S interface systems as well as by extensive theoretical investigations lm3.But until today, the mechanisms for the interface formation and several unique properties of the M/S interfaces, e.g. Schottky barrier formation, Fermi level pinning, and the role of the interface chemical reactions, etc., have not been thoroughly understood. Therefore, a wide range of works aimed at further understanding of the M/S interfaces are still in progress. The palladium/semiconductor interfaces, e.g. Pd/Si, Pd/GaAs, and Pd/InP, have attracted considerable attention and have been widely studied for their specific electronic properties and validity in device technologies” ‘. The reported theoretical and experimental results have revealed that Pd is a mild reactive metal. It can form several compounds with the substrate elements in the interfaces. Many interface processes, such as Pd depositioninduced surface disruption, substrate cations release into and intermixing with the growing Pd overlayers, and anion segregations etc., have been observed in the Pd/semiconductor systems. In this paper, we present a SR photoemission study on the valence bands of the Pd/GaAs( 1 lo), Pd/InP( 1 lo), Pd/InSb(l 10) and Pd/PbS(lOO) systems with sequential Pd depositions. By extending the peak decomposition and curve fitting method to the Pd 4d band, evolution of the valence band with respect to increasing Pd coverages is investigated. With this study, some
insight into the physical processes has been gained.
in the M/S interface
formation
2. Experimental The experiments were performed on four semiconductor substrates : GaAs (Si doped at 5 x 1O-. I8 cm ‘), InP (Sn doped at 4x lo-” cmm3), InSb (Te doped at 4x IO-l5 cmm3) and PbS crystal. The former three samples were oriented within 0.1 u along the [1 lo] direction while the PbS was oriented along the [loo] direction. Clean surfaces were obtained by cleaving the samples in an uhv chamber at a pressure of about 5 x 1O‘- ’ ’ torr. Metal Pd was evaporated from a resistively heated W boat and the evaporation rates, monitored by quartz crystal microbalances, were maintained stable at about 1 8, min- ‘. The SR photoemission study was performed with radiation from the Grasshopper Mark II monochromator and beam line at the Aladdin electron storage ring at the Wisconsin Synchrotron Radiation Center. The valence band spectra of the M/S interface systems were obtained with a double pass cylindrical mirror analyser in a vacuum system described in detail elsewherea. Photon energies were chosen to improve surface sensitivity, so that the photoelectron mean free path i was kept at - 3.5 8, and the probing depth was - 10 A for the sampled surfaces. The valence band energy distribution curves (EDCs) of the Pd/semiconductor systems were produced with a compatible IBM computer. Difference curves were taken for each of the spectra by subtracting the substrate contribution from the spectra to emphasize the changes due to the interface formation. Then, peak decomposition and curve fitting were performed for the 587
Nan Li et al: Valence band studies
Pd valence band EDCs. The Voigt function, a convolution of Gaussian and Lorentzian functions, was used to fit the Pd 4d peaks, and a one-side power-law function was taken to account for the asymmetric line shapes of the Pd 4d peaks due to the screening of free electrons at the Fermi level to the photoelectrons. With suitably chosen parameters, the best fits were obtained for the Pd 4d bands at each stage of Pd deposition. 3. Results and discussion 3.1. Basic structure of the valence hand EDCs. The SR photoemission spectroscopies were measured on all four interfaces, Pd/GaAs(l IO), Pd/InP(l lo), Pd/InSb(llO) and Pd/PbS(lOO) systems. A representative result from the Pd/GaAs( 110) system is given in Figure 1, where each curve in the figure was taken after subtracting the substrate contributions. The results obtained from the other interfaces are very similar. It can be seen in Figure 1 that at Pd coverages from -0.3 to - 40 A, the valence band EDCs is composed, as the basic feature of the systems, of two peaks, P, and Pz, and a shoulder, A, located between the main peak structure and the Fermi level. The main structure, P, and P1, can usually be attributed to the contribution of the Pd 4d band, as the Pd 4d orbital has a relatively high photoionization cross-section (Pd 04,, is about one order of magnitude larger than G,,~of s and p orbitals for Pd or other elements’), and that the two peaks finally developed into the bulk Pd 4d band at the highest Pd coverages. The shoulder A located near the Fermi level, on the other hand, can be attributed to the interaction ofPd with the substrate.
Rubloff et al“ observed a similar shoulder near the Fermi level in the valence band of Pd/Si. They associated it with the contribution of the antibonding u orbital formed with the interaction of Pd 4d orbital and Si 2p orbital. Kendelewitz et a/‘” have also given a similar result in the valence band of Pd/InP. Russo et al’ ’ further demonstrated that Pd can combine with most elements in the IV A group forming Pd compounds. Their calculation showed that the highest occupied valence orbital in all of these compounds was the 12 a2 antibonding orbital, which was formed with about 0.20.4 4d orbital and 0.7-I .O 5s orbital from Pd, as well as 0.74.9 p orbital from the IV A elements. With the similarity of the covalent bond character between the III and V element and the IV A element, we suppose that Pd can also combine with the III-V compounds with a small part of Pd 4d electron intermixed with the p-electrons of IIILV compounds. forming a similar antibonding CJorbital which will contribute to the shoulder A near the Fermi level. With increasing coverages, the shoulder overlapped gradually with the main structure of P, and P, which shifted towards the Fermi edge. and was finally covered by it, indicating that the interaction layer was gradually buried by the growing Pd overlayer and finally covered by the overlayer below the probing depth of the photoemission measurement. 3.2. Evolution of the Pd 4d peaks. The results of the peak decomposition and curve fitting for the Pd 4d band of the four systems are given in Figures 24. The evolution of the peak parameters for the Pd 4d band, i.e. the peak positions, the full widths at half maximum (FWHM) and the peak splittings, of P, and Pz with increasing coverages are illustrated in the figures. It can be seen in the figures that at low coverages, the FWHM of the peaks become narrower compared with that of bulk Pd metal, the peak centre shifts about IL2 eV towards high binding energy, and the peak splittings become smaller than that of bulk Pd metal by about 0.64.7 eV. With increasing coverages, the centres of the peaks shift towards lower binding energy, the FWHM and the peak splittings are increased again ; up to 0 - 50 A (0 - 80 8, for Pd/PbS), all the peak shape parameters become close to those of the bulk Pd 4d band.
_..(-
_c*-
t
s -3.2 .8 a 3 2
..__. _______._-----0 q ---A -.---
-4.
v
cl em surf ace ’ -8
-i
-6
-‘i
Binding
\ -4
-3
-2
L!
I
Figure 1. Synchrotron radiation photoemission spectroscopies at various Pd coverages: (a) Pd/GaAs(l IO), hv = 60 eV; (b) Pd/InP(I lo), hv = 55 eV; (c) Pd/InSb(llO), hv = 56 eV; (d) Pd/PbS(lOO), hv = 55 eV. 588
-..-..
FM/G& Pd/[$’ WI,&, PdIPbS
-4.8 0
10
20
-1 Ef-0
Energy (eV)
/... _
30
a
Pd coverage
50
60
70
1 @iI
( 11
shifts at various Pd coverages : (O---) Pd/GaAs(llO); (a---) Pd/InP(llO); (a-.) Pd/lnSb(llO) and (77 .. -,,) Pd/PbS(lOO). The shifts of P, and P, are represented by the low binding energy curve and the high binding energy curye, respectively.
Figure 2. Pd 4d peak position
Nan Li et al: Valence band studies 2.7,
I
o
-
1
d
Pd/GaAa
I
I
of the observed Pd 4d peak splittings can be considered to be very small, and then, the observed peak splittings can be attributed mainly to the Pd-Pd nearest neighbour overlap interaction. Shevchik ’ 3proposed a simpler molecular orbital model that gave the energy splitting of two originally degenerate orbitals due to the neighbour overlap interaction as -JLb = -(fSV)/(l
1.2f 0
10
20
30
40
Pd coverage
50
60
70
I 80
( 1)
Figure 3. Pd 4d peak FWHM changes at various Pd coverages : (0-j Pd/GaAs(llO); (o---) Pd/InP(llO); (&-.-) Pd/InSb(llO) and (V-..-..) Pd/PbS(lOO). The upper and the lower curves represent the changes of P, and P,, respectively.
The origin of the band narrowing and peak shifting can be usually attributed to the initial and final state effects in photoemission. At low coverages, the initial state effects, which may include the influence of atomic rearrangement and interactions on the initial electronic states of the systems, just appear as d band localization and related energy shifts. The final state effects, on the other hand, may affect the peak positions and widths through the Coulomb interaction of free electrons and the final state holes as well as the lifetime broadening. For the valence band spectra of metals, the final state effects can be more significant. The peak splitting of the Pd 4d band can be attributed mainly to the 4d spin-orbital splitting and the Pd-Pd nearest neighbour overlap interaction. But it turns out that the spin-orbital splitting in the Pd 4d band is only -0.44 eV”, that is even smaller than the measured value of N 1.0 eV at the lowest coverage of -0.2 A. Furthermore, as the spin-orbital splitting is usually dominated by the inner atomic potential and therefore can hardly be affected by the atomic rearrangement or changes in chemical environment due to the growing Pd overlayers, its contribution to the evolution
I 0
10
20
30 40 Pd coverage
50
60
70
80
(i 1
Figure 4. Pd 4d peak splitting changes at various Pd coverages : (O-) Pd/GaAs(llO) ; (O----) Pd/InP(llO) ..) Pd/PbS( 100).
; (A-.-.-)
Pd/InSb(llO)
and (O-
-IS)
a-(-),
b_(+>
where a and b represent the antibonding and bonding orbitals, respectively, Vis an interaction potential and S is the total charge density d-d overlap integral. This relation indicates that larger energy splitting will be formed as two neighbour atoms are brought closer to each other, where their interaction and charge density overlap are strong. Using this model, Riley et ~1’~ explained the transition from Ag 4d spin-orbital splitting (- 0.6 eV) to metal Ag 4d band splitting (- 2.2 eV) with increasing Ag atomic concentration in Ag-Cd and Ag-In alloy systems. Thus, the results shown in Figures 24 reflect that with increasing surface Pd concentrations, both the initial and the final state effects, as well as the Pd-Pd neighbour overlap interaction, tend to be intensified in the interface systems. This leads to the evolution and transition of the Pd 4d peak parameters to the values of the bulk Pd metal. 3.3. Interface processes. A further analysis of the evolution of the Pd 4d peak shape parameters indicates that two important interface processes, namely surface segregation of the substrate elements and chemical interaction between Pd and the substrates, are included in the Pd/compound semiconductor systems studied here. For each interface system, the contributions of the two processes are different. This can be explored by analysing the evolutions of the Pd 4d peak shape parameters. For the Pd/GaAs system, the changes of the Pd 4d peak shape parameters at lower coverages, compared with those at the highest coverage, are smaller than that for the other three systems, as shown in Figures 24. This indicates that for each Pd coverage, the initial and the final state effects as well as the Pd-Pd overlap interaction in this system are stronger than that in the other three systems. This implies further that for Pd/GaAs, the surface Pd concentration is relatively high, and the two interface processes mentioned above, which may dilute the surface Pd concentration, have little effect. The successive and monotonous variations of the Pd 4d peak shape parameters with increasing coverages, seen from the figures, can also imply the minor possibility of forming a peculiar Pd atom arrangement due to Pd and substrate interaction. By the relative atomic size and the surface energies3 of the interface systems, the consideration that surface segregations of As and Ga atoms are very weak compared with those of Sb and In etc., can be supported, too. The variation trends of the Pd 4d peak shape parameters for Pd/InSb are similar to those for Pd/GaAs, except that the peak positions and the FWHM at each coverage move towards the d band localization direction further, and the peak splitting is smaller than that in Pd/GaAs, at least in the coverage range 0 < 35 A. These results indicate decrease of the Pd atomic concentration and weakening of the Pd-Pd neighbour overlap interaction on the surface of the system can be associated with the surface segregation of Sb and In atoms. Considering the relative atomic size and surface energies of Sb and In”, the strong tendency of their surface segregation can be supported. Meanwhile, the results that the Pd 4d peak splitting for Pd/InSb increases linearly with increasing Pd coverages after an initial rapid ascend589
Nan Li et al: Valence
band studies
ing (0 > 2-3 A), as shown in Figure 4, also imply the smooth variation of the surface Pd concentrations and no peculiar configurations of Pd-In or Pd-Sb compounds being formed. This can be realized from the relatively low reactivity of Sb in InSb compared with that of As in GaAs. The case for the Pd/InP system is different from and more complicated than that in the above-mentioned systems, as shown in Figures 224. At the lowest coverage range (0 - 0.2-2.0 A), the Pd 4d peak FWHM and splitting increase rapidly with coverage. After that, a plateau appears in both the variation curves in the range of 2.0-12 A. Beyond this range, they show an ascending trend again. The appearance of the plateau implies that the PdPd neighbour overlap interaction and the related surface Pd concentrations may be kept constant with increasing covet-ages. This can be attributed to a chemical interaction limited process, as the stoichiometry of a particular chemical compound must be constant. Considering the stronger chemical reactivity of phosphorus in the substrate, the chemical interaction limited process can be supported. Then, the rapid rising of the Pd 4d peak FWHM and splitting in the lowest coverages can be considered as the quick transition from the initial stage of a very small amount Pd deposition to the formation of the above-mentioned compound state, while their rerising after the plateau (0 > l215 A) indicates a transition of the surface Pd concentration from a chemical interaction limited process to an atomic diffusion limited process. The chemical interaction between Pd and the substrate anions, P, may lead to a decrease in the surface Pd concentrations. but the decrease may not be so serious as that caused by the surface segregation of Sb and In in Pd/InSb. Hence, an order such as GaAs < InP < InSb for the changes of the Pd 4d peak shape parameters from the values of bulk Pd metal can be observed. For Pd/PbS, the evolution of the Pd 4d peak shape parameters is very similar to that for Pd/InP, except that the evolution proceeds more slowly, and the parameter values are smaller than those for Pd/InP, indicating a much lower surface Pd concentration and a weakened Pd-Pd neighbour overlap interaction. Considering the similarity between Pd/PBs and Pd/InP in their evolution of the Pd 4d peak shape parameters, as well as the differences between these two systems and Pd/GaAs and Pd/InSb, a chemical interaction limited process between Pd and the PbS substrate can also be inferred. This further demonstrates that chemical interaction limited interface process takes place in the Pd/semiconductor systems with the more reactive substrate anions, S and P. For the small value and the slow development of the Pd 4d peak shape parameters in Pd/PbS, we can attribute them to the large difference between the atomic size of Pb and Pd, which may lead to strong surface segregation of Pb and cause a decrease in the surface Pd concentration. 4. Conclusions The valence band spectra of the Pd/compound semiconductor systems are studied with SR photoemission spectroscopy. Peak decomposition and curve fitting are performed for the Pd 4d band, and the evolution of the peak parameters for the interfaces is illustrated. Detailed analyses on these results led to the following conclusive remarks. (1) Pd can interact with all the four semiconductor substrates in some degree; wherein a small part of the Pd 4d electrons combines with the substrate p electrons forming an antibonding 0 orbital, which appears as a shoulder near the Fermi level in the 590
valence band; the main structure of the valence band EDCs is associated with the Pd 4d band ; with increasing coverage, the contribution of the surface Pd to the photoemission is increased. while the shoulder which reflects the Pd-substrate interactions is gradually covered by the bulk Pd 4d band. (2) The Pd 4d peak shape parameters, including the peak positions, the FWHM and the peak splittings, as well as their evolution are mainly determined by the initial and the final state effects in the photoemission process and Pd-Pd neighbour overlap interaction, and these factors can be associated with the surface Pd atom distributions. (3) Two interface processes. namely the surface segregation and chemical interaction of Pd with the substrate, are included in the interfaces studied here ; the interface processes, as well as their different contributions to each interface system, can be demonstrated by analysing the evolution of the Pd 4d peak shape parameters. (4) According to the different chemical reactivity of the anions in the substrates studied here, the interface systems can be divided into two categories : one is Pd/GaAs and Pd/InSb, in which the surface Pd concentrations vary in a monotonous and successive way with increasing Pd coverage, indicating a surface scgrcgation and interdiffusion limited interface process ; the other is Pd/InP and Pd/PbS. in which a plateau was observed after an initial ascending in the evolution of the Pd 4d peak shape parameters, indicating that a chemical interaction process is present in the interface systems. (5) For the very localized states in the transition metals even near the Fermi level, the peak decomposition and curve fitting technique can be available for the valence band spectra analysis. Acknowledgements This work was supported by the US Office of Naval Research under Contracts Nos ONR-N00014-87-K-0029 and ONRNOOOO14-86-K-0427, and by the Chinese National Science Foundation. The synchrotron radiation experiment was done at the Wisconsin Synchrotron Radiation Center, and the support by its staff is gratefully acknowledged. References ’ See, e.g.. L J Brillson,
Su$xe
Sci Rc>p,2, 123 (1983).
2R H Williams, In f'h~aics and Chemistry of’ 111~1”Cowipowzc/ SwniconductorIntwfuces (Edited by C W Wilmsen). Plenum, New York (1985). ‘Zhangda Lin, F Xu and J H Weaver, Phys Rcc, B36,5777 (1987). ‘G W Rubloff: P S Ho, J F Freeouf and J E Lewis. P/IL..FRec.. B23.4183 (1981). ‘T Kendelewitz, W G Petro, 1 Lindau and W E Spicer. Phj,.v Rev. B28, 3618 (1983). ‘R Ludeke and G Landgren, Ph_r.v Rer, B33, 5526 (1986). ‘T Sands, V G Keramidas. R Gronsky and J Washburn, 7’/& Solicl Fi/nl.v, 136, 105 (1986). ‘C M Aldao. I M Vitomirov. F Xu and J H Weaver, Phvs Rw. B37, 6019 (1988). ‘J C Fuggle, F U Hillebrecht, R Zeller. Z Zolmierek and P A Bennett, Phr.v Rcr, B27, 2 I45 (1982). ‘“T Kendelewitz, R S List, K A Bertness, M D Williams. I Lindau and W E Spicer, J Vuc Sci Tcchnol, B4,959 ( 1986). ’ ’ N Russo, J Andzelm and D R Salahub, Clzem PhJ>s, 114, 33 I (I 987). “C E Moore. Atomic Energy Levels. US Natn Bur Stand Circ, No 467, US GPO, Washington, DCyi958). ’ ’ N J Shevchik, J Phys, FS, 1860 (1975). I4 J D Riley, R C G Leckey, J G Jenkin, J Liesegang and R T Poole J Phys, F6, 293 (1976). “F Boscherini, Yoram Shapira, C Capasso, C M Aldao and J H Weaver, J Vcrc Sci Tcchnol, B5, 1003 ( 1987).
5-7fpages
587 to 590/l
992
0042-207x/92$5.00+.00 Pergamon Press Ltd
Synchrotron radiation photoemission studies the palladium/compound semiconductor interfaces : valence band studies Nan Li and Zhangda Beijing
100080,
Lin, laboratory
for Surface Physics, Institute
of Physics, Chinese Academy
on
of Sciences,
P R China
and
Yongli
Gao and J H Weaver,
Minnesota,
Minneapolis,
Department MN 55455, USA
of Chemical
Engineering
and Material
Science,
University
of
A synchrotron radiation (SR) photoemission study on the valence band of the PdlGaAs(1 IO), PdflnP(7 lo), PdllnSb (110) and Pdf PbS( 100) systems has been presented. It was observed in the study that at Pd coverages ranging from 0.2 to 40 A, all of the valence bands of the interface systems are composed of a main structure characterizing the Pd 4d band, and a shoulder near the Fermi level representing the antibonding states coming from the interaction of Pd with the substrates. With increasing Pd coverages, the main peak in the valence bands developed gradually into the bulk Pd 4d band, and the shoulder appeared to merge into the main band. Two interface processes, namely surface segregation and chemical interaction, have been investigated for the interface systems with the help of peak decomposition and curve fitting for the Pd 4d band, which revealed the evolution of the band shape parameters at various coverages. It is found that for the semiconductor substrate with more reactive anions, chemical interaction will dominate the interface processes at the first stage of interface formation.
1. Introduction The formation and properties of metal/semiconductor (M/S) interfaces have been studied a great deal in past decades for their scientific and technological importance. Abundant experimental results have been reported on many M/S interface systems as well as by extensive theoretical investigations lm3.But until today, the mechanisms for the interface formation and several unique properties of the M/S interfaces, e.g. Schottky barrier formation, Fermi level pinning, and the role of the interface chemical reactions, etc., have not been thoroughly understood. Therefore, a wide range of works aimed at further understanding of the M/S interfaces are still in progress. The palladium/semiconductor interfaces, e.g. Pd/Si, Pd/GaAs, and Pd/InP, have attracted considerable attention and have been widely studied for their specific electronic properties and validity in device technologies” ‘. The reported theoretical and experimental results have revealed that Pd is a mild reactive metal. It can form several compounds with the substrate elements in the interfaces. Many interface processes, such as Pd depositioninduced surface disruption, substrate cations release into and intermixing with the growing Pd overlayers, and anion segregations etc., have been observed in the Pd/semiconductor systems. In this paper, we present a SR photoemission study on the valence bands of the Pd/GaAs( 1 lo), Pd/InP( 1 lo), Pd/InSb(l 10) and Pd/PbS(lOO) systems with sequential Pd depositions. By extending the peak decomposition and curve fitting method to the Pd 4d band, evolution of the valence band with respect to increasing Pd coverages is investigated. With this study, some
insight into the physical processes has been gained.
in the M/S interface
formation
2. Experimental The experiments were performed on four semiconductor substrates : GaAs (Si doped at 5 x 1O-. I8 cm ‘), InP (Sn doped at 4x lo-” cmm3), InSb (Te doped at 4x IO-l5 cmm3) and PbS crystal. The former three samples were oriented within 0.1 u along the [1 lo] direction while the PbS was oriented along the [loo] direction. Clean surfaces were obtained by cleaving the samples in an uhv chamber at a pressure of about 5 x 1O‘- ’ ’ torr. Metal Pd was evaporated from a resistively heated W boat and the evaporation rates, monitored by quartz crystal microbalances, were maintained stable at about 1 8, min- ‘. The SR photoemission study was performed with radiation from the Grasshopper Mark II monochromator and beam line at the Aladdin electron storage ring at the Wisconsin Synchrotron Radiation Center. The valence band spectra of the M/S interface systems were obtained with a double pass cylindrical mirror analyser in a vacuum system described in detail elsewherea. Photon energies were chosen to improve surface sensitivity, so that the photoelectron mean free path i was kept at - 3.5 8, and the probing depth was - 10 A for the sampled surfaces. The valence band energy distribution curves (EDCs) of the Pd/semiconductor systems were produced with a compatible IBM computer. Difference curves were taken for each of the spectra by subtracting the substrate contribution from the spectra to emphasize the changes due to the interface formation. Then, peak decomposition and curve fitting were performed for the 587
Nan Li et al: Valence band studies
Pd valence band EDCs. The Voigt function, a convolution of Gaussian and Lorentzian functions, was used to fit the Pd 4d peaks, and a one-side power-law function was taken to account for the asymmetric line shapes of the Pd 4d peaks due to the screening of free electrons at the Fermi level to the photoelectrons. With suitably chosen parameters, the best fits were obtained for the Pd 4d bands at each stage of Pd deposition. 3. Results and discussion 3.1. Basic structure of the valence hand EDCs. The SR photoemission spectroscopies were measured on all four interfaces, Pd/GaAs(l IO), Pd/InP(l lo), Pd/InSb(llO) and Pd/PbS(lOO) systems. A representative result from the Pd/GaAs( 110) system is given in Figure 1, where each curve in the figure was taken after subtracting the substrate contributions. The results obtained from the other interfaces are very similar. It can be seen in Figure 1 that at Pd coverages from -0.3 to - 40 A, the valence band EDCs is composed, as the basic feature of the systems, of two peaks, P, and Pz, and a shoulder, A, located between the main peak structure and the Fermi level. The main structure, P, and P1, can usually be attributed to the contribution of the Pd 4d band, as the Pd 4d orbital has a relatively high photoionization cross-section (Pd 04,, is about one order of magnitude larger than G,,~of s and p orbitals for Pd or other elements’), and that the two peaks finally developed into the bulk Pd 4d band at the highest Pd coverages. The shoulder A located near the Fermi level, on the other hand, can be attributed to the interaction ofPd with the substrate.
Rubloff et al“ observed a similar shoulder near the Fermi level in the valence band of Pd/Si. They associated it with the contribution of the antibonding u orbital formed with the interaction of Pd 4d orbital and Si 2p orbital. Kendelewitz et a/‘” have also given a similar result in the valence band of Pd/InP. Russo et al’ ’ further demonstrated that Pd can combine with most elements in the IV A group forming Pd compounds. Their calculation showed that the highest occupied valence orbital in all of these compounds was the 12 a2 antibonding orbital, which was formed with about 0.20.4 4d orbital and 0.7-I .O 5s orbital from Pd, as well as 0.74.9 p orbital from the IV A elements. With the similarity of the covalent bond character between the III and V element and the IV A element, we suppose that Pd can also combine with the III-V compounds with a small part of Pd 4d electron intermixed with the p-electrons of IIILV compounds. forming a similar antibonding CJorbital which will contribute to the shoulder A near the Fermi level. With increasing coverages, the shoulder overlapped gradually with the main structure of P, and P, which shifted towards the Fermi edge. and was finally covered by it, indicating that the interaction layer was gradually buried by the growing Pd overlayer and finally covered by the overlayer below the probing depth of the photoemission measurement. 3.2. Evolution of the Pd 4d peaks. The results of the peak decomposition and curve fitting for the Pd 4d band of the four systems are given in Figures 24. The evolution of the peak parameters for the Pd 4d band, i.e. the peak positions, the full widths at half maximum (FWHM) and the peak splittings, of P, and Pz with increasing coverages are illustrated in the figures. It can be seen in the figures that at low coverages, the FWHM of the peaks become narrower compared with that of bulk Pd metal, the peak centre shifts about IL2 eV towards high binding energy, and the peak splittings become smaller than that of bulk Pd metal by about 0.64.7 eV. With increasing coverages, the centres of the peaks shift towards lower binding energy, the FWHM and the peak splittings are increased again ; up to 0 - 50 A (0 - 80 8, for Pd/PbS), all the peak shape parameters become close to those of the bulk Pd 4d band.
_..(-
_c*-
t
s -3.2 .8 a 3 2
..__. _______._-----0 q ---A -.---
-4.
v
cl em surf ace ’ -8
-i
-6
-‘i
Binding
\ -4
-3
-2
L!
I
Figure 1. Synchrotron radiation photoemission spectroscopies at various Pd coverages: (a) Pd/GaAs(l IO), hv = 60 eV; (b) Pd/InP(I lo), hv = 55 eV; (c) Pd/InSb(llO), hv = 56 eV; (d) Pd/PbS(lOO), hv = 55 eV. 588
-..-..
FM/G& Pd/[$’ WI,&, PdIPbS
-4.8 0
10
20
-1 Ef-0
Energy (eV)
/... _
30
a
Pd coverage
50
60
70
1 @iI
( 11
shifts at various Pd coverages : (O---) Pd/GaAs(llO); (a---) Pd/InP(llO); (a-.) Pd/lnSb(llO) and (77 .. -,,) Pd/PbS(lOO). The shifts of P, and P, are represented by the low binding energy curve and the high binding energy curye, respectively.
Figure 2. Pd 4d peak position
Nan Li et al: Valence band studies 2.7,
I
o
-
1
d
Pd/GaAa
I
I
of the observed Pd 4d peak splittings can be considered to be very small, and then, the observed peak splittings can be attributed mainly to the Pd-Pd nearest neighbour overlap interaction. Shevchik ’ 3proposed a simpler molecular orbital model that gave the energy splitting of two originally degenerate orbitals due to the neighbour overlap interaction as -JLb = -(fSV)/(l
1.2f 0
10
20
30
40
Pd coverage
50
60
70
I 80
( 1)
Figure 3. Pd 4d peak FWHM changes at various Pd coverages : (0-j Pd/GaAs(llO); (o---) Pd/InP(llO); (&-.-) Pd/InSb(llO) and (V-..-..) Pd/PbS(lOO). The upper and the lower curves represent the changes of P, and P,, respectively.
The origin of the band narrowing and peak shifting can be usually attributed to the initial and final state effects in photoemission. At low coverages, the initial state effects, which may include the influence of atomic rearrangement and interactions on the initial electronic states of the systems, just appear as d band localization and related energy shifts. The final state effects, on the other hand, may affect the peak positions and widths through the Coulomb interaction of free electrons and the final state holes as well as the lifetime broadening. For the valence band spectra of metals, the final state effects can be more significant. The peak splitting of the Pd 4d band can be attributed mainly to the 4d spin-orbital splitting and the Pd-Pd nearest neighbour overlap interaction. But it turns out that the spin-orbital splitting in the Pd 4d band is only -0.44 eV”, that is even smaller than the measured value of N 1.0 eV at the lowest coverage of -0.2 A. Furthermore, as the spin-orbital splitting is usually dominated by the inner atomic potential and therefore can hardly be affected by the atomic rearrangement or changes in chemical environment due to the growing Pd overlayers, its contribution to the evolution
I 0
10
20
30 40 Pd coverage
50
60
70
80
(i 1
Figure 4. Pd 4d peak splitting changes at various Pd coverages : (O-) Pd/GaAs(llO) ; (O----) Pd/InP(llO) ..) Pd/PbS( 100).
; (A-.-.-)
Pd/InSb(llO)
and (O-
-IS)
a-(-),
b_(+>
where a and b represent the antibonding and bonding orbitals, respectively, Vis an interaction potential and S is the total charge density d-d overlap integral. This relation indicates that larger energy splitting will be formed as two neighbour atoms are brought closer to each other, where their interaction and charge density overlap are strong. Using this model, Riley et ~1’~ explained the transition from Ag 4d spin-orbital splitting (- 0.6 eV) to metal Ag 4d band splitting (- 2.2 eV) with increasing Ag atomic concentration in Ag-Cd and Ag-In alloy systems. Thus, the results shown in Figures 24 reflect that with increasing surface Pd concentrations, both the initial and the final state effects, as well as the Pd-Pd neighbour overlap interaction, tend to be intensified in the interface systems. This leads to the evolution and transition of the Pd 4d peak parameters to the values of the bulk Pd metal. 3.3. Interface processes. A further analysis of the evolution of the Pd 4d peak shape parameters indicates that two important interface processes, namely surface segregation of the substrate elements and chemical interaction between Pd and the substrates, are included in the Pd/compound semiconductor systems studied here. For each interface system, the contributions of the two processes are different. This can be explored by analysing the evolutions of the Pd 4d peak shape parameters. For the Pd/GaAs system, the changes of the Pd 4d peak shape parameters at lower coverages, compared with those at the highest coverage, are smaller than that for the other three systems, as shown in Figures 24. This indicates that for each Pd coverage, the initial and the final state effects as well as the Pd-Pd overlap interaction in this system are stronger than that in the other three systems. This implies further that for Pd/GaAs, the surface Pd concentration is relatively high, and the two interface processes mentioned above, which may dilute the surface Pd concentration, have little effect. The successive and monotonous variations of the Pd 4d peak shape parameters with increasing coverages, seen from the figures, can also imply the minor possibility of forming a peculiar Pd atom arrangement due to Pd and substrate interaction. By the relative atomic size and the surface energies3 of the interface systems, the consideration that surface segregations of As and Ga atoms are very weak compared with those of Sb and In etc., can be supported, too. The variation trends of the Pd 4d peak shape parameters for Pd/InSb are similar to those for Pd/GaAs, except that the peak positions and the FWHM at each coverage move towards the d band localization direction further, and the peak splitting is smaller than that in Pd/GaAs, at least in the coverage range 0 < 35 A. These results indicate decrease of the Pd atomic concentration and weakening of the Pd-Pd neighbour overlap interaction on the surface of the system can be associated with the surface segregation of Sb and In atoms. Considering the relative atomic size and surface energies of Sb and In”, the strong tendency of their surface segregation can be supported. Meanwhile, the results that the Pd 4d peak splitting for Pd/InSb increases linearly with increasing Pd coverages after an initial rapid ascend589
Nan Li et al: Valence
band studies
ing (0 > 2-3 A), as shown in Figure 4, also imply the smooth variation of the surface Pd concentrations and no peculiar configurations of Pd-In or Pd-Sb compounds being formed. This can be realized from the relatively low reactivity of Sb in InSb compared with that of As in GaAs. The case for the Pd/InP system is different from and more complicated than that in the above-mentioned systems, as shown in Figures 224. At the lowest coverage range (0 - 0.2-2.0 A), the Pd 4d peak FWHM and splitting increase rapidly with coverage. After that, a plateau appears in both the variation curves in the range of 2.0-12 A. Beyond this range, they show an ascending trend again. The appearance of the plateau implies that the PdPd neighbour overlap interaction and the related surface Pd concentrations may be kept constant with increasing covet-ages. This can be attributed to a chemical interaction limited process, as the stoichiometry of a particular chemical compound must be constant. Considering the stronger chemical reactivity of phosphorus in the substrate, the chemical interaction limited process can be supported. Then, the rapid rising of the Pd 4d peak FWHM and splitting in the lowest coverages can be considered as the quick transition from the initial stage of a very small amount Pd deposition to the formation of the above-mentioned compound state, while their rerising after the plateau (0 > l215 A) indicates a transition of the surface Pd concentration from a chemical interaction limited process to an atomic diffusion limited process. The chemical interaction between Pd and the substrate anions, P, may lead to a decrease in the surface Pd concentrations. but the decrease may not be so serious as that caused by the surface segregation of Sb and In in Pd/InSb. Hence, an order such as GaAs < InP < InSb for the changes of the Pd 4d peak shape parameters from the values of bulk Pd metal can be observed. For Pd/PbS, the evolution of the Pd 4d peak shape parameters is very similar to that for Pd/InP, except that the evolution proceeds more slowly, and the parameter values are smaller than those for Pd/InP, indicating a much lower surface Pd concentration and a weakened Pd-Pd neighbour overlap interaction. Considering the similarity between Pd/PBs and Pd/InP in their evolution of the Pd 4d peak shape parameters, as well as the differences between these two systems and Pd/GaAs and Pd/InSb, a chemical interaction limited process between Pd and the PbS substrate can also be inferred. This further demonstrates that chemical interaction limited interface process takes place in the Pd/semiconductor systems with the more reactive substrate anions, S and P. For the small value and the slow development of the Pd 4d peak shape parameters in Pd/PbS, we can attribute them to the large difference between the atomic size of Pb and Pd, which may lead to strong surface segregation of Pb and cause a decrease in the surface Pd concentration. 4. Conclusions The valence band spectra of the Pd/compound semiconductor systems are studied with SR photoemission spectroscopy. Peak decomposition and curve fitting are performed for the Pd 4d band, and the evolution of the peak parameters for the interfaces is illustrated. Detailed analyses on these results led to the following conclusive remarks. (1) Pd can interact with all the four semiconductor substrates in some degree; wherein a small part of the Pd 4d electrons combines with the substrate p electrons forming an antibonding 0 orbital, which appears as a shoulder near the Fermi level in the 590
valence band; the main structure of the valence band EDCs is associated with the Pd 4d band ; with increasing coverage, the contribution of the surface Pd to the photoemission is increased. while the shoulder which reflects the Pd-substrate interactions is gradually covered by the bulk Pd 4d band. (2) The Pd 4d peak shape parameters, including the peak positions, the FWHM and the peak splittings, as well as their evolution are mainly determined by the initial and the final state effects in the photoemission process and Pd-Pd neighbour overlap interaction, and these factors can be associated with the surface Pd atom distributions. (3) Two interface processes. namely the surface segregation and chemical interaction of Pd with the substrate, are included in the interfaces studied here ; the interface processes, as well as their different contributions to each interface system, can be demonstrated by analysing the evolution of the Pd 4d peak shape parameters. (4) According to the different chemical reactivity of the anions in the substrates studied here, the interface systems can be divided into two categories : one is Pd/GaAs and Pd/InSb, in which the surface Pd concentrations vary in a monotonous and successive way with increasing Pd coverage, indicating a surface scgrcgation and interdiffusion limited interface process ; the other is Pd/InP and Pd/PbS. in which a plateau was observed after an initial ascending in the evolution of the Pd 4d peak shape parameters, indicating that a chemical interaction process is present in the interface systems. (5) For the very localized states in the transition metals even near the Fermi level, the peak decomposition and curve fitting technique can be available for the valence band spectra analysis. Acknowledgements This work was supported by the US Office of Naval Research under Contracts Nos ONR-N00014-87-K-0029 and ONRNOOOO14-86-K-0427, and by the Chinese National Science Foundation. The synchrotron radiation experiment was done at the Wisconsin Synchrotron Radiation Center, and the support by its staff is gratefully acknowledged. References ’ See, e.g.. L J Brillson,
Su$xe
Sci Rc>p,2, 123 (1983).
2R H Williams, In f'h~aics and Chemistry of’ 111~1”Cowipowzc/ SwniconductorIntwfuces (Edited by C W Wilmsen). Plenum, New York (1985). ‘Zhangda Lin, F Xu and J H Weaver, Phys Rcc, B36,5777 (1987). ‘G W Rubloff: P S Ho, J F Freeouf and J E Lewis. P/IL..FRec.. B23.4183 (1981). ‘T Kendelewitz, W G Petro, 1 Lindau and W E Spicer. Phj,.v Rev. B28, 3618 (1983). ‘R Ludeke and G Landgren, Ph_r.v Rer, B33, 5526 (1986). ‘T Sands, V G Keramidas. R Gronsky and J Washburn, 7’/& Solicl Fi/nl.v, 136, 105 (1986). ‘C M Aldao. I M Vitomirov. F Xu and J H Weaver, Phvs Rw. B37, 6019 (1988). ‘J C Fuggle, F U Hillebrecht, R Zeller. Z Zolmierek and P A Bennett, Phr.v Rcr, B27, 2 I45 (1982). ‘“T Kendelewitz, R S List, K A Bertness, M D Williams. I Lindau and W E Spicer, J Vuc Sci Tcchnol, B4,959 ( 1986). ’ ’ N Russo, J Andzelm and D R Salahub, Clzem PhJ>s, 114, 33 I (I 987). “C E Moore. Atomic Energy Levels. US Natn Bur Stand Circ, No 467, US GPO, Washington, DCyi958). ’ ’ N J Shevchik, J Phys, FS, 1860 (1975). I4 J D Riley, R C G Leckey, J G Jenkin, J Liesegang and R T Poole J Phys, F6, 293 (1976). “F Boscherini, Yoram Shapira, C Capasso, C M Aldao and J H Weaver, J Vcrc Sci Tcchnol, B5, 1003 ( 1987).