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Solid State Communications, Vol. 76, No. 2, pp. 113-116, 1 9 9 0 . Printed in Great Britain.
STUDIES OF Au INTERACTION ON Si(100)
0038-1098/9053.00+.00 Pergamon Press plc
BY PHOTOEMISSION SPECTROSCOPY
Z.H. Lu, T.K. Sham, K. Griffiths and P.R. Norton Interface Science Western and Department of Chemistry, The University of Western Ontario, London, Ontario Canada N6A 5B7 (Received 20 June 1990 by R. Barrie) The interaction of Au on Si(100) surface has been studied by synchrotron radiation photoemission spectroscopy and low energy electron diffraction. Detailed studies of the interaction between Au and Si dangling bonds at submonolayer Au coverages are reported. The nature and consequences of such interactions to the Au/Si interface properties are discussed.
The physicochemical properties of metal/semiconductor interfaces have been actively researched for more than half a century, due to their importance in applications in electronic devices 1. It is also a very interesting system in which to study how the d-orbitals of transition metals interact with the unpair .~1 sp3 orbitals of the semiconductor surface atoms. This interaction is believed to produce the metal-induced gap states (MIGS)2. Current tight-binding calculations of MIGS have not so far incorporated metal d-orbitals2'3. The Au/Si interface is probably the most intensively studied system. Most of the publications were based on the studies of the (111) surface4' 5 and there are very few reports on the (100) surface°'/. A detailed picture of the interactions at initial stages of Au-Si interface formation on either" surface, however, is still not available. In this paper, we report studies of interaction of Au on a Si(100) surface by synchrotron radiation photoemission spectroscopy. The experiments were carried out in ultra high vacuum (UHV) chambers with base vacua of lxl0 "t0 torr. An n-type Si(100) sample was used. The details of the experiment have been described elsewhere6's. Photoemission spectroscopy measurements were performed on the grasshopper beamline at the Canadian Synchrotron Radiation Facility (CSRF) located at the Synchrotron Radiation Center (SRC), University of _ Wisconsin-Madison, The photons were incident at 30°. A Leybold 1800 hemi-spherical electron analyzer was used to collect the photoelectrons in an angle integrated mode. The angle of emission was 60o . In Fig. 1, we show the evolution of valence band spectra as a function of Au coverage. General observations on the Au 5d band are: (1) an increase of the spin-orbit splitting from 1.5 eV to 2.2 eV, the value at saturation, with increasing Au coverages, and (2) an increase of the relative intensity of 5d5/2 to 5d3/2 with increasing Au coverages. Since at high Au coverage (z 1ML), the interaction between Au and Si is complicated by the increasing Au-Au interaction, we will restrict our discussions of Au-Si interactions to submonolayer gold
coverages. The studies of high gold coverages have been reported elsewhere6. In Fig. 2(a), we show the photoemission spectrum of pure silicon (solid curve), and a representative spectrum of silicon after deposition of about 0.3 monolayer (ML) gold-(dashed~ne). The general features of the energy distribution curve (EDC) of pure c-Si are composed of, (I) one 3p-like band at energies between 0 and 5 eV, and (H) two 3s-like bands in the energy range 5-14 eV. The shoulder in the 3p-band at an energy of about I eV is due to the surface dangling bond (DB). After the deposition of gold, two extra peaks are observed at energies of 6.4 and 4.9 eV, and are attributed to quasi Au 5d3/2 and 5d5/2 bands respectively. The more important observation is that DB states are drastically decreased, and a new density of states is produced above the Si valence band edge (the
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114
STUDIES OF Au INTERACTION ON Si(100)
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as hydrogen thereby leading to the breakdown of the Si dimers'. This suggests that Au atoms may be located at the hollow sites and interact with the nearby silicon dangling bonds. With further increase of Au coverage, the l x l structure became diffuse and disappeared at about 3.5 ML. In order to have a clear understanding of the interaction between Si dangling bonds and Au atoms. We made a comparative study on hydrogen passivated Si(lO0) surface. In Fig. 1Co), we show the EDCs of pure cSi(lO0) and c-Si(lO0) passivated with hydrogen (c-Si:H). The H-passivation was obtained by introducing 100 l.angmuir (L; = 10-6 torr.s) of hydrogen at room temperature in the presence of a heated tungsten filament close to the crystal surface. From measurements reported elsewhere8 under similar conditions we estimate the hydrogen coverage to lie in the range 0.1 to 0.3 ML. From Fig. 2 ~ ) , we can see that DB states disappear after the exposure to hydrogen. This indicates that DBs have been replaced by Si-H bonds. The energy of Si-H bonds generally lies between 4 and 14 eV 1°. This gives an intensity increase in the two 3s-like bands as shown in the dashed curve of Fig. 2(b). EDCs shown in Fig. 2(c) are recorded from e-Si:H and e-Si:H covered with 0.3 ML gold. No additional density of statesabove the valence band edge is observed. The two quasi Au 5d3/2 and 5d5/2 peaks are located at 6.3 and 4.8 eV, which give a spin-orbitsplittingof 1.5 eV. The difference between Au/c-Si and Au/c-Si:H is more evident from the difference EDCs shown in Fig. 3, which are obtained from Fig. 2(a) and (c), by subtracting the E D C of Au/c-Si (Au/c-Si:I-I)from the "background" E D C of c-Si (c-Si:I-I).Before subtraction, spectral intensitiesin the Si 3p-like bands are normalized to the
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shaded region indicated by an arrow). This is interpreted as being caused by interactions of Au via 6s-5d hybridization with Si sp3 DB. This will be discussed in detail in the following text. From Si 2p core level studies, we found that the Fermi level was not changed at Au coverages s 0.3 ML. No significant change in the core level due to Au-Si interaction is found on these surfaces, while two peaks due to reacted and unrcacted silicon species were clearly identified at higher coverages. The studies of high coverages were reported elsewhere6. From LEED studies, we found that the surface structure remained 2xl at submonolayer Au coverages. With increasing Au coverage, the 2xl structure became diffuse and a l x l structure appeared at about 1 ML Au overlayer. It is known that the clean Si(100) 2xl structure is a consequence of surface Si dimer formation. It is also known that 2xl can be transformed into ",he l x l structure if the dangling orbital is bonded to a foreign adatom such
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Vol. 76, NO. 2
STUDIES OF Au INTERACTION ON Si(100)
same value. To justify the consideration of the EDC of cSi(:I-I) as a "background', we observe that the structures of both 3p-like and one of the 3s-like band at an energy of about 11 eV are basically unchanged after the deposition of Au. It is well known that the two 3s-like bands are related to the presence of the 6-fold ring in a diamond-cubic crystalline silicon lattice 1°. Therefore the structure of the 3s-like band at art energy of about 7 eV should be conserved as is the one at about 11 eV. The distinct features of Au on c-Si are clearly shown a s : (a) replacement of DB states (shaded region) by new states above the valence band edge (shaded region indicated by an arrow), and Co) the "abnormal" Au 5dband structure with intensity ratio I(5d5/2)/I(5d3/2) of 0.8 ± 0.1, in contrast to the ratio of 1.5 expected for atomiclike gold. For Au on hydrogen passivated silicon (c-Si:H), no state above the valence band edge is observed. The 5dband, which has a spin-orbit splitting of 1.5 eV and an intensity ratio I(5d5/2)/I(5d3/2) of 1.4 ± 0.1, is very similar to atomic gold 5d-orbital structure with a spinorbit splitting of 1.5 eV and degeneracy ratio of 1.5. This observation suggests that gold on c-Si:H is atomic-like, i.e. there is no, or very weak Au-Au and Au-Si interactions at submono-layer coverages. In a separate experiment studies by Rutherford backscattering and nuclear reaction analysis (NRA), we found that hydrogen concentration on SiQ00) surface was not changed upon the deposition of Au v. In contrast to pure silicon surface, the 2xl LEED pattern of a hydrogen passivated surface was not transformed to lxl until an Au coverage of about 3.5 ML, a coverage at which no LEED pattern was observed on pure c-Si(100) surface. This again suggests that the presence of hydrogen on the silicon surface reduces the interaction between dangling bonds and Au atoms, even though the hydrogen coverage is far from saturationS. The formation of a Schottky barrier on c-Si:H surface is most likely by direct diffusion of Au into silicon, as we proposed elsewhere6. From the above experimental results, it is evident that Au interacts with Si DB on the clean surface. The result of the interaction is the replacement of DB states by a new density of states above the valence band edge. As compared to the Au/c-Si:H interface, the Fermi level is pinned during early stages of Schottloj barrier formation at the Au/c-Si interface6. This indicates that Au-induced surface states above the valence band edge are loca!i~l and occupie~l electron states. This
observation also suggests that there might exist unoccupied states or antibonding states below the conduction band edge. The existence of metal-induced extrinsic states on silicon surface around the conduction band edge has been reported by Rowe et. al. 1! in an electron energy-loss experiment. As to the nature of the Au-Si bond in our studies, it can not be simpl~ described as the interaction between Au 6s and Si DB (sp.~) electrons as suggested by Iwami et al. 12 and MOnth 13. It is known that the interaction between Au and a metalloid always involves 6s-5d hybridization ]4. As we have described above, the intensity ratio I(ds/2)/I(d3/2) of Au 5d-band is about 0.g, a value far from the normal ratio of 1.5. This indicates that the Au 5d electrons are directly involved, most probably through s-d hybridization. This type. of interaction has been found in various gold alloys 14. The most notable feature is that the overall charge transfer is onto the Au site as expected on the basis of electronegativity considerations, but this charge transfer is accomplished through a gain of s-ch~ge compensated by a depletion of d-charge at Au site 14"16. Our observations that the d5/2band is more affected by Au-Si interactions than is d3/2, are consistent with previous studies of Au-metaUoid interaction 14"16, in which the d5/2 state appears to be more actively involved in chemical bonding than the d3/2 state. In conclusion, we have carried out photoemission and LEED measurements on pure c-Si(100) and hydrogen passivated c-Si(100). Upon deposition of submonolayers of gold on pure c-Si(100), the dangling bond states are replaced by new surface states above the valence band edge. This Au-induced surface state is attributed to the chemical bonding between Au s-d and Si DB sp3 hybrids. For H-passivated Si(100), the DBs were replaced by Sill bonds and no Au-induced surface states were found.
Acknowledgement - We wish to acknowledge Drs. Kim H. Tan and J.M. Chen of the Canadian Synchrotron Radiation Facility at the Synchrotron Radiation Center (SRC), University of Wisconsin-Madison, for their technical assistance. SRC is supported by the U.S. Natural Science Foundation (NSF). This work is supported by the Ontario Centre for Materials Research (OCMR) and the Natural Science and Engineering Research Council of Canada (NSERC). Support from Alcan International, Kingston, Canada, is also gratefully acknowledged.
References 1 See, for example, S.M. Sze, "Physics of Semiconductor Devices', 2nd edn., (Wiley, New York) 1981, Chap.5, p.245. 2 For a recent review, see W. M~nch, Rep. Prog. Phys. 53, 221, (1990). 3 J.E. Klepeis and W.A. Harrison, Phys. Rev. _B 40, 5810, (1989). 4 See, for example, L.J. Brillson, Surf. Sci. Rep. 2, 123 (1982); in "Handbook on Synchrotron Radiation', edited by G.V. Marr (Elsevier,
115
Amsterdam) 1987, v01. 2, p. 541, and references therein. 5 L. Braicovich, C.M. Garner, P.R. Skeath, S.Y. Su, P.W. Chye, I. Lindau, and W.E. Spicer, Phys. Rev. B 2Q, 5131 (1979). 6 Z.H. Lu, T.K. Sham, and P.R. Norton, Appl. Phys. Lett. July 1990 (in press). 7 K. Hricovini, J.E. Bonnet, B. Carri~re, J.P. Deville, M. Hanb~icken, and G. Le Lay, Surface. Sci. 211/212, 630 (1989).
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STUDIES OF Au INTERACTION ON Si(100)
8 Z.H. Lu, K. Griffiths, T.K. Sham, and P.R. Norton, unpublished. 9 Y.J. Chabal, Surf. Sci. Rep. 8, 211 (1988), and references therein. 10 L. Ley, M. Cardona, and R.A. Pollak, in "Photoemission in Solids II', edited by L. Ley and M. Cardona, (Springer-Verlag, New York) 1979, p. 11, and references therein. 11 J.E. Rowe, S.B. Christman, and G. Margaritondo, Phys. Rev. Lett. 35, 1471 (1975).
vol. 76, NO. 2
12 M. Iwami, T. Terada, H. Tochihara, M. Kubota, and Y. Murada, Surf. Sci. 197, 115 (1988). 13 W. M~neh, Europhys. Lett. 7, 275 (1988). 14 T.K, Sham, M.L. Perlman, and R.E. Watson, Phys. Rev. B 19, 539 (1979). 15 T.K. Sham, R.E. Watson, and M.L. Perlman, Phys. Rev. B 20, 3552 (1979). 16 T.K. Sham, Solid State Comm. 64, 1103 (1987).