Electronic structure and geometry of group IV semiconductor surfaces

Physica 117B& 118B(1983) 767-770 North-HollandPublishingCompany

767

ELECTRONIC STRUCTURE AND GEOMETRY OF G R O U P IV S E M I C O N D U C T O R S U R F A C E S F. J. Himpsel IBM Thomas J. Watson Research Center Box 218 Yorktown Heights, NY 10598, U S A Photoemission results for the electronic structure of group IV semiconductor surfaces (C, Si, Ge) are critically reviewed and implications on the surface geometry are discussed. Data are presented for surface states and for core level shifts of surface atoms. The electronic and geometric structure of group IV semiconductor surfaces has been and continues to be a controversial subject. There is no consensus on the atomic arrangement for any group IV semiconductor surface. C o m m o n l y accepted models such as the buckling model (Ref.1) have recently been questioned (Ref.2). Experimental results are difficult to reproduce since well-ordered group IV semiconductor surfaces are difficult to prepare. The cleavage surfaces are metastable and tend to form badly reproducible domain structures stabilized by cleavage steps. The annealed surfaces are irreversibly damaged by i o n - b o m b a r d m e n t cleaning and exhibit several energetically very close structures (e.g., thermally quenchend and impurity-stablized phases). In the following, reasonably wellestablished photoemission results for the electronic structure will be shown and their implications on the surface geometry discussed. Surface-sensitive (~5 A depth) photoemission spectra from Si are shown in Fig.1 which represent the density of states by integrating over several unit cells in momentum space (1.8 steradians acceptance at 50 eV photon energy). Different Si surfaces exhibit a variety of surface states (tic marks) near the top of the valence band which are quenched when a monolayer of hydrogen saturates the dangling bonds. Looking at the angular ( = m o m e n t u m ) distribution of these surface states one finds very characteristic patterns which are shown in Figs 2 and 3. These figures represent both the traditional way to look at energy distributions with the angle as a parameter and a novel way to look at angular distributions for different energies. The display spectrometer used for this work [3] Projects the unit cell in the two-dimensional momentum space as shown in Figs 2 and 3. The surface states on the Si(lll)-(2xl) surface (Fig.2) exhibit a band dispersion which cannot be explained in a buckling model unless the band picture is given up and highly correlated localized states are assumed [2]. Calcula0 378-4363/83/0000-0000/$03.00 © 1983 North-Holland

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tions were performed for such correlated states [5-7] using buckling models and the energetically more favorable relaxed (1 x 1) surface with antiferromagnetic spin ordering. Two surface states were obtained which can explain the two features observed at J, (the peak at E v - 0.15 eV and the shoulder at E v - 0.7 eV in Fig.2). Pandey [8] proposed a chain model With band-like surface states.

F.J. Himpsel / Electronic structure and geometry o f group I V semiconductor surfaces

768

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Energy and angular distributions are quite characteristic for a surface structure. Even two polymorphs of the S i ( l l l ) surface, i.e., cleaved Si(lll)-(2xl) (see Refs 2,9,11) and annealed S i ( l l l ) - ( 7 x 7 ) (see Refs 10-12) have different surface states such as the lowest surface state on the (7 x 7) surface which has no obvious counterpart on the ( 2 x 1) surface (see Fig.l). The middle state on the ( 7 x 7 ) surface has a symmetry similar to the lower state on the ( 2 × 1 ) surface [10,11]. Some of these states are sensitive to sample preparation, e.g., the uppermost state on the ( 7 x 7 ) surface. It has been confirmed [12] recently that this state is intrinsic and not due to metallic impurities. This state has a characteristic m o m e n t u m distribution around the M / 2 points as shown in Fig.3.

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With such a variety of states for Si, it is interesting to see that surface states are quite similar on analogous Si and Ge surfaces [12] (or Si and C surfaces [13]). For example, the annealed G e ( l l l ) - ( 2 × 8 ) surface has two surface states with energy spectra and angular dependences (Fig.3). very similar to the lower two states of the annealed Si(111)-(7 x 7 ) surface despite the different long-range order. The angular dependence is characterized by a ( I x l)

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In this model, the states at E v - 0.7 eV at F and at E v - 0.15 eV at J are critical points of a single ~r band and the extra structure at J is explained by multiple ( 2 x 1) domains or defects. A recent photoemission study [9] claims experimental evidence for a single surface state band on S i ( l l l ) - ( 2 x l ) despite the fact that the second structure at J is seen as a peak at E v - 0.75 eV with the same intensity as the shoulder in our previous data. While the interpretation of the data is still ambiguous the two data sets (Refs 2,9) agree with each other remarkably well (e.g., peak positions agree within 0.07 eV for all ]~ points, see Fig.2).

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Angular distributions of surface states for annealed Ge(111)-(2x8) and Si(111)-(7 x7).

769

F.J. Himpsel / Electronic structure and geometry o f group I V semiconductor surfaces

unit cell. Thus, photoemission seems to probe the local bonding. One can hope to sort out a local bonding geometry from such data and then to use different long-range arrangements of a local building block for Ge and Si. The surface states of annealed diamond ( C ( 1 1 1 ) - ( 2 x 1 ) / ( 2 x 2)) appear to be similar to the Si(111)-(2 x 1) surface [ 13] which could be a hint that surfaces have a stronger tendency to ~r bonding [8,14] than the bulk where only C has stable ~r bonds. By laser quenching or impurity stabilization, one can prepare S i ( l l l ) and G e ( l l l ) surfaces which exhibit a (1 x 1) electron diffraction pattern. These surfaces are similar to the thermally annealed surfaces (see F i g . l ) in photoemission. If these (1 x 1) surfaces were ideal truncated bulk [15,16] structures (see below) one would expect highly localized surface states for the (1 x 1) surfaces as well as for the annealed surfaces because an ideal (111)-(1 x 1) surface has the dangling bond orbitals isolated from each other on second nearest-neighbor atoms.

Very direct but limited structural information is obtained from surface core level shifts [17]. Figure 4 shows spin-orbit decompsed Si2p3/2 lineshapes where signals from surface atoms can he resolved at the low binding energy side (arrows). The intensities are calibrated in fractions of a monolayer using the H-terminated S i ( l l l ) surface where a full monolayer is chemically shifted. S i ( 1 0 0 ) - ( 2 x 1) has half a layer of surface atoms with shifted core levels which rules out symmetric pairing. S i ( l l l ) - ( 2 x l ) has very small surface core level shifts ( < 0.4 eV) showing that this surface is not ionic (a full electron charge transfer gives about 2-3 eV core level shift). Si(lll)-(lxl) and S i ( l l l ) - ( 7 x 7 ) exhibit about 1 / 6 - 1 / 4 layer of surface atoms with strongly shifted core levels [10,15]. This is not compatible with an ideal relaxed surface geometry but there could be small unreconstructed islands which are disordered versions of the terraces proposed [18-20] for S i ( l l 1)-(7 ×7). REFERENCES: [1]

D. Haneman, Phys. Rev. 121 (1961) 1093.

[2]

F.J. Himpsel, P. Heimann, and D.E. Eastman, Phys. Rev. B24 (1981) 2003.

[3]

D.E. Eastman, J.J. Donelon, N.C. Hien, and F.J. Himpsel, Nuclear Instrum. and Methods, 172 (1980) 327.

[4]

This plot differs somewhat from Ref.2 where broad peaks have been decomposed into two structures. All energies are measured with respect to the Fermi level E F and are referenced to the top of the valence band E v using E F - E v = 0.33 eV for S i ( l l l ) - ( 2 x l ) from F.G. Allen and G.W. Gobeli, Phys. Rev. 127 (1962)150. C. Sebenne, et al. (Phys Rev. B12 (1975) 3280) have obtained E F E v = 0.48 eV and recent core level measurements (F.J. Himpsel, G. Hollinger, and R.A. Pollak, unpublished) give E F - E v = 0.41 eV. Relative to S i ( l l l ) - ( 2 x l ) , E F - E v is 0.18 eV larger for Si(111)-(7x7) and Si(lll)-(l×l) and 0.01 eV larger for S i ( 1 0 0 ) - ( 2 x 1) (see Refs. 15,17).

[5]

A Redondo, W.A. Goddard III, T.C. McGiI1, F.J. Himpsel, and D.E. Eastman, submitted to Phys. Rev. Lett. (August 1981).

[6]

R. Del Sole and D.J. Chadi, Phys. Rev. B24 (1981) 7430.

[7]

J.E'. Northrup, J. Ihm, and M.L. Phys. Rev. Lett. 47, (1981) 1910.

[8]

K.C. Pandey, 1913.

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Surface sensitive Si2P3/2 core level spectra showing shifted core levels for surface atoms (after Ref.17).

Cohen,

Phys. Rev. Lett 47 (1981)

770 [9]

[10]

[11]

[12]

[13]

F.J. Himpsel / Electronic structure and geometry o f group I V semiconductor surfaces R.I.G. Uhrberg, G.V. Hansson, J.M. NichoUs, and S.A. Flodstr~m, Phys. Rev. Lett 48 (1982) 1032. F.J. Himpsel, D.E. Eastman, P. Heimann, B. Reihl, C.W. White, and D.M. Zehner, Phys. Rev. B24 (1981) 1120. F. Houzay, G.M. Guichar, R. Pinchaux, and Y. Petroff, Jrnl. Vac. Sei. Technol. 18 (1981)-860. T. Yokotsuka, S. Kono, S. Suzuki, and T. Sagawa, Solid State Commun. 39 (1981) 1001. F.J. Himpsel, D.E. Eastman, P. Heimann, and J.F. van der Veen, Phys. Rev. B24 (1981) 7270.

[14]

K.C. Pandey, Phys. Rev B25 (1982) 4338.

[15]

D. Zehner, C.W. White, P. Heimann, B. Reihl, F.J. Himpsel, and D.E. Eastman, Phys. Rev. B (Rapid Commun.) 24 (1981) 4875.

[16]

D.E. Eastman, F.J. Himpsel, J.F. van der Veen, Solid State Commun. 35 (1980) 345.

[17]

F.J. Himpsel, P. Heimann, T.-C. Chiang, D.E. Eastmann, Phys. Rev. Lett. 45 (1980) 1112.

[18]

M.J. Cardillo, Phys. Rev. B23 (1981) 4279.

[19]

J.C. Phillips, Phys. Rev. Lett 45 (1980) 905.

[20]

E.G. McRae and C.W. Caldwell, Phys. Rev. Lett. 46 (1981) 1632.