Chemisorption of sulfur on Ge(100)

Surface Science 201 (1988) 245-256 North-Holland, Amsterdam

245

CHEMISORPTION OF SULFUR ON Ge(lO0) T. WESER, A. BOGEN, B. KONRAD, R.D. SCHNELL *, C.A. SCHUG, W. MORITZ and W. STEINMANN Sektion Physik der Universitiit Mi~nchen, $chellingstrasse 4, D-8000 Miinchen 40, Fed. Rep. of Germany Received 11 November 1987; accepted for publication 2 March 1988

The adsorption of elementary sulfur on Ge(100) has been studied with AES, LEED and various surface photoemission spectroscopy techniques. The adsorption of sulfur reaches saturation after a few langmulr exp ~sure and yields an ordered (1 x 1) structure with one sulfur atom per surface unit cell bonded on a buR-like bridge site. Sulfur deposition exceeding one monolayer is possible, probably due to defects. During room temperature submonolayer adsorption no sulfur islands are formed. The adsorbed single atoms are bonded in well-defined bridge sites as in the case of full monolayer coverage. The valence band structure of the Ge surface is changed under sulfur adsorption: The surface states of clean Ge(100)(2 x 1) disappear, for S/Ge(100X1 x 1) one strongly and two weakly dispersing sulfur induced states with Ee ffi 2-4 eV, EB = 5 eV and Ea = 7.5 eV, respectively, are identified. The resulting two-dimensional band structure confirms the idea of covalent S-Ge bonds with (1 x 1) periodicity. The system S/Ge(100)(1 x 1) cart be regarded as an ideally terminated surface.

1. Introduction

The crystal lattice of clean and adsorbate-covered semiconductor surfaces differs sometimes drastically from the bulk lattice. The reason is the tendency of the atoms to tetrahedric coordination and the existence of unsaturated bonds of surface atoms. Stoichiometric saturation of all surface valences may be performed by adsorption of monatomic layers of suitable elements. This may result in a (1 x 1) reconstruction of high order and stability with bulk-like adsorption geometry: Such surfaces are called "ideally terminated", they ar:~ simple model systems for the investigation of the bonding mechanisms which determine the reconstruction of semiconductor surfaces. Furthermore, these ideally (1 × 1) reconstructed surfaces might have applications in future MBE layer growing techniques. As first realizations of ideal (1 × 1) terminations of (111) surfaces were reported: As/Ge(111)(1 × 1) [1], As/Si(111)(1 × 1) [2,3] and CI/Ge(111)C, x 1) [4]. * Present address: Siemens Corporate Research and Development, Otto-Hahn-Ring 6, D-8000 Mtinchen 83, Fed. Rep. of German)..

0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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I". Weser et al. / Chemisorption of sulfur on Ge(lO0)

Sulfur and higher chaleogens are prospective candidates as adsorbates to obtain an ideal (1 x 1) termination of the bivalent (1.00) surfaces of silicon and germanium; oxygen is known to penetrate aad form SiO2 and GeO2 instead [5-7]. Recently we have investigated the interaction of sulfur with these two surfaces. Whereas the adsorption system S/Si(100) does not realize an ideal surface termination [8], the system S/Ge(100) does [9]. For a description of the chemisorption system the atomic arrangement and the electronic structure at the surface have to be known. In this paper we present AES, LEED, core-level photoemission and angle-integrated as well as angle-resolved valence band photoemission studies of the adsorption behaviour of elementary sulfur on Ge(100)(2 x 2). A structural model can be established using Ge 3d surface core-level photoemission spectroscopy, the sulfur induced surface ~.~tates are characterized by determination of their two-dimensional band structures.

2. Experimental Much work has been done on sulfur adsorption on metal surfaces where the standard preparation technique uses H2S. Applying this method to semiconductor surfaces leads to coadsorpfion of H, HS and S. Hydrogen, however, cannot be removed selectively. These mixed systems do not show any well-defined long range order. Some investil~ations have been performed on such systems [10,11] concerning the short range order. In order to obtain pure sulfur adsorption we prepared our surfaces by exposure to a molecular beam of elementary sulfur. We used germanium samples of llow n-type doping concentrations to minimize band bending effects within the escape depth of the photoelectrons [12.,13]. Cleaning of the G~100) surfaces was performed by repeated cycles of mild sputtering (600-800 eV At+) and heating to about 600 ° C. This resulted in a (2 × 1) LEED pattern. A molecular beam of sulfur ($2) was produced in situ trader UHV conditions by dissoc~.ation of Ag2S in a solid state electrochemical cell [14,15] in a separate chamber connected to the vacuum system. All surfaces were checked before vLnd after sulfur adsor~.i,.,a by LEED and AES analysis. The AES-intensity of sulfur LMM emission reaches saturation for room temperature adsorptien after an exposure of a few langmuirs (1 L = 1 0 - 6 Tort s). The sulfur covered surface shows a (1 x 1) LEED pattern. The quality of the LEED patten can be improved by carefully annealing the sample. The sulfur covered surface is inert with respect t~ residual g~s contamination and does not show any detectable changes after ~ 48 h in U HV. The photoemission experiments were carried out using an unpolarized He I discharge lamp (h~,= 2!.22 eV) as weU as linearly pdafized synchrotron

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radiation at the dedicated storage ring BESSY (Berliner Elektronenspeicherdng-Gesellschaft ftir Synchrotronstrahlung, Berlin, Germany) in a photon energy range of 15-60 eV. The sync~rotron radiation was dispersed by the toroidal-grating monochromator TGM3 [16]. The photoelectrons were detected with an ellipsoidal-mirror display analyzer (EMDA) [17,18]. This type of analyzer provides angle-integrated measurements within an acceptance cone of 90 ° and angle-resolved measurements simultaneously within the whole acceptance cone with an angular resolution of 3 °. The overall energy resolution (monochromator + analyzer) was typically 300-400 meV for all spectra presented in this paper. Energies were measured relative to the Fermi energy Er which was determined by photoemission from the sample holder. For the clean Ge(100)(2 x 1) surface of moderately doped samples the position of the top of the bulk valence band Eva is 100 meV below EF [19]. After sulfur adsorption the distance E~ - Evn is increased by 0.21 ± 0.02 eV as we found by the shift of the bulk Ge 3d core level fine.

3. Sulfur-induced surface core-level shifts

Surface coreqevel spectroscopy is a powerful tool to investigate the local bonding configuration of adsorbates. In particular, for nonmetals on semico~ductors several coexisting bond geometries according to different oxidation states of surface atoms can be detected [20-23]. We used this experimental technique in our recent study of sulfur adsorption on Si(100) [8] and Ge(100) [9]. The core-level experiments have been reported in greater detail there. In this paper we give a short outline in order I,o present the necessary context for the valence band results. Ge 3d electrons were excited in the photon energy range hv = 40-60 eV with hp = 60 eV giving the best surface sensitivity. Experimental spectra are decomposed into spin-orbit partners, and the inelastic background is subtracted. After substraction of the 3d3/2 contribution we fit the remaining Ge 3d5/2 spectra using a Voigt lineshape (Lorentzian line width: 190 meV full width at half maximum (FWHM); Gaussian broadening: 390 meV FWHM). Fig. 1 shows experimental and fitted Ge 3d5/2 core-level spectra of (a) the clean and (b) the sulfur-terminated surface. The surface binding energy shift AE and the intensity ratio R = I s / I t o t of the surface emission (Is) relative to the total 3d5/2 emission (/tot) are determined from the spectra. From these results we conclude on the basis of the discrete layer model [24,25] that the complete first layer of Ge atoms contdoutes to the shifted surface peak for S/Ge(100)(1 x 1) in fig. lb. The s:.iagle Ge 3d surface emission line of S/Ge(100)(1 × 1) in fig. lb indicates that all surface atoms are in the same oxidation state; in other words, all Ge surface atoms are bonded to the same number of sulfur atoms. The

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T. Weser et at / Chemisorption of sulfur o n Ge(lO0)

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oxidation state of the first Ge layer can be identified by detection of other oxidation states at a stepped surface. Fig. 2a shows a Ge 3d5/2 spectrum taken from a sulfur-covered 6 o vicinal surface. The surface emission of this spectrum is assigned to four different oxidation states as has been shown in ref. [9]. Consequently, the oxidation state of the smooth surface (fig. lb) is identified as 2+, which corresponds to a sulfur coverage of one atom per Ge surface atom. With another experiment the adsorption geometry of the adsorbed sulfur is determined. As it is well known that S - G e compounds have covalent bonding character, we exclude any other adsorption geometry than those shown in fig. 3. As the bridge site corresponds to single bonds ( S - G e ) and the top site corresponds to double bonds (S=Ge), the two different models can be distinguished by investigation of low coverage (submonolayer) adsorption: One bat ~:,~ly o~e (2 -i- ) in the case oi top site adsorption. T ~ coneh~sion implies that the adsorbed sulfur does not form large islands. Otherwise the ~pe::i ~, would even in the case of bridge site adsorption be dominated by the 2 + oxidation sta~e - with only a weak 1 + contribution dae to the borders of the

7". Weser et ai. / Chemisorption (~ sulfur on Ge(lO0)

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surface, (b) submonolayer sulfur coverageon Ge(100). islands in the case of bridge site adsorption. Submonolayer sulfur coverages were prepared by low exposure at room temperature. Fig. 2b shows a corresponding Ge 3d5/2 spe,;trum. As has been discussed in ref. [9] it is domLqated BRIDGE SITE

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7'. Weser et al. / Chemisorption of suljilr on Ge(lO0)

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by a surface contribution of the oxidation state i + . Thus we conclude: (a) the sulfur is bridge-bonded, and (b) no large sulfur islands are formed. We sum up the major findings resulting from core-level spectroscopy: A structural model of S/Ge(100)(1 x 1) is established by measuring the Ge 3d surface core-level shifts according to different oxidation states of the first layer of Ge atoms. For S/Ge(100)(1 x 1) the sulfur coverage is one atom per ge surface atom, the sulfur being bonded on bridge sites. Stoichiometdc saturation of all surface valences is achieved, and the adsorbed sulfur atoms are bonded in "bulk-like" Ge positions. Therefore, S/Ge(100)(1 x 1) can be regarded as an ideally terminated surface.

4. Band structure

The valence band structure of the sulfur covered Ge(100) surface was investigated by angle-integrated and angle-resolved valence band photoemission spectroscopy (UPS/ARUPS). These techniques have been used in a wide range of applications to obtain information on surface structures and chemical bonds of adsorbate systems on metal and semiconductor surfaces. One example is our recent study of C1/Si(111)(7 x 7) [22]; for this system we found that the chlorine atoms are sited on top of all silicon surface atoms in areas with locally ideal (1 x 1) configuration. Other examples are the investigations of the ideally terminated (111) surfaces As/Ge(111)(1 x 1) [1] and As/Si(111)(1 x 1) [2,3]. In this study of sulfur adsorption on Ge(100) the valence band photoemission experiments provide additional evidence for the structural model established on the basis of core-level spectroscopy above. Furthermore, sulfur-induced surface states are characterized by the determination of their two-dimensional band dispersions. The UPS spectra of the Ge(100) surface are drastically changed after sulfur adsorption. Fig. 4 shows angle integrated valence band spectra for sulfur coverages of 0 = 0, 6 = 0.3 and 0 - 1 (in monolayers) in the binding energy range 0-6 eV below Eva. These spectra were taken using synchrotron radiation with a photon energy hv of 18 eV. Sulfur adsorption leads to enhanced photoemission intensity in the whole binding energy range below 2 eV. The spectrum of S/Ge(100)(1 x 1) shows two sulfur-induced emissions at binding energies of about 3 and 5 eV (marked). Note the coverage independence of the binding energy of the 5 eV state. This confirms a well defined bonding geomet~ for submono!ayer coverages ( h e i n o i d e n t i c a l -x,i t h t h a m n n n l u v ~ . r ad.:~crption geometry), which was presupposed in the previous discussio~-I of the core-level spectra. The emission in the binding energy range 0-2 eV from surface states of the clean Ge(100)(2 x 1) surface vanishes after sulfur adsorption. All spectra contain a feature (labelled "B") at a binding energy of about .

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3-4 eV; this can be characterized as emission from bulk states as its binding energy varies with photon energy (not shown here). The sulfur induced states are investigated using angle-resolved photoemission spectroscopy. Fig. 5 shows the surface Brillouin zone (left-hand side) for S/Ge(100)(1 x 1) and its orientation relative to the real space surface lattice due to the structure modelm (dgh___~thand side) determined in the previous ---.:

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experimental realization the directions r J and F J' are always probed simultaneously due to the coexistence of two domains rotated to each other by 90 ° caused by the presence of monatomic steps on the surface. Angle-resolved photoemission spectra were taken at the photon energies hn - 18 eV (synchrotron rs-tiation) and hp - 21.22 eV (He discharge lamp). As an example, in fig. 6 He I spectra of the clean and sulfur covered Ge(100) surface in the direction F K are plotted. The polar angles increase in steps of 3 ° starting from normal emission in the lowest spectra. In addition to the sulfur-induced states shown in fig. 4 a further emission is detected at a binding energy of about 7.5 eV. This spectral feature is rather broad and shows only weak dispersion. We would like to point out that the width of the sulfur induced emission line at about 5 eV binding energy varies with the angle of emission. This effect is particularly clean along F K, as this direction is equivalent for both domain orientations. The smaller width at higher emission angles is due to a band gap of the projected bulk band structure in this region [26,27]. In the overlap region (low polar angles), however, the hybridization of the sulfur-induced state with bulk states of suitable symmetry leads to a broadening of the emission. The evaluation and determination of the band structure was performed in single particle approximation under the assumption of direct transitions using the common relation

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structures and amounts up to 300 meV for the emission with EB -- 7.5 eV. The symmetry of the experimental band dispersion around J,J' corresponds to the (1 × 1) periodicity of the surface. The large energetic separation of the sulfur-induced states shows the covalent bonding character of the adsorption and the qualitative distinction between binding and nonbinding states [28-30].

T. Weser et al. / Chemisorption of sulfur on Ge(lO0)

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Since there are no relevant theoretical investigations we attempt a tentative interpretation. The dispersion of the state with 2 - 4 eV binding energy along 1' J,J'l" is consistent with the properties of a pure px-derived state of the free sulfur layer in nearest-neighbour LCAO approximation [31,32] (z denotes the direction of the surface normal). As the interaction with other states (especiaUy with Ge-derived states) may he neole~ted for thiq gimnlo cmnlitatlvo description and as this is the sulfur-induced state with lowest binding energy, this emission might be ascribed to non-bonding sulfur., derived states (lone pairs). The major results of the valence band photoemission study can be summarized as follows: The surface valence band structures of Ge(100)(2 × 1) and -

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T. Weser et al. / Chemisorption of sulfur on Ge(lO0)

255

S/Ge(100)(1 x 1) (ideal surface termination) are drastically different: After sulfur adsorption the surface states of clean Ge(100)(2 x 1) disappear (this is consistent with the saturation of all dangling bonds), the terminated surface shows sulfur induced emissions at about 2-4, 5 and 7.5 eV below the valence band edge. The energy separation and dispersion of these states confirm the covalent S-Ge bonding character and the (1 × 1) periodicity. Submonolayer adsorption takes place in the same bonding configuration as monolayer adsorption.

Acknowledgements We would like to thank the Staff of BESSY (Berliner Elektronenspeicherring-Gesellschaft far Synchrotronstrahlung, Berlin, Germany) for support. We thank D. Rieger, F.J. Himpsel and K. Wandelt for valuable discussions. This work has been supported by the Bundesministerium fiir Forschung und Teclmologie (Bonn, Fed. Rep. of Germany) from research funds for synchrotron radiation; one of us (T.W.) has received support from the Studienstiftung des Deutschen Volkes (Bonn, Fed. Pep. of Germany).

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