Si(110)16 × 2 and Si(110)2 × 3-Sb surfaces studied by photoemission and optical spectroscopy

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Applied Surface Science 104/105 (1996) 118-123

Si(110) 16 X 2 and Si(ll0)2 x 3-Sb surfaces studied by photoemission and optical spectroscopy A. Cricenti

a,*,

p. Perfetti a, B. Nesterenko b, G. LeLay c, C. Sebenne d

a lstituto di Struttura della Materia, CNR, Via Enrico Fermi 38, 1-00044 Frascati, Italy b Institute of Semiconductor Physics, Prospekt Nauki 45, 252650 Kiev 28, Ukraine c CRMC2, UPR CNRS 7251, Campus de Luminy, 13288 Marseille Cedex 9, France d Laboratoire de Physique des Solides, URA CNRS 154, Universit£ P. et M. Curie, 75252 Paris Cedex 05, France

Received 28 June 1995; accepted 14 November 1995

Abstract

The electronic properties of clean Si(ll0)16 X 2 and Si(110)2 X 3-Sb surfaces have been studied by angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and surface differential reflectivity (SDR). For the clean 16 x 2 surface four surface states have been recognized by ARUPS and their dispersions have been mapped along the main symmetry lines in the surface Brillouin zone. SDR experiments revealed transitions between filled and empty surface states at ~ 1.8, 2.4 and 2.9 eV. Antimony has been subsequently evaporated (about one monolayer) thus obtaining a 2 × 3 reconstruction. The surface electronic structure resulted to be strongly modified with three surface state bands observed in ARUPS along the [111] direction and no optical transitions detected by SDR in our energy range (1.3-3.5 eV).

I. Introduction

As compared to the (111) and (100) face of silicon, the Si(110) face has been less studied. The first low energy electron diffraction (LEED) study [1] of the Si(110) surface has shown the presence of several reconstructions, (4 X 5), (2 × 1) and (5 x 1) and several experiments [2-9] have been performed afterwards. However, with the help of auger electron spectroscopy (AES) and LEED, Ichinokawa et al. [10] proved that the Si(ll0)-(4 X 5), (2 X 1) and (5 X 1) phases are stabilized by Ni-contamination (2-7% of a monolayer). Experiments performed in

* Corresponding author.

very clean conditions have shown that the clean Si(110) face is characterized by only one superstructure, the '16 X 2'. These conclusions were confirmed by reflection high energy electron diffraction (RHEED) [11] and scanning tunneling microscopy (STM) [12,13] and some geometrical models have been proposed [7,8] to explain the experimental data. Due to this recent discovery and the experimental difficulties of the surface preparation, the electronic properties of the clean S i ( l l 0 ) 1 6 X 2 phase are scarcely known. Photoemission yield spectroscopy [14] has revealed a filled band of electronic states near the valence band maximum with a width of approximately 1 eV, and field effect kinetics [15] experiments have shown that the (16 X 2) surface is semiconducting with a gap of 0.4 eV between filled

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A. Cricenti et al. / Applied Surface Science 104 / 105 (1996) 118-123

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and empty surface states. Further optical transitions [16] were detected at 2.4, 3.2 and 3.8 eV by means of differential reflectivity experiments. The interaction of Sb atoms with Si(100) and S i ( l l l ) surfaces has been the subject of much research in recent years because antimony is the most commonly used donor dopant in Si-MBE. Depending on Sb coverage several reconstructions can be obruined on the Si(111) face [17] while on the Si(100) face only a Sb-induced 2 X 1 superstructure is observed [18,19]. In contrast just few studies are devoted to the study of the interaction of Sb atoms with the Si(110) face [20,21]. In particular Rich et al. [20] have shown that 1 ML of Sb deposited onto the Si(110) substrate held at 350°C forms a 2 x 3 structure. Different phases have been observed by Zotov et al. [21] as a function of Sb coverage. In this work we present a study of the electronic properties of the clean Si(ll0)16 X 2 and of Si(110)2 x 3-Sb surfaces with angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and surface differential reflectivity (SDR). On the clean surface ARUPS spectra showed the existence of four surface state bands whose dispersions have been mapped along the main symmetry lines in the surface Brillouin zone (SBZ) showing similarities with the Si(111)7 X 7 electronic properties [22]. SDR resuits showed, in the energy range between 1.4 and 3.3 eV, the presence of three optical transitions at 1.8, 2.4 and 2.9 eV. The Si(ll0)2 X 3-Sb surface shows a semiconducting behavior with the highest occupied surface-state band observed around 1.7 eV below the Fermi level ( E F) in normal emission. SDR experiments showed no optical transitions up to an energy of 3.5 eV indicating that the minimum energy position of the lowest empty band must be at least 1.8 eV above E F.

He I light was 100 meV and the angular resolution of the hemispherical analyzer was + 1°. The SDR experiment consists of shining light at normal incidence onto the surface of the Si(110) in UHV conditions and measuring the intensity of the reflected light with an optical multichannel array; a dummy silicon sample is used as reference. The results are given in terms of A R/R, i.e., the change in sample reflectivity after gas adsorption [23,24]: A R/R = ( g c l e a n - Ro)/R o. gclea n corresponds to the clean surface and R 0 to the surface after atomic hydrogen adsorption. For photon energies below the silicon direct bulk gap (3.5 eV), A R/R gives directly the imaginary part of the surface dielectric function [25]. The samples (n-type, 1014 carriers/cm 3) were mechanically polished and etched in the CP-type etchant [26]. They were thoroughly outgassed at 500°C in ultrahigh vacuum and the final cleaning was performed by heatings with direct current (1000-1200°C, 10 min, slow cooling) in a vacuum of less than 3 × 10 - 9 Torr. Quite distinct two-domain '16 X 2' LEED pattern was observed. Sb was evaporated from thoroughly outgassed Knudsen cell at a rate equivalent to 0.5 ML/min, as monitored with a quartz microbalance. 1 ML of Sb is defined as the site density for the unreconstructed surface which is 9.6 X 1014 atoms/cm 2. Pressures during Sb deposition and sample heating did not exceed 1.0 X 10 - 9 Torr. The presented spectra for the Si(110)2 X 3-Sb surface were recorded from a surface obtained by evaporating 1.2 ML of Sb onto a clean 16 X 2 surface (prepared according to the procedure described above) held at about 650°C. This produced a surface with a sharp 2 X 3 LEED pattern with very low background. All temperatures were measured with an infrared pyrometer.

2. Experimental set-up

3. Experimental results

ARUPS spectra were recorded in a Vacuum Generators VG-450 ultrahigh-vacuum (UHV) chamber at a pressure of less than 2 X 10 -~° Torr. Unpolarized 21.2 eV radiation from a helium discharge lamp was used. The estimated total energy resolution as determined by the analyzer voltages and the width of the

Fig. I(a) is a scheme of the two-domain Si(ll0)16 X 2 LEED pattern showing the integerorder spots (large circles) and the fractional-order spots (small circles) together with the geometry of the Si(ll0) surface Brillouin zone (SBZ) with the main symmetry lines. Two families of equidistant

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A. Cricenti et al. // Applied Surface Science 104 / 105 (1996) 118-123

parallel lines drawn by the spots can be easily distinguished on the diagram. This type of pattern is explained by the presence of two domains, each domain being associated to one family of parallel lines respectively oriented in the [711] and [~ll] directions. It is generally admitted that this reconstruction is named '16 X 2' according to the 16 spots along the two diagonals of the unit mesh in the reciprocal lattice in the [11 l] directions. Fig. l(b) is a scheme of the Si(110)2 x 3-Sb LEED pattern showing the integer-order spots (large circles) and the fractional-order spots (small circles) together with the geometry of the Si(110) surface Brillouin zone (SBZ) with the main symmetry lines. Fig. 2 shows ARUPS spectra recorded from the clean Si(ll0)16 X 2 surface for various angles of emission along the [ l l l ] direction. They contained several distinct structures: the one (S l) closest to the Fermi level has a binding energy of 0.9 eV at normal emission. The structure then moves towards E F for larger angles of emission with maximum energy of - 0.30 eV for (9e = 25 ° and a total bandwidth of 0.6 eV. We point out that a similar state with a larger bandwidth (1 eV) has been observed by photoemission yield spectroscopy [14]. The structure S 2 is

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observed as a weak shoulder around 1.7-1.9 eV below E F. The feature S 3 is observed around 2.8-2.9 eV below E F at F point and shows a parabolic downward dispersion for larger angles of emission with a total bandwidth of 1.8 eV. Structure S 4 is observed around 3.9-4.0 eV below E F at F point, then it shows un upward dispersion of 0.75 eV with maximum energy for 19e = 25 °. The ARUPS spectra obtained along the I l l l] azimuth from the Si(110)2 × 3-Sb surface are shown in Fig. 3 and present three Sb-induced structures, Sb 1, Sb 2 and Sb 3. Sb 1 is identified in normal emission at 1.7 eV below E F and it shows a downward dispersion (0.4 eV) with minimum energy at 61e = 10° and then an upward dispersion for larger angles of emission. A prominent peak (Sb 2) is present at F around 2.1-2.2 eV below EF, the peak moves downward for larger angles of emission with minimum energy of - 4 . 9 eV for 19~ = 30 ° and a total bandwidth of 2.7 eV. The s t r u c t u r e S b 3 is observed at 19e = 25 ° and disperses upwards 0.7 eV up to ~9~ = 40 °"

A. Cricenti et al. / Applied Su.rface Science 104/105 (1996) 118-123

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The surface origin of the previous features is supported by their strong sensitivity to the surface ordering and contamination. In practice on the S i ( l l 0 ) 1 6 X 2 surface ten hours at a pressure of 2 × 10 t0 Torr quenched all the structures around the bulk valence band maximum. For this reason we had to refresh the surface by mild annealing at 700°C every two hours. Similar results are obtained for the Si(110)2 X 3Sb surface: Fig. 4 shows ARUPS spectra taken in normal emission for a clean surface (Fig. 4(a)) and after 1000 L of excited molecular hydrogen (Fig.

4(b)). Fig. 5 shows the experimental energy dispersions of the different structures reported in Figs• 2 and 3. Similar features with similar dispersion are observed along the [710] direction. It can be immediately noticed that the surface states S 2, S 3 and S 4 present on the clean surface have their corresponding ones (Sb l, Sb 2 a n d Sb 3) on the 2 X 3-Sb surface. A

Fig. 4. Photoemission spectra for a clean (a) and contaminated (b) Si( 110)2 x 3-Sb surface.

structure corresponding to S 1 has not been observed on the 2 X 3-Sb surface. This can be explained by the fact that S~ is presumably due to dangling bonds A 0

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A. Cricenti et al. /Applied Surface Science 104 / 105 (1996) 118-123

122

states on the clean 16 X 2 surface; on the 2 X 3-Sb surface, if we take into account the trimer model by Zotov et al. [21], no dangling bonds are left unsaturated. It is worth to note that these surface states had distributions in k-space consistent with a 1 X 1 SBZ, i.e. a mirror symmetry around the Brillouin zone boundary. Similarly in the photoemission studies on the Ge(l I 1)c(2 X 8) [27] and Si(111)7 X 7 surfaces [28], the surface state distributions have not been observed to show the periodicity of the small superstructure Brillouin zone. Rather the observed surface state bands have been found to correspond to (1 X 1) [27] or (2 X 2) [28] surface cells, indicating that the photoemission experiments, therefore, seem to be more sensitive to the short range ordering of the surface, in contrast to, e.g. LEED which reveals the long range periodicity of the surface geometry. The SDR spectrum for the Si(110)16 X 2 surface is shown in Fig. 6. It contains rather broad ( ~ 0.5 eV) but quite distinct peaks centered at ~ 1.8, 2.4 and 2.9 eV. Peaks at 2.4 and 2.9 eV are in good agreement with previous data from Ref. [16] (2.4 and 3.2 eV, respectively) while the peak at ~ 1.8 eV was not observed before. The interpretation of the

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SDR results needs additional information on the empty electronic states and their dispersions. On the basis of energy conservation and excluding excitonic effects we can predict from our filled bands that the transition at 1.8 eV arises between the S~ band and an empty final state located between 0.85 and 1.5 eV above the Fermi level. The transition at 2.4 eV could involve the same SI initial band and a final empty band between 1.45 and 2.1 eV above E F, or it could involves the S 2 band with an empty final state around 0.6 above E F. The interpretation of the 2.9 eV peak is more difficult since also the S~ band can be involved. SDR experiments did not show any optical transition on the 2 X 3-Sb surface. From the ARUPS spectra of Fig. 3 and from energy considerations the minimum energy position of the lowest empty band must be at least 1.8 eV above E F. In this picture the empty band needs to be mapped all along the symmetry lines. The interconnection between atomic and electronic structures of semiconductor surfaces needs information on their geometry and according to general theoretical considerations [22] the presence of features in the ARUPS spectra of semiconductor surfaces is caused by several structural peculiarities. For example, the rich ARUPS-spectrum of the Si(111)-(7 X 7) phase [22] is based on the variety of its structural elements (adatoms, dimers, rest atoms, stacking faults, corner holes) [29,30]. More or less detailed analysis have been performed on Si(110)16 X 2 surface [8] on the ground of experimental results available and semi-empirical tight-binding total energy calculations for concrete structural elements. Structural measurements [12,13] have suggested that the 16 X 2 unit cell of terrace shape could be built with the help of silicon adatoms, dimers and rest atoms just as the well-known DAS-model [30] of the Si(lll)-(7 X 7) reconstruction. For the 2 X 3-Sb phase, Sb adatoms contributions based on the trimer model of Zotov et al. [21], must be taken into account. Detailed discussion of the surface electronic spectrum of the Si(110)16 X 2 and of Si(110)2 X 3-Sb phases needs to be done on the ground of modem theoretical treatment. It could clarify the participation of each structural element in the surface bands formation.

A. Cricenti et al. /Applied Surface Science 104/105 (1996) 118-123

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