Si(100) ordered interfaces

surface science ELSEVIER

Surface Science 374 (1997) 251 258

Electronic properties of Sn/Si(100) ordered interfaces M. Pedio, A. Cricenti * Istituto di Struttura della Materia del CNR, V.E. Fermi 38, 00040 Frascati, Italy Received 6 July 1996; accepted for publication 18 October 1996

Abstract

We studied the optical and electronic structures of the different ordered superstructures of Sn grown on Si(100)2 x 1 by means of angular resolved photoemission (ARUPS), surface differential reflectivity (SDR), Auger and low energy electron diffraction (LEED). Five different reconstructions, showing a semiconductor character, have been found. For the (5 x 1)Sn/Si(100) interface one dispersive surface state is identified, while for the other superstructures, c(4 × 4), (6 × 2) and (4 × 1), the ARUPS spectra show two non-dispersive tin-induced states. For the c(8 x 4) reconstruction three surface states are clearly identified: one non-dispersive state at 0.9 eV below the Fermi level, one state at the border of the surface Brillouin zone with a binding energy of 1.7 eV and one highly dispersive state between 1.6 and 2.8 eV below the Fermi level. On this surface SDR revealed four optical transitions in the energy range between 1.3 and 3.5 eV. © 1997 Elsevier Science B.V. All rights reserved.

Keywords: Angle resolved photoemission; Low index single crystal surfaces; Silicon; Surface electronic phenomena; Tin; Visible/ultraviolet absorption spectroscopy

1. Introduction There are few elements that do not form compounds with Si surfaces, namely those elements for which no alloy phase exists between the substrate and adsorbate elements [ 1]. Due to their negligible solubility with Si and their growth onto low-index silicon surfaces, Group IV elements, like tin and lead, are examples of these non-silicide-forming elements and can be considered model systems for unreactive epitaxial interfaces. These systems present complex phase diagrams with a high number of superstructures. A surface reconstruction, caused by the non-alloy-forming adsorbate and substrate elements, is a realization of a new surface compound that shows new interesting properties and *Corresponding author. E-mail: [email protected]

its study is extremely useful for further understanding the correlation between electronic and structural properties. For example, the difference in the Schottky barrier heights [2] of two different surface phases of Pb/Si(111) is due to the correlation between the geometric and the electronic structure in these systems and stresses the importance of a detailed knowledge of all the properties of the Si/IV Group elements interfaces. The bulk Sn presents two stable phases that differ in crystalline structure and electronic properties: gray tin, semiconducting (~-Sn), and white tin, metallic (fl-Sn). The 7-Sn phase can be stabilized at higher temperatures by epitaxial growth on different substrates I-3]. The mismatch is > 10% for all different low-index Si surfaces. Though the growth of the c~-Sn should not take place, the Sn/Si(100) system presents peculiar electronic and structural properties. In fact the epitaxial interface

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M. Pedio, A. Cricenti/Surface Science 374 (1997) 251-258

of Sn on Si(100) presents a phase diagram with a large variety of surface reconstructions [4]. The c(4 x 4), (6 x 2), c(8 x 4) and (5 x 1) LEED patterns are observed increasing the Sn amount and after thermal treatment. A scanning tunneling microscopy (STM) study interpreted the local arrangement of all the interfaces, except the (5 x 1), with a dimer local model for the Sn-Si and Sn-Sn bonds [4]. For Sn/Si(100) grown at room temperature literature results [5,6] show that the onset of the metallicity takes place for tin coverage above 1 ML (monolayer). In the IPES spectra of Sn/Si(100) after annealing [7], there is a clear evolution of the empty states as a function of the tin coverage and of the kind of superstructure observed by LEED. In particular the c(8 x 4) phase is semiconducting, with a small gap that has been estimated to be slightly larger than the e-Sn gap. In the IPES spectrum of a c(8 x4)Sn/Si(100) surface, a clear and pronounced state is present at about 1.1 eV above the Fermi level and its origin has been assigned, with the help of STM studies [4], to Sn-Sn asymmetric dimers. Though this phase is semiconducting, it is different from the c~-Sn and should be typical of the Sn growth on substrates having a strong lattice mismatch (> 10%). The (5 x 1) Sn/Si(100) superstructure, corresponding to tin coverage above 1 ML, has a slight metallic character, in agreement with direct photoemission results [5]. Unfortunately no theoretical band calculation for the Sn/Si(100) system is available. An STM study [8], performed on the Sn/GaAs(110) system, shows that it is possible to distinguish a chain structure of the tin overlayer, similar to that observed for the c(8 x 4)Sn/Si(100) interface, and a clear empty state at energy below 1.2 eV. From theoretical calculation it is possible to assign the state detected in IPES to the Sn-Sn empty dangling bonds. In this paper we present a study of the superstructures of Sn grown on Si(100)(2 x 1) by means of angular resolved photoemission (ARUPS), surface differential reflectivity (SDR), Auger and LEED, in order to characterize the electronic states of the different overlayer phases. For the (5 x 1)Sn/Si(100) interface one surface state is iden-

tiffed, while for all the other superstructures the spectra show at least two clear surface states. In particular for the c(8 x 4) reconstruction three surface states are clearly identified and SDR reveals four optical transitions in the energy range between 1.4 and 3.5 eV. The results are discussed and compared with previous IPES measurements [7].

2. Experimental set-up The samples (n-type, 1014 carriers/cm 3) were mechanically polished and etched in the CP-type etchant [9]. They were thoroughly outgassed at 500°C in ultrahigh vacuum and the final cleaning was performed by heating with direct current (1000-1200°C, 10 min, slow cooling) in a vacuum of less than 3 x 10 -9 Torr. A quite distinct twodomain (2 x 1) LEED pattern was observed. Tin was evaporated from a thoroughly outgassed tungsten basket at a rate equivalent to 0.5 ML/min, as monitored with a quartz microbalance. 1 ML of Sn is defined as the site density for the unreconstructed surface that is 6.8 x 1014 atoms/cm z. Pressure during Sn deposition and sample heating did not exceed 1.0x 10-9Torr. The presented spectra for the Si(100)-Sn surface were recorded from a surface obtained by evaporating different amounts of Sn onto a clean 2 x 1 surface (prepared according to the procedure described above) held at about 650°C. The reconstructions have been observed by LEED. ARUPS spectra were recorded in a Vacuum Generators VG-450 ultrahigh-vacuum (UHV) chamber at a pressure of less than 2 x 10- lo Torr. Unpolarized 21.2 eV radiation from a helium discharge lamp was used. The estimated total energy resolution as determined by the analyser voltages and the width of the He I light was 100 meV and the angular resolution of the hemispherical analyzer was _ 1°. All the ARUPS spectra have been collected along the [010] direction that is the common direction of the two Si(100) domains (see Fig. 1). The SDR experiment [10] consists of shining light, at normal incidence, onto the Si(100) surface, 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 AR/R, i.e. the


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(b) Fig. 5. (a) Angular resolved photoemission spectra of c(8 x 4)Sn/Si(100) interface, taken with h v - 2 1 . 2 eV, for angles of emission Oo along the [010] direction, ranging between - 10 ° and 30 ° at 5° intervals. (b) Experimental energy dispersion of the filled surface states for the c(8 x 4)Sn/Si(100) reconstruction along the I010] direction of the SBZ. The full line shows the projected bulk band edge for the 1 × 1 SBZ, from Ref. [,12].

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(b) Fig. 6. (a) Angular resolved photoemission spectra of (5 × 1)Sn/Si(100) interface, taken with by=21.2 eV, for angles of emission O~ along the I010] direction, ranging between - 10 ° and 30 ° at 5 ° intervals. (b) Experimental energy dispersion of the filled surface states for the (5 × 1)Sn/Si(100) reconstruction along the [,010] direction of the SBZ. The full line shows the projected bulk band edge for the 1 x 1 SBZ, from Ref. [12].

256


that was found to show metallic character in previous studies [ 5 - 7 ] . However, we want to point out that there is evidence of strong but structureless emission close to the Fermi level. Fig. 4a shows the ARUPS spectra from the (4 x 1)Sn/Si(100) surface, obtained along the [010] azimuth, while Fig. 4b shows the dispersion of the two surface states. Such superstructure is observed for a tin coverage of 0.6 ML, i.e. between (6 x 2) (0.45 ME) and c ( 8 x 4 ) (0.7-1 ML) reconstructions. This ( 4 x 1) superstructure has never been detected in the literature, though it is present in the phase diagram of Pb/Si(100) at higher coverage with respect to the Sn/Si(100) [ 13]. In the ARUPS set of spectra it is possible to single out the dispersion of two tin-induced surface states ($1 and S2) for the (4 x 4), (6 x 2) and (4 x 1) superstructures (Figs. 2, 3 and 4, respectively), i.e. for the epitaxial interfaces related to low tin coverage (