PHYSICAL REVIEW 8 VOLUME 43, NUMBER 11 15 APRIL 1991-I Cation surface excitons in Sb/III-V interfaces M. Pedio, Maria Grazia Betti, C. Ottaviani, C. Quaresima, and M. Capozi (Received 31 October 1990) A complete study of the evolution of the surface cation excitons in III-V semiconductor com- pounds as a function of the Sb coverage is presented. Electron-energy-loss and yield spectroscopies (constant initial state, constant final state) on the Sb/GaAs(110) and Sb/InP(110) interfaces have been performed. The excitons for Sb/InP(110) and Sb/GaAs(110) systems are still present up to coverages of at least one monolayer, confirming an unreactive growth mechanism. The different growth morphology, depositing Sb on GaAs(110) and InP(110) in the submonolayer coverage, is dis- cussed. The surface-exciton behavior is monotonically decreasing for Ga, in agreement with the scanning-tunneling-microscopy growth-morphology picture. For the Sb/InP system, the In and P sites are not equivalent in different stages of the monolayer formation. INTRODUCTION In the past few years antimony chemisorption on the cleavage surface of III-V semiconductor compounds has attracted interest because it is a model system of an epit- axial and unreactive interface. The electronic and the structural properties of these systems have been the topic of extensive experimental and theoretical studies. ' ' At room temperature (RT) antimony deposition on GaAs (110) and on InP (110) gives rise to an ordered p(1 X 1) structure. The Sb atoms are periodically arranged along zigzag chains resembling the geometry of cation and anion atoms at the nearly unrelaxed topmost semicon- ductor layer. The Sb/GaAs(110) growth morphology has been stud- ied by several techniques. In particular scanning tun- neling microscopy (STM) brought into evidence that, for submonolayer coverages, the antimony forms large or- dered terraces. At the completion of one monolayer, a well-ordered (1 X 1) structure was observed in agreement with the low-energy electron-diffraction (LEED) analysis. ' Thermal-desorption experiments pointed out the high stability of the Sb monolayer. From bulk thermodynamics there is no evidence of P Sb & alloy formation, while the analogous As„Sb, compounds are well known to react. In general for the Sb/InP(110) system, a higher degree of disorder in the submonolayer coverage is expected with respect to the Sb/GaAs(110) interface, in relation to the differences in size and in electronegativity between the cation and anion substrate elements. The Sb/InP(110) interface has been studied by LEED, the Auger electron spectroscopy' (AES) technique, and recently by photoemission, "' Raman spectroscopy, ' ' and high-resolution electron-energy-loss spectroscopy' (HREELS). On the basis of Schottky-barrier measurements by means of Raman' and photoemission spectroscopy"' it was suggested that the Sb — P bond should not take place, or that the Sb — In bond could be highly preferential, while LEED (Ref. 9) experiments indicated an expitaxial growth involving both In and P sites. Furthermore, from LEED analysis, when a monolayer is formed on the InP(110) surface, the Sb overlayer presents a higher rela- tive displacement of the two Sb atoms (b, &i)— respectively bonded with In and P — with respect to the GaAs(110) surface in the analogous interface. Theoretical tight-binding band-structure calculations lay stress on the importance of the A, i parameter to determine the energy position of Sb-induced states. A larger A&i has the effect of lowering the energy level of the Sb-anion bonds. In particular, band-structure calcu- lations, using the A&i deduced by the LEED data, found the empty Sb — P bond state in the semiconductor bulk gap, in agreement with recent high-resolution electron- energy-loss experiments. ' The aim of this paper is to investigate the role of the cation substrate in the Sb/III-V interface formation by means of the analysis of the surface cation dangling bonds. The study of cation core-level excitons in the cleaved surfaces of III-V compounds is a powerful tool to bring into evidence the differences between the growth mechanism in these systems as a function of the Sb cover- age in the submonolayer range. The cation surface exci- ton in III-V compounds is a Frenkel exciton' ' generat- ed by the cation dangling-bond orbital at the surface. It is a very sensitive tool to detect the fraction of free sur- face cation atoms because of the extremely localized and intra-atomic character of the electron-hole pair. ' Several authors have used the cation surface exciton detected with the EELS technique„' and sometimes with yield spectroscopies, ' to determine the degree of reactivity at the interface. The amplitude of the excitonic excitations can be correlated with the amount of atoms deposited on the cations in the topmost semiconductor layer, and for this reason it is a highly sensitive indicator of the effective surface coverage. A complete study of the evolution of the III-V semi- conductor surface cation excitons versus the Sb coverage is still lacking. As far as it concerns the Sb overlayer on 43 9070 1991 The American Physical Society