Large anisotropy in thermal atomic vibrations at theInSb(111)A−(2×2)surface

Abstract

Transmission electron diffraction and reflection high-energy electron diffraction have been used to study thermal vibrations at the InSb(111)A-(2ϫ2) surface. The surface In and Sb atoms show a large anisotropy in their vibrational amplitudes. In particular, contrary to common belief, the Debye temperature of surface In ͑Sb͒ atoms is larger than the corresponding bulk value in the directions normal ͑parallel͒ to the surface. We show that thermal vibrations in these directions are suppressed so as to maintain the sp 2 -type (p 3 -type͒ bonding character of surface In ͑Sb͒ atoms with their nearest neighbors.Thermal vibrations at solid surfaces are an important issue in surface science, because they relate closely with the dynamical processes on the surfaces, such as phase transition, adsorption/desorption, and epitaxial growth. It is generally assumed that thermal fluctuations are enhanced at surfaces, because surface atoms are usually less coordinated. Demuth, Marcus, and Jepsen performed a low-energy electrondiffraction analysis for Ni surfaces, and found that surface Debye temperatures are ϳ70% of the bulk value. 1 Similar trends were reported for ͑001͒ surfaces of NaCl and KCl. 2 In these studies isotropic Debye temperatures are reasonably assumed.Reduced surface Debye temperatures in the direction normal to the surface were also found for semiconductors such as Ge͑111͒ ͑Ref. 3͒ and Si͑111͒. 4 On the other hand, anisotropy in thermal vibrations at semiconductor surfaces, which is due to strong directional properties of the covalent bond, was confirmed theoretically 5 and experimentally. 6,7 Thus, thermal vibrations on semiconductor surfaces could not be described in a way similar to the case for metals and ionic crystals, and require a more appropriate description. In addition, what makes the situation more complex is a surface reconstruction, which occurs on the majority of semiconductor surfaces. Indeed, different values of vibrational amplitudes were reported for different surface atomic geometries. 5,7 This paper reports evidence that the atomic geometry of the InSb(111)A-(2ϫ2) surface causes a large anisotropy in the atomic vibrations. We found that the surface In͑Sb͒ atoms have larger ͑smaller͒ and smaller ͑larger͒ vibrational amplitudes than in the bulk in the directions parallel and normal to the surface, respectively. The present results are closely related to the atomic arrangement of this surface: the InSb(111)A-(2ϫ2) surface has the In-vacancy buckling structure, 8-10 as shown in Fig. 1. We show that the sp 2 -type (p 3 -type͒ bonding configuration of surface In ͑Sb͒ atoms with their nearest neighbors plays a key role in the reduction in the vibrational amplitude at this surface.We used transmission electron diffraction ͑TED͒ and reflection high-energy electron diffraction ͑RHEED͒ in this study. The TED analysis enables us to obtain vibrational amplitudes in the lattice planes perpendicular to an incident electron beam, i.e., in the directions parallel to the surface. It is of great advantage that TE...