Electronic structure of Au-Sn compounds grown on Au(111)

Abstract

The electronic structure of Au-Sn intermetallic layers of different compositions grown on Au(111) to the thickness of several nanometers has been studied in this paper. The layer, the interface, and the substrate related components in the Au $4f$ and Sn $4d$ core-level spectra obtained using x-ray photoelectron spectroscopy (XPS) vary with deposition parameters to reveal the details of the Au-Sn formation. While AuSn is grown by deposition at room temperature, Au rich compounds form as a result of heat treatment through interdiffusion of Au and Sn. Deposition at high temperature forms more Au rich compositions compared to postannealing at the same temperature due to the kinetic energy of the impinging Sn atoms in the former case. Postannealing, on the other hand, stabilizes the bulk phases such as AuSn and ${\mathrm{Au}}_{5}\mathrm{Sn}$ and exhibits an activated behavior for transition from the former to the latter with increasing temperature. The XPS valence-band spectra of AuSn and ${\mathrm{Au}}_{5}\mathrm{Sn}$ layers show good agreement with the density functional theory calculation, indicating that these have the bulk structure reported in literature. However, the influence of antisite defects is observed in ${\mathrm{Au}}_{5}\mathrm{Sn}$. Low-energy electron-diffraction study reveals that although the AuSn layer is ordered its top surface is disordered at room temperature. Surface order is obtained by annealing or deposition at elevated temperatures and dispersing bands are observed by angle-resolved photoemission spectroscopy. Both electronlike and holelike bands are evident for the ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ensuremath{\circ}}$ phase, while a nearly free electronlike parabolic surface state is observed for the $p(3\ifmmode\times\else\texttimes\fi{}3)R{15}^{\ensuremath{\circ}}$ phase.