Surface core-level shifts ofGaAs(100)(2×4)from first principles

Abstract

First-principles calculations show that measured surface core-level shifts (SCLSs) of the $\mathrm{Ga}\mathrm{As}(100)(2\ifmmode\times\else\texttimes\fi{}4)$ surfaces can be described within the initial state effects. The calculated $\mathrm{As}\phantom{\rule{0.2em}{0ex}}3d$ and $\mathrm{Ga}\phantom{\rule{0.2em}{0ex}}3d$ SCLSs for the $\ensuremath{\beta}2$ and $\ensuremath{\alpha}2$ reconstructions of the $\mathrm{Ga}\mathrm{As}(100)(2\ifmmode\times\else\texttimes\fi{}4)$ surfaces are in reasonable agreement with recent measurements. In particular, the results confirm that both the lower and the higher binding energy SCLSs, relative to the bulk emission in the $\mathrm{As}\phantom{\rule{0.2em}{0ex}}3d$ photoelectron spectra, are intrinsic properties of the $\mathrm{Ga}\mathrm{As}(100)(2\ifmmode\times\else\texttimes\fi{}4)$ surfaces. The most positive and most negative As shifts are attributed to the third layer As atoms, which differs from the previous intuitive suggestions. In general, calculations show that significant SCLSs arise from deep layers, and that there are more than two SCLSs. Our previously measured $\mathrm{As}\phantom{\rule{0.2em}{0ex}}3d$ spectra are fitted afresh using the calculated SCLSs. The intensity ratios of the SCLSs, obtained from the fits, show that as the heating temperature of the $\mathrm{Ga}\mathrm{As}(100)(2\ifmmode\times\else\texttimes\fi{}4)$ surface is increased gradually, the area of the $\ensuremath{\alpha}2$ reconstruction increases on the surface, but the $\ensuremath{\beta}2$ phase remains within the whole temperature range, in agreement with previous experimental findings. Our results show that the combination of the experimental and theoretical results is a prerequisite for the accurate analysis of the SCLSs of the complex reconstructed surfaces.