Scanning tunneling microscopy images of Al0.2GA0.8As-{1 1 0}: Influence of applied bias voltage and comparison between filled- and empty-states images

Bias-voltage dependent images of Al_0.2Ga_0.8As-{1 1 0} are presented. The images show both the filled- and empty-states of the surface. Apart from voltage-dependent changes in the apparent direction of the atomic rows also observed for binary III-V semiconductors, bright and dark areas about 2 nm in diameter appear at small voltage in filled-states images, and small ridges along [−1 1 0] appear at small voltage in empty-states images. The spatial extent of the bright and dark areas observed in filled-states images is thought to be determined by the electron-electron interaction. It is also shown that when a given patch of Al_0.2Ga_0.8As-{1 1 0} surface is imaged simultaneously in the filled- and empty-states mode, the locations and spatial extent of the alloy-related minima (the “dark patches”) do not coincide. This casts doubt on the assumption that a locally decreased tunneling probability represents an increased local content of Al.