Scanning tunneling microscopy and x-ray photoelectron spectroscopy studies of atomic level structure and Fermi level pinning on GaAs(110) surfaces grown by molecular beam epitaxy

Abstract

The relationship between atomic level structure and Fermi level pinning on n-type GaAs(110) surface grown by molecular beam epitaxy (MBE) is studied in situ by scanning tunneling microscopy (STM)/scanning tunnel spectroscopy (STS) and x-ray photoelectron spectroscopy (XPS). Fermi level by XPS on our MBE-grown n-GaAs(110) surfaces is found to lie at around EC−0.5 eV, indicating the presence of band bending. Among a few characteristic features observed by STM in addition to the basic (1×1) relaxation structure, the dominant holelike structure in the empty state image is correlated with band bending. It shows a gradual decrease of the apparent height over the several atomic distances toward the center in the empty state STM image and gradual increase of the apparent height in the filled state image, indicating the presence of negative charge. It accompanies random site deviations of surrounding atoms. The density of the holelike feature agrees roughly with the required number density of surface pinning centers to cause the observed band bending. This holelike structure originates most likely from a Ga vacancy. However, in STS spectroscopy measurements, a U-shaped continuous gap state distribution extending from the band edges, rather than a peaked discrete deep acceptor state distribution, is observed with a local downward shift of the surface Fermi level. A picture is presented where generation of Ga vacancy introduces local bond disorder and acts like a pinning center having a gap state continuum in accordance with a disorder-induced gap state model.