Effects of gap states on scanning tunneling spectra observed on (110)- and (001)-oriented clean surfaces and ultrathin Si layer covered surfaces of GaAs prepared by molecular beam epitaxy

Abstract

In an attempt to understand and control Fermi level pinning on GaAs surfaces, an ultrahigh vacuum (UHV) scanning tunneling spectroscopy (STS) study was made on (110) and (001) clean surfaces and Si covered (001) surfaces of n-type GaAs prepared by molecular beam epitaxy. Normal STS spectra showing conductance gaps corresponding to GaAs energy gap and anomalous spectra showing much larger gaps coexisted on all samples. The rate of finding normal spectra was very low on the initial surfaces, but it greatly increased after Si deposition particularly on the c(4×4) surface. A previous explanation of the gap anomaly by tip-induced electrostatic bend bending change is invalid. A new model based on a band bending change due to tip-induced local charging of surface states is presented where tunneling proximity makes occupancy of surface states in equilibrium with the scanning tunneling microscopy (STM) tip. Spots with anomalous spectra correspond to Fermi level pinning centers where the tip directly “writes” or “erases” single or a few electrons to and from the surface states. Away from the pinning center, such charge transfer does not take place, and normal STS spectra are obtained with Fermi level positions consistent with macroscopic band bending measured by x-ray photoelectron spectroscopy and by an UHV contactless capacitance–voltage system. No direct one-to-one correlation existed between the pinning center and any specific visual STM defect features such as vacancies, dimer-desorbed holes, dimer kinks, step etc. Pinning centers make up inhomogeneous distributions of spatially extended pinning areas of universal nature surrounding any kind of structural disorder. Si deposition is shown to be very effective in reducing the number, spatial extension and state density of such pinning areas, particularly on the initially c(4×4) reconstructed surface.