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

In an attempt to understand and control Fermi level pinning on GaAs surface s, an ultrahigh vacuum (UHV) scanning tunneling spectroscopy (STS) study wa s 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 c onductance gaps corresponding to GaAs energy gap and anomalous spectra show ing much larger gaps coexisted on all samples. The rate of finding normal s pectra was very low on the initial surfaces, but it greatly increased after Si deposition particularly on the c(4x4) surface. A previous explanation o f the gap anomaly by tip-induced electrostatic bend bending change is inval id. A new model based on a band bending change due to tip-induced local cha rging of surface states is presented where tunneling proximity makes occupa ncy of surface states in equilibrium with the scanning tunneling microscopy (STM) tip. Spots with anomalous spectra correspond to Fermi level pinning centers when the tip directly "writes" or "erases" single or a few electron s 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 Fer mi level positions consistent with macroscopic band bending measured by x-r ay photoelectron spectroscopy and by an UHV contactless capacitance-voltage system. No direct one-to-one correlation existed between the pinning cente r and any specific visual STM defect features such as vacancies, dimer-deso rbed holes, dimer kinks, step etc. Pinning centers make up inhomogeneous di stributions of spatially extended pinning areas of universal nature surroun ding any kind of structural disorder. Si deposition is shown to be very eff ective in reducing the number, spatial extension and state density of such pinning areas, particularly on the initially c(4x4) reconstructed surface. (C) 2000 American Vacuum Society. [S0734-211X(00)02704-9].