Direct and indirect causes of Fermi level pinning at the SiO∕GaAs interface

Abstract

The correlation between atomic bonding sites and the electronic structure of SiO on Ga As ( 001 ) - c ( 2 × 8 ) ∕ ( 2 × 4 ) was investigated using scanning tunneling microscopy(STM), scanning tunneling spectroscopy (STS), and density functional theory(DFT). At low coverage, STM images reveal that SiO molecules bond Si end down; this is consistent with Si being undercoordinated and O being fully coordinated in molecular SiO. At ∼ 5 % ML (monolayer) coverage, multiple bonding geometries were observed. To confirm the site assignments from STM images, DFT calculations were used to estimate the total adsorption energies of the different bonding geometries as a function of SiO coverage. STS measurements indicated that SiO pins the Fermi level midgap at ∼ 5 % ML coverage. DFT calculations reveal that the direct causes of Fermi level pinning at the SiO Ga As ( 001 ) - ( 2 × 4 ) interface are a result of either local charge buildups or the generation of partially filled dangling bonds on Si atoms.