Band bending and oxygen-induced defects in a-Si:H

Abstract

Total yield photoelectron spectroscopy and Kelvin probe measurements are used to monitor the adsorption of activated oxygen onto clean hydrogenated amorphous silicon (a-Si:H) surfaces at submonolayer coverages. Oxygen adsorption induces up to 3×1012 cm−2 occupied near-surface gap states in a band centered 5.15 eV below the vacuum level and results in an 0.25-eV downward band bending at both slightly n-type and intrinsic a-Si:H surfaces. It is proposed that the downward band bending is due to the transfer of electrons from overcoordinated surface oxygen atoms to neutral defects native to a-Si:H, which form negatively charged deep states in a near-surface accumulation layer. The surface donor character of adsorbed oxygen is contrary to its usual strong electronegativity but is analogous to the donor character of overcoordinated phosphorus atoms in the a-Si:H bulk, which also lead to the creation of negatively charged deep states. Doping and deep defect formation in a-Si:H are discussed.