Photoionization and thermal activation of compound semiconductor MOS interfaces and origin of interface states

Abstract

Dynamic properties of GaAs and InP anodic MOS interfaces are studied, using photocapacitance transient spectroscopy (PCTS) and deep level transient spectroscopy (DLTS) techniques. Measured PCTS spectra possess a gate bias dependent portion corresponding to U-shaped continuous distribution of interface states and a bias-independent portion with a large photoionization rate. The photoionization cross section of a state continuum increases monotonically with photon energy, similarly to Si MOS, but very differently from bulk deep traps. Anomalously reduced DLTS activation energies of interface states are observed for electron emission in GaAs and for hole emission in InP, and they are correlated to a bias-independent portion of PCTS spectra. Measured results are explained by coexistence of an energetically and spatially distributed localized state continuum region and a partially delocalized state region. Surface disorder is proposed to be the origin of such composite interface state structure rather than the surface-vacancy based model.