Interface states in MOS structures with 20–40 Å thick SiO 2 films on nondegenerate Si

Abstract

MOS structures having different metal contacts on 20–40 Å thick SiO 2 films and non-degenerate Si have been investigated. These structures show large frequency dispersion of the measured admittance curves caused by carrier recombination in interface states. However, build-up of a Si inversion layer is not observed because the minority carriers are drained away by tunneling through the oxide into the metal. Interface state density distributions across the whole Si band gap are obtained by evaluating measured capacitance and conductance curves. Characteristic peaks of 0.12–0.30 eV half width are found for the metal contacts, Mg at 0.54 eV, Cu at 0.23 eV, Cr at 0.18 eV and 0.52 eV, and Au at 0.15 eV and 0.97 eV above the valence band edge. The position of a peak seems to be related to the valence of the metal. Interface states caused by Cr, Cu and Mg contacts are of acceptor type. Their capture cross-section obtained from conductance measurements varies between 10 −7 and 10 −18 cm −2. A rapid increase of the state density with decreasing oxide thickness or sample annealing indicates that the states are caused by metal diffusion through the oxide into the interface. An observed saturation of the state density at about 2 × 10 13 cm −2 V −1 is possibly due to a limited solubility of metal impurities in the oxide.