Ultrahigh Vacuum Preparation and Passivation of Abrupt SiO2/Si(111) Interfaces

Abstract

Compositionally and structurally abrupt Si/SiO2 interfaces were prepared under ultrahigh vacuum conditions by RF plasma oxidation of Si(111) substrates with thermalized neutral oxygen atoms. The chemical, structural and electronic properties of the interface were analyzed and discussed with respect to a possible application in photovoltaic Si/SiO2 quantum well structures. The benefits of using neutral atomic oxygen were explored and turned out to be manifold: ultrathin SiO2 layers (thickness: 1–2 nm) can be precisely grown, the formation of suboxides is mostly suppressed and abrupt Si/SiO2 interfaces are obtained even at moderate substrate temperatures of 300 to 600 °C. Due to the perfect Si/SiO2 interfaces, the SiO2 layers allow thermal post-oxidation treatment up to 1000 °C without significant change in oxide thickness or stoichiometry. This is an essential prerequisite for the envisaged realization of Si/SiO2 superlattices with high crystallinity and low strain. It was shown that a post-oxidation annealing step lowers the strain and disorder at the interface resulting in lower intrinsic density of interface states. A further decrease of the density of interface states was achieved by hydrogen passivation as a result of saturation of dangling bonds.