Ultrathin SiO2 layers on Si(111): preparation, interface gap states and the influence of passivation

Abstract

An essential prerequisite for the successful application ofSi/SiO2 nanostructures in photovoltaics is the realization of well-defined and abruptinterfaces with low densities of interface gap states. Here, a complete in situprocess from preparation and hydrogen passivation to interface gap stateanalysis by near-UV photoelectron spectroscopy without breaking ultrahighvacuum (UHV) conditions is introduced. It is demonstrated that by RF plasmaoxidation of Si(111) substrates with thermalized neutral oxygen atoms, ultrathinSiO2 layers can be realized with compositionally and structurally abruptSi/SiO2 interfaces and a minimal amount of intermediate oxidation states bridging the transition from Si toSiO2. Plasmaoxidized samples have significantly lower interface gap states than samples oxidized by thermal oxidationat 850 °C. Interface gap state densities were further reduced by in situ hydrogen plasma passivationwith nearly thermalized H atoms. The resulting reduction of interface recombinationvelocity and the increase of effective majority and minority carrier lifetimes are revealed byconstant photocurrent measurements and quasi-steady-state photoconductance,respectively.