Bandgap control of silicon based material provides a promising way towards next generation photovoltaic devices such as tandem solar cells, what can be realized by nanostructures consisting of Si SiO2 quantum wells or superlattices. However, due to increased interface to volume ratios at reduced dimensions, charge carrier recombination and scattering at Si SiO2 interfaces might dominate the photoelectrical properties and gain critical impact on mobility lifetime products amp; 956; amp; 964; and thus internal quantum efficiencies [1]. To circumvent this drawback, the effect of hydrogen treatment on charge carrier recombination and electronic densities of states at the interface of ultrathin oxides layers is analyzed. Samples with structurally and chemically well defined interfaces were prepared by plasma oxidation of crystalline Si with atomic oxygen under ultrahigh vacuum conditions [2]. It is demonstrated, that Si SiO2 interface states can be passivated under appropriate conditions in forming gas H2 N2 and in hydrogen plasma. As a result, the photoelectrical performance of such structures is clearly improved. This is verified by i estimation of mobility lifetime products from photocurrent measurements, ii analysis of interface densities of states by means of surface photovoltage measurements SPV , and iii deducing densities of occupied states in the band gap as elucidated from UV excited constant final state yield spectroscopy CFSYS
