Subnanometer thin silicon oxide films, applied as interlayer between crystalline silicon absorbers and functional layers, have demonstrated to improve interface passivation properties. Here we compare the interface defect density as well as the fixed charge of simple native air oxides to wet-chemical oxides on silicon substrates of different doping type, with different crystal orientations and after different chemical pre-treatment processes. We show that optimized wet-chemical pre-treatment and wetchemical oxidation leads to strong improvement in terms of interface defect density as compared to simple oxidation in air. Furthermore we show that such subnanometer thin layers can contain large positive fixed charges of up to 1011 cm-2. Due to excellently low defect densities for thin layers <0.5 nm, tunneling transport and thus application for solar cells with passivated contacts should be possible. While on p-type substrates such layers feature a high positive charge and would thus support an emitter band bending, on n-type substrates the charge is smaller and can even be negative due to electrons trapped in mid gap defects.