Structure and photoelectrical properties of SiO2/Si/SiO2 single quantum wells prepared under ultrahigh vacuum conditions

SiO2/Si/SiO2 single quantum wells (QWs) were prepared under ultrahigh vacuum conditions in order to study their structural, chemical and photoelectrical properties with respect to a possible application in photovoltaic devices. Amorphous silicon (a-Si) layers (thickness <10 nm) were deposited onto quartz glass (SiO2) substrates and subsequently oxidized with neutral atomic oxygen at moderate temperatures of 600 °C. Under these conditions, the formation of suboxides is mostly suppressed and abrupt Si/SiO2 interfaces are obtained. Crystallization of a-Si QWs requires temperatures as high as 1000 °C resulting in a nanocrystalline structure with a small amorphous fraction. The spectral dependence of the internal quantum efficiency of photoconductivity correlates well with the nanocrystalline structure and yields mobility lifetime products of <10−7 cm2 V−1. This rather low value points towards a strong influence of Si/SiO2 interface states on the carrier mobility and the carrier lifetime in Si QWs. Electronic passivation of interface states by subsequent hydrogen treatment in forming gas enhances the internal quantum efficiency by nearly one order of magnitude.