High-quality oxide formed by evaporation of SiO nanopowder: Application to MOSFETs on plastic substrates and GaN epilayers

Although good gate oxide of SiO2 is usually formed by high-temperature thermal oxidation, lowering the temperature for formation of SiO2 is mandatory for future Si VLSIs, in particular, for flexible ICs, the demand for which has been increasing every year. Vacuum evaporation of SiO powder is an ideal technique not only to form oxide at low temperature but also to form an abrupt interface with the substrate. The latter feature of evaporation is suitable to form thin gate oxide for Si MOSFETs and gate oxide on compound semiconductors. High-quality SiO2 on compound semiconductors helps development of MOSFETs made of compound semiconductors, which were longed for to be commercially available. The evaporation is not much used to form SiO2 for MOSFETs in spite of its many advantages, because quality of SiO2 formed by evaporation of SiO is too poor to be used as gate oxide. Unlike the commercial SiO powder, the newly developed SiO nanopowder, made by thermal CVD using SiH4 and O2, consists of spherical particles with sizes less than 50 nm. It does not contain any Si nanocrystals but small molecular Si networks. Such molecular Si networks are easily thermally or optically decomposed. This makes the deposited oxide more free from Si nanocrystals, which usually degrade the insulating property of the oxide. The SiO2 thin films formed by evaporation of the SiO nanopowder have demonstrated great potential for application to MOSFETs on plastic substrates and GaN epilayers.