Tadahiro Ohmi

Partially crystalline structures in thin SiO₂ films formed by thermal oxidation of atomically flat Si surface were investigated by performing X-ray reflectometry and grazing incidence X-ray diffraction measurements. The densities of the SiO₂ films were higher than those of amorphous SiO₂ films, especially at the SiO₂/Si interface. The structure of thermally grown SiO₂ has been expected to be amorphous, but sharp diffraction peaks were observed in GIXRD measurements. The diffraction peaks tended to reflect a cristobalite-like structure with preferential orientations to the Si substrate. In addition, the lattice spacing of the cristobalite-like structure was estimated from the GIXRD measurement and found to be isotropically distorted.

We investigated the SiN/GaN interface properties formed by the microwave-excited plasma-enhanced chemical vapor deposition (PECVD) with SiH₄/N₂/H₂ gases. The interface and insulating properties of SiN films on GaN, formed by the microwave-excited PECVD, strongly depend on SiH₄ flow rate. Although the interface trap density is lower than 10¹¹ cm^-2 eV^-1 at the relatively high SiH₄ flow rate of 1.0 standard cm³/min (sccm), the breakdown electric field is very low (approximately 1 MV/cm). Using the SiH₄ plasma pretreatment before the stoichiometric SiN deposition, both low interface trap density and high breakdown voltage greater than 2 MV/cm were obtained. In this case, the clearly ordered Ga bonding near the SiN/GaN interface was estimated by electron energy loss spectroscopy. The formation of SiN film on GaN using the microwave-excited PECVD is a very useful technique for the high-quality interface properties.