A stacked gate insulator consisting of photooxide and PECVD film prepared from SiH4 and N2O for use in low-temperature poly-Si thin-film transistors has been developed. The rate of photooxidation using a Xe excimer lamp is the same for (100) and (111) single-crystal Si wafers, and SiO2 with good coverage of the poly-Si can be formed by photooxidation. By applying low-RF power conditions, under which the ion impact damage is smaller, the interface trap density can be improved in a thin oxide film with a thickness of several nanometers. When a PECVD film is formed from SiH4 and N2O, N is accumulated in the SiO2/Si surface, but the surface N can be reduced if the film is formed at the surface by oxidation. In the stacked gate insulator film consisting of photooxide and PECVD films constructed under these conditions, a good SiO2/Si interface with an interface trap density of 2 to 3 × 1010cm−2eV−1 can be formed by photooxidation with a substrate temperature of 200 to 300 °C. This interface trap density is equivalent to that of a thermal oxidation film formed at 950 °C, and is an improvement of approximately 1/10 compared to the PECVD film. The constructed stacked gate insulator film exhibited a flat-band voltage shift in reliability tests. This is attributed to hole traps produced by N taken up by the SiO2 film during the formation of the PECVD film. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 86(11): 21–28, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10100