Study on Initial Stage of Oxidation of Hydrogen-Terminated Silicon Surface and Structure of Interface between Ultra-thin Metal Film and Ultra-thin Silicon Oxide Film

In order to form high quality silicon oxide and SiO_2/Si interface in atomic scale, it is important to prepare ultra-clean and atomically flat Si surface. Namely, in the case of Si(111) surface it can be realized by the treatment in 40% NH_4F solution, while in the case of Si(100) surface it can be realized by the epitaxial growth of Si in hydrogen gas at 1100ﾟC.In either case Si surface is terminated with hydrogen and is atomically flat. Because hydrogen-terminated Si surface is chemically stable, the passivation of Si surface with native oxide is expected to be replaced by that with hydrogen. In the present study initial stage of oxidation of atomically flat hydrogen-terminated Si(111) in 1 Torr dry oxygen at 300ﾟC was studied in details by X-ray photoelectron spectroscopy. By comparing this experimtal results with simulated results of oxidation process, it was found that the oxidation proceeds non-uniformly and layr by layr growth of oxide proceeds locally. However, if the oxidation is performed in 1 Torr dry oxygen at 600 and 800ﾟC through 0.5 nm thick oxide formed in 1 Torr dry oxygen at 300ﾟC, the periodic changes in interface structure appeared with the progress of oxidation. This can be understood by the layr by layr growth of thermal oxide. These results can be understood such that below the critical thickness of 0.5-0.8 nm oxidation proceeds non-uniformly, while above this critical thickness the atomic steps at the interface flow uniformly to result in layr by layr growth of thermal oxide. In order to confirm this important discovery, only a little time could be spent for the formation of metal /Si interface and the measurement of valence band spectra of this system. By the way it was found from the study on the initial stage of oxidation in 1 Torr dry oxygen at 200, 300 and 400ﾟC that the oxidation proceeds more uniformly at lower oxidation temperature.