Photoinduced self-limited low-temperature growth of ultra-thin silicon-oxide films with water vapor

Abstract

The growth of ultra-thin (<2 nm) silicon-oxide films was investigated on Si(100):H, Si(111):H, and a-Si:H surfaces in a pure water atmosphere (0.1–10 Pa) at low temperatures of 30–250 °C. Oxidation was induced photochemically by pulsed F2-laser radiation at 157 nm. The thickness and composition of the growing oxide films were monitored in real time by spectroscopic ellipsometry in the photon energy range of 1.15–4.75 eV. The mechanism of laser-induced silicon oxidation in a H2O atmosphere is shown to differ fundamentally from the classical Deal–Grove mechanism of thermal oxidation at 900–1200 °C, as well as from the photoinduced low-temperature oxidation in an O2 atmosphere. In particular, the film thickness essentially does not depend on temperature below 250 °C. A kinetic model is developed for low-temperature silicon oxidation in a H2O atmosphere. According to this model, the growth is limited at small thicknesses by the oxidation reaction and at larger thicknesses by reactions of the diffusing oxidizing species in the oxide layer. Very good agreement is established between this kinetic model and the ellipsometric measurements and the temperature and pressure dependence of the water oxidation process.