Amorphous silicon oxide thin films were deposited at 25 degrees C by thermal evaporation as a function of oxygen partial pressure (0-9 x 10(-4) pa). The optical properties show a dependence on the increasing oxygen partial pressure: the near-infrared refractive index decreases from 1.95 to 1.80, and the optical gap increases from 2.2 to 2.6 eV. The optical data indicates the presence of distributed voids in thin films evaporated at higher oxygen partial pressures, and X-ray photoelectron spectroscopy indicates negligible differences in short-range bonding configuration between the films. Chemical etching trials reveal fast localised etching of the thin films in 30 wt.% hot (70 degrees C) aqueous potassium hydroxide solution, which is indicative of a highly defective structure. (c) 2006 Elsevier B.V. All rights reserved.