Optical Properties and Color-Center Formation in Thin Films of Molybdenum Trioxide

Abstract

The optical absorption spectrum of thin films of MoO3 has been measured over the temperature range from 77° to 290°K. The fundamental absorption edge occurs at 3300 Å with an absorption coefficient of 105 cm−1. A stoichiometric film of MoO3 shows three absorption peaks at 3350, 2880, and 2700 Å, respectively, and these are suggested to be due either to exciton formation or to transitions involving a split valence band. Values for the refractive index have been measured in the wavelength range 0.42–1.2μ. The low-frequency dielectric constant (18.0±1) has been measured and the comparison with the high-frequency dielectric constant (5.70) indicates that the bonding is predominantly ionic. The electrical conductivity has been measured on a polycrystalline sample in the temperature range 298° to 900°K and the activation energies of conduction found to be 1.83 eV (intrinsic) and 0.56 eV (impurity). On irradiating a thin film (0.2μ) of MoO3 with light of wavelength shorter than 3300 Å, three color-center bands having maxima around 5000, 6250, and 7700 Å are formed. The same type of color centers are also formed on heating the film in oxygen in inert atmosphere. The color centers thus formed cannot be bleached optically. However, they can be bleached thermally in oxygen at 300°C. The growth and decay of color centers has been studied in detail. The formation of color centers is associated with increased dark conductivity of the MoO3. Very weak photoconductivity and photovoltaic effects with threshold energy corresponding to the fundamental absorption edge at 3300 Å have been observed. An ESR signal characteristic of Mo5+ state has been observed in a film of MoO3. There is a small increase of ESR signal on color-center formation which disappears completely on heating in air at 300°C. An energy-level diagram has been proposed to account for the optical and electrical properties as well as the color-center formation in MoO3.