Optical characterization of low temperature amorphous MoOx, WOX, and VOx prepared by pulsed laser deposition

Transition metal oxides are materials combining properties of electrical conductivity, optical transparency, and catalytical function. They are widely used in applications including solar cells, flat panel displays, and detectors. In particular, high work function oxides such as MoO3, WO3, and V2O5 have become popular. In many applications, low deposition temperatures are required, leading to amorphous structure. In this study, thin films of amorphous MoOX, WOX, and VOX were prepared by pulsed laser deposition, and their optical properties and work function were determined. Samples of polycrystalline ZnO were also prepared for comparison. Substrate temperature was varied in the range of 25 degrees C-100 degrees C and oxygen pressure was varied in the range of 10-20 Pa during the process. Effect of pressure durign sample cool-down and chamber venting was also observed.. Optical characterization was based on photothermal deflection spectroscopy, which is a non-contact and non-destructive method for measuring directly absorptance spectra with sensitivity down to 10(-4). Absorptance in the band gap serves as an indication of the presence of defects such as oxygen vacancies or metallic phases. Our optimized films achieved a sub-bandgap absorption coefficient as low as 10(3) cm(-1) for MoOX, VOX, and 10(2) cm(-1) in the case of the WOX. From the gradient of the absorption edge, Urbach energy was obtained, evaluating disorder in the semi-conductor material. The work function of each material was obtained by Kelvin probe, and a slight correlation with Urbach energy was found. X-ray photoelectron spectroscopy indicated successful stochiometric transfer mainly for the lowest pressure and highest temperature samples.