A study of some thin transition metal oxide films
This thesis analyses the effect of varying the compositions of co-evaporated V205/TeO2, W03/CeO2, SiO/TeO2 and W03/TeO2 amorphous thin films on their electrical and optical properties. Some information about the electronic properties of these oxides may be obtained by comparison of the results. In the oxide systems containing transition metal ions the expression for hopping energy at low temperatures contains a term due to the hopping energy of polarons in addition to that due to the disorder. In the dielectric SiO/TeO2 thin films the distortion of 'the molecule is thought to be quite weak and thus the carriers do not form polarons. They would move by hopping at the band edge at low temperatures and by excitation to a mobility edge at high temperatures. The electrical conductivity of V205/TeO2 amorphous thin films is discussed in the light of the Mott(1968) theory. The optical absorption edge was found to obey the direct forbidden transitions equation cc ico=B(fiw-EOP)3'2. The frequency-squared dependence of the conductivity of W03/CeO2 thin films (high content of CeO2) in the frequency region where the capacitance is constant is associated with the lead resistance according to Street et al (1971). The optical energy gap of the films varies with the composition in same way as in doped crystalline semiconductors. The value of the, optical W03/Ce02 was calculated using the Davis and Mott (1970) formula for non-direct optical transitions. The capacitance of SiO/TeO2 thin films is found to be almost independent of frequency as well as of temperature. This is due to a strong ionic bonding which characterises a good insulator. The optical absorption edge of SiO/TeO2 is found to be sharper than that of W03/ CeO2 and very similar to that found in most crystalline solids. The value of the optical energy gap is calculated using the same formula as in W03/CeO2. The systematic change of the optical gap with composition is observed only in a limited range of compositions. The a. c electrical properties of W03/Te02 amorphous thin films are described using the Springett(1974) and Elliott(1977) models. The optical absorption edge of WO3/TeO2 is found to lead to new arguments about the origin of the Urbach edges.The a. c electrical conductivity shows a frequency dependence of the form as ca wS in all samples studied in the present work. The mechanism of conduction at low temperatures with the index varying from 0.5 to I is thought to be due to hopping of electrons between localized states in the gap. At high fields the d. c current shows a non-linear dependence on the applied electric field. This is thought to be due to either space charge or Schottky effects in the oxides containing transition metal ions. In SiO/TeO2 dielectric films, the non-linear dependence of current on the electric field is thought to be due to either the Poole-Frenkel effect or at slightly lower fields it could be due to impurities.