الفهرس 
ِAcknowledgementOnly 14 pages are availabe for public view
 |  | from | 96 |  |  |
| | | from | 96 | | |
Abstract
Deposition of Tungsten oxide W03 thin films on fused silica DStrates by sol-gel technique using precursor based on non-alkoxide aterials has been performed in this study. Preparation of precursor roxotungstic acid (sol) of enhanced stability is carried out by olving tungsten metal with excess hydrogen peroxide under titrolled conditions of temperature. Peroxotungstic acid (PTA) was acterized by TGA, DTA, XRD and FTIR . W03 thin films were asited on fused silica substrates by spin coating technique using a ion of PTA. Infrared studies showed that the water molecules are ciated with tungsten oxygen networks of the precursor PTA as a er of hydration and as a hydroxyl group. Thermal analysis showed ■ the complete transformation of amorphous dried gel (xerogel) of TA to crystalline phase of WQ3 was found to be at 430°C. X-ray diffraction investigation revealed that the formation of fconoclinic W03 phase starts by annealing xerogel of PTA at 300°C for 2b in air. A complete transformation to the monoclinic phase of W03 was aed by annealing the xerogel of PTA at temperature of 400°Cfor 2h atmospheric air. The XRD studies showed that the crystallinity sed with increasing the annealing temperature. The as-deposited
in film has been shown to be amorphous in structure. Annealing the as-deposited films at temperature 500°C for 2h in air gave the monoclinic fform ofWOs with preferred orientation along (200) direction. r Scanning electron microscope photographs showed that the film ■mealed at 500°C has uniform surface morpholpogy.
The direct allowed energy gap (Eg) of the annealed films of WO3 was found to be in the range 2.72-3.71 eV depending ori the annealing temperature. For thin film annealed at 700°C,the oxygen nonbonding |«fbitals which cause a considerable density of states in the gap affect greatly the lowering of Eg for this film.
The changes in the refractive index (n) values of WO3 thin films was reted in the light of the existence of dispersive and nondispersive regions in the films. The activation energy for conduction (Ea) has been measured from teture dependence of electrical conductivity measurements using us equation. Activation energy values of WO3 pellets and films found to be in the range 0.93 to 1.58 eV depending on the annealing fir. We interpret our data in the light of defect band model which s that surface oxygen vacancies introduces defect levels in the gap of WO3. With increasing annealing temperature the surface Abstract oxygen vacancy concentration increases and lead to the formation of deep lying defect levels in the gap of W03.