Effect of film thickness on structural and optical properties of sol-gel spin coated aluminum doped zinc oxide (Al:ZnO) thin films

Al doped ZnO (Al:ZnO, 1 at%) thin films of varying thickness are deposited on silicon (Si) substrates using sol-gel spin coating method. The thermally annealed films are characterized for structural, surface morphology, vibrational and optical properties. The room temperature x-ray diffraction (XRD) patterns reveal polycrystalline hexagonal wurtzite structure (space group: P63mc) having compressive stress. The root mean square interface width and fractal dimension of AFM images are used to quantify the surface morphology and is found to be decreasing with increasing film thickness. The refractive index increases with increasing film thickness. A correlation between surface morphology and refractive index has been noticed and is attributed to the densification of the deposited films with corresponding increase in film thickness. The presence of characteristic E 2 high mode in Raman spectrum confirms the formation of hexagonal wurtzite phase in deposited films. The IR spectral analysis reveals incorporation of Al in to ZnO host lattice. The optical band gap decreases (3.24 eV to 3.15 eV), whereas the Urbach energy as a measure of disorder in deposited films increases with increasing film thickness.