Effect of sol temperature on the structure, morphology, optical and photoluminescence properties of nanocrystalline zirconia thin films

Transparent nanocrystalline zirconia thin films were prepared by sol-gel dip coating technique using Zirconium oxychloride octahydrate as source material on quartz substrates, keeping the sol at room temperature (SET I) and 60 A degrees C (SET II). X-ray diffraction (XRD) pattern shows the formation of mixed phase [tetragonal (T) + monoclinic (M)] in SET I and a pure tetragonal phase in SET II ZrO2 thin films annealed at 400 A degrees C. Phase transformation from tetragonal to monoclinic was achieved in SET II film annealed at 500 A degrees C. Atomic force microscopy analysis reveals lower rms roughness and skewness in SET II film annealed at 500 A degrees C indicating better optical quality. The transmittance spectra gives a higher average transmittance > 85% (UV-VIS region) in SET II films. Optical spectra indicate that the ZrO2 films contain direct-band transitions. The sub- band in the monoclinic ZrO2 films introduced interstitial O(-)defect states above the top of the valance band. The energy bandgap increased (5.57-5.74 eV) in SET I films and decreased (5.74-5.62 eV) in SET II films, with annealing temperature. This is associated with the variations in grain sizes. Photoluminescence (PL) spectra give intense band at 384 and 396 nm in SET I and SET II films, respectively. A twofold increase in the PL intensity is observed in SET II film. The Red shift of SET I films and Blue shift of SET II films with annealing temperature, originates from the change of stress of the film due to lattice distortions.