Single layer polycrystalline ZnO-Al2O3 nanocomposite thin films:: Optical and electrical properties

Nanometric size dependent optical absorption coefficients, dispersive nature of the dielectric constants and ac conductivity are observed for the single layer ZnOx(Al2O3)(1-x) (x = 0.20 and 0.50) nanocomposites thin films. The sol-gel prepared thin films with thickness approximately 119 nm, contains randomly dispersed ZnO nanocrystallites in the Al2O3 matrix. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) revealed homogeneous distributions of the nanoparticles in the matrix. The shifting and broadening of the optical absorption peaks to the lower wavelength region at higher annealing temperatures was analyzed in order to relate the particle-particle interactions and the sizes of the dispersed nanoparticles in the matrix. AC impedance spectroscopy in the frequency range of 500 Hz to 5 MHz was measured at room temperature (300 K), 325 K, 395 K, and 450 K. The dielectric relaxations in each case were found to be of Cole-Cole type. The semicircular pattern of the ac capacitance in the complex plane showed contributions from the grain and grain boundary subject to concentration of ZnO in the matrix. The appearance of the negative capacitance at higher frequency was also analyzed.