Nanocrystalline CuO thin films: synthesis, microstructural and optoelectronic properties

Nanocrystalline copper oxide (CuO) thin films have been synthesized by a sol-gel method using cupric acetate Cu (CH3COO) as a precursor. The as prepared powder was sintered at various temperatures in the range of (300-700 A degrees C) and has been deposited onto a glass substrates using spin coating technique. The structural, compositional, morphological, electrical optical and gas sensing properties of CuO thin films have been studied by X-ray diffraction, Scanning Electron Microscopy (SEM), Four Probe Resistivity measurement and UV-visible spectrophotometer. The variation in annealing temperature affected the film morphology and optoelectronic properties. X-ray diffraction patterns of CuO films show that all the films are nanocrystallized in the monoclinic structure and present a random orientation. The crystallite size increases with increasing annealing temperature (40-45 nm).The room temperature dc electrical conductivity was increased from 10(-6) to 10(-5) (Omega cm)(-1), after annealing due to the removal of H2O vapor which may resist conduction between CuO grain. The thermopower measurement shows that CuO films were found of n-type, apparently suggesting the existence of oxygen vacancies in the structure. The electron carrier concentration (n) and mobility (mu) of CuO films annealed at 400-700 A degrees C were estimated to be of the order of 4.6-7.2 x 10(19) cm(-3) and 3.7-5.4 x 10(-5) cm(2) V-1 s(-1) respectively. It is observed that CuO thin film annealing at 700 A degrees C after deposition provide a smooth and flat texture suited for optoelectronic applications. The optical band gap energy decreases (1.64-1.46 eV) with increasing annealing temperature. It was observed that the crystallite size increases with increasing annealing temperature. These modifications influence the morphology, electrical and optical properties.