Highly conductive and transparent ZnO thin films prepared by spray pyrolysis technique

Thin layers of pure and Al-doped zinc oxide of 0.2 μm thickness have been prepared by spray pyrolysis of aqueous solution of ZnCl2 on borosilicate slides at temperature of 430 °C. Doping is achieved by adding AlCl3 to the solution (by weight ratio), which is mixed thoroughly prior to spraying, using the air as the carrier gas. Substrate temperature has been found to be the most important film preparation parameter. The optimum substrate temperature was obtained by looking for maximum electrical conductivity accompanied by good optical properties. This substrate temperature was found to be 430 °C, giving conductivities of 0.023(Ω cm)−1 and 0.3(Ω cm)−1, for pure and 0.3% Al-doped films, respectively, and having optical transmittance at solar maximum of 85 and 70%, for pure and doped samples, respectively. The ZnO films prepared are polycrystalline but retain würtzite structure with preferred orientations of (0 0 2) and (1 0 0). The effect of doping and annealing on the crystalline structure was studied, by investigating the X-ray patterns obtained for films with different doping before and after annealing at 400 °C in Argon for 40 min. The film sensitivity to CO and C4H10 gases was evaluated by studying the electrical conductivity as a function of gas molar concentrations.