Resistivity Depends on Preferred Orientation for Transparent Conductive Thin Films

An Al-doped ZnO (AZO) thin film is an important and useful material for optoelectronic devices. In this paper, pure ZnO and AZO thin films were deposited on glass substrates by using pulsed laser deposition (PLD) at different substrate temperatures from room temperature to 650°C. The effect of substrate temperature on the crystallization behavior and optoelectronic properties of the AZO thin films were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), transmittance spectra and resistance measuring system, and so on. The XRD results indicated that all the samples were polycrystalline with a hexagonal wurtzite structure. As the substrate temperature was increased from RT to 650°C, the preferred orientation of the AZO thin films undergone a gradual change that affect the resistivity of the thin film. The AFM morphology also showed the same growth pattern. The average optical transmittance of over 90% in the visible range was obtained at a substrate temperature of 400°C. In addition, with increasing substrate temperature, the optical band gap decreased first and then increased, reaching a minimum at 400°C, which is the same trend as the resistivity. These results indicate that the atoms can easily diffuse from one position to another at the proper substrate temperature. The atoms trapped in non-equilibrium positions can shift to positions closer to equilibrium, which causes the Al atoms to be evenly disperse on the surface of the ZnO thin film to improve the transmittance and the conductivity.