Structural, electrical, and optical characterizations of epitaxial Zn(1-x)Ga(x)O films grown on sapphire (0001) substrate

In this paper we report the structural, electrical, and optical properties of epitaxial Zn1−xGaxO films (x=0-0.05) grown on single crystal sapphire (0001) substrate by pulsed laser deposition technique. Structural and elemental analysis was performed using high-resolution x-ray diffraction (θ-2θ and Φ scan) and energy dispersive x-ray spectroscopy. Temperature dependent electrical resistivity and thermoelectric power measurements were performed over the temperature range of 77-300 K and 296-373 K, respectively. Hall effect and optical transmission measurements were preformed at room temperature. All these studies showed that the structural, electrical as well as the optical characteristics of Zn1−xGaxO films depend very sensitively on the Ga contents. As the Ga doping concentration is increased, initially an increase in carrier concentration and optical band gap is observed (until x=0.04), which is followed by a decrease at higher concentrations. These features were attributed to the combined effect of band filling (urstein-Moss effect), electronic correlation, and epitaxial strain present in the system. Above parameters also affected the electrical properties of the films quite significantly. Zn1−xGaxO films with 1% of Ga doping (x=0.01) showed metal-like electrical resistivity. However, for higher doping levels, enhanced scattering potential, arising from randomly distributed impurity atoms, resulted in the Anderson localization of electronic states.