Electrical and optical properties in O-polar and Zn-polar ZnO films grown by pulsed laser deposition

O-polar and Zn-polar ZnO films were grown on c-sapphire by pulsed laser deposition. Positron annihilation spectroscopy study reveals that the V-zn-related defects in the ZnO films with different polarities are different in structure and their thermal evolution is different. Hall effect measurement and luminescence spectroscopy reveal that the electrical and optical properties and their corresponding thermal evolution are strongly dependent on the polarity of the film. The luminescence spectra of the as-grown Zn-polar ZnO film is signified by a negligible green defect emission (at similar to 2.4 eV) and strong near band edge emission as compared with the O-polar film. The as-grown Zn-polar film exhibited a lower electron concentration (2 x 10(18) cm(-3)) than that of the O-polar film (6 x 10(18) cm(-3)); this difference is attributed to their different H concentrations. For the O-polar film, the electron concentration decreased with annealing temperature T-anneal, reaching a minimum at 700 degrees C and then increased to 4 x 10(18) cm(-3) at T-anneal = 900 degrees C. In comparison, the electron concentration of the Zn-polar ZnO film monotonically decreased with T-anneal attaining a value of similar to 1 x 10(17) cm(-3) at T-anneal = 900 degrees C, 40 times smaller than that of the O-polar film. The cause for the differences in the optical and electrical properties for the O-polar and Zn-polar films is explained by the presence of different defects in these films.