Electrical and Structural Properties of Semi-Polar-ZnO/ a -Al 2 O 3 and Polar-ZnO/ c -Al 2 O 3 Films: A Comparative Study.

In this work, the properties of ZnO films of 100 nm thickness, grown using atomic layer deposition (ALD) on a -(100) and c -(001) oriented Al 2 O 3 substrate are reported. The films were grown in the same growth conditions and parameters at six different growth temperatures (T g ) ranging from 100 °C to 300 °C. All as-grown and annealed films were found to be polycrystalline, highly (001) oriented for the c -Al 2 O 3 and highly (101) oriented for the a -Al 2 O 3 substrate. The manifestation of semi-polar-(101) and polar (001)-oriented ZnO films on the same substrate provided the opportunity for a comparative study in terms of the influence of polarization on the electrical and structural properties of ZnO films. It was found that the concentration of hydrogen, carbon, and nitrogen impurities in polar (001)-oriented films was considerably higher than in semi-polar (101)-oriented ZnO films. The study showed that when transparent conductive oxide applications were considered, the ZnO layers could be deposited at a temperature of about 160 °C, because, at this growth temperature, the high electrical conductivity was accompanied by surface smoothness in the nanometer scale. On the contrary, semi-polar (101)-oriented films might offer a perspective for obtaining p-type ZnO films, because the concentration of carbon and hydrogen impurities is considerably lower than in polar films.