LAUSR

ZnO has attracted much attention due to its wide bandgap (3.2 eV) and high exciton binding energy of 60 meV. These properties make ZnO a highly desirable material for high frequency devices that can work in harsh environment. We have grown ZnO thin films at different temperatures ranging from 100 ◦C to 500 ◦C. We have observed that surface roughness is first decreased with the increase in the growth temperature but then by further increasing the growth temperature beyond 300 ◦C, results in increased surface roughness of the grown samples, whereas grain size of the samples increases with the increase in the growth temperature. Crystalline quality of the films is also improved with the increase in the growth temperature but then degrades by further increase beyond 200 ◦C. We achieved the highest Hall mobility for the ZnO sample grown at 200 ◦C. The optimum growth condition of ZnO thin films on sapphire (0001) in our RF/DC magnetron-sputtering unit were achieved for the films grown at 200 ◦C. Subsequently, we performed pre-growth treatment to the sapphire substrate then grew ZnO films at 200 ◦C. Pre-growth treatment involved heating the substrate at 500 ◦C for about half an hour and then etching the sapphire surface with nitrogen plasma. We have observed that pre-growth heating and nitridation of the sapphire substrate results in bigger grain size whereas no improvement was observed in the crystallinity of the film.