Co-Ga codoping effect on preferred growth orientations and properties of ferromagnetic ZnO thin films

In this study, we demonstrate an effective route towards the tuning of growth behavior for ZnO thin film grown on amorphous substrates (quartz and glass) via Co-Ga codoping. It is revealed that the mono-doping of Co (5 at%) into ZnO initiates the preferential growth direction along the (101¯1) semi-polar direction rather than along the c-axis (0002), which is the well-known growth direction of pure ZnO. The additional introduction of Ga as a dopant (1 at%) significantly promotes this preferential growth on glass substrates, where the (101¯1) orientation is predominant. This was achieved by varying the growth conditions including oxygen pressure (maintained within a crucial narrow window) and growth temperature to tune the growth velocity anisotropies on different planes. The Zn(Co,Ga)O (ZCGO) films exhibit enhanced n-type conduction with electron concentrations of 1016-1017 cm−3, which is two orders of magnitude higher than that of ZnO:Co (ZCO). Optical band gap variations were observed for the ZCO and ZCGO films due to the sp-d exchange interactions. Magnetic measurements showed that ZCGO films are ferromagnetic systems with an anisotropic easy axis, indicating its intrinsic origin. It is expected that there are oxygen vacancies in an n-type environment that would be responsible for the observed ferromagnetism.