Growth of Ga-doped ZnO thin film prepared by MOCVD for TCO application

The group-III elements, such as Al, Ga and In, are possible dopants for ZnO to improve the electric conductivity of ZnO thin film. Since Ga has lower cost than In and has higher oxidation resistance than Al, it becomes the preferred dopants for ZnO thin film for transparent conducting oxide (TCO) application. In the research, we used MOCVD method with Ga doping to prepare the Ga-doped ZnO thin film (GZO) on the corning-1737 glass substrate. We changed the amount of Ga doping into ZnO thin films to increase the carrier concentration and decrease the electric resistivity of ZnO thin film. We used dimethylzinc (DMZn) and oxygen as Zn and O sources, respectively, and kept both of them constant. The trimethylgallium (TMGa) was used as Ga source, and the Ga/Zn was controlled in the range of 0.8% to 2.8% by adjusting TMGa flow rate. From the top views of GZO thin films, we observed that there are small grains arranged loosely on the surface of the undoped ZnO thin film. After doping Ga into ZnO thin films the grains become bigger, and the arrangement of grains becomes compact. However, when the Ga/Zn flow ratio is over 1.6%, the grains of the GZO thin film become small again. The variation trend of grain size observed by SEM is similar with the results of FWHM in XRD spectra. The FWHM decreases as Ga/Zn flow ratio increases to 1.6%, but then the FWHM increases as Ga/Zn flow ratio increases from 1.6% to 2.8%. It implies the crystallinity of GZO thin films was improved with the Ga/Zn flow ratio and has the best crystallinity at 1.6% of the Ga/Zn flow ratio. Afterwards the crystallinity of GZO thin film worsens when the Ga/Zn ratio increases from 1.6% to 2.8%. The Hall measurement shows the electrical resistivity decreases with increasing Ga/Zn flow ratio, and the carrier concentration increases with Ga/Zn flow ratio. In addition, we found the mobility is still about 29 cm/Vs even as the carrier concentration has the highest value around 7.1×10 cm. It reveals that the Ga doping can significant improve the conductivity and increase the carrier concentration. Since the site which the Ga occupies in ZnO thin films can influence the electric behavior and the structure, we will use XPS to determine the Ga bond for determining the site of Ga occupied. Because the transparency is an important factor for TCO application, we will also show the transparency of GZO by the transmission spectra.