Influence of grain size and room-temperature sputtering condition on optical and electrical properties of undoped and Ga-doped ZnO thin films

The optical and the electrical properties of undoped zinc-oxide (ZnO) thin films of various thicknesses were compared with those of Ga-doped (GZO) thin films. Transparent, high-quality undoped ZnO and GZO films were deposited successfully using radio-frequency (RF) sputtering at room temperature. The films were polycrystalline with a hexagonal structure and a strongly preferred orientation along the c-axis. The films had an average optical transmission >85 % in the visible part of the electromagnetic spectrum. The undoped ZnO thin films were more transparent than the GZO thin films. The ZnO thin-film transistors (TFTs) were operated in the enhancement mode with a threshold voltage of 2.5 V. In contrary, the Ga-doped ZnO TFTs were operated in a depletion mode with a threshold voltage of-3.4 V. We successfully demonstrated undoped and the Ga-doped ZnO TFTs by using conventional SiO₂ gate insulators at room temperature. We postulate that undoped ZnO films, which have not been treated to improve the optical properties, can be used, instead of doped ZnO films, in transparent devices for next generation optoelectronic devices.