Al-doped ZnO thin films deposited by confocal sputtering as electrodes in ZnO-based thin-film transistors

Abstract

Aluminum doped zinc oxide (AZO) films were deposited by confocal RF magnetron sputtering at different substrate temperatures. AZO films with a transparency up to 90% in the visible spectrum were obtained. Polycrystalline AZO films, with planes in the (002) and (103) orientations of the zincblende (hexagonal) structure, were obtained at substrate temperatures higher than 60°C. The best AZO films, obtained at 75°C, showed an electrical resistivity of 8.8×10−4Ω-cm, with a carrier concentration of 4×1020cm−3 and mobility of 20cm2/V-s. These are appropriate values for solar cell applications. In addition, ZnO-based thin-film transistors (TFTs) were fabricated for evaluating the behavior of the AZO films as source and drain contacts on the transistors. The field effect mobility and the threshold voltage of the fabricated devices were 20cm2/V-s and 7V, respectively. The TFTs showed an Ion/Ioff ratio of up to 9 orders of magnitude. A specific contact resistance of approximately 0.06Ω-cm2 was determined for the AZO/ZnO interface. This result corresponds to the first report of the ZnO/AZO specific contact resistance obtained with a maximum processing temperature of 100°C. Therefore, this TFT technology is fully compatible with flexible substrates and can be used for transparent and large area electronics.