Reactive Radiofrequency Sputtering-Deposited Nanocrystalline ZnO Thin-Film Transistors

The structural and electrical properties of ZnO films deposited by reactive radiofrequency sputtering with a metallic zinc target are systematically investigated. While the as-deposited ZnO film is in a poly-crystalline structure when the partial pressure of oxygen (pO₂) is low, the grain size abruptly decreases to a few nanometers as pO₂ increases to a critical value, and then becomes almost unchanged with a further increase in pO₂. In addition, the resistivity of the ZnO films shows a non−monotonic dependence on pO₂, including an abrupt transition of about seven orders of magnitude at the critical pO₂. Thin−film transistors (TFTs) with the nanocrystalline ZnO films as channel layers have an on/off current ratio of more than 10⁷, an off−current in the order of pA, a threshold voltage of about 4.5 V, and a carrier mobility of about 2 cm²/(V⋅s). The results show that radiofrequency sputtered ZnO with a zinc target is a promising candidate for high-performance ZnO TFTs.