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(pO_(2))is low,the grain size abruptly decreases to a few nanometers as pO_(2)increases to a critical value,and then becomes almost unchanged with a further increase in pO_(2).In addition,the resistivity of the ZnO films shows a non-monotonic dependence on pO_(2),including an abrupt transition of about seven orders of magnitude at the critical pO_(2).Thin-film transistors(TFTs)with the nanocrystalline ZnO films as channel layers have an on/off current ratio of more than 10^(7),an off-current in the order of pA,a threshold voltage of about 4.5 V,and a carrier mobility of about 2 cm^(2)/(V.s).The results show that radiofrequency sputtered ZnO with a zinc target is a promising candidate for high-performance ZnO TFTs.