Improvement in the Device Characteristics of Tin Oxide Thin-film Transistors by Adopting Ultralow-Pressure Sputtering

摘要

Recently, transparent oxide semiconductor (TOS) TFTs like as ZnO, In_2O_3-ZnO, In_2O_3-Ga_2O_3-ZnO, and ZnO-SnO_2 have been intensively studied the back-plane of active matrix organic light emitting diode, active matrix liquid crystal display, and flexible display because they exhibit high mobility (> 5 cm~2/Vs) and good transparency in the visible range compared to the counterpart of amorphous Si TFTs. (1-4) Among of a various kind of TOSs, tin oxide have been firstly studied to apply the TFTs because of a proper band gap of 3 - 4 eV and a high optical transparency (> 85 %) in the visible light range. A few studies only are reported to the application of TFTs since 1964, their reports also were focused on the device characteristics. That is, all these tin oxide TFTs are depletion-mode devices, requiring the application of a gate voltage to turn them off. (5-7) Therefore, its electron density was more important factor to use the active channel of TFTs. In particular, lots of studies reported the electron density (N_d) of sputtered tin oxide film for the application of transparent conductive oxide has been related to the crystalline structure, oxidation state, and oxygen deficiency. (8-10) We have to try the novel approach to decrease Nj. According to the recently report of authors on the electrical properties of indium doped tin oxide (ITO) films as a function of sputtering pressure, their properties were closely related to the kinetic energy of sputtered particles. In this article, we report the fabrication of TFTs with the SnO_x film as channel layer by adopting ultralow-pressure sputtering. An ultralow-pressure sputtering (ULPS) method means that the sputtering pressure is below 1.3 * 10~(-1) Pa compared to an conventional sputtering pressure (CSP) of over 6.7*10~(-1) Pa. Decreasing sputtering pressure from 6.7*10~(-1) to 6.7*10~(-2) Pa significantly decreased the N_d from 10~(19) to 10~(17) cm~(-3), thereby producing a transistor