Preparation And Characteristics Investigation Of InSnZn Oxide Thin Film Transistors

As pixel switch, TFT is used widely in flat panel display and flexible display such as liquid crystal displays, active matrix organic light emitting diode, electronic paper and so on. There are high requests for TFT due to the display products needing to exhibit superior performances in viewing angle, response time, screen size, contrast ratio, power consumption and other aspects. In recent the last 7 years, ITZO TFT has attracted more attention owing to its high mobility, low sub-threshold voltage, and it has been expected to work as the core unit of the display field, however, researches about ITZO TFT are insufficient, deposition methods and process parameters are in exploration. Films deposited by ratio frequency magnetron sputtering system have good uniformity and repeatability, but systematic studies of ITZO TFT which prepared by the technique have not been reported. Therefore, in this thesis, ITZO TFT was prepared by RF magnetron method on silicon substrate at room temperature, influences of the process parameters and the active layer thickness on device performance in electricity and stability were investigated. The stoichiometry ratio of ITZO target is In2O3:SnO2:ZnO=88:10:2 wt%.(1) Effects of sputtering power on ITZO TFT electrical performance were studied, power variation range was 50-80 W. The results reveal that the current on to off ratio is limited by lower power; on the contrary, device is invalid for the large close state current at higher power. When the sputtering power is 60 W, ITZO TFT shows minimum off state current of 10-10 A, and a high current on to off ratio of 105.(2) ITZO TFT was fabricated at working pressure of 0.4,0.6,0.8,1.0 Pa, respectively, and the effects of pressure on the device properties were investigated. As the pressure increase, the mobility decreases gradually and the negative threshold voltage decreases firstly and then increases, which possibly because of the dominance of the carriers concentration and interfacial defects density. A suitable pressure to prepare ITZO TFT is 0.6 Pa.(3) ITZO TFT was deposited with oxygen partial pressure of 12%,17% and 19 %, respectively. With the increasing of oxygen partial pressure, the threshold voltage changes from negative to positive, and the mobility increases initially and then decreases. ITZO TFT exhibits optimized properties at oxygen partial pressure of 17%, while a mobility of 6.28 cm2/Vs, a current on to off ratio over 105, and a sub-threshold swing of 0.60 V/decade.(4) Electrical properties of ITZO TFTs with different active layer thickness were investigated. Fabrication conditions were set as follows:sputtering power of 60 W, working pressure of 0.6 Pa and oxygen partial pressure of 17%, thickness of the ITZO layer was 16,25,36,45,55 nm, respectively. With the increase of thickness, TFT mobility changes into an increase-decrease way, but the threshold voltage changes in the opposite trends against mobility; all TFTs show excellent performance except the one with thickness of 16 nm, whose current on to off ratio higher than 105 and the sub-threshold swing below 0.8 V/decade.(5) The stability of devices were studied. The TFTs with different active layer thickness were placed in atmosphere for 20 days and then studied their properties. It’s found that device with 16 nm active layer has the worst output characteristics; the threshold voltage shift reached 12.51 V for the TFT with 36 nm layer; but the drift value is only 1.07 V for ITZO TFT with 55 nm layer, so it has good stability.It’s found that:after heating at 60 degrees in air atmosphere for 360 s, ITZO TFTs with active layer thickness of 45 and 55 nm have good electrical properties, the mobilities improve more than 3 times, practically reaching the original value; the threshold voltages increase slightly; the sub-threshold swings are below 0.8 V/decade, which have a little changes compared with the initial ones. It can be concluded that: increasing the active layer thickness can weaken the erosion caused by air and water on the TFT; and improve the stability of device; heating is an effective method to inhibit bad effect of air on the device and to enhance the TFT performance in threshold voltage.