Investigation Of Tungsten Doped Indium Zinc Oxide Thin Film Transistor

The thin film transistors (TFT) using amorphous tungsten doped indium-zinc oxide (a-IZWO) thin film as channel layer and aluminium oxide as dielectric layer have been prepared in order to explore novel oxide semiconductor channel materials. The channel layer was prepared by radio frequency magnetron sputtering. The source, drain and gate electrodes are all prepared by flash evaporation. The influence of oxygen partial pressure, sputtering power and tungsten doping concentration on the properties of a-IZWO thin films and the corresponding performance of the TFT devices have been investigated for obtaining the optimal preparation conditions. Because the radio frequency magnetron sputtering and flash evaporation are operated under room temperature without the heating of the glass substrate, the a-IZWO-TFTs are compatible with the flexible electronics.The X-ray diffraction analysis shows that IZWO thin films prepared by radio fre-quency magnetron sputtering stay in amorphous state in spite of the oxygen partial pres-sure and tungsten doping concentration. The measurement of UV spectrophotometer shows that the films have a high average optical transmission over 80%(including glass substrate) in the visible range (400-700 nm). The results show that the transparency in-crease with the increase of the oxygen partial pressure. The tungsten incorporation acts as an oxygen vacancy suppressor in the a-IZWO thin film. X-ray photoelectron spec-troscopy of the a-IZWO thin film shows that the area of Ols peak associating with the oxygen-deficient regions decreases as tungsten doping concentration increases.The a-IZWO-TFTs using aluminium oxide as dielectric layer have been prepared by radio frequency magnetron sputtering. The results show that as the oxygen to argon ratio increases from 15% to 25%, the current on-off ratio of the devices first increase and then decrease. Both the currenton-off ratio and mobility of the device first increase and then decrease with the increasing of the tungsten doping concentration. These results are consistent with the conclusions of the XPS spectra of the a-IZWO thin films which also demonstrate that the oxygen deficiencies could be suppressed by W doping. At tungsten molar ratio of 6.2 at%, oxygen to argon ratio of 22.5% and sputtering power of 20 W, the optimized a-IZWO-TFTs with the saturation mobility of 11.1 cm2/Vs, current on-off ratio of 107, and subthreshold swing of 0.31 V/decade was obtained. TFTs with a-IZWO as channel layer and PVP as insulator layer have been prepared by pulsed plasma deposition, which also proves that it is applicable to use a-IZWO as semiconductor layer.The stability of the TFTs has been investigated. The hysteresis curve and threshold voltage shift of the devices under different time of positive gate voltage bias have been measured. According to the positive bias stability measurement, a-IZWO-TFTs show a threshold voltage shift about 6.0 V under 10 V gate voltage of 1500 s. In order to improve stability of the a-IZWO-TFTs, the devices have been annealed in vacuum and organic passivation layers have been prepared. The result shows that poly-4-vinylphenol (PVP) passivation layer is helpful in improving the performance of the devices. Fully Transparent TFTs have successfully been prepared with IZWO as transparent electrode and aluminium oxide as dielectric layer. The devices show current on-off ratio of 104, subthreshold swing of 0.8 V/decade, field effect mobility of 2.6 cm2/Vs and threshold voltage of 1.6 V, which verify the feasibility of the preparation of fully transparent devices with a-IZWO.