Exploration Process And Performance Optimization Of IGZO-TFT Devices

Because of the low preparation temperature, high mobility, good transparency and low production cost, InGaZnO-TFTs have attracted world-wide attentions, and may replace the traditional silicon-based TFTs as the mainstream of display technology in the next-generation. Currently, the IGZO target with the atomic ratio of In:Ga:Zn=2:2:1 or 1:1:1(the percentage of In and Ga even more high) often be used to prepare amorphous IGZO-TFTs. However, In and Ga are rare elements and suffer from serious shortage of the mineral resource and rising prices. Recently, Sharp Corporation reported superior device performance with extremely low off-state current(about 10-24A) using the crystalline IGZO-TFTs with a target of In:Ga:Zn=1:1:1. However, the preparation process is quite complicated. To obtain crystalline IGZO-TFTs, one method is to reduce of the In and Ga doping amount. Therefore, this thesis present detailed studies on preparation of IGZO active layers with low doping content of In and Ga(In:Ga:Zn=0.42:0.25:1) for IGZO-TFTs, and to optimize the preparation process to improve the device performance.In this thesis, bottom-gate top-contact TFT devices were fabricated by depositing ZnO active layer through RF magnetron sputtering method onto the SiO2/p+-Si substrate. The resulting device had heavily doped p-Si substrate as the gate electrode and the SiO2 as insulating layer. Since large-sized ZnO-TFTs prepared by the metal mask technology have a broad area of conducting channel, the leakage current of device is usually large. On the contrary, small-sized IGZO-TFTs prepared by the photolithography process can generally reduce the leakage current and improve the success rate of the devices. Therefore photolithography process was used to prepare small-sized TFT devices in this thesis.The IGZO active layer prepared without oxygen usually contains a large number of donor defects(VO, Zni, etc.), leading to high carrier concentration, poor crystalline quality,rough active layer surface and large surface defect state density of the conductive channel,which will decrease the device performance of IGZO-TFT. Hence, adding a small amountof oxygen during the sputtering process can generally facilitate the carrier transport in the conducting channel. The device prepared with 1.0sccm oxygen flow rate shows the best performance with a mobility of 4.49cm2/Vs, a threshold voltage of 13.01 V, an on/off current ratio of 2.08×10~7, and a subthreshold swing of 2.126V/Dec. Increasing oxygen flow rate would accumulate adsorbed oxygen and acceptor-like defects, which would result in poor device performance of IGZO-TFTs because of the poor crystallinity of IGZO active layer,increased defect states density and enhanced trapping and scattering of the carriers.In order to further improve the performance of IGZO-TFT devices, air annealing treatment is performed for the active layer before depositing source and drain electrodes.With the increase of annealing temperature, the performance of the TFT devices improved.The best performance was obtained at 400°C with a mobility of 8.96cm2/Vs, an on/off current ratio of 1.23×108, a threshold voltage of 0.98 V, and a subthreshold swing of1.460V/Dec. This is because the oxygen is absorbed by the active layer and gradually diffuses into the film to reduce the donor defects when increasing the annealing temperature.Meanwhile, the surface roughness of the IGZO thin film is reduced and the density of defect states in the channel is decreased resulted in the enhanced charge transport properties and improved device performance. However, when the annealing temperature is higher than400°C, the device performance would gradually decrease because the acceptor defects in the IGZO active layer and the active layer surface roughness increased. On the other hand,after applying gate bias of 20 V for 60 min, the IGZO-TFT without air annealing shows a threshold voltage shift of 13.03 V, while the device annealed at 400°C shows a threshold voltage shift of 8.46 V. This indicated that the bias stress stability is enhanced at the annealing temperature of 400°C due to the reduction of the surface defect density, which weakened trapping effect of the free charge.In order to further reduce the subthreshold swing of IGZO-TFT devices, we studied the effect of active layer thickness on the performance of TFT devices. With the increase of the active layer, the threshold voltage decreases, and the mobility firstly increased and then decreased, while the subthreshold swing increases gradually. The IGZO-TFT with an active layer thickness of 30 nm shows the best performance with a mobility of 10.05cm2/Vs, a threshold voltage of 0.38 V, an on/off current ratio of 9.53×10~7, a subthreshold swing of0.672V/Dec. The threshold voltage decreased because of more free charges in the thicker active layer. The charge transport process would easily be affected by the scattering effects from the inner surface of active layer when the active layer is too thin. If the active layer is too thick, the number of defect states will increased and the carriers are more likely to be scattered, which will decrease the mobility. With the increase of the thickness of the active layer, the density of the defect states at the conductive channel surface are increased, and the trapping and scattering effects of the carriers are enhanced, which will lead to the deterioration of the subthreshold.