Research On Preparation Process And Structure Optimization Of ZnO-based TFT Devices

ZnO based transparent oxide thin film field effect transistor?TFT? has becomes the first choice to drive the active matrix organic light emitting display?AMOLED? due to its advantages such as low fabrication temperature, high transparent properties in the visible light region and so on. However,ZnO-TFT still has the problems of high threshold voltage, low mobility, and high subthreshold swing etc. This dissertation aims to improve and enhance the performance ofZnO based TFT devices by optimizing the technology and structure of the devices. The main contents and results are as the followings:1. MgZnO thin films were grown on heavily doped n-type Si substrate with a layer of SiO2?SiO2/n+-Si? by pulsed laser deposition method?PLD?. A ceramic target of Mg0.1Zn0.9O was used as the source materials. The substrate temperature of 300?, oxygen pressure of 1Pa and target-substrate distances of 60 mm was selected as the growth condition. The optimization of micro-processing technologies, such as masking, lithography, baking, etching etc., was studied. A bottom gate type MgZnO-TFT device was designed and fabricated by using the lithography technology. The device off current?Ioff? reaches the order of 10-11 A, on/off current ratio is 7.2×105, and the rate of finished MgZnO-TFT devices is higher than 90%.2. Al₂O₃ thin films with a high dielectric constant were fabricated on n+-Si substrates by PLD technology. The effect of oxygen pressure on the crystallization of the Al₂O₃ thin films was studied. The Al₂O₃ thin films prepared under high oxygen pressure exhibit a completely amorphous structure. The research on the preparation of MgZnO thin films on Al₂O₃ substrate with different crystallization degrees is carried out. The results show that the MgZnO active layer prepared on the amorphous Al₂O₃ has a relatively complete crystalline and dense structure. The performance of MgZnO-TFT device with an amorphous Al₂O₃ as insulating layer was significantly improved. The threshold voltage downs to 0.7V, and the mobility reaches 6.7 cm2V-1s-1. In particular, the sub-threshold swing is obviously reduced to 0.138V/decade, this indicates that the regulation ability of the gate voltage on the channel current is significantly enhanced.3. The effect of active layer growth process and structure optimization on the performance of TFT device was studied. It is found that under a high-power laser condition, a lower growth temperature can ensure the crystal quality of the active layer, in which the interfacial defects between the active layer and the insulating layer can be reduced, leading to a lower subthreshold swing. The device with an active layer using MgZnO thin film grown at 300 ? has the best performance. The characterization ofZnO/MgZnO heterostructure fabricated by PLD method was studied. The absorption fromZnO layer and MgZnO layer was observed by the absorption spectra at room temperature, indicating that the formation of the heterostructures. The highest mobility ofZnO/MgZnO heterostructures is 53.69 cm2V-1s-1 at 300 K obtained by Hall effect measurements. A significant increase of the mobility is considered to be related to the formation of two-dimensional electron gas?2DEG? in the interface of the heterogeneous structure. UsingZnO/MgZnO heterostructures with differentZnO thicknesses as the active layer, bottom gate typeZnO/MgZnO-TFT device was fabricated. It is noted that the device performance can be improved obviously, in which the obtained best performance parameters is as follows: the threshold voltage is 1.16 V, on/off ratio up to 4.25 x 107, the mobility reaches 16.5 cm2V-1s-1, the subthreshold swing reduces to 0.126V/decade.4. To further improve the performance of MgZnO-TFT devices, the optimization of the device structure is discussed. Compared with the performance parameters of the device with different channel width to length?W/L? ratios, it is found that in the same source drain voltage and gate voltage, the source drain current of the devices increases with the increase of the W/L ratio, while the threshold voltage decreases with the increase of the W/L ratio. Bottom-gate MgZnO-TFT with a layer Al₂O₃ as the protective films was fabricated. The devices with and without protective layers were placed in the same environment conditions for 3 months, then they were measured again. The result shows that the performance stability of TFT device with the protective layer is better than that of the device without protective layers. Top-gate MgZnO-TFT using Al₂O₃ as insulating layer is designed and fabricated. The on/off current ratio is the orders of ¹03, the subthreshold swing is 1V/decade, and the mobility is as low as ¹cm2V-1s-1. This will lay the experimental foundation to further studyZnO/Zn MgO heterostructure Schottky barrier field-effect transistor?MESFET?.