Study On Preparation Of Dual-active-layer TFT Devices Based On MgZnO/ZnO

Thin film transistor?TFT?has been widely used in active matrix display and large area sensor arrays.ZnO-based thin film transistor has the advantages of high optical transmittance,low growth temperature,high breakdown voltage and high electron mobility,which can meet the requirements of large area and high resolution display screen.Mobility is an important factor for the application of ZnO-based thin film transistors.However,polycrystalline ZnO-based TFTs come with a problem of low mobility due to the electron scattering at the grain boundaries.This study aims to optimize the active layer and its structure of the ZnO-based TFT devices in order to enhance their mobility and other performances,by designing aMgZnO/ZnO dual-active-layer TFT device structure which combines the advantages of both the ZnO and MgZnO materials.The main contents of this dissertation and results are shown as follows:?1?Study on the MgZnO thin-film preparation processMgZnO thin filmswere prepared by a solution method with different post annealing temperatures.The thin film samples prepared at 500?have crystallized structure withuniformdistribution of the grainsat the surface,and the luminescence peak of defects is weak.The XPS test further shows that the 2p peak of Mg is strong after annealing at 500?.Therefore,the film prepared at 500?has the best overall quality.MgZnO thin films with different Mg content have also been prepared.It has been found that doping amount of Mg by the solution method has a limit.When the content of Mg in the precursor solution is high,the crystallinity of the film decreases sharply,indicating not all the Mg in the precursor solution can be doped into the ZnO lattice.?2?Fabrication and properties of MgZnO TFT devicesMg_0.2Zn_0.8O thin films were prepared by the solution method with annealing at 450,500or 550?and used as active layers.A bottom gate top contact type TFT device was fabricated on as p-Si substrate by thermal evaporation deposition with Al as source and drain electrode.The results show that the overall performance of the device at 500?is superior,but the mobility is still too low,and the on/off current ratio is only 1.4×10³.The influence of spin coating process was studied for device performance optimization.It has been found that the uniformity and compactness of the thin films can be improved by using low-concentration solutions along with spin coating for multiple times.However,the performance of the device decreases due to the influence of the interface between the multilayers.It has been found that the Mg_0.2Zn_0.8O-TFT prepared by spin coating two layers has the better performance.The mobility reaches 0.03 cm²V^-1s^-1,which is 10 times higher than that of the monolayer counterpart.The on/off current ratio of the device has also increased by an order of magnitude to 1.1×10⁴.Due to the poor compactness of the films prepared by solution method,the mobility of the films is relatively low,which is similar to that of the literature(0.1 cm²V^-1s^-1).?3?Pulse laser deposition of ZnO thin films and study of the device performancesZnO thin films were prepared by the PLD method.The influence of oxygen pressure on the electrical properties of ZnO thin films was studied.The results show that with the increase of oxygen pressure,the interstitial oxygen?Oi?of acceptor-type defects increases,resulting in the decrease of the carrier concentration in the ZnO thin films.In the ZnO-TFT devices,it is found that the mobility of the devices is generally higher than that of MgZnO,but theon/off current ratio is relatively low.With the increase of oxygen pressure,the on/off current ratio of the device increases only slightly.The overall performance of ZnO-TFT device under the condition of 6 Pa is better.The device shows a mobility,threshold voltage,subthreshold swing and on/off current ratio of 4.42 cm²V^-1s^-1,3.7 V,8.7 V/Dec and 7.5×10³,respectively.However,there is a certain gap between the ZnO-TFT prepared by us and our peers.?4?Fabrication and performance of MgZnO/ZnOdual-active-layer TFT devicesThe MgZnO/ZnO double-layer structure was fabricated by employing the PLD and solution method for the preparation of the ZnO and MgZnO thin films respectively.The influence of the film thickness as well as fabrication process on performances of the dual-active-layer Mg_0.2Zn_0.8O/ZnO-TFT devices is discussed.It has been found that that Mg_0.2Zn_0.8O samples prepared by a single spin coating by using low-concentration solution and annealing at550?have lower porosity and better interfacial properties,which is beneficial to the fabrication of high-performance devices.The appropriate thickness of the ZnO film can effectively relax the lattice mismatch,reduce the off-state current of the dual-active-layer TFT,and increase the on/off current ratio.When the thickness of ZnO is 46.5 nm,the overall performance of the dual-active-layer device is better,with the mobility of 1.10 cm²V^-1s^-1,the threshold voltage of 9.1 V,the subthreshold swing of 3.4 V/Dec,and theon/off current ratio of2.2×10⁵.?5?Fabrication process optimization and performance study of the MgZnO/ZnO-TFT devicesThe dual-active-layer Mg_0.2Zn_0.8O/ZnO-TFT devices were fabricated by lithography.As compared to the devices fabricated by the shadow-mask-assisted deposition technique,the leakage current of the devices is effectively reduced due to the effective isolation of the active layer and the reduction of thewidth to length ratioofthe channel in the devices.The parasitic effect between different devices has also been avoided,so that the mobility is greatly improved.The characteristic parameters of the best-performance devices,including the mobility,subthreshold swing,and the on/off current ratio can achieve 2.11×10¹ cm²V^-1s^-1,9.9×10^-1V/Dec,and 1.5×10⁸,respectively.Comparisons with peers,the whole performance of dual-active-layer MgZnO/ZnO-TFT devices we fabricated are better.The on/off current ratio of the devices is close to the level reported in the literature.At the same time,the mobility of the device can achieve 2.11×10¹ cm²V^-1s^-1.The MgZnO/ZnO-TFT devices reported in the literatures:on the one hand,while the mobility is high,the on/off current ratio is relatively low(2.40×10² cm²V^-1s^-1?10²);On the other hand,the mobility is low,but the on/off current ratio is very high(7.56 cm²V^-1s^-1?10⁸).It is found that our results not only have high switching ratio but also improve the mobility of MgZnO/ZnO-TFT devices,which is of great significance to the research of dual-active-layer TFT devices.