Fabrication And Properties Of Low-temperature Solution-processed Metal Oxide Thin Film Transistors

Display is one of the most effective terminals for processing and disseminating information.Thin film transistors,as the current driving units of the display circuits and switching elements of the pixel units,are widely used in the fields of flat panel displays,flexible electronics,and transparent electronics.Oxide TFTs have received extensive attention in the past few years because of their merits such as high mobility,high transparency to visible light,good chemical stability,and so on.Solution process for oxide TFTs has many advantages including low cost,simple process,and controllable chemical composition.However,most of the metal oxide TFTs require a high temperature annealing process(?400?)to realize decomposition of precursors and removal of impurities,which is incompatible with flexible substrates.Therefore,it is necessary to seek a simple low-temperature fabrication process to prepare highperformance metal oxide TFTs.In this thesis,a series of researches have been carried out on the solution-processed low-temperature oxide TFTs.The conversion process of metal precursors to metal oxides,microstructures of oxide thin films,and the properties of the dielectric/active layer interfaces have been intensively studied.Low-temperature lightwave annealing technology has been developed and the fabrication process has been optimized to obtain high-performance metal oxide(IGZO,ZTO,In2O3)TFTs.It opens up a new way for the preparation of flexible electronic devices in the future.The main results are as follows:(1)Considering the high temperature of solution-processed IGZO TFTs,we proposed a new low-temperature annealing process of IGZO TFTs based on lightwave annealing,and systematically studied the effect of lightwave annealing time,Ga element composition,and stabilizer content on the properties of IGZO thin films and TFTs.Lightwave annealing can induce transformation of metal precursors to metal oxides at 230 ?.Compared to traditional high temperature annealing(400?),IGZO thin films prepared by lightwave annealing exhibited more uniform surface morphology,lower trap density and higher metal oxygen bond content.With the increase of lightwave annealing time,impurities in the films were gradually removed,mobility of the device increased,and threshold voltage was negatively shifted.The increase of Ga content can effectively reduce oxygen vacancy concentration in the films and reduce off-state current of the device.Based on above results,optimized fabrication condition of IGZO TFTs was:irradiation time of 60 min,In:Ga:Zn ratio of 3:1:2.Low-operatingvoltage IGZO TFTs were fabricated by using solution-processed AlOx thin film as dielectric layer.With operating voltage of 4 V,mobility of the device reached 37.5 cm2V-1s-1,threshold voltage was 0.78 V,and on/off current ratio was about 106,realized the high-performance IGZO TFTs.(2)Due to the scarcity of In resources,In-based TFTs show limitation in low-cost electronic device application,so the development of In-free oxide TFTs is extremely important.Besides,among the commonly used metal precursors,chloride precursors show high stability and are benefits to prepare high-quality films.However,metal chlorides are difficult to be converted into oxides at low temperatures(?400?)at present.To solve above problems,a low-cost low-temperature solution-processed ZTO TFTs preparation scheme was designed with ethanol as solvent and metal chlorides as precursors.The results showed that lightwave annealing can not only convert chloride precursors into metal oxides,but also effectively remove Cl residues in ZTO films,besides,the electrical performance of fabricated ZTO TFTs was better than that of ZTO TFTs with high temperature annealing(400?).On this basis,effects of lightwave annealing time and Zn:Sn ratio on the properties of ZTO films and TFTs were systematically studied.With the increase of Sn content,electron and oxygen vacancy concentrations of the ZTO films increased.As lightwave irradiation time was 60 min and the ratio of Zn:Sn was 5:5,devices showed best performance.AlOx as dielectric layer,low operating voltage ZTO TFTs were fabricated.Carrier mobility of the ZTO device obtained under optimal preparation conditions was 30.4 cm2V-1s-1,threshold voltage was 0.88 V,and the on/off current ratio was larger than 105,.Under 1 V gate bias voltage for 5400 s,threshold voltage offset was+0.23 V,showing good bias stability.(3)In order to solve the problem that In2O3 TFTs cannot take into account both high mobility and low off-current,dual active layers InO3/IGZO TFTs and IGZO/In2O3 TFTs were designed and fabricated by introducing IGZO channel layer.Firstly,In2O3 thin films with high density and low impurity content were prepared by lowtemperature lightwave annealing process,and In2O3 TFTs with high carrier mobility were successfully fabricated.Secondly,the effect of active layer thickness on In2O3 films and TFTs was studied.With the decrease of active layer thickness,the number of carriers in the films gradually decreased,and carrier mobility decreased obviously.Based on SiO2 dielectric layer,carrier mobility of 7 nm In2O3 TFTs was 11.1 cm2V-1s-11,on/off current ratio was?105,and threshold voltage was-2.98 V.IGZO active layer can provide higher electron barrier for In2O3 active layer and reduce carrier concentration in active layers.Prepared dual-active layer TFTs realized the regulation of off-state current and threshold voltage of monolayer In2O3 TFTs.Off-state current was reduced by an order of magnitude,the devices worked in enhanced mode,and stability was significantly improved.In IGZO/In2O3 TFTs structure,low carrier concentration in IGZO active layer effectively regulated off-state current of the device,and protected the device by acting as a passivating layer.Mobility of IGZO/In2O3 TFTs was 8.05 cm2V-1s-1,on/off current ratio was higher than 105,threshold voltage was 3.78 V,and threshold voltage offset was+3.41 V under 40 V gate bias voltage for 5400 s,exhibited high-performance dual-layer TFTs.