The Preparation And Research Of High Performance Oxide Thin Film Transistors

Wide-bandgap oxide semiconductors are special materials that combine electrical conductivity with optical transparency.They are widely used in electronic and optoelectronic devices,such as:thin film transistors,gas sensors,solar-blind detectors,solar cells,light emitting diodes,and transparent electronics.At present,oxide thin film transistors are mainly used in pixel switches,drivers and transparent electronic circuits of flat panel displays.Among its numerous material fabrication methods,the solution method is a promising fabrication technique for the fabrication of metal oxide thin film transistors,due to its environmentally friendly process with high output,low cost,controllable composition and simple operation.However,the development of aqueous solution-processed oxide thin film transistors has been limited due to the low performance and stability issues.At present,researchers are committed to improving the carrier mobility and stability of thin film transistors by using element doping,molecular doping,new material preparation,and device structure optimization.This paper mainly focuses on doping engineering,heterojunction engineering,quantum well engineering and high dielectric constant engineering to improve the mobility and stability of metal oxide films.In order to the key problems of high-power consumption and high-operating voltage of traditional structure thin film transistors and promote the the development of their practical applications,we fabricated the oxide thin film transistor based on the traditional Si/SiO2 substrate structure,and achieved low power consumption and low operating voltage.The main research contents and innovative achievements of this paper are listed as follows:(1)High-performance InGaO thin film transistors were fabricated by an aqueous solution method.Five types of IGO films with different molar ratios were prepared by doping engineering and aqueous solution method,and the corresponding AFM,XPS,XRD and other material characterizations were carried out.With the increase of In concentration,the IGO films exhibit the decreased and larger carrier concentration,which is mainly caused by the increased oxygen vacancies.Corresponding thin-film devices were prepared to explore the electrical properties.In dopant can activate gallium oxide thin film transistors at low temperature,and greatly reduce the annealing temperature of gallium oxide alloy thin films.The TFT based on IGO film annealed at 450℃ with Cln=50%exhibits good electrical performance with μsat of 3.63 cm2V-1s-1、Ion/off of 106、SS of 1.38 V dec-1 and Nt of 2.02 ×1012 cm-2eV-1.The device also demonstrates good bias stress stability.Under the action of+20 V and-20 V gate bias for 3000 s,the ΔVTH is+2.27 V and-1.95 V,respectively.(2)Preparation of high-performance In2O3/IGO heterojunction thin film transistors based on an aqueous solution method.In order to solve the problems of high annealing temperature and low mobility of single-layer IGO thin films,the corresponding IGO/In2O3 and In2O3/IGO heterojunction thin film transistors were fabricated with using heterojunction engineering.The study found that the carrier transport mode of the heterojunction thin-film transistors changed from trap-limited conduction(TLC)to percolation conduction(PC),and finally the PC dominates the channel carrier transport.The In2O3/IGO heterojunction interface is similar to two-dimensional electron gas structure,resulting in the accumulation of carriers at the interface,and this heterojunction exhibits the largest carrier mobility.The IGO/In2O3 heterojunction TFT showed a carrier mobility of about 14 times higher than single-layer IGO TFT.It is worth noting that the ΔVTH of IGO/In2O3 and In2O3/IGO heterojunction TFTs are+0.54 V and+2.93 V,under the gate bias of+10 V(t=2400 s).While the ΔVTH of the single layer In2O3 TFT is+3.52 V,and the ΔVTH of single layer IGO is+5.01 V at the same conditions.It is obviously that the heterojunction structures enhanced the device stabilities.(3)Heterojunction and quantum well engineering synergistic optimization of thin film transistors.Using heterojunction and quantum well engineering,six types of thin film transistors were prepared,which include single layer In2O3,single layer IZO,In2O3/IZO heterojunction,IZO/In2O3 heterojunction,In2O3/IZO/In2O3 quantum well and IZO/In2O3/IZO quantum wells.The internal polarization of the oxide,the carrier transport model,and the contact resistance are analyzed.The In2O3/IZO heterojunction TFT exhibits the best electrical properties with μsat of 30cm2V-1s-1,VTH of 10 V,and Ion/off of 107.(4)AGO-based low-power thin film transistors were fabricated by an aqueous solution method.Firstly,the film preparation and characterization of five types of AGO thin films by solution method were carried out.Al dopant could increase the bandgap and improve the breakdown field strength of the thin films.The MIM structure was prepared,and the capacitance and breakdown field strength were statistically analyzed.The corresponding thin film transistors were prepared by using the AGO thin film.Moreover,the effects of the annealing temperature of the insulating layer AGO thin film and the IZO annealing temperature of the active layer on the device performance were investigated.Finally the thin film transistor with the optimal performance was obtained.The A0.5G0.5O(450℃)/IZO(300℃)thin film transistor exhibits the best performance with μsat of 55.4 cm2V-1s-1,VTH of 0.1 V,Ion/off of～104,SS of 0.29 V dec-1,and Nt of 7.68 × 1012 cm-2eV-1.Under the gate biases of+1 V and-1 V(t=3200 s),this device also exhibits good bias stability with ΔVTH of+0.17 V and-0.20 V,respectively.(5)Fabrication and performance exploration of low-power thin film transistors based on ferroelectric HZO.Firstly,the ferroelectricity of the HZO film was investigated with PFM characterization.From the transfer characteristics of the devices,it can be seen that the ferroelectric thin film transistors with 20 nm series exhibited better device performance in terms of leakage compared to with that of the 10 nm series.Different bias voltages were applied to pre-polarize the ferroelectric materials,and the effects of polarization voltage on the threshold voltage,current switching ratio and subthreshold swing of the device were analyzed.With the increase of gate polarization voltage,the subthreshold swings of all devices showed a decreasing trend,and the switching characteristics of the devices became more and more ideal.The threshold voltages of all devices further increased with the increase of the gate polarization voltage,which is beneficial to effectively control the threshold voltages required by the devices in the future experiments.