Study On Novel Materials And Processes For Oxide Thin-Film Transistors

Thin film transistors (TFTs) based on amorphous oxide semiconductors (AOSs) have attracted extensive attention for their potential applications in liquid crystal displays, organic light-emitting diodes, transparent devices and flexible electronics, due to their high carrier mobility, high transparency in the visible region and low temperature processability. Although AOS-based TFTs have been considered as the most promising candidate to replace the silicon-based TFTs in display area, to obtain better electrical performance and stability, there is still a lot of work to be investigated in oxide channel, gate dielectric material and preparation process research. In this dissertation, our research works mainly focus on the development of novel oxide semiconductor and gate dielectrics, new process technology of infrared-ray irradiation annealing and oxygen plasma treatment for sol-gel processed AOSs. Main research work and conclusions are summarized as follows:Amorphous high-valence tungsten doped indium oxides (a-IWO) were prepared by direct current magnetron sputtering for the first time. The a-IWO films exhibit great transparency in the visible range (400-700 nm) with an average transmittance above 85%, and excellent smoothness and uniformity with the largest average roughness of 0.436 nm. a-IWO-TFTs with a-IWO films as channel layers and spin-coated poly(4-vinylphenol) (PVP) as gate dielectric were fabricated. The measurement suggests that tungsten plays the role of electron suppressant, and a moderate amount of tungsten is of benefit to the device performance. The optimal performance with a saturation mobility of 8.3 cm2/Vs, subthreshold swing of 0.58 V/dec, on/off current ratio of 5.23 ×105 was obtained at tungsten doping of 7.5 at % in a-IWO channel layer. And a small hysteresis of 0.47 V indicated the good stability of the TFT device. It is found that appropriate atmosphere annealing of a-IWO thin films promotes the oxidization of low valence state of tungsten ions and repairs the oxygen vacancies in the shallow region of top surface. Due to the decrease of trap state, top surface provides a high mobility path for electron movement. So the mobility of a-IWO-TFTs increased from 8.3 cm2/Vs to 12.6 cm2/Vs after annealing. However, longer time annealing will drop the total carrier concentration and roughen the topography of the channel, and results in small mobility and large subthreshold swing. This work implies that high-valence doped a-IWO film is a promising channel material with the potential applications in high-performance and transparent TFTs due to its enhanced semiconductor property.Solution-processed AOS-TFTs normally show inferior performance. Aiming at solving this problem, process study was carried out. The impact of oxygen plasma treatment on the solution-processed amorphous indium-gallium-zinc oxide (a-IGZO) TFTs was explored. It was observed that the oxygen plasma treatment significantly promoted the mobility and the on/off current ratio, increased from 0.072 cm2/Vs and 8.23 X 104 to 1.07 cm2/Vs and 2.99 X 104, respectively, while the subthreshold swing showed a monotonical growth with plasma treatment time as well, from 0.84 V/dec to 2.01 V/dec. According to the photoluminescence (PL) measurement, there were two components in oxygen plasma, atomic oxygen and oxygen molecule ion O2+. Atomic oxygen reacted with oxygen vacancies in channel layer resulting in an improved mobility, and O2+ tended to aggregated at the surface acting as trapping states, causing the growth of subthreshold swing. Consequently, in order to achieve the expected device performance, the atomic oxygen should be encouraged and the O2+ be avoided as far as possible.Generally, solution-processed AOSs require high-temperature annealing (>400℃). In order to lower the annealing temperature, a new annealing process of infrared-ray (IR) irradiation was proposed. It was demonstrated by Fourier Transform Infrared Spectroscopy (FTIR) that IR irradiation was six times as strong as oven annealing on the effect for removal of organic species and decomposition of metal hydroxides. When raising the irradiation power density and extending the irradiation time, it’s conducive to reduce the residual organic groups and metal hydroxides, and promote the formation of metal oxides,. The optimized a-IGZO-TFT with a spin-coated PVP as gate dielectric and a-IGZO film annealed using a power density of 2.54 W/cm2 exhibited a saturation mobility of 2.04 cm2/Vs, subthreshold swing of 0.84 V/dec, on/off current ratio of 1.52×106, and the process temperature was only 300 ℃. Our results indicate that IR annealing is a novel effective annealing method.High-k materials generally suffer from the poor thermal stability, easy to crystallization, narrow bandgap, and low breakthrough voltage. To solve those problems, a hybrid high-k material of titanium-aluminum oxide (TAO) was proposed to be prepared by sol-gel method. TAO films with a permittivity between 8.7-33.4, and an optical bandgap between 5.6-3.8 eV were obtained by varying the components of the films. Besides, TAO films showed great thermal stability and stayed in amorphous even annealed at 800 ℃. TAO films with 40 at% Ti exhibited a remarkable comprehensive performance, a permittivity of 16.5, and a leakage current density of 4×10-7 A/cm2 at 2 MV/cm. Solution-processed a-IZO-TFT with T0.4A0.6O as gate dielectric showed a field-effect mobility of 3.92 cm2/Vs, subthreshold swing of 0.56 V/dec, on/off current ratio of 7.7×106, and threshold voltage of 1.76 V. The small leakage current of 1.74x 10-7 A reflected the enhanced dielectric properties.Based on the aforementioned research, further study of solution-processed Magnesium titanium oxide (MTO) dielectric films were proposed. The influence of Ti content on films’ permittivity, bandgap, and leakage current density had been investigated in detail. The permittivity and leakage current density were proportion to the Ti content, while the bandgap did the reverse. A permittivity of 17.2 could be obtained in Mg0.6Ti0.4O film, meanwhile the leakage current density was ～9.1×10-7 A/cm2 at 20 V. Solution-processed a-IZO-TFT employing Mg0.6Ti0.4O as gate dielectric exhibited an improved subthreshold swing Of 0.32 V/dec, a moderate field-effect mobility of 3.41 cm2/Vs, the on/off ratio and threshold voltage were ～6×106 and -0.9 V, respectively.These results demonstrate the feasible use of solution-processed TAO and MTO thin films as a promising gate dielectric in oxide-TFTs. For different applications, trade-off needs to be made carefully among bandgap, permittivity and leakage properties.