Electrical characteristics of oxide thin film transistors fabricated by a sol-gel process

Abstract

Despite relatively short history, oxide-semiconductor-based thin-film transistors (TFTs) are a promising technology for applications in large-area electronics and high definition. Especially, zinc oxide (ZnO) based materials such as indium zinc oxide (IZO), and indium gallium zinc oxide (IGZO) have been considered as alternatives to a-Si and poly-Si TFTs. Various methods, such as sputtering, pulsed laser deposition, chemical solution deposition, and chemical vapor deposition, have been proposed for the fabrication of oxide semiconductor TFTs. Among these methods, chemical solution depositions including a sol-gel are much more effective due to their advantages in cost-effectiveness, and large area and mass production capabilities. However, solution-processed oxide-semiconductor TFTs suffer from severe bias-stress instability for device operation. The oxide TFTs based on IGZO systems including indium and gallium have been regarded as the best candidates to achieve high channel mobility and stability. In spite of their excellent performance, the alternatives to substitute indium and gallium are still needed to overcome the limitation of reserves. In this thesis, I attempted to describe the electrical characteristics of oxide thin film transistors fabricated by using a sol-gel process and evaluate the feasibility of oxide thin film transistors substituted for indium and gallium. The IZO TFT was also studied to determine whether it could improve the stability of the sol-gel process. TFTs utilizing titanium-indium-zinc-oxide (TIZO) channel layers with different titanium (Ti) molar ratios were fabricated by using a solution process to investigate the effect of Ti incorporation in the IZO system as a substitute for gallium. X-ray photoelectron spectroscopy (XPS) spectra exhibited by the TIZO film have revealed that the vacancy-related oxygen 1s peak intensity decreased with increasing Ti molar ratio and the Ti 2p1/2 peak intensity increased. The addition of Ti atoms lead to decrease O2- ions that are related to the carrier concentration in the TIZO film due to the removal of oxygen deficiencies, resulting in a positive shift of the threshold voltage (VTH) and in a decrease of the Ioff. The on/off current ratio of the TFTs with a 10% Ti molar ratio was as large as 0.21 × 107, which is an indicative level of excellent device performance. TFTs utilizing titanium oxide (TiO2) were also fabricated by using a sol-gel process to verify the feasibility of TiO2 channel layer and investigate the electrical properties of TFTs according to various annealing temperature and ozone treatment. The TiO2 showed amorphous phase after thermal annealing at 300oC, but it represented a mixture of rutile and anatase after thermal annealing at 450oC. The on-current levels were definitely different between 300oC and 450oC and the mobility of the films annealed at 450oC was five times larger than that of the films annealed at 300oC. Furthermore, the TiO2 film was exposed to the ozone generated by using UV light to enhance the device performance, resulting in a shift of the VTH and in a decrease of the Ioff. The amount related to the variation in the concentration of oxygen vacancies and the trap levels were decreased due to the ozone treatment. The Ion/Ioff (3.85 × 105) of the TFTs was also significantly improved. TiO2 film after ozone treatment showed more uniform morphology and was more suitable in performance. As a result, TFTs based TiO2 for a channel layer had the relatively sharper subthreshold slope (SS) and fast mobility Finally, TFTs with IZO channel layers were fabricated by using a solution process to study long-term stability of sol-gel based IZO thin films. The channel and the SiO2 insulator layers were exposed to ozone to investigate the effect of ozone treatment generated by using UV. The enhancement of the SS was dominantly attributed to a decrease in the defect density and an increase in the adhesion by application of the ozone treatment on the SiO2 insulator layer. The results of positive-bias temperature stress for the ozone-treated and untreated IZO thin films showed that the VTH shift of untreated thin films was more positive than that of ozone treated films. The SS variation of the TFTs with an UV-ozone-treated IZO thin film was smaller than that of the TFTs with an as-deposited IZO thin film, which was indicative of an enhancement in the bias-stress stability. XPS spectra showed that both the number of the oxygen vacancies and trap sites for the as-deposited IZO film were larger than those for the UV-ozone-treated IZO film.