Sun-Jae Kim

Anion control as a strategy to achieve high-mobility and high-stability oxide thin-film transistors

Ultra-definition, large-area displays with three-dimensional visual effects represent megatrend in the current/future display industry. On the hardware level, such a “dream” display requires faster pixel switching and higher driving current, which in turn necessitate thin-film transistors (TFTs) with high mobility. Amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O are poised to enable such TFTs, but the trade-off between device performance and stability under illumination critically limits their usability, which is related to the hampered electron-hole recombination caused by the oxygen vacancies. Here we have improved the illumination stability by substituting oxygen with nitrogen in ZnO, which may deactivate oxygen vacancies by raising valence bands above the defect levels. Indeed, the stability under illumination and electrical bias is superior to that of previous AOS-based TFTs. By achieving both mobility and stability, it is highly expected that the present ZnON TFTs will be extensively deployed in next-generation flat-panel displays.

High performance gallium-zinc oxynitride thin film transistors for next-generation display applications

High speed thin film transistors (TFTs) are in great need for next-generation TVs which will employ ultra high definition resolution (3840×2160) panels and possibly include multi-view autostereoscopic 3D technology which will negate the use of glasses for 3D viewing mode. In order to achieve high mobility devices, various types of metal oxide semiconductors have been extensively studied, including the most popular In-Ga-Zn-O, with typical field effect mobilities ranging between 10 to 30 cm2/Vs. Although these numbers are much higher than that of conventional amorphous silicon (0.5~1.0 cm2/Vs) TFTs, there is a strong demand for even higher mobility semiconductors which can exhibit excellent uniformity over a large area.

High mobility zinc oxynitride-TFT with operation stability under light-illuminated bias-stress conditions for large area and high resolution display applications

We have investigated material and electrical properties of ZnON based on 1st principle calculations and TFT evaluations. Theoretically, ZnON has high mobility characteristics and band-structure for high stability. Fabricated TFTs exhibited high mobility (100 cm2/Vs), good uniformity, and stable operation performance such as -2.87 V of Vth-shift under light illuminated bias-stress condition. As a new approach to overcome the performance limit of oxide-semiconductors, ZnON technology is strongly promising to achieve high mobility and operation stability required for next generation displays.

Improvement of photo-induced negative bias stability of oxide thin film transistors by reducing the density of sub-gap states related to oxygen vacancies

The optical absorption in the sub-gap region of amorphous indium zinc oxide films and the photo-induced negative bias stability of the resulting thin film transistors were studied. As the indium ratio increases, optical absorption via sub-gap states increases, and the threshold voltage degradation under negative bias temperature stress (NBTS) with light illumination becomes more severe. By applying high pressure anneal treatments in oxygen ambient, the density of sub-gap states is reduced by an order of magnitude compared to air-annealed devices. Consequently, significant improvements are observed in the threshold voltage shifts and the stretched exponential parameters under NBTS with light illumination.

Defect Control in Zinc Oxynitride Semiconductor for High-Performance and High-Stability Thin-Film Transistors

The fabrication of thin-film transistor devices incorporating active semiconductors based on zinc oxynitride (ZnON) compound is presented. It is demonstrated that the addition of appropriate dopant, gallium, in ZnON, suppresses the formation of shallow donor, nitrogen vacancies, and significantly improves electrical characteristics of the resulting TFT. The Ga:ZnON devices with field-effect mobility values exceeding 50 cm2/Vs are achieved, which makes them suitable as switching or driving elements in next-generation flat-panel displays.