Fabrication and Characterization of Amorphous InGaZnO Thin Film Transistor for Flexible Devices

Abstract

The current interest in transparent amorphous oxide semiconductor (AOS) thin film transistors was performed in operation of amorphous indium gallium-zinc oxide, a-IGZO, TFTs on flexible, room temperature and polymer substrate. AOS area has rapidly devel-oped, with a-IGZO TFT addressed active matrix displays. AOS are a new area of materials with electrical and optical characteristics uniquely suited to transparent, flexible and large area electronics. The a-IGZO TFTs have considerable attracted interest due to a room tem-perature processing, a visible transparency and a large area uniformity. These are ideal prop-erties for using them as active layers in TFTs which form the backbone of active matrix dis-play and large area electronic applications. Nevertheless, for these devices to be commercial-ly viable for flexible electronics, new device design concepts with an electrically analysis of device operation and reliability should be considered. In this dissertation, a-IGZO TFTs on various substrates were studied in the respect of material characteristics, the design of de-vice and its electrical performances. One of the key features of a-IGZO TFTs have a much higher field-effect mobility (μFE = >10 cm2/V·s) compared to a-Si:H TFTs, a low threshold voltage exhibiting in en-hancement mode operation, excellent switching properties, and a small parasitic series re-sistance employing IZO as source/drain electrode without the additional contact doping process. Process optimization of a-IGZO TFT was performed in scaling channel length, temperature-dependence and oxygen dispersion on active surface. A bendable a-IGZO TFTs and inverter circuits was demonstrated on a thin glass sub-strate. From the bending tests on the TFTs, VTH was negatively shifted as an increase of the bending strain for the symmetric gate overlap sample, while the TFTs showed relatively sta-ble operation against mechanical strain for the asymmetric gate overlap sample. Owing to the high temperature thermal annealing process, the a-IGZO TFTs showed very good bias stress stability under prolonged positive and negative stress test. Therefore, transparent, flexible, and stable TFTs can be realized using the a-IGZO TFTs on the thin glass substrate which can open a new topic for flexible display applications. Finally, we demonstrated high performance and flexible a-IGZO TFTs with hole-array on polyimide substrate and investigated the variation in the electrical characteristics as a function of hole area and radius. The a-IGZO TFTs with hole-array device performance shows good electrical characteristics and the mechanical strain reduced remarkably in hole-array structure as compared with the TFT without hole-array. Electrical stability measure-ments of the flexible devices with hole-array structure under tensile and compressive me-chanical strain showed no appreciable change in the I-V characteristics during bending. The electrical characteristics under mechanical bending suggest that carrier transport was unaf-fected during mechanical strain. Testing under dc gate bias conditions, the electrical stabil-ity of the TFTs showed a positive VTH shift of 3.8 V after 3600 s without any change in subthreshold-swing (S.S.). The a-IGZO TFTs with hole array structure exhibits high on/off ratio of >106 and field effect mobility of >6 cm2/V·s even after high bending radius. The bending radius was set to 100 mm by considering minimum bending radius (tensile strain of 0.22 % perpendicular to the channel current flow). The a-IGZO TFTs with hole-array re-markably reduced more electrical failure than TFT without hole-array samples since discon-nected micro-cracks induced the release of mechanical strain. Thus, proposed hole-array structure of a-IGZO TFTs can give an important merit to use flexible devices. ⓒ 2017 DGIST