Study On Illumination Characteristics Of Amorphous Indium-gallium-Zinc Oxide Thin Film Transistors

Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide band-gap semiconductors, where oxides of different origins play an important role, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In the field of flat panel display, there are two solutions:Hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs) and low-temperature polysilicon thin film transistors (LTPS TFTs), which have already been applied to the market, such as active matrix organic light emitting diode display (AMOLED) and active matrix liquid crystal display (AMLCD). However, as the increasing electronic products performance requirements, the above methods can no longer meet market demand.As compared to a-Si:H TFTs and LTPS TFTs, amorphous indium-gallium-zinc oxide thin film transistors (a-IGZO TFTs), have more prominent advantages, such as high optical transparency, lower off-state leakage current, relatively low preparation temperature, high field-effect mobility, etc. When a-IGZO TFTs are integrated in the circuits, the most critical problem faced is the electrical stability, especially for the back illumination in panel displays and UV irradiation in UV detectors. In this thesis, we present the fabrication and illumination characteristics of a-IGZO TFT devices. The main work accomplished includes the following:1. A high performance inverted-staggered a-IGZO TFT device is fabricated on a heavily doped n-type silicon substrate. Under a positive gate-bias stress (PBS), the device degrades over time and the threshold voltage shift is measured to be ～1.62 V after a 1500-s stress. As compared, the threshold voltage shift under the application of positive gate-bias illumination stress (PBIS) is reduced to 1.16 V. Here the illumination is provided by a white light emitting diode (LED). The corresponding variations of field effect mobility can be also observed. After PBS and PBIS, the decrements of drop are 1.44 cm2/V.s and 1.15 cm2/V.s, respectively. To further understand the illumination-induced electrical instability, the recovery processes of the a-IGZO TFT after PBS are studied under dark or the similar illumination condition as used for the PBIS measurement. It is found that the white light illumination can cause a faster recovery process.2. The electrical stability of a-IGZO TFTs under UV illumination is studied based on a staggered bottom-gate structure on a glass substrate with channel width/length (W/L)= 100 μm/5 μm. The device shows reasonable performance with a turn-on voltage Von of～2.43 V, a sub-threshold swing (SS) of～0.58 V/dec and a field-effect mobility of 20.56 cm2/V.s. A 266 nm UV pulse laser is chosen as a relatively strong UV illumination source to the channel region, with an effective radiation dosage ranging from 6 J/cm2 to 96 J/cm2. Von decreases almost linearly from 2.43 V to-5.56 V as a function of radiation dose. Meanwhile, SS degrades from 0.58 V/dec to 0.89 V/dec, and the field effect mobility of the TFT drops at a higher UV radiation dose. To investigate the detailed chemical effect of UV treatment, the bonding properties of a-IGZ O films are analyzed by x-ray photoelectron spectroscopy (XPS). The relative quantity of oxygen vacancies within the a-IGZO films increases from 29.4% to 43.3% after the UV illumination. To further understand the UV radiation related electrical instability, the electrical properties of the a-IGZO TFT is studied as a function of time after the UV illumination is removed. After 2 months storage, the main performance parameters of the a-IGZO TFT can be recovered to its initial values before the UV treatment.