The Characteristics and Reliability of In-Ga-Zn-O Thin-Film Transistors on Glass and Flexible Polyimide Substrate under Temperature and Illumination Stress

Abstract

Recently, flexible displays have attracted considerable attention in the emerging electronic device market. Flexible plastic substrates have the advantages such as flexibility, ruggedness and light-weight and its low cost, compared to glass substrate. Indium-Gallium-Zinc-Oxide thin-film transistors (IGZO TFTs) are promising candidates for next generation display backplane due to high mobility, good uniformity, and low process temperature, which suitable for flexible display. In this thesis, the characteristics and reliability of flexible IGZO TFTs were presented and discussed. Firstly, the electrical characteristics and reliability of IGZO TFTs on glass substrate are discussed. The IGZO TFTs were fabricated on a glass substrate with an inverted staggered structure. The initial electrical characteristics and gate bias induced instability was investigated. And drain bias induced instability is investigated. Unique degradation phenomenon was observed under the high drain bias stress. After the high drain bias stress, the drain current, measured at the low drain bias, was significantly decreased. Based on the experimental results, I proposed a degradation model for the high drain bias induced degradation. And light-induced hysteresis of IGZO TFTs is investigated. Hysteresis was observed under the 450-nm illumination, and was increased with temperature. And hysteresis was increased with wavelength decrease. Light-induced hysteresis occurs due to increased sub-band gap states at the interface between the gate insulator layer and the active layer. Also, bias illumination stress induced instability is investigated. The transfer curve did not change after positive bias illumination stress. However, the transfer curve shifted to a negative direction after negative bias illumination stress. The transfer curve could be shifted to the negative direction after negative bias illumination stress due to the increase of VO2+ states. Secondly, the electrical characteristics and reliability of IGZO TFTs on flexible substrate are discussed. The IGZO TFTs were fabricated on a polyimide (PI) substrate with an inverted staggered structure. An inorganic buffer layer, composed of SiO2 and SiNx multi-layer, was employed, in order to prevent the environmental stress, such as water or oxygen molecules. The effects of PI and inorganic buffer layer on the characteristics and reliability of IGZO TFTs were investigated. And the effects of passivation layer on the electrical stability of IGZO TFTs with single passivation layer and double passivation layer fabricated on PI substrate were investigated. The positive bias stress and negative bias stress were applied to the IGZO TFTs at various temperatures from 20 oC to 80 oC. The threshold voltage shift of double passivation device was larger than that of single passivation device under NBTS. The threshold voltage shift of double passivation device was slightly less than that of single passivation device under PBTS. The threshold voltage shift of NBTS is considerably increased than that of PBTS at high temperature due to the difference between conduction band offset and valence band offset. Lastly, the effects of mechanical bending on the electrical stability of flexible IGZO TFTs were investigated.