Instability of Amorphous Indium Gallium Zinc Oxide Thin Film Transistors under Light Illumination

The electrical and optical properties of amorphous indium gallium zinc oxide (a-IGZO) thin films and thin film transistors (TFTs) were investigated for their applications in flat panel displays (FPD). The device characteristics were found to be sensitive to gate dielectric and interlayer film material. TFTs using SiO(2) as a gate dielectric and passivation showed very good characteristics when measured in the dark, with mobility above 10 cm(2) V(-1) s(-1), on/off ratio of approximate to 10(8), sub-threshold swing less than 0.2 V/decade and the threshold voltage (V(th)) 0.9 V. However when exposed to light of wavelength below 420 nm V(th) shifted markedly. The shift in V(th) increased with increasing power density, irradiation time, and decreasing wavelength. The shift is attributed to trapping of photogenerated holes in the light induced traps in SiO(2) close to the IGZO interface. It is postulated that these traps are created due to the unstable IGZO surface. Shielding TFTs from light stabilizes the characteristics, however, stabilization of the IGZO surface is required to realize stability for practical applications. (C) 2009 The Japan Society of Applied Physics