A study on the competition between bias-induced charge trapping and light-induced instability in In-Ga-Zn-O thin-film transistors

Thin-film transistors (TFTs) were fabricated using In-Ga-Zn-O (IGZO) semiconductor layers deposited under different oxygen partial pressures. The devices were subjected to negative bias stress (NBS), negative bias illumination stress (NBIS), positive bias stress (PBS) and positive bias illumination stress (PBIS). While device degradation is negligible under NBS, negative shifts in the threshold voltage (V_th) are observed in the presence of light (NBIS), of which the magnitude (ΔV_th) decreases with increasing oxygen partial pressure during IGZO growth. Under PBS, the devices undergo positive V_th shifts, which become more severe with increasing oxygen content in IGZO. However, negative ΔV_th values are observed under PBIS, of which the magnitude decreases with increasing oxygen content in the semiconductor. When positive gate bias is applied, the trapping of negative charge by interstitial oxygen atoms in IGZO is presumed to be the driving force inducing positive V_th shifts. On the other hand, when light is present, the generation of photo-induced excess carriers from oxygen-deficient defect sites is anticipated to be the driving force inducing negative V_th shifts. A balance between the competing mechanisms inducing either positive or negative V_th shifts must therefore be established when the devices are subjected to PBIS, for example in operating active matrix organic light emitting diode (AMOLED) displays using transparent panel arrays.