Improvement in the Photo-Induced Bias Instability of Oxide TFT by Controlling Sub-Gap States

Oxide thin film transistors (TFT) are in the spotlight as the switching device of the next generation active matrix display, owing to their high mobility, uniform characteristics and low production cost in the large size substrate. The last remaining issue is the bias instability, especially when the TFTs are exposed to light. In this paper, we show the impact of sub-gap states of oxide semiconductor on the photo-induced bias instability of TFTs. Indium zinc oxide (IZO) film as the semiconductor material was used because the formation energy of oxygen vacancy of indium oxide (3.06eV) is lower than those of other metal oxides such as zinc oxide (3.75eV) or gallium oxide (4.18eV). This means that the density of vacancy related sub-gap states can be widely varied by controlling the indium ratio. An inverted staggered structure was used for the TFTs. Both the gate and source-drain electrodes were formed by a 200 nm thick Mo film. A 350 nm thick SiNx film and 50nm thick SiO2 film were sequentially deposited by plasma enhanced chemical vapor deposition (PECVD) at 350°C as the gate insulator. 50nm IZO films with various indium ratio ([In] / [In+Zn]) were deposited using RF magnetron co-sputtering at room temperature with two ceramic targets of In2O3 and ZnO. Single IZO film was also prepared for the optical and Hall measurements. A 100nm SiO2 film was adopted as an etch stop layer. Finally the devices were annealed at 250°C for 1hr. To distinguish the effects of thermal anneal and the reduction of sub-gap states, one sample was annealed in air and the other was annealed under 5atm of O2 atmosphere. Fig.1 (a) and (b) show the optical and the electrical characteristics as a function of In ratio, respectively. The increase of Urbach tail and the deep state absorption with increasing In ratio is attributed to the rise of the oxygen vacancy concentration. Above the 40% indium ratio, all of Hall mobility, field effect mobility, and carrier concentration increase as the In ratio rises. Therefore it is estimated that the density of sub-gap states related to oxygen vacancy increases as the In ratio increases. In our previous paper, we reported that the photo-induced instability of oxide TFT under negative bias is due to the trapping of photo-generated holes. Photo generation process of wide bandgap (>3eV) material under visible light is presumed to be mainly from excitation of electrons via sub-gap states. Therefore it is expected that oxide TFT with low density of sub-gap states will show excellent VTH stability. This can be identified in Fig.2. Even though the VTH shifts without illumination are negligible regardless of In ratio, the magnitude of VTH shift under light illumination increases with In ratio. To reduce the density of oxygen vacancy related sub-gap states, O2 high pressure anneal (HPA) was applied to IZO TFT of 60% In ratio. The absorption spectra in Fig.3 (a) show that the sub-gap states absorption is reduced considerably with O2 HPA. The decrease of sub-gap states was also observed from the Photo Excited Charge Collection Spectroscopy (PECCS) measurement, as in Fig.3 (b), which utilizes the photo-induced VTH response of TFT device through the photon energy scanning. Fig.4 exhibits the variation of VTH shift as a function of the stress time. All the measurements fit the stretched exponential equation. It is clearly shown that the photoinduced instability of device processed by O2 HPA was dramatically reduced, compared with that of the air annealed device. In summary, the effect of sub-gap states on the photo induced instability was investigated using IZO TFTs. By applying the high pressure anneal in the oxygen ambient, the reduction of the sub-gap states and the resultant reduction of the photo-induced instability were observed. Reference [1] J.F.Wager et al. J.SID, 19, p.745 (2010) [2] K.S.Son et al. IEEE Electron Device Lett., 32, p.164 (2011) [3] Kimoon.Lee et al. Adv.Mater., 22, p.3260 (2010)