Threshold Voltage Instability and Persistent Photo-Current in HIZO Thin Film Transistors

Abstract: Electrical bias and light stressing followed by natural recovery of hafnium-indium-zinc-oxide (HIZO) thin film transistors (TFTs) with a silicon oxide (SiOx)/silicon nitride (SiNx) dielectric stack reveals defect density changes, charge trapping and persistent photoconductivity (PPC). In absence of light, the polarity of bias stress controls the magnitude and direction of the threshold voltage shift (∆VT), while under light stress VT consistently shifts negative. In all cases, there was no significant change in field-effect mobility. Light stress gives rise to a PPC with a wavelength- dependent recovery with a time-scale of days. We observe that the PPC becomes more pronounced at shorter wavelengths. Localization of holes at vacancy sites stemming from ionization of oxygen at wavelengths as high as green is suggested as the origin of the PPC.