Abnormal Positive Shift of Threshold Voltage in Praseodymium-Doped InZnO-TFTs Under Negative Bias Illumination Temperature Stress

This article presents a comprehensive investigation of the abnormal positive shift phenomenon of the threshold voltage ( ${V}_{\text {th}}$ ) in praseodymium (Pr)-doped indium zinc oxide thin-film transistors (PrIZO-TFTs) under negative bias illumination temperature stress (NBITS). It has been demonstrated that the presence of acceptor-like defects near the conduction band minimum (CBM) increases with Pr doping, leading to a lower carrier concentration. These TFTs also exhibit significant suppression of light-induced instability by reducing oxygen vacancies (Vo). However, our examination of NBITS revealed an abnormal positive shift of ${V}_{\text {th}}$ in PrIZO-TFTs. Further analysis uncovered that the positive shift of ${V}_{\text {th}}$ primarily occurred under 2.17-eV illumination, which aligns with the defect position of 2.30 eV from the conduction band in oxygen vacancy theory. We propose the “dipole theory” to explain this contradictory positive shift, suggesting that Pr $^{{4}+}$ gains two electrons from Vo under illumination. This results in the generation of a dipole composed of negatively charged Pr $^{{3}+}$ and positively charged Vo $^{{2}+}$ , leading to lattice distortion under negative bias. Consequently, positive charges accumulate near the gate insulator (GI), resulting in a positive ${V}_{\text {th}}$ shift.