Control of positive and negative threshold voltage shifts using ultraviolet and ultraviolet-ozone irradiation

The precise control of the threshold voltage (Vth) required to turn a metal-oxide semiconductor field-effect transistor on/off at the designed voltage was important to ensure that the Vth was constant for all the transistors in the circuit. In this study, Vth was adjusted in both the negative and positive directions by manipulating the number of O vacancies on the surface of an oxide nanowire channel using ultraviolet (UV) and UV-ozone (UVO) irradiation. UV irradiation shifted the Vth in the negative direction by an amount that increased with increasing exposure time (−0.96 V at 1 min, −1.42 V at 2 min, and −1.70 V at 3 min), whereas UVO irradiation shifted the Vth in the positive direction by an amount that increased with increasing exposure time (+0.37 V at 3 min, +0.69 V at 5 min, and +1.07 V at 10 min). The shifted values of Vth were maintained by passivating the channel region of the oxide nanowire with an octadecylphosphonic acid self-assembled monolayer.