Back-Channel Defect Termination by Sulfur for p-Channel Cu₂O Thin-Film Transistors

The absence of a high-performance p-channel oxide thin-film transistor (TFT) is the major challenge faced in the current oxide semiconductor device technology. Simple solution-based back-channel subgap defect termination using sulfur was developed for p-channel cuprous oxide (Cu₂O)-TFTs. We investigated the origin of poor device characteristics in conventional Cu₂O-TFTs and clarified that it was mainly because of a back-channel donor-like defect of ∼2.8 ×10¹³ cm^-2 eV^-1, which originated from the interstitial Cu defect. Sulfur ion treatment using thiourea effectively reduced the back-channel defect down to <3 × 10¹² cm^-2 eV^-1 and demonstrated the Cu₂O-TFT with a saturation mobility of 1.38 ± 0.7 cm² V^-1 s^-1, a s-value of 2.35 ± 1.22 V decade^-1, and an on/off current ratio of ∼4.1 × 10⁶. The improvement of device characteristics was attributed to the reduction of back-channel defect by the formation of a thin CuSO₄ back-channel passivation layer by the chemical reaction of interstitial Cu with S and O ions. An oxide-based complementary inverter using a p-channel Cu₂O-TFT and a n-channel a-In-Ga-Zn-O-TFT was demonstrated with a high voltage gain of ∼230 at V_DD = 70 V.