Switching Enhancement via a Back-Channel Phase-Controlling Layer for p-Type Copper Oxide Thin-Film Transistors

P-type copper oxide (CuO) thin-film transistors (TFTs) with enhanced switching characteristics were fabricated by introducing a sputter-processed capping layer capable of controlling the back-channel phase (labeled as phase-controlling layer, PCL). By optimizing the processing conditions (the deposition power and postdeposition annealing parameters), the switching characteristics of the TFTs achieved a subthreshold swing of 0.11 V dec^-1, an on/off current ratio (I_on/I_off) of 2.81 × 10⁸, and a field-effect mobility (μ_FET) of 0.75 cm² V^-1 s^-1, a considerable enhancement in performance compared to that of CuO TFTs without the PCL. Through optical/electrical analyses and technology computer-aided design simulations, we determined that the performance improvements were because of the CuO back-channel phase reconstruction through PCL deposition and subsequent annealing. The two factors that occurred during the process, sputtering damage and heat treatment, played key roles in creating the phase reconstruction by inducing a local phase transition that sharply reduced the off-current via controlling back-channel hole conduction. As a sample application, we fabricated a complementary metal oxide semiconductor inverter based on our optimized CuO TFT and an InGaZnO TFT that demonstrated a large inverter voltage gain of >14. The proposed approach opens up advancements in low-power circuit design by expanding the utilization range of oxide TFTs.