In situ one-step synthesis of p-type copper oxide for low-temperature, solution-processed thin-film transistors

Abstract

Solution-processed n-type oxide semiconductors have received great interest in thin-film transistor (TFT) applications. However, solution-processed p-type oxide semiconductors are not as successful as their n-type counterparts because of the lack of material choice and their complicated fabrication procedures. In this study, a simple one-step synthetic method was employed to fabricate a p-type Cu₂O thin film via in-situ reaction of a CuI film in aqueous NaOH solution at room temperature. The structure, morphology, and component variations as a function of annealing conditions (≤350 °C) were investigated. The analysis indicates that the phase evolution of Cu₂O → Cu₂O + CuO → CuO occurred at higher temperatures and the pure CuO phase was achieved at 250 °C. The crystallinity, average grain size, and surface morphology of the Cu_xO thin films were found to increase in slope as the annealing temperature increased. To explore the possible applications of the obtained Cu_xO films as semiconducting channel components, bottom-gated TFTs on SiO₂ gate dielectrics were constructed and examined. The hole mobility of the optimized device was calculated to be 0.32 cm² V⁻¹ s⁻¹, along with an on/off current ratio of 5 × 10⁴, and a subthreshold swing of 1.1 V dec⁻¹. The further integration of the Cu_xO film on an Al₂O₃ high-k dielectric achieves an improved device performance at 2.5 V. This work successfully demonstrates a simple method to fabricate p-type Cu-based thin films and TFTs via a solution route, which represents a great step towards the development of low-cost and all-oxide complementary metal oxide semiconductor electronics.