Effects of post-deposition vacuum annealing on film characteristics of p-type Cu$_{2}$O and its impact on thin film transistor characteristics

Abstract

Annealing of cuprous oxide (Cu$_{2}$O) thin films in vacuum without phase conversion for subsequent inclusion as the channel layer in p-type thin film transistors (TFTs) has been demonstrated. This is based on a systematic study of vacuum annealing effects on the sputtered p-type Cu$_{2}$O as well as the performance of TFTs on the basis of the crystallographic, optical, and electrical characteristics. It was previously believed that high-temperature annealing of Cu$_{2}$O thin films would lead to phase conversion. In this work, it was observed that an increase in vacuum annealing temperature leads to an improvement in film crystallinity and a reduction in band tail states based on the X-ray diffraction patterns and a reduction in the Urbach tail, respectively. This gave rise to a considerable increase in the Hall mobility from 0.14 cm$^{2}$/V·s of an as-deposited film to 28 cm$^{2}$/V·s. It was also observed that intrinsic carrier density reduces significantly from 1.8 × 1016 to 1.7 × 10$^{13}$ cm$^{-3}$ as annealing temperature increases. It was found that the TFT performance enhanced significantly, resulting from the improvement in the film quality of the Cu$_{2}$O active layer: enhancement in the field-effect mobility and the on/off current ratio, and a reduction in the off-state current. Finally, the bottom-gate staggered p-type TFTs using Cu$_{2}$O annealed at 700 °C showed a field-effect mobility of ∼0.9 cm$^{2}$/V·s and an on/off current ratio of ∼3.4 × 102.