Effects of post-deposition vacuum annealing on film characteristics of p-type Cu2O and its impact on thin film transistor characteristics

Abstract

Annealing of cuprous oxide (Cu2O) 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 Cu2O 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 Cu2O 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 cm2/V·s of an as-deposited film to 28 cm2/V·s. It was also observed that intrinsic carrier density reduces significantly from 1.8 × 1016 to 1.7 × 1013 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 Cu2O 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 Cu2O annealed at 700 °C showed a field-effect mobility of ∼0.9 cm2/V·s and an on/off current ratio of ∼3.4 × 102.