Crystal phase control of copper oxide thin films by process pressure during high power impulse magnetron sputtering

High-performance P-type cuprous oxide (Cu2O) film was prepared at room temperature by high power impulse magnetron sputtering. Optical emission spectra revealed that the ratio of Cu radicals/ions in the plasma significantly decreased with increasing process pressure due to the reduction of sputtering yield as the Cu target surface was oxidized by the increased oxygen radicals/ions. In addition, the increase of self-sputtering yield for Cu cations with increasing process pressure reduced the arrival ratio of Cu species at the substrate surface. As a result, the film crystal phase transformed from Cu2O to Cu4O3 and CuO with increasing process pressure. X-ray photoelectron spectra and Hall effect test results revealed that the oxygen vacancy defects in the films were passivated by the increased oxygen species at higher process pressure, leading to the inhibition of electron background and the emergence of net hole concentration. A mobility of 37.3 cm2/V·s was achieved, which is very high for room-temperature-deposited Cu2O film and comparable to high-temperature (300–600 °C) deposited/post-annealed Cu2O film. Finally, Cu2O thin film transistors (TFTs) exhibited reasonable switching characteristic with a low off-current of 0.3 nA without post-annealing treatment, showing advantage in low-temperature preparation.