Preparation of sputter-deposited CuO_x thin film with p-type conductivity and application as thin film transistor

This paper explored the effect of deposition conditions on the characteristics of copper oxide (CuO_x) thin films prepared by direct current (DC) magnetron sputtering. X-ray diffraction exhibited that CuO with n-type conductivity was the main composition regardless of the DC magnetron sputtering power whereas the phase transition from n-type CuO to p-type Cu₂O was observed with decreasing the oxygen pressure (OP) from 40 to 20%. The optical band gap ranges of 1.6-1.9 eV, which are characteristic of n-type CuO, were determined for samples prepared with OPs of 30-40% while the optical band gap of 2.3 eV, which is characteristic of p-type Cu₂O, was measured for samples prepared with an OP of 20%. In addition, only Cu⁺ X-ray photoelectron spectroscopy (XPS) peak at the ~932.6 eV position exists in the films deposited with an OP of 20%, whereas only Cu²⁺ XPS peaks at ~934.2 eV and in the range of 940-945 eV are observed in the films deposited with an OP of 40%. Furthermore, as a result of XPS depth profile analysis, it was confirmed that the composition ratio of the sample prepared at an OP of 20% was Cu₂O, whereas the composition ratio of the sample prepared at an OP of 40% was CuO. These suggest that the CuO_x thin films could be constantly converted from n-type CuO to p-type Cu₂O by decreasing the oxygen partial pressure. Thin film transistors with Cu₂O deposited at 20% OP revealed p-type characteristics such as onset voltage (V_ON) of -3 V, saturated hole mobility of 8 cm²/Vs at V_GS = -28 V, subthreshold swing of 0.86 V/decade at V_GS-V_ON = -0.5 V, and on/off ratio of 1.14 × 10³.