Wide-Gap Zn_1-xNi_xO Alloy: A Transparent p -Type Oxide

The development of transparent bipolar oxide devices is largely hampered by the lack of oxides with reliable p-type conductivity. Recent calculations show that rock salt (RS) structured ZnO alloys are promising candidates for transparent p-type oxides. Here, we synthesize wide-gap Zn_1-xNi_xO alloy thin films over the entire composition range on glass substrates by means of radiofrequency magnetron sputtering. We find that the Zn_1-xNi_xO alloy thin films undergo a phase transition from wurtzite (WZ) to RS structure as the Ni content x increases to x ∼0.27. Interestingly, the band gap of RS-Zn_1-xNi_xO is about 4.6 eV at the WZ to RS transition composition (x ∼0.27) and decreases with x to the value of RS-NiO (∼3.8 eV). Nominally undoped alloy thin films sputtered in pure Ar are semi-insulating, while O-rich RS-Zn_1-xNi_xO thin films with relatively high x (e.g., x ≥ 0.5) sputtered in Ar⁺ 1.4% O₂ and/or with Cu doping exhibit good p-type conductivity at room temperature. The hole transport in these RS-Zn_1-xNi_xO alloy thin films follows a small polaron hopping process, with activation energy ranges from 0.19 to 0.32 eV. Drastic differences in the electronic band structure of the WZ and RS Zn_1-xNi_xO alloys are also observed with a type II band offset for alloys at the WZ to RS transition composition. The valence band maximum (VBM) of the RS phase is >1 eV above that of the WZ phase, making them energetically more favorable for the formation of native acceptor defects. The much higher VBM position of RS alloys also favors their extrinsic p-type doping efficiency.