Controlling the p-Type Conductivity and Composition Range for Bipolar Conduction in Ni<i><sub>x</sub></i>Cd<sub>1–<i>x</i></sub>O Alloys by Acceptor Doping
Kingsley Egbo, Chao Ping Liu, Mohammad Kamal Hossain, Chun Yuen Ho, Cheuk Kai Gary Kwok, Sujit Kumer Shil, Chioma Vivian Ezeh, Ying Wang, K. M. Yu
Abstract
Oxide-based transparent p–n homojunctions are desirable for the development of transparent electronics. However, most oxide semiconductors are intrinsically n-type and to achieve p-type wide-gap oxide is still challenging. Previously, we have demonstrated that alloying a high-mobility n-type material such as CdO with p-type NiO can provide an avenue for electronic band engineering and consequently achieve bipolar conductivity in the midalloy composition. In this study, we synthesized O-rich NixCd1–xO alloys (NixCd1–xO1+δ) over the entire composition with Li and Cu doping using radio frequency (RF) magnetron sputtering at room temperature. We show that by Li and Cu doping the conductivity in the p-type regime of these alloys is improved, which also leads to a wider composition window for bipolar doping in NixCd1–xO. Specifically, by Li doping, the p-type alloy composition can be extended to x ≥ 0.3 so that the bipolar doping window is expanded to 0.30 ≤ x ≤ 0.52. Detailed measurements on electrical, structural, optical, and electronic properties suggest that Li is an effective acceptor, offering a promising way to improve the p-type conduction in Ni-rich NixCd1–xO alloys as well as to regulate the bipolar conductivity in this alloy system. A p-Ni0.7Cd0.3O:Li/n-Ni0.45Cd0.55O quasi-homojunction was fabricated and a rectification ratio ∼102 with an ideality factor of ∼2.9 was obtained. The demonstrated quasi-homojunction structure also showed >60% transmittance in the visible spectrum.