Room temperature sputtered Cu doped NiO1+δ: p-type conductivity, stability of electrical properties and p-n heterojunction

Nickel oxide is one of the few wide gap p-type materials which has been applied in many optoelectronic devices. It is well known that the p-type conductivity in NiO arises from Ni vacancy V-Ni (and/or Oxygen interstitial) acceptors in Oxygen-rich NiO (NiO1+delta). However, due to the instability of these native defects, NiO1+delta is unstable, and its p-type conductivity degrades gradually over time, even at room temperature. In this work, we investigate the effects of copper doping on the optoelectronic properties and the stability of the electrical properties of NiO1+delta (CuxNi1-xO1+delta) synthesized at room temperature by magnetron sputtering. Our results show that with up to 8% Cu doping, the p-type resistivity of NiO1+delta decreases almost by an order of magnitude from 3.3 Omega-cm to 0.4 Omega-cm. Variable temperature Hall measurements show that the hole conduction mechanism can be described by small polaron hopping (SPH) conduction. X-ray photoemission spectroscopy (XPS) confirms that Cu is incorporated as Cu+ acceptors in NiO1+delta. Since the p-type conductivity of CuxNi1-xO1+delta films come from both V-Ni and the more stable Cu acceptors, the electrical properties of these films are found to be more thermally stable compared to NiO1+delta. P-type CuxNi1-xO1+delta films with x = 0.08 exhibit a reasonable resistivity of similar to 30 Omega-cm with a transmittance of 60% after annealing at 400 degrees C. Furthermore, a p-CuxNi1-xO1+delta and an n-type ZnO p-n heterojunction was fabricated, and the structure shows good rectification. XPS reveals that the p-CuxNi1-xO1+delta/n-ZnO heterojunction has a type-II band alignment with a valence band and conduction band offsets of 1.61 +/- 0.1 eV and 1.8 +/- 0.1 eV, respectively. (C) 2020 Elsevier B.V. All rights reserved.