Vacancy defects induced changes in the electronic and optical properties of NiO studied by spectroscopic ellipsometry and first-principles calculations

Abstract

Native defects in semiconductors play an important role in their optoelectronic properties. Nickel oxide (NiO) is one of the few wide-gap p-type oxide semiconductors and its conductivity is believed to be controlled primarily by Ni-vacancy acceptors. Herein, we present a systematic study comparing the optoelectronic properties of stoichiometric NiO, oxygen-rich NiO with Ni vacancies (NiO:VNi) and Ni-rich NiO with O vacancies (NiO:VO). The optical properties were obtained by spectroscopic ellipsometry while valence band spectra were probed by high-resolution X-ray photoelectron spectroscopy. The experimental results are directly compared to firstprinciples DFT + U calculations. Computational results confirm that gap states are present in both NiO systems with vacancies. Gap states in NiO:Vo are predominantly Ni 3d states while those in NiO:VNi are composed of both the Ni 3d and O 2p states. The absorption spectra for the NiO:VNi sample show significant defect-induced features below 3.0 eV compared to NiO and NiO:VO samples. The increase in sub-gap absorptions in the NiO:VNi can be attributed to gap states observed in the electronic density of states. The relation between native vacancy defects and electronic and optical properties of NiO are demonstrated, showing that at similar vacancy concentration, the optical constants of NiO:VNi deviate significantly from those of NiO:VO. Our experimental and computational results reveal that although VNi are an effective acceptors in NiO, they also degrade the visible transparency of the material. Hence, for transparent optoelectronic device applications, an optimization of native VNi defects with extrinsic doping is required to simultaneously enhance p-type conductivity and transparency.

Light absorption and emission by defects in doped nickel oxide

“…The higher-order transition at approximately 5.5 eV is unaffected. Similar observations have been made by Egbo et al 9 by spectroscopic ellipsometry on a V Ni -doped NiO thin film. A broad absorp- V Ni -doped (Ref.…”

Inferring relative dose-dependent color center populations in proton irradiated thoria single crystals using optical spectroscopy

“…This technique has been used previously to evaluate vacancy defects-induced changes in optical properties of nickel oxide, a wide band gap semiconductor. 57 As opposed to transmission mode optical absorption spectroscopy that is predominantly sensitive to optical properties through the bulk of the sample, spectroscopic ellipsometry in glancing incidence reflection mode offers high sensitivity to surface-confined properties. 58 This technique is therefore well-suited for measuring changes in micrometer-thick surface damage layers created by ion irradiation.…”

Improving the p-Type Conductivity and Transparency of Pure Phase SnO by Ga and Na Doping

“…It is worth noting that for p-type wide gap oxides such as NiO, T vis is typically rather low (in the range of 40 to 60%), 39,40 presumably due to the presence of Ni vacancies. 44 Hence, the T vis for our highly conducting SnO can be considered as relatively high among p-type oxides. Structural, optical and electrical results suggest that while SnO:Na films can achieve a lower resistivity, SnO:Ga films have a higher T vis with resistivity still lower than undoped SnO after RTA.…”