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Native defects in semiconductors play an important role in theiroptoelectronic properties. Nickel oxide (NiO) is one of the few wide-gap p-typeoxide semiconductors and its conductivity is believed to be controlledprimarily by Ni-vacancy acceptors. Herein, we present a systematic studycomparing the optoelectronic properties of stoichiometric NiO, oxygen-rich NiOwith Ni vacancies (NiO:VNi), and Ni-rich NiO with O vacancies (NiO:VO). Theoptical properties were obtained by spectroscopic ellipsometry, while valenceband spectra were probed by high-resolution x-ray photoelectron spectroscopy.The experimental results are directly compared to first-principles densityfunctional theory + U calculations. Computational results confirm that gapstates are present in both NiO systems with vacancies. Gap states in NiO:Vo arepredominantly Ni 3d states, while those in NiO:VNi are composed of both Ni 3dand O 2p states. The absorption spectra of the NiO:VNi sample show significantdefect-induced features below 3.0 eV compared to NiO and NiO:VO samples. Theincrease in sub-gap absorptions in NiO:VNi can be attributed to gap statesobserved in the electronic density of states. The relation between nativevacancy defects and electronic and optical properties of NiO are demonstrated,showing that at similar vacancy concentration, the optical constants of NiO:VNideviate significantly from those of NiO:VO. Our experimental and computationalresults reveal that although VNi are effective acceptors in NiO, they alsodegrade the visible transparency of the material. Hence, for transparentoptoelectronic device applications, an optimization of native VNi defects withextrinsic doping is required to simultaneously enhance p-type conductivity andtransparency.

journal of applied physics

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