Identification of Li$_{\text{Ni}}$ and V$_{\text{Ni}}$ acceptor levels in doped nickel oxide

Nickel oxide, in particular in its doped, semiconducting form, is an important component of several optoelectronic devices. Doping NiO is commonly achieved either by incorporation of lithium, which readily occupies Ni sites substitutionally, producing the Li$_{\text{Ni}}$ acceptor, or by supplying reactive oxygen species during NiO film deposition, which leads to the formation of Ni vacancies (V$_{\mathrm{Ni}}$). However, the energetic position of these acceptors in the NiO band gap has not been experimentally determined until today. In this work, we close this knowledge gap by studying rectifying n$^{++}$p heterojunctions of NiO on top of fluorine-doped tin oxide. These structures show sufficient rectification to perform electric characterization by defect spectroscopic techniques, specifically capacitance-voltage and thermal admittance spectroscopy. Using these methods, the (0/-) charge transition levels are determined to be 190meV and 409meV above the valence band edge for the Li$_{\text{Ni}}$ and the V$_{\text{Ni}}$ acceptor, respectively.