Identification of Li_Ni and V_Ni acceptor levels in doped nickel oxide

Nickel oxide, in particular in its doped, semiconducting form, is an important component of several optoelectronic devices. Doped NiO is commonly achieved either by incorporation of lithium, which readily occupies Ni sites substitutionally, producing the Li-Ni acceptor, or by supplying reactive oxygen species during NiO film deposition, which leads to the formation of Ni vacancies (V-Ni). However, the energetic position of these acceptors in the NiO bandgap has not been experimentally determined until today. In this work, we close this knowledge gap by studying rectifying n(++)p heterojunctions consisting of NiO thin films grown 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 190 meV and 409 meV above the valence band edge for the Li-Ni and the V-Ni acceptor, respectively.