Abstract
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 LiNi acceptor, or by supplying reactive oxygen species during NiO film deposition, which leads to the formation of Ni vacancies (VNi). 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 LiNi and the VNi acceptor, respectively.
Highlights
Nickel oxide is one of the rare examples of a p-type semiconducting metal oxide
Even though there exist some studies that estimate the positions of the charge transition levels (CTLs) associated with these acceptors, no experimental verification of these estimates has been published to date
We show that n++-SnO2:F/p-NiO heterocontacts possess the desired properties in that they lead to a considerable depletion layer width of 25 nm–35 nm at 0 V bias, and a band bending of roughly 0.5 V, making capacitance–voltage measurements and thermal admittance spectroscopy and the determination of the acceptor charge transition levels possible
Summary
Nickel oxide is one of the rare examples of a p-type semiconducting metal oxide. Because of its high bandgap of 3.8 eV, it is transparent in the visible spectral range, making it interesting for a variety of optoelectronic bipolar devices, such as organic and perovskite solar cells, light-emitting diodes, electrochromic devices, and resistive-switching elements. For many of these applications, doping is a key element in obtaining functional layers and devices because pure, stoichiometric NiO is known to be an insulator. Acceptor doping of NiO is typically achieved by either incorporating Li into the NiO lattice, which substitutes for Ni and contributes a single hole to the electronic system, or by growing the sample under an oversupply of oxygen, which renders the specimen Ni-deficient, producing nickel vacancy (VNi) double acceptors In both cases, the holes are rather tightly bound to the acceptors, and it has been shown that the carriers are localized mainly in the O 2p states of the immediate ionic neighborhood.. Even though there exist some (mostly theoretical) studies that estimate the positions of the charge transition levels (CTLs) associated with these acceptors (see Table I), no experimental verification of these estimates has been published to date In our view, this is a result of a lack of rectifying NiO-based structures, i.e., Schottky or heterodiodes, with a depletion layer located within the NiO layer that enables the use of conventional defect spectroscopical techniques, such as thermal admittance spectroscopy (TAS). This is mainly due to the low electron affinity of NiO of only around 1.5 eV, limiting the choice of materials with even lower work functions to achieve hole depletion
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