Nickel Oxide–Based Heterostructures with Large Band Offsets

Abstract

The authors present research on the electronic transport in heterostructures based on p-type nickel oxide (NiO) with the n-type oxide semiconductors zinc oxide (ZnO) and cadmium oxide (CdO). NiO is a desirable candidate for application in (opto)electronic devices. Because of its small electron affinity, heterojunctions with most n-type oxide semiconductors exhibit conduction and valence band offsets at the heterointerface in excess of 1 eV. ZnO/NiO junctions exhibit a so-called type-II band alignment, making interfacial electron–hole recombination the only process enabling vertical current flow through the structure. These heterojunctions are nevertheless shown to be of practical use in optoelectronic devices, as exemplified here by UV-converting transparent solar cells. These devices, although exhibiting high conversion efficiencies, suffer from two light-activated recombination channels connected to the type-II interface, one of which the authors analyze in more detail here. Furthermore, CdO/NiO contacts are studied—a heterostructure with even larger band offsets, achieving a type-III band alignment which theoretically enables the development of a 2D electronic system consisting of topologically protected states. The authors present experiments demonstrating that CdO/NiO heterostructures indeed hosts a conductive layer absent in both materials when studied separately.