Electronic Properties of ZnO: Reconciling Multiple Techniques

Significant progress has been made recently in our understanding of the electronic properties of ZnO and their physical origins, and yet we are far from being in possession of a complete picture despite decades of effort. Given the ultraviolet bandgap of this semiconductor (essentially the same as GaN), its easy synthesis using a wide variety of crystal growth techniques, the ready availability of high-quality single crystals, and intrinsic highly efficient luminescence, it is a material of great interest for a wide variety of device applications. Still, reliable p-type doping remains a considerable obstacle to realizing junction devices, and luminescence features often attributed to acceptor related transitions in fact have alternative physical origins. The role of impurities is reasonably well-understood, but a detailed understanding of dominant defects is somewhat elusive, although several techniques based directly and indirectly on Schottky contacts have provided some illumination on the topic. In this talk, I will summarize recent results in the field, and outline some of the key issues to which definitive answers are desirable if ZnO is going to be commercially competitive with GaN.