Doping Asymmetry Problem in ZnO: Current Status and Outlook

ZnO has gained considerable interest recently as a promising material for a variety of applications. To a large extent, the renewed interest in ZnO is fuelled by its wide direct band gap (3.3 eV at room temperature) and large exciton binding energy (60 meV) making this material, when alloyed with, e.g., Cd and Mg, especially attractive for light emitters in the blue/ultraviolet (UV) spectral region. Unfortunately, as with other wide-gap semiconductors, ZnO suffers from the doping asymmetry problem, in that the n-type conductivity can be obtained rather easily, but p-type doping proved to be a formidable challenge. This doping asymmetry problem (also dubbed as the p-type problem in ZnO) is preventing applications of ZnO in light-emitting diodes and potential laser diodes. In this paper, we provide a critical review of the current experimental efforts focused on achieving p-type ZnO and discuss the proposed approaches which could possibly be used to overcome the p-type problem.