Co-doping: an effective strategy for achieving stable p-type ZnO thin films

ZnO is one of the most important functional materials with a wide direct band gap, large exciton binding energy, and facile growth of high quality nanostructures, which make it very promising for various optoelectronic applications, especially in light-emitting diodes, piezotronics and nanoelectronics. However, ZnO suffers from the doping asymmetry problem, and p-type doping is not stable, which is major hurdle for its application in optoelectronics. Co-doping, which is defined as doping acceptor with other impurities simultaneously, was proposed and attempted to address the p-type doping problem by enhancing the solubility and reducing the ionization energy of acceptor dopants. During the past two decades, extensive studies on co-doping have shown positive effects on improving the p-type conductivity of ZnO. The success was made mainly in ZnO films and nanowires. In this article, we provide an overview of the experimental efforts for growing p-type ZnO nanocrystalline films by the co-doping method, which are mainly classified as acceptor-donor co-doping, dual-acceptor co-doping, acceptor-hydrogen co-doping, and acceptor-isovalence co-doping. In addition, a few works on co-doped p-type ZnO nanowires are also summarized. Finally, we discuss the remaining problems of p-type ZnO by co-doping.