Recent Developments of ZnO-Based p-type Transparent Conductive Oxide Thin Films

Nowadays, high performance p-type transparent conductive oxide (TCO) thin films have gained tremendous intersts, and the fact is that if p-type TCOs with high electrical conductivity and optical transmittance can be fabricated, transparent p-n junctions can be obtained and "invisible electronics" be realized, and hence the use area of TCOs will be highly broadened. A lot of work have been done on non-stoichiometric and doped versions of p-type TCOs in the last few years to improve the optical and electrical properties by various deposition techniques. ZnO-based thin films were thought to be the most promising candidate for p-type TCOs based on the fact that ZnO has advantages over the others, so in this paper the development of ZnO-based p-type TCOs has been discussed. Firstly, the reasons why p-type ZnO-based TCOs are difficult to synthesize were discussed, and then the general ways now used to produce p-type ZnO-based TCOs were summerized, including intrinsic p-type ZnO, doping of group I elements, codoping of V and III elements, doping of group V elements, the origin of p-type conductivity and the feasibility of each way, and the state-of-the-art optical and electrical properties were presented. Finally, the specific shortcomings in producing high quality p-type TCOs were discussed. Based on the comparision, it is believed that the doping of group I elements in ZnO may be the most pronising way in realizing p-type TCO.