Shallow donor formation in phosphorus-doped ZnO thin films

ZnO is a direct band gap semiconductor that is attractive for a number of applications, including high-temperature electronics, transparent electronics, and blue to UV optoelectronics. The most significant impediment to the exploitation of ZnO is the difficulty in achieving effective p-type carrier doping. Attractive acceptor-state candidates for ZnO are the group-V elements substituted on the oxygen site. In this paper, the doping behavior of phosphorus in as-deposited ZnO crystalline thin films grown by pulsed-laser deposition is reported. The experimental work reported here indicates that phosphorus doping significantly increases the electron carrier density in as-deposited films, making the material heavily n-type. These results indicate the formation of a shallow donor level associated with the phosphorus impurity, resulting from either substitution on the Zn site or formation of a compensating complex. Based on X-ray photoelectron spectroscopy measurements, the phosphorus-dopant ions in the films occupy multiple sites yielding oxidation states of -3 and +5. This would explain the compensated n-type behavior for group-V doping in the II-VI host material.