Improved p-type conductivity and acceptor states in N-doped ZnO thin films

N-doped, p-type ZnO (ZnO : N) thin films were prepared by magnetron sputtering using NO as the N-doping source. The introduction of Ar in the growth ambient could evidently improve the p-type conductivity and crystal quality of the ZnO : N films, with the lowest room-temperature resistivity of 3.51 Ω cm obtained at an optimal Ar partial pressure of 30%. The p-type ZnO : N films have high optical quality, as suggested by temperature-dependent photoluminescence spectra. The NO substitution acceptor state with an energy level of 180 meV was identified from the free-to-neutral-acceptor (e, A 0) transition. The NO-acceptor bound-exciton binding energy was derived to be 17 meV from the neutral-acceptor-bound-exciton and free exciton emissions. The Haynes factor was about 0.094 for the NO acceptor in ZnO. Besides the NO acceptor, a zinc vacancy (VZn) acceptor state with an energy level of 255 meV was also identified in ZnO : N from the (e, A 0) transition, which might also contribute to the observed p-type conductivity.