Electron paramagnetic resonance (EPR) is used to characterize the 67Zn hyperfine interactions associated with neutral nitrogen acceptors in zinc oxide. Data are obtained from an n-type bulk crystal grown by the seeded chemical vapor transport method. Singly ionized nitrogen acceptors (N−) initially present in the crystal are converted to their paramagnetic neutral charge state (N0) during exposure at low temperature to 442 or 633 nm laser light. The EPR signals from these N0 acceptors are best observed near 5 K. Nitrogen substitutes for oxygen ions and has four nearest-neighbor cations. The zinc ion along the  direction is referred to as an axial neighbor and the three equivalent zinc ions in the basal plane are referred to as nonaxial neighbors. For axial neighbors, the 67Zn hyperfine parameters are A‖ = 37.0 MHz and A⊥ = 8.4 MHz with the unique direction being . For nonaxial neighbors, the 67Zn parameters are A1 = 14.5 MHz, A2 = 18.3 MHz, and A3 = 20.5 MHz with A3 along a [10ˉ10] direction (i.e., in the basal plane toward the nitrogen) and A2 along the  direction. These 67Zn results and the related 14N hyperfine parameters provide information about the distribution of unpaired spin density at substitutional neutral nitrogen acceptors in ZnO.
Journal of Applied Physics
E.M. Golden, S.M. Evans, L.E. Halliburton, and N.C. Giles, J. Appl. Phys. 115, # 103703 (2014). https://doi.org/10.1063/1.4867736