Electron paramagnetic resonance was used to investigate molecular N and isolated N acceptors in single crystals. The samples were grown by using the seeded chemical vapor transport method, with N2 added to the gas stream. A 5-line electron paramagnetic resonance spectrum was observed at low temperatures in the as-grown bulk crystals and was attributed to N2- molecules which substituted for O. This structure arose from nearly equal hyperfine interactions with two N nuclei (14N, 99.63% abundant, I = 1). The spin Hamiltonian parameters for the N2- center were g|| = 2.0036, g = 1.9935, A|| = 9.8MHz, and A = 20.1MHz, with the unique directions parallel to the c-axis. Laser excitation at 9K, with 325 or 442nm light, eliminates the N2- spectrum (when the N2- convert to N20) and independently introduces the electron paramagnetic resonance spectrum due to isolated N acceptors (when N– acceptors converted to N0). Removing the laser light and warming to approximately 100K restored the crystal to its pre-illuminated state. In separate experiments, heating between 600 and 800C increased the number of N2- and N0 acceptors that could be observed. It was suggested that the activation of these N acceptors occurred when complexes of H and N thermally dissociated. Further heating above 800C drove the two N acceptors into inactive forms.

Molecular Nitrogen (N2-) Acceptors and Isolated Nitrogen (N–) Acceptors in ZnO Crystals. Garces, N.Y., Wang, L., Giles, N.C., Halliburton, L.E., Cantwell, G., Eason, D.B.: Journal of Applied Physics, 2003, 94[1], 519-24