Molecular nitrogen (N2-) acceptors and isolated nitrogen (N-) acceptors in ZnO crystals

Electron paramagnetic resonance (EPR) has been used to investigate molecular nitrogen and isolated nitrogen acceptors in single crystals of ZnO. These samples were grown by the seeded chemical vapor transport method with N-2 added to the gas stream. A five-line EPR spectrum is observed at low temperature in the as-grown bulk crystals and is assigned to N-2(-) molecules substituting for oxygen. This structure arises from nearly equal hyperfine interactions with two nitrogen nuclei (N-14, 99.63% abundant, I=1). The spin Hamiltonian parameters for the N-2(-) center are g(parallel to)=2.0036, g(perpendicular to)=1.9935, A(parallel to)=9.8 MHz, and A(perpendicular to)=20.1 MHz, with the unique directions parallel to the c axis. Laser excitation at 9 K, with 325 or 442 nm light, eliminates the N-2(-) spectrum (when the N-2(-) convert to N-2(0)) and independently introduces the EPR spectrum due to isolated nitrogen acceptors (when N- acceptors convert to N-0). Removing the laser light and warming to approximately 100 K restores the crystal to its preilluminated state. In separate experiments, heating between 600 and 800 degreesC increases the number of N-2(-) and N-0 acceptors that can be observed. We suggest that the activation of these nitrogen acceptors occurs when complexes of hydrogen and nitrogen thermally dissociate. Further heating above 800 degreesC drives the two nitrogen acceptors to inactive forms. (C) 2003 American Institute of Physics.