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

Abstract

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 N2 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 N2− molecules substituting for oxygen. This structure arises from nearly equal hyperfine interactions with two nitrogen nuclei (14N, 99.63% abundant, I=1). The spin Hamiltonian parameters for the N2− center are g∥=2.0036, g⊥=1.9935, A∥=9.8 MHz, and A⊥=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 N2− spectrum (when the N2− convert to N20) and independently introduces the EPR spectrum due to isolated nitrogen acceptors (when N− acceptors convert to N0). 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 °C increases the number of N2− and N0 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 °C drives the two nitrogen acceptors to inactive forms.

Recharging behavior of nitrogen-centers in ZnO

“…¼ 20:1 MHz for the hyperfine parameters, assuming a spin of S ¼ 1/2 for the system. 9 Comparing these parameters to those of the N O centers (g k ¼ 1:995; g ? ¼ 1:963 and A k ¼ 81:1 MHz; A ?…”

“…By EPR, it was also possible to identify N 2 -pair centers which were considered to be acceptors with probably deep level character. 9 This situation motivated us to investigate the recharging behavior of the N 2 centers in more detail by photo-EPR. We find N O and N 2 centers being present in our samples and upon photon irradiation with energies larger than 1.9 eV both signals were greatly enhanced.…”

Electron paramagnetic resonance and photo-electron paramagnetic resonance investigation on the recharging of the substitutional nitrogen acceptor in ZnO

“…Indeed isolated nitrogen acceptors (N O ) were identified by electron paramagnetic resonance (EPR) spectroscopy. [1][2][3] The EPR signals of the isolated nitrogen centers were first observed in nominally undoped ZnO single crystals and later also in ammonia treated ZnO powders. 4,5 To our knowledge, the isolated nitrogen acceptors were never observed in intentionally doped epitaxial films or single crystals, although evidence for the presence of high concentrations of nitrogen was obtained by other characterization methods.…”

“…[6][7][8] For the EPR experiments on the N centers the samples had to be illuminated by light to convert them to the paramagnetic charge state (N 0 ). [1][2][3] This illumination was needed because residual shallow donors were present in the samples causing the nitrogen centers to be in the negative EPR inactive charge state (N À ). It was noticed that photon energies of about 2.4 eV were already sufficient to carry out this conversion process and in comparison to the band gap energy of 3.4 eV it might give already a hint that isolated nitrogen is a deep level center, rather than being a shallow acceptor.…”

Effects of Nitrogen Irradiation on Photoluminescence around 1.54 μm from Er-containing ZnO

“…It has been reported 8) that, according to the XAFS results, the optically active Er in ZnO (ErO 6 ) was at the center of an oxygen octahedron with unequal edge lengths, while showing the C 4v symmetry. Also, there have been many reports [16][17][18][19] on the N-doping into ZnO to explore the p-type conductivity. At this moment, it is however difficult to distinguish a N-doping effect from synergistic oxygen effects, since, in those reports, the N-doped ZnO specimens were also annealed in air or oxygen atmosphere at high temperatures.…”