Yellow Emission from Zinc Oxide giving an Electron Spin Resonance Signal at g=1.96

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Abstract
ZnO polycrystals doped with Li, Cu or Al were prepared by solid-state reactions and their cathodoluminescence (CL) and electron spin resonance (ESR) spectra were measured. A strong yellow emission centered at 2.0 eV was observed for the Al-doped specimen and its intensity was higher than that of the green emission observed for the Al-free samples. The intensity of the green emission was independent of the annealing atmosphere. On the other hand, the intensity of the yellow emission was enhanced by annealing in Ar gas but was suppressed by annealing in O2 gas. The increase of the intensity of the ESR signal at g≈1.96 was parallel to the increase in the yellow emission intensity. The effect of Al-doping upon CL spectra was discussed on the basis of the assignment that the yellow emission is due to a donor-acceptor pair transition.