Origins of green band emission in high-temperature annealed N-doped ZnO

Nitrogen-doped ZnO sample has been annealed in O₂ ambient at high temperature (1000 °C) to improve its photoluminescence property. Low-temperature photoluminescence spectra of the sample are dominated by three near-band-edge emissions at 3.377, 3.362, and 3.332 eV, which are ascribed to free exciton emission (FX_A), and neutral donor-bound exciton (D⁰X), and its two-electron satellite (TES), respectively. With increasing temperature in low temperature region, the intensity of FX_A increases and the green band (GB) shows a negative thermal quenching effect resulting from thermal dissociation of D⁰X with more free excitons and neutral donors formed. The doublet structure with energy space ∼30 meV and repeated separation of longitudinal-optical phonon energy of 72 meV are observed in GB at low temperatures. The temperature independent energy position of GB indicates a typical recombination characteristic within strongly localized complexes. The doublet structures are considered to originate from the ground and exited states of shallow donors recombining with deep acceptors such as zinc vacancies.