We explore the temperature dependence of the photoluminescence (PL) and photoluminescence excitation characteristics of the green emission band in undoped ZnO crystals. We find that there exists a thermally assisted luminescence channel, which tends to dominate over the existing luminescence channel especially under below-band-gap excitation. Shallow donor electrons are likely to contribute to the thermally assisted luminescence process by being thermally excited from the donor levels to the conduction band. We hence suggest that the shallow donor centers, which are presumably attributed to zinc interstitials and will not act as emitting centers by themselves, are indirectly responsible for the green PL emission. The most probable candidate of the emitting center is the neutral oxygen vacancy, showing the green emission either via a direct internal emission process or an indirect thermally assisted emission process in combination with shallow donor centers. V C 2012 American Institute of Physics. [http://dx.

Since the seminal observation of room-temperature laser emission from ZnO thin films and nanowires, numerous attempts have been carried out for detailed understanding of the lasing mechanism in ZnO. In spite of the extensive efforts performed over the last decades, the origin of optical gain at room temperature is still a matter of considerable discussion , .We show that ZnO microcrystals with a size of a few micrometers exhibit purely excitonic lasing at room temperature without showing any symptoms of electron-hole plasma emission. We then present the distinct experimental evidence that the room-temperature

The mechanism of the commonly observed green photoluminescence (PL) emission in ZnO is still controversial and is being actively discussed. Herein, through comprehensive time-resolved and excitation-energy-dependent PL measurements on variously annealed ZnO samples, we show that the PL decay profile, which is well fitted to a tripleexponential function, depends not only on temperature but also on excitation energy. Under excitation with photons near the band-gap energy, the decay profile is governed by three decay components with decay times of ∼2, ∼10, and ∼50 μs. The contribution from the slower ∼10 and ∼50 μs decay components becomes dominant when the photoexcitation energy is ∼100 meV below the band-gap energy of bulk ZnO. It has also been revealed the highest green PL intensity is achieved under excitation of photons with energies ∼100 meV below the band-gap energy of bulk ZnO as well. These PL features are interpreted in terms of the formation of sub-band-gap states, band edge perturbation, and the related thermally activated trapping/detrapping processes, which can simultaneously increase the intensity and decay time of the green PL emission.

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