We report the observation of optically pumped lasing in ZnO at room temperature. Thin films of ZnO were grown by plasma-enhanced molecular beam epitaxy on (0001) sapphire substrates. Laser cavities formed by cleaving were found to lase at a threshold excitation intensity of 240 kW cm−2. We believe these results demonstrate the high quality of ZnO epilayers grown by molecular beam epitaxy while clearly demonstrating the viability of ZnO based light emitting devices.

The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kW cm−2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kW cm−2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.

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We have investigated the optical and structural properties of high-quality ZnO films grown on epitaxial GaN (epi-GaN) by plasma-assisted molecular-beam epitaxy employing low-temperature buffer layers. High-resolution x-ray diffraction for both symmetric and asymmetric reflexes shows that crystalline defects in ZnO films have a similarity to epi-GaN used as a substrate. The quality of ZnO epilayers grown on epi-GaN is basically determined by epi-GaN. The photoluminescence (PL) spectrum at 10 K exhibits very sharp exciton emission with a linewidth of 1.5 meV, while deep-level emission is negligible, indicative of small residual strain. At 77 K, PL is dominated by a free-exciton emission line in the low-excitation regime, while it is overtaken by a new emission band due to biexcitons at its low-energy side as the excitation intensity increases. This biexciton emission band emerges even under the intermediate excitation regime of 100 W/cm2, which is 100 times smaller than the previously reported threshold for bulk ZnO. The biexciton binding energy is estimated to be 15 meV, in agreement with previous results. At the higher excitation regime, the emission line due to exciton–exciton scattering dominates the PL spectrum.

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