We have investigated the structural and optical properties of Ga-doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy. The carrier concentration in Ga-doped ZnO films can be controlled from ${\text{1.33×10}}^{18}/{\mathrm{cm}}^3$ to ${\text{1.13×10}}^{20}/{\mathrm{cm}}^3.$ Despite high Ga incorporation, the linewidth of (0002) ω-rocking curves of Ga-doped ZnO films still lies in the range from 5 to 15 arc min. Photoluminescence (PL) spectra of Ga-doped ZnO films show dominant near-bandedge emission with negligibly weak deep-level emission, independent of carrier concentration. The PL spectrum exhibits a new emission line at 3.358 eV, which corresponds to exciton emission bound to a Ga donor. To avoid degradation of the PL intensity, the maximum dopability of Ga in ZnO is determined to be around ${\text{2.6×10}}^{19}/{\mathrm{cm}}^3.$

By introducing a thin MgO buffer, layer-by-layer growth of ZnO epilayers on ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}\mathrm{(0001)}$ substrates is achieved by plasma-assisted molecular beam epitaxy. The MgO buffer is very effective on the improvement of surface morphology during the initial growth stage, which eventually leads to an atomically flat surface. As a result, $\text{(3×3)}$ surface reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Structural analysis indicates that the twin defect with a 30° in-plane crystal orientation misaligned is completely eliminated, while the total dislocation density is reduced. Free exciton emissions at 3.3774 eV ${\mathrm{(X}}_A)$ and 3.383 eV ${\mathrm{(X}}_B)$ are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers.

We report the results of an experimental investigation on lasing mechanisms in optically pumped ZnO epilayers at room temperature. High-quality ZnO epilayers grown on sapphire by plasma-assisted molecular-beam epitaxy employing an MgO buffer were used. Free exciton emissions and their phonon replicas dominate the photoluminescence from low excited samples. Inelastic exciton–exciton scattering contributes to the mechanism of stimulated emission mainly at intermediate excitation. By using the variable stripe length method, we measured the near threshold optical gain spectrum of the ZnO epilayers. Different from the interband transition governed mechanisms, exciton–exciton scattering gives rise to a nearly symmetric gain spectrum with the peak at 3.17 eV. The electron-hole plasma emerges to contribute to the optical gain when excitation exceeds 220 kW/cm2.

The authors fabricated GaN-based light-emitting diodes (LEDs) on two different GaN templates with the same LED structure. One on thin GaN template $(\sim 2\mu \mathrm{m})$ with high dislocation density [low $\left({10}^9{\mathrm{cm}}^{-2}\right)$ ] grown by metal-organic vapor-phase epitaxy (sample A) and the other on thick GaN template $(\sim 20\mu \mathrm{m})$ with comparatively low dislocation density [high $\left({10}^8{\mathrm{cm}}^{-2}\right)$ ] by hydride vapor-phase epitaxy (sample B). In order to understand the mechanism of leakage current in LEDs, the correlation between current-voltage characteristics and etch pit density of LEDs was studied.