We presented a comparative investigation on the morphological and structural properties of the produced ZnO nanowires (NWs) on c-Al₂O₃ substrates under different preparation conditions. The effects of a low-temperature nucleation layer, reaction pressure and light irradiation on the alignment, diameter and growth rate of ZnO NWs were evaluated in detail. It was found that the low-temperature nucleation layer acting as a supporting layer favored the formation of a transition layer featuring initially a rough surface and a high density of grain boundaries, guiding and facilitating the subsequent homoepitaxy of ZnO NWs grown on the top of developed (0001) facets of the new activation sites. The crystallinity and vertical alignment of epitaxial ZnO NWs were further optimized to achieve the best value, as the second growth stage was conducted with a higher reaction pressure and light irradiation, such that a relatively low full width at half-maximum (701 arcsec) of the rocking curve could be obtained. The thermodynamical growth kinetics of the produced ZnO NWs via the two-step growth process was also investigated by monitoring the morphology evolution at different growth stages. Coaxial n-ZnO/MgO/p-NiO-core/shell NWs heterostructured light-emitting diodes (LEDs) were then fabricated and characterized, and a unique tunability of the electroluminescence (EL) spectra depending on the bias polarities was observed. We studied in detail the bicolor EL characteristic of the bidirectional direct-current driven LEDs and tentatively proposed carrier tunneling and hole generation models to explain such interesting features.

Vertically aligned ZnO/MgO coaxial nanowire (NW) arrays were prepared on sapphire substrates by metal–organic chemical vapor deposition combined with a sputtering system. We present a comparative investigation of the morphological and optical properties of the produced heterostructures with different MgO layer thicknesses. Photoluminescence measurements showed that the optical performances of ZnO/MgO coaxial NWs were strongly dependent on the MgO layer thickness. The intensity of deep-level emission (DLE) decreased monotonously with the increase of MgO thickness, while the enhancement of ultraviolet (UV) emission showed a critical thickness of 15 nm, achieving a maximum intensity ratio (∼226) of I_UV/I_DLE at the same time. The significantly improved exciton emission efficiency of the coaxial NW structures allows us to study the surface passivation effect, photogenerated carrier confinement and transfer in terms of energy band theory. More importantly, we achieved an ultralow threshold (4.5 mA, 0.58 A cm⁻²) electrically driven UV lasing action based on the ZnO/MgO NW structures by constructing an Au/MgO/ZnO metal/insulator/semiconductor diode, and the continuous-current-driven diode shows a good temperature tolerance. The results obtained on the unique optical properties of ZnO/MgO coaxial NWs shed light on the design and development of ZnO-based UV laser diodes assembled with nanoscale building blocks.

Vertically aligned ZnO nanowall networks were grown on p-GaN/sapphire substrates by metal-organic chemical vapor deposition, and further heterojunction light-emitting diodes based on n-ZnO/p-GaN were fabricated. Highly efficient ultraviolet-blue emission was observed from the diode under forward bias, and it operated continuously for 8.5 h with decay of only 8.08% under a continuous current of 12 mA. The diode exhibited low emission onset and good stability. In addition, temperature-dependent electroluminescence and current-voltage behaviors of the diode were investigated to examine the thermal effects on light output power, spectral line shift, and temperature sensitivity of the resulting voltage.

To obtain high-quality n-type doped β-Ga₂O₃ films, silane was used as an n-type dopant to grow Si-doped β-Ga₂O₃ films on (100) β-Ga₂O₃ substrates by metal-organic chemical vapor deposition (MOCVD). The electron concentrations of the Si-doped β-Ga₂O₃ films obtained through experiments can be stably controlled in the range of 6.5 × 10¹⁶ cm⁻³ to 2.6 × 10¹⁹ cm⁻³, and the ionization energy of Si donors is about 30 meV, as determined by analysis and calculation. The full width at half maxima of the rocking curves of the (400) crystal plane of all doped films was less than 500 arcsec, thus showing high crystal quality, while the increase of the doping concentration increased the defect density in the β-Ga₂O₃ films, which had an adverse effect on the crystal quality and surface morphology of the films. Compared with heteroepitaxial Si-doped β-Ga₂O₃ films, homoepitaxial Si-doped β-Ga₂O₃ films exhibited higher quality, lower defect density, and more stable electron concentration, which make them more conductive for preparing Ga₂O₃-based power devices.

Electrically pumped lasing action has been realized in ZnO from an n-MgZnO/i-ZnO/SiO₂/p-Si asymmetric double heterostructure, an ultralow threshold of 3.9 mA was obtained. The mechanism of the laser is associated with the in-plane random resonator cavities formed in the ZnO films and the elaborate hollow-shaped SiO₂ cladding pattern, which prevent the lateral diffusion of injection current and ultimately lower the threshold current of the laser diode. In addition, a waveguide mechanism due to different refractive indices of three epilayers enhances the guided optical field on the ZnO side, resulting in an improved light extraction efficiency.

β-Ga₂O₃ films were grown on sapphire (0001) substrates with various O/Ga (VI/III) ratios by metal organic chemical vapor deposition. The effects of VI/III ratio on growth rate, structural, morphological, and Raman properties of the films were systematically studied. By varying the VI/III ratio, the crystalline quality obviously changed. By decreasing the VI/III ratio from 66.9 × 10³ to 11.2 × 10³, the crystalline quality improved gradually, which was attributed to low nuclei density in the initial stage. However, crystalline quality degraded with further decrease of the VI/III ratio, which was attributed to excessive nucleation rate.