We have grown nitrogen-doped Mg x Zn 1−x O : N films on Zn-polar ZnO single crystal substrates by molecular beam epitaxy. As N-sources, we employed NO-plasma or NH 3 gas itself. As x increased, optimum growth temperature window for smooth film morphology shifted to higher temperatures, while maintaining high N-concentration ͑ϳ1 ϫ 10 19 cm −3 ͒. The heterosructures of Mg x Zn 1−x O:N ͑0.1Յ x Յ 0.4͒ / ZnO were fabricated into light emitting diodes of 500-m-diameter. We observed ultraviolet near-band-edge emission ͑ ϳ 382 nm͒ with an output power of 0.1 W for a NO-plasma-doped LED and 70 W for a NH 3-doped one at a bias current of 30 mA.

Quantum-confined Stark effects (QCSEs) in a polarization-free m-plane In0.15Ga0.85N∕GaN multiple quantum well (MQW) blue light-emitting diode fabricated on the low defect density (DD) freestanding GaN substrate were investigated. The electroluminescence (EL) peak at 2.74eV little shifted to the higher energy with the increase in current because of the absence of the polarization fields. The effective radiative lifetime increased and the nonradiative lifetime decreased with the increase in the junction field, and the results were quantitatively explained in terms of field-induced QCSE including tunneling escape of holes from the MQW. As a result of the use of the low DD substrate, the equivalent internal quantum efficiency, which was approximated as the spectrally integrated EL intensity at 300K divided by that at 150K, of 43% was achieved.

The helicon-wave-excited-plasma sputtering epitaxy (HWPSE) method was exemplified to grow atomically-smooth semiconductor epilayers of good structural and optical qualities. For a case study, ZnO homoepitaxy was carried out. According to the surface damage-free nature, the Zn-polar ZnO epilayers grown above 950 °C exhibited a smooth surface morphology with 0.26-nm-high monolayer atomic steps, of which tilt and twist mosaics were comparable to those of the substrate. Their room temperature photoluminescence (PL) spectrum was dominated by the free-excitonic emission. Clearly split PL peaks originating from A-exciton polaritons and sharp peaks due to the first excited-state excitons were observed at low temperature.