We report on the optical properties of ZnO/͑Mg, Zn͒O multiple quantum wells ͑MQWs͒ on lattice-matched ScAlMgO 4 substrates fabricated by laser molecular-beam epitaxy. As the well layer thickness decreased down to 7 Å, the photoluminescence ͑PL͒ and absorption peaks showed a systematic blueshift, consistent with the quantum-size effect. Moreover, a bright PL of free excitons could be observed even at room temperature. As a result, the PL could be tuned in the energy range of 3.3-3.6 eV by choosing the appropriate barrier height and well layer thickness. The widest tunability on the room-temperature luminescence of the excitons could be attained on the basis of the ZnO quantum structure. These favorable properties could not be attained in the MQWs on lattice-mismatched sapphire substrates.

Optical properties in undoped-ZnO epilayers grown by the laser-molecular-beam epitaxy method on lattice-matched ScAlMgO4 substrates were investigated. The absorption spectrum at 5 K has two sharp peaks, both of which are attributed to resonances of A and B excitons, which reflect a small nonradiative damping constant of excitons as well as high film crystallinity accomplished by the virtue of lattice matching. The coupling strengths of exciton-acoustic phonon and of exciton–longitudinal-optical phonon were directly determined from the temperature dependence of exciton absorption spectra independently for A and B excitons, which are close in energy and obey the same selection rule for each other.

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 mechanism of ultraviolet stimulated emission was investigated in ZnO/ZnMgO multiquantum wells. Stimulated emission induced by exciton–exciton scattering occurred throughout a range of temperatures from 5 K to room temperature. At temperatures higher than 160 K, stimulated emission due to electron-hole plasma recombination was also observed with a higher excitation threshold than that of exciton–exciton scattering. The exciton binding energies of multiquantum wells were larger than that of bulk ZnO and increased with a decrease in the well widths. This enhancement of exciton binding energy is due to the quantum-confinement effect and is favorable for the stability of exciton states.

Time-resolved photoluminescence studies have been performed on ͑Cd,Zn͒O/͑Mg,Zn͒O multiquantum wells, which are almost perfectly lattice matched ͑0.034%͒, grown by laser molecular-beam epitaxy on a ScAlMgO 4 substrate. Radiative recombination of excitons in the wells exhibits a significant spectral distribution of times. This distribution was interpreted in terms of localization of excitons by potential fluctuations due to alloy disorder and to well width and depth variations. The temperature dependence of the radiative lifetime of excitons was deduced from the measurement of both the photoluminescence decay time and intensity. We found that the radiative lifetime increases linearly with temperature, showing a two-dimensional feature of excitons in the quantum wells.Recently, as an oxide wide-band-gap semiconductor, ZnO is expected to be one of the main candidates for ultraviolet optical applications such as light-emitting diodes or laser diodes. The large exciton binding energy ͑EBE͒ of 59 meV 1 allows us to observe the excitonic absorption and recombination even at room temperature ͑RT͒, which makes this material appealing. Excitonic laser oscillation with a very low threshold (24 kW/cm 2 ) at RT was confirmed in 50 nm thin films on sapphire͑0001͒ substrates. 2-4 Such observations indicate that an exciton-related recombination process can be utilized as an optoelectronic device operatable at RT. A lower pumping threshold can be expected, in principle, if an exciton-related recombination, rather than a recombination of an electron-hole plasma, is used. Fabrication and characterization of alloyed semiconductors such as ͑Mg-,Zn͒O or ͑Cd,Zn͒O are important from the viewpoint of band-gap engineering as well as a p -n junction. The alloyed oxide ͑Mg,Zn͒O has been found to be a suitable material for the barrier layers of ZnO/͑Mg,Zn͒O multiquantum wells ͑MQWs͒ owing to its larger band-gap energy as revealed in previous studies. 5,6 On the other hand, a ͑Cd,Zn͒O mixed crystal is expected to expand the wavelength tunability by virtue of its narrower band gap. A ͑Cd,Zn͒O/͑Mg,Zn͒O ͑CZM͒ quantum well having a perfect ͑in-plane͒ lattice match can be achieved by choosing an appropriate combination of cadmium and magnesium concentrations. 7 This is one of the advantages compared to ͑In,Ga͒N/͑Al,Ga͒N quantum wells. In the latter case, an internal electric field induced by lattice strain makes the excitonic properties complicated.There have been few experimental studies on the optical properties of this alloyed epitaxial film. 8 In this letter, we report on a combined analysis and the deduced results of temperature dependences of the photoluminescence ͑PL͒ decay time and the integrated PL intensity. The relevant analysis was performed in order to separate the radiative and nonradiative recombination times in CZM MQWs on ScAlMgO 4 substrates grown by laser molecular-beam epitaxy ͑LMBE͒.MQW of ten periods, ͓Cd 0.004 Zn 0.996 O/ Mg 0.12 Zn 0.88 O͔ 10 , was grown by the LMBE under high vacuum condition 9 on a ScAlMgO 4 (0001) substra...

The excitonic properties of high-quality ZnO/Zn0.88Mg0.12O multiquantum wells grown by laser-molecular-beam epitaxy were investigated using temperature-dependent optical absorption spectra from 5 K to room temperature. The strength of exciton-longitudinal-optical (LO) -phonon coupling was deduced from the temperature dependence of the linewidth of the fundamental excitonic peak. Effective reduction of the exciton-LO-phonon coupling with decreasing the well width was observed, which is consistent with the confinement-induced enhancement of the exciton binding energy. The thermal shift of the lowest excitonic energy is independent of well width, indicating that the strain effect is negligible for this material.

We report on temperature dependence of excitonic photoluminescence ͑PL͒ from ZnO/͑Mg, Zn͒O multiple quantum wells ͑MQWs͒. Two kinds of MQWs having different barrier heights grown by laser molecular-beam epitaxy showed significantly different temperature dependences of PL spectra; in ZnO/Mg 0.27 Zn 0.73 O MQWs, the PL peak energy at 50-200 K was a monotonically increasing function of temperature, which was opposite to that ascribed by band gap shrinkage. Moreover, spectra taken at 95-200 K encompassed two peaks, both of which originated from recombination of localized excitons. The temperature-induced shift ͑redshift-blueshift-peak duplication-redshift͒ at 5-300 K is caused by a change in the exciton dynamics with increasing temperature due to inhomogeneity and the exciton localization effect. On the other hand, the corresponding dependence in ZnO/Mg 0.12 Zn 0.88 O MQWs ͑lower barrier height͒ was similar to that in bulk II-VI semiconductors.

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