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The temperature-dependent behavior of excitonic photoluminescence observed in ZnO/MgZnO multiple quantum wells (MQWs) in the temperature range of 5-300 K is described. In a ZnO/Mg[SUB0.27]Zn[SUB0.73]O MQW grown by laser molecular-beam epitaxy, the luminescence was dominated by localized exciton (LE) emission throughout the whole temperature range studied. Luminescence of free excitons (FEs) was not observed. A simple rate equation is used to describe the quenching of LE emission. The activation energy for LE luminescence quenching is of the order of the localization energy of excitons, suggesting that the thermionic emission of the LEs out of the localization potentials leads to nonradiative recombination. In a ZnO/Mg[SUB0.12]Zn[SUB0.88]O MQW having lower barriers, the luminescence was dominated by LE emissions at low temperatures, while the FE transition was dominating emissions at temperatures above 175 K. A rate equation assuming one nonradiative recombination channel is used to describe the quenching of the transitions observed. The activation energy for LE luminescence quenching deduced in this sample is also of the order of the localization energy of excitons. The temperature dependences of FE emission intensities are also discussed by using a simple rate equation in which a thermal release effect of LEs toward FEs is taken into account. [ABSTRACT FROM AUTHOR]

The photoluminescence properties of ZnO/ZnMgO multiquantum wells have been investigated by continuous and time-resolved photoluminescence spectra at various temperatures. Strong first-order and second-order longitudinal optical (LO)-phonon replicas were observed in the emission spectra. Both the zero-phonon emission and its LO-phonon replicas can be resolved into two components with different temperature dependence. The observation of LO-phonon replicas provided an additional way to characterize the recombination mechanism in quantum wells. Two kinds of excitons are responsible for the resolved emission components and have different coupling strength with LO phonons: one is donor bound excitons which dominated at low temperatures, the other one is the localized excitons due to the well width fluctuation which took over the dominated emission at high temperatures. [ABSTRACT FROM AUTHOR]

Provides information on an experimental study reporting on the orientation dependence of strained ZnSe/ZnS(h11) single quantum well luminescence. Methodology used to conduct the experiment; Results of the experiment; Information on optical characterizations; Discussion on the results.

A set of ZnO/Mg[sub 0.12]Zn[sub 0.88]O multiple quantum wells (MQWs) with well widths, L[sub w], varying from 6.91 to 46.5 Å has been grown by laser molecular-beam epitaxy. We estimated the L[sub w] dependence of the radiative and nonradiative recombination times of localized excitons at 5 K. Suppression of quantum efficiency can be avoided even in the MQWs having small L[sub w]s studied in this work. Effects of excitonic localization are discussed in order to explain the L[sub w] dependence of radiative recombination time at 5 K. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

ZnO/Mg0.22Zn0.78O quantum wells were studied by excitation-intensity-dependent luminescence at 10 K. The samples were grown by laser molecular-beam epitaxy on ScAlMgO4 substrates to evaluate the well width dependence (1 to 10 nm) of exciton transition energies. Under weak excitation, the photoluminescence shows a quantum-confined Stark effect for the wide wells. The well width dependences of the experimental transition energies are compared with previously reported calculations to evaluate the electric field due to spontaneous and piezoelectric polarizations. The internal electric field is comparable with 650 kV/cm. With an increase in excitation intensity, blueshift of the luminescence was observed, suggesting photoexcitation screening of electric fields. [ABSTRACT FROM AUTHOR]

Photoluminescence study of ZnO/Mg0.1Zn0.9O quantum wells with graded well width (Lw) was carried out at 4.2 K. The emission evolution from quantum confinement regime to quantum-confined Stark regime was observed clearly. For large Lw, the emission splits into two peaks which are attributed to the emissions of ZnO band edge and separately localized carriers, respectively. The internal electric field in the well layer was estimated to be ∼0.3 MV/cm, being similar to previous reports. The results are useful in designing ZnO QW based optoelectronic devices. [ABSTRACT FROM AUTHOR]

By employing a nanosecond pump-probe method, biexciton formation process was investigated in ZnO/Zn[sub 1-x]Mg[sub x]O (x=0.26) multiple quantum wells (MQWs) grown on ScAlMgO[sub 4] substrate by laser molecular-beam epitaxy. Bleaching of absorption due to the saturation of excitonic states, and induced absorption related to the exciton-biexciton transition were observed in their spectra. It is demonstrated that the pump-probe method allows us to precisely determine binding energies of exciton complexes even applied to the semiconductor quantum structures where the localization effect are not negligible. This is because a transition from free-excitonic states to free-biexcitonic states is involved in the induced absorption process. The biexciton binding energy is a monotonically decreasing function of well width (L[sub w]). For the MQWs with L[sub w] smaller than 2.5 nm, the biexciton binding energy is larger than 25 meV, comparable to the thermal energy of room temperature. [ABSTRACT FROM AUTHOR]

We studied photoluminescence (PL) properties of eighteen samples of wurtzite ZnO/Mg[sub x]Zn[sub 1-x]O multiple quantum wells (x=0.12 and 0.27) with various well widths (L[sub w]) of 0.7-4.65 nm. Radiative recombination of the electron-hole pairs that are spatially separated due to the quantum-confined Stark (QCS) and Franz-Keldish (QCFK) effects was observed in two thicker samples at 5 K. This PL band is located ≈40 meV in energy below the emission band of the localized excitons and ≈60 meV below the absorption energy of the free exciton transition. One can not observe such kind of luminescence unless both of the following conditions are accomplished: (1) higher Mg concentration (x=0.27) and (2) L[sub w]≥4.23 nm. These experimental findings do not contradict the following two characteristic features for the QCS and QCFK effects; the magnitude of the electric field due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the L[sub w], respectively. The coupling strength with longitudinal-optical phonons, which is determined from the relative luminescence intensities of the phonon replicas, is significantly larger than that between the localized excitons and phonons. It is considered that the strong electric field increases the distance between electron and hole charge distributions from that determined by the Coulomb force and leads to the enhancement in the phonon interaction. [ABSTRACT FROM AUTHOR]

Luminescence due to the radiative recombination of localized biexcitons has been observed at low temperature (5 K) in ZnO/Zn[sub 0.74]Mg[sub 0.26]O multiquantum wells grown by laser-molecular-beam epilaxy on a lattice-matched ScAlMgO[sub 4] substrate (0001). The emission components due to the recombination of localized excitons and biexcitons and due to the exciton-exciton scattering were verified by examining their relative energy positions and intensity dependence on excitation power density. The excitation threshold for biexciton emission was significantly lower than that for exciton-exciton scattering. The binding energy of biexcitons in multi-quantum wells is largely enhanced by quantum confinement effect. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The excitonic properties of high-quality ZnO/Zn[sub 0.88]Mg[sub 0.12]O 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. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on temperature dependence of excitonic photoluminescence (PL) from ZnO/(Mg, Zn)O multiple quantum wells (MQWs). 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[sub 0.27]Zn[sub 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-redshift) 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[sub 0.12]Zn[sub 0.88]O MQWs (lower barrier height) was similar to that in bulk II-VI semiconductors. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence studies have been performed on (Cd,Zn)O/(Mg,Zn)O multiquantum wells, which are almost perfectly lattice matched (0.034%), grown by laser molecular-beam epitaxy on a ScAlMgO[sub 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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the optical properties of ZnO/(Mg, Zn)O multiple quantum wells (MQWs) on lattice-matched ScAlMgO[sub 4] substrates fabricated by laser molecular-beam epitaxy. As the well layer thickness decreased down to 7 Å, the photoluminescence (PL) 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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have developed a concurrent method of optical measurements using optical fiber bundles and charge coupled devices (CCDs), especially equipped for application to combinatorial samples. This approach can also be used for experiments performed at cryogenic temperatures. This combinatorial characterization method will be explained in detail. The dependence of the optical properties in combinatorial samples of ZnO/(Mg,Zn)O multi-quantum wells prepared by laser molecular-beam epitaxy upon well width was found to be smooth compared with the samples grown by a conventional method. The use of lattice-matched ScAlMgO4 substrates greatly improved the properties of quantum wells and, as a result, a bright photoluminescence (PL) of free excitons could be observed even at room temperature. In addition, the PL could be tuned in the energy range 3.3–3.6 eV by choosing the appropriate barrier height and well layer thickness. [Copyright &y& Elsevier]

Spectroscopic studies have been conducted for wurtzite ZnO/Mg0.27Zn0.73O multiple quantum wells. Internal electric fields at interfaces are found to have an influence on the photoluminescence (PL) properties for well width (Lw) greater than 4.2 nm. These experimentally observed features are characteristic of the quantum-confined Stark effects; the magnitude of the electric field due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the Lw, respectively. Results of the time-resolved PL study are also presented. [Copyright &y& Elsevier]

We proposed a vertically integrated one-chip-two-wavelength light source which consists of a separate confinement single-quantum-well (SCH-SQW) ZnCdSe/ZnSe/ZnMgBeSe heterostructure for blue–green light emitters grown on SCH-MQW InGaP/InGaAlP for red light-emitting devices.We investigated, firstly, the effect of a thin low-temperature-grown ZnSe buffer layers (LT-ZnSe) in improving ZnSe crystallinity by inserting it between the high-temperature-grown ZnSe epilayer and the GaAs substrate, secondly, the growth optimization of LT-ZnSe on tilted GaAs (0 0 1) substrate, and lastly, the molecular beam epitaxy growth and characterization of ZnCdSe/ZnSe/ZnMgBeSe quantum well structures on metal organic chemical vapor deposition (MOCVD) grown III–V red light emitters.Optically pumped lasing is achieved from II–VI and III–V laser structures on one chip at room temperature. The present results clearly show the feasibility of epitaxial integration of II–VI and III–V laser structures. [Copyright &y& Elsevier]