C. H. Chia

Room-temperature luminescence of excitons in ZnO/(Mg, Zn)O multiple quantum wells on lattice-matched substrates

We report on the optical properties of ZnO/(Mg, Zn)O multiple quantum wells (MQWs) on lattice-matched ScAlMgO4 substrates fabricated by laser molecular-beam epitaxy. As the well layer thickness decreased down to 7 A, 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.

Exciton spectra of ZnO epitaxial layers on lattice-matched substrates grown with laser-molecular-beam epitaxy

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.

Radiative and nonradiative recombination processes in lattice-matched (Cd,Zn)O/(Mg,Zn)O multiquantum wells

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 ScAlMgO4 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.

Temperature dependence of near ultraviolet photoluminescence in ZnO/(Mg, Zn)O multiple quantum wells

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/Mg0.27Zn0.73O 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/Mg0.12Zn0.88O MQWs (lower barrier height) was similar to that in bulk II–VI semiconductors.

Confinement-enhanced biexciton binding energy in ZnO/ZnMgO multiple quantum wells

By employing a nanosecond pump-probe method, biexciton formation process was investigated in ZnO/Zn1−xMgxO (x=0.26) multiple quantum wells (MQWs) grown on ScAlMgO4 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 (Lw). For the MQWs with Lw smaller than 2.5 nm, the biexciton binding energy is larger than 25 meV, comparable to the thermal energy of room temperature.

Temperature quenching of exciton luminescence intensity in ZnO/(Mg,Zn)O multiple quantum wells

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/Mg0.27Zn0.73O 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/Mg0.12Zn0.88O 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 o...

Layer-by-layer growth of high-optical-quality ZnO film on atomically smooth and lattice relaxed ZnO buffer layer

The growth mode of ZnO thin films can be well regulated in a molecular layer-by-layer growth by employing a ZnO buffer layer deposited on a lattice-matched ScAlMgO4 substrate and annealed at high temperature. The annealed buffer layer has atomically flat surface and relaxed (strain-free) crystal structure. The intensity oscillation of reflection high-energy electron diffraction persisted for more than a 100-nm film deposition under optimized conditions on such a buffer layer. Thus prepared thin films show free exciton emissions in a 5 K photoluminescence spectrum and excited-state exciton resonance structures in a reflection spectrum, both indicating very high optical quality.

Emission from the higher-order excitons in ZnO films grown by laser molecular-beam epitaxy

Epitaxial ZnO thin films were grown by laser molecular-beam epitaxy on lattice-matched ScAlMgO4 substrates following the deposition and annealing of suitable buffer layers. The samples were characterized by low-temperature photoluminescence (PL), absorption, and reflectivity measurements. PL from higher order (n=2) excitons (A exciton) was observed at temperatures lower than 40 K. The absorption spectrum contained lines and the reflection spectrum exhibited anomalies that were assigned to the excited-states (n=2,3) of A and B excitons. The optical quality could be improved dramatically by using annealed ZnO or MgZnO buffer layers.

Effect of MgZnO-layer capping on optical properties of ZnO epitaxial layers

Photoluminescence (PL) and reflectivity spectra of ZnO epilayers capped with MgxZn1−xO layers (up to x=0.36) are reported. These capped films were epitaxially grown on lattice-matched ScAlMgO4 substrates by laser molecular-beam epitaxy. A photoluminescence spectrum from the ZnO layer taken at 5 K shows emission bands at 3.389, 3.376, and 3.362 eV. The two higher bands are due to A- and B-free exciton emissions and the lowest band is due to emission of a neutral-acceptor bound exciton (I6). The linewidth of the I6 emission in our uncapped sample (0.8 meV) is significantly smaller than that in the capped one (6 meV). This is probably due to strain applied across the ZnO layer because of the difference in lattice constant between the two layers. The spectral assignment of the free exciton emissions is strengthened by a comparison with a temperature-dependent PL study and a reflectivity study. With an increase in temperature, the intensity of the bound exciton emission line decreased drastically and became co...

Optical spectra in ZnO thin films on lattice-matched substrates grown with laser-MBE method

Optical properties in undoped ZnO thin films grown with laser-molecular beam epitaxy method on lattice-matched ScAlMgO 4 substrates have been investigated. The transmission spectrum on this substrate at 6 K has two sharp peaks both of which are attributed to resonances of A- and B-excitons, which reflects small nonradiative damping constant of excitons as well as high film crystallinity accomplished by virtue of lattice matching. This is, to our best knowledge, the first observation of exciton absorption and its direct estimation of parameters related to A- and B-excitons.