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Optoelectronic properties of undoped ZnSe/ZnMgSSe multiple quantum wells (MQWs) grown by molecular beam epitaxy on (001) GaAs substrates have been investigated by photoluminescence, photoconductivity, and persistent photoconductivity (PPC) measurements. The features related to the band-to-band excitonic and impurity-related transitions of ZnSe/ZnMgSSe MQWs are observed and discussed. In addition, the parameters that describe the temperature dependence of the interband transition energy and broadening function of the excitonic feature are evaluated. PPC has been observed in undoped ZnSe/ZnMgSSe MQWs. The decay kinetics of the PPC effect can be described by a stretched-exponential function, Ippc(t)=Ippc(0)exp[−(t/τ)β], (0

We present photoluminescence (PL) and time-resolved photoluminescence measurements in GaN/Al0.2Ga0.8N superlattices grown by metalorganic chemical vapor deposition under the optimal GaN-like growth conditions. We found that the carrier confinement of our samples is better than most of the previous reports. The dependence of the PL emission energy and intensity on temperature, in the low temperature regime, is consistent with recombination mechanisms involving localized states attributed to a small degree of interface fluctuations. Picosecond time-resolved photoluminescence spectroscopy has been employed to probe the well-width dependence of the lifetime of these superlattices. We see that the recombination lifetime increases with the decrease of well width. This behavior can be interpreted by the fact that the effect of localization keeping carriers away from nonradiative pathways can be enhanced by a decrease in the well width. This explanation is consistent with the temperature-dependent PL data.

The optical studies of SiO2 mesoporous materials with hierarchical tubules-within-tubule structure have been investigated by photoluminescence and Fourier-transform infrared transmittance (FTIR). Our results suggest that the radiative intensity can be strongly enhanced by annealing the samples in N2 environment. From the FTIR spectra, we have pointed out that the origin responsible for the strong emission is Si–OH complexes located on nanotube surface. It has been observed that after turning off the pumping laser, the photoluminescence signal of SiO2 nanotubes can persist for several seconds, which is much longer than that of most materials performed under similar conditions. We have found that the decay of the photoluminescence signal is due to the quantum tunneling process. These are triplet and singlet states of Si–OH complexes that are responsible for the observed persistent photoluminescence.

The rapid solidification processes of Ca50Mg20Cu30 liquid alloy have been simulated by first principle molecular dynamics simulation based on the density functional theory. The local structural evolution of the alloy is analysed using Honeycutt Andersen (HA) bond-type index and bond-angle distribution methods. The electronic properties of this amorphous solid are also studied. The simulated coordination numbers are very close to the theoretical values according to the efficient cluster packing model (ECP) model. The interaction between Ca-Cu atomic pairs is strongest in this alloy. The HA bond-type result shows that a large quantity of pentagonal bipyramids are formed in undercooled alloy liquid and become most common polyhedral local structures of the amorphous solid, which suppress the crystallization and increase the glass forming ability of Ca-Mg-Cu alloy. The bond-angle distribution indicates that the close packing of the three neighbor atoms and the pentagon configurations become the primary short r...

Si/Ge core-shell nanowires, which possess uniform diameters around 100 nm, were synthesized at low temperature using a chemical vapor deposition process. The radial structures of Si/Ge nanowires were investigated via cross-sectional transmission electron microscopy analysis. The data from energy dispersive x-ray spectroscopy confirmed the coaxial structures of our nanowires, which consistently determined the core to be polycrystalline Si and the shell to be crystalline Ge. The optical properties of Si/Ge core-shell nanowires were also discussed from Raman measurement. The method presented in this study will allow efficient fabrication of core-shell nanostructures and may be promising for future device application.

Understanding dislocation core structures at the atomic level is of significant theoretical and technological importance because of the role dislocations play in the electronic/optical properties of materials. In this paper, we report our aberration-corrected scanning transmission electron microscopy study on misfit dislocation core structures at non-polar (112¯0)ZnO/(11¯02)Al2O3 (a-ZnO/r-Al2O3) interface. The atomic configuration of the core structure is found to be closely related to the preferred interfacial bonding configuration. A significant number of these misfit dislocations have undergone a core structure modification involving the incorporation of Zn in the Al2O3 side of the dislocation.

The authors studied the concentration dependence of carrier localization in InN epilayers using time-resolved photoluminescence (PL). Based on the emission-energy dependence of the PL decays and the PL quenching in thermalization, the localization energy of carriers in InN is found to increase with carrier concentration. The dependence of carrier concentration on the localization energy of carriers in InN can be explained by a model based on the transition between free electrons in the conduction band and localized holes in the deeper tail states. They suggest that carrier localization originates from the potential fluctuations of randomly located impurities.

Structural and optical properties of Mg-doped AlGaN∕GaN superlattices have been investigated by photoluminescence, scanning electron microscopy, cathodoluminescence (CL), and transmission electron microscopy (TEM). We found that the edge blue-band emission shows a strong optical anisotropy. Through the combination of the CL and TEM images, we clearly establish that the underlying microstructure responsible for the blue luminescence in Mg-doped AlGaN∕GaN arises from the pyramidal defects. The observed optical anisotropy is due to the well aligned pyramidal structure. Our result provides one of the model systems to demonstrate the correlation between optical transition and structural defect in a solid.

x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectrometry, and cathodoluminescence measurements have been employed to study the correlation between optical and structural properties in InGaN epitaxial films. In-rich quantum dots were found to be dispersed throughout the film. By the combination of these measurements, we clearly identify that brighter luminescence arises from In-rich regions while dimmer luminescence corresponds to the Ga-rich matrix regions.

We have investigated the crystal orientation dependence of optical properties in InxGa1−xN∕GaN multiple quantum wells. The spectral peaks and intensity of the microphotoluminescence signal for different crystal orientations were found to have six-fold symmetry. Quite interestingly, the refractive index, obtained from the interference pattern, also varies with the crystal orientation. The 60° periodic anisotropy of electronic transitions as well as optical parameters was interpreted in terms of the Pockels effect induced by the strong built-in field in nitride heterojunctions. The linear dependence of the change of the refractive index on electric field is consistent with the prediction of the Pockels effect. Our result provides an alternative solution to improve the designs of photonic and electronic devices based on nitride semiconductors.