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Sharp and discrete emission lines from single localized states in ZnCdS ternary alloys were clearly observed from selectively grown mesa structures. This is a demonstration that compound semiconductor alloy systems, which usually show broad emission spectra due to alloy fluctuations, are able to exhibit emissions from discrete energy levels with quasi-zero-dimensional density of states in limited mesa areas where limited number of deeper localized states will contribute. Introduction of ZnCdS/MgS short-period superlattices is found to play a significant role for the exciton migration enhancement from shallower to deeper localized states, which makes the observation of the emission lines from the single localized states possible in the ZnCdS alloy layers.

In this letter, we report the growth of single-crystalline rocksalt CdO layers on (001) GaAs substrates using ZnS buffer layers. The growth processes of CdO layers were studied by reflection high-energy electron diffraction (RHEED), and the grown CdO layers were evaluated with atomic force microscopy (AFM), and x-ray diffraction (XRD) measurements. After an initial growth delay, the formation of polycrystalline CdO was observed in RHEED measurements during the initial growth of very thin CdO layers. With the increase of the CdO layer thicknesses, streaky RHEED patterns were observed, which indicate the formation of single-crystalline cubic-phase CdO layers. Surface morphology of the CdO layers observed by AFM was atomically flat with root-mean-square roughness of ∼1 nm. The crystalline structures were elucidated from XRD measurements by the determination of the lattice constant to be 4.686±0.001 A, indicating the single-phase rocksalt CdO structure.

A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.

We have investigated the effects of In doping on the optical properties of GaN films grown by gas-source molecular-beam epitaxy. Time-resolved photoluminescence was carried out to study the transient optical properties of the epitaxial films. In comparison to the undoped GaN film, the spontaneous emission lifetime was prolonged from below 20 to 70 ps by doping with In. Under high-excitation density, stimulated emission was observed from both samples. The threshold excitation density was found to be reduced in the In-doped sample. These significant improvements of the optical properties are attributed to the effective suppression of the formation of the nonradiative recombination centers caused by a change of the growth kinetics induced by a small amount of In supplied during growth of the GaN films.

Self-organized ZnSe quantum dots (QDs) were grown on (100) ZnS/GaAs surfaces to study the relation of the size dispersion and luminescence. The exact dot sizes were obtained by measurements of atomic force microscope with its tip calibration and transmission electron microscope. The average dot size was 2.0 nm high and 11 nm in its diameter and the density was 1×1010 cm−2. Transition energies of ZnSe QDs were calculated using these measured dot sizes. These calculated peaks were in reasonable agreement with measured photoluminescence (PL) peaks. It was also revealed that the broadening of the PL spectra from ZnSe QDs were consistently explained by the dot size distribution.

Here we describe a strategy toward constructing semiconductor photonic dots in the ultraviolet to blue region. An array of ZnS dots was grown on a GaAs substrate with a selective growth method. The ZnS dots have a pyramidal structure with the base plane of 800 nm square and the height of 300 nm. The {034} crystallographic planes form the sidewalls of the pyramids. Therefore, the size of the pyramidal dots is uniquely determined by the mask patterning. The optical reflection spectra showed clear resonance peaks which are reasonably assigned by the calculation of the resonance modes. Each resonance showed the Q values on the order of 160–300, a reasonable value to observe the modification of the total spontaneous emission rate in this kind of photonic dots.

Optical properties of a ZnSe/MgS superlattice (SL) were studied by reflection and photoluminescence (PL) spectroscopies. Excitonic structures were clearly observed in the reflection spectra up to room temperature, and the exciton absorption peak energy and linewidth were well coincident with the PL peak energy and linewidth. The origin of the lowest luminescence peak was indicated to be C1-HH1 exciton up to room temperature. The lineshape broadening due to longitudinal-optical-phonon scattering of excitons is much decreased in this SL compared to bulk ZnSe.

We report the growth of zinc-blende ZnSe/MgS superlattices (SLs) on GaAs (001) substrates. The SLs were grown with metalorganic vapor phase epitaxy by selecting appropriate precursors for Mg and S. MgS naturally forms rocksalt structures, but zinc-blende MgS layers were grown. The lattice constant of MgS was estimated to be 5.59 Å. X-ray diffraction measurements show that the ZnSe/MgS SLs are grown coherently to the GaAs substrates up to the total thicknesses of ~ 3000 Å.

Metallic nanocavities have been actively studied for realizing nanolasers with low threshold. Presence of resonance modes with high cavity Q values is the indication of low internal loss that leads to low threshold lasing. However, cavity Q values observed in metallic nanocavities below lasing threshold remain low at present on the order of 100 to 500. We study the possibility to realize higher resonance Q values with a metallic nanocavity. For probing purpose of cavity modes we propose to employ broad mid-gap-state optical emission of n-type GaAs. With this method we report the observation of a resonance mode with the high Q value of 3800 at room temperature with the metallic nanocavity. The cavity mode is identified as a whispering-gallery mode with finite-element- method simulation.

We report on the experimental demonstration of a single-photon source based on an InAs quantum dot (QD) on InP grown by molecular-beam epitaxy emitting in the telecommunication band. We develop a method to reduce the QD density to prevent inter-dot coupling via tunneling through coupled excited states. A single InAs QD embedded in an as-etched pillar structure exhibits intense and narrow emission lines. Photon antibunching is clearly observed using superconducting single-photon detectors with high sensitivity, and further improvement of the generated single-photon purity is demonstrated with below-barrier-bandgap excitation. (C) 2013 AIP Publishing LLC.

High photon-extraction efficiency is strongly required for a practical single-photon source. We succeed in fabricating metal (sliver)-embedded nanocone structure incorporating an InAs quantum dot. Efficient photon emission of similar to 200 000 photons per second is detected and single-photon emission is demonstrated using autocorrelation measurements. The photon-extraction efficiency as high as 24.6% is obtained from the structure. (C) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4801334]

We report the observation of an anomalous antibunching dip in intensity autocorrelation function with photon correlation measurements on a single-photon emitter (SPE). We show that the anomalous dip observed is a manifestation of quantum nature of SPEs. Taking population dynamics in a quantum two-level system into account correctly, we redefine intensity autocorrelation function. This is of primary importance for precisely evaluating the lowest-level probability of multiphoton generation in SPEs toward realizing versatile pure SPEs for quantum information and communication., 10 pages including 3 figires

The axial orientation of molecular-beam-epitaxy (MBE)-grown Fe3Si(111)∕Ge(111) hybrid structures was investigated by Rutherford backscattering spectroscopy. We confirmed that during MBE above 300°C, the interdiffusion of Fe and Ge atoms results in a composition change and the epitaxial growth of FeGe in Fe3Si. Low-temperature (

Low-temperature (60°C) molecular beam epitaxy (MBE) of Fe3Si layers on Ge substrates was investigated. From x-ray diffraction and transmission electron microscopy measurements, it was shown that Fe3Si layers including the DO3 type were epitaxially grown on Ge(110) and Ge(111), while polycrystal Fe3Si was formed on Ge(100). Although the Fe3Si∕Ge(110) interface was slightly rough (∼1nm), the Fe3Si∕Ge(111) interface was atomically flat. Such atomically controlled MBE of Fe3Si on the Ge(111) substrate can be employed to realize Ge channel spin transistors, which can be integrated with Si large-scale integrated circuits.