Hong-Wen Ren

In0.5Ga0.5As quantum dot intermixing and evaporation in GaAs capping layer growth

Our study of GaAs growth over self-assembled In0.5Ga0.5As quantum dots grown by metalorganic vapor-phase epitaxy showed that GaAs capping layer surface morphology at the onset strongly depended on temperature. Incompletely capped In0.5Ga0.5As islands were elongated toward [110], indicating anisotropy in intermixing. During higher-temperature growth interruption, islands show craters in quantum dot centers. Craters become hexagonal holes whose depth matches GaAs capping layer thickness. Postannealing photoluminescence spectra show no peak corresponding to overly large quantum dot radiation, indicating that growth interruption after capping layer formation at a certain thickness eliminates overly large quantum dots.

Lateral Composition Modulation Induced Optical Anisotropy in InP/GaInP Quantum Dot System

GaInP lattice matched to GaAs (001) and InP/GaInP self-assembled quantum dots were grown by gas-source molecular beam epitaxy. Transmission electron microscopy and photoluminescence reveal that the composition-modulated lateral superlattice along the [110] direction dominates over Cu–PtB type long-range ordering on determining the strong spectral polarization of GaInP along the [110] direction. InP dots embedded in GaInP with the lateral superlattice show the same polarization in the photoluminescence spectra, the degree of polarization of quantum dots increases with that of the surrounding GaInP matrix to as high as 40%. Although the shape elongation of InP dots along the [110] direction is also observed, lateral composition modulation in the GaInP matrix is considered to be the major origin for the strong optical anisotropy in the InP/GaInP quantum dot system.

Imaging and single dot spectroscopy of InP self-assembled quantum dots

Abstract Optical properties of InP self-assembled quantum dots (SADs) embedded in Ga0.5In0.5P were studied under various conditions by means of the micro-spectroscopy. A strong optical anisotropy of the InP SADs was found in the macro-photoluminescence spectra and micro-photoluminescence (μ-PL) images showing two-fold symmetry, which reflects the anisotropic structure of the Ga0.5In0.5P matrix. Successive red-shifts of the μ-PL peaks and a recovery of the PL intensity with the increase of the temperature were clearly observed. The fluorescence intermittency from a single quantum dot was also investigated. The on–off switching rate is drastically enhanced by a weak near-IR laser beam irradiation. The influence of the matrix on the optical properties of the SADs is discussed.

Spontaneous one-dimensional lateral alignment of multistacked InGaAs quantum dots on GaAs (n 1 1)B substrates

In0.45Ga0.55As/GaAs multistacking quantum dot (QD) structures were fabricated on a GaAs (n11)B (n=2–4) substrate by metalorganic vapor-phase epitaxy. QDs spontaneously aligned in the [0 1 1] direction were observed on stacked QDs, whereas QDs were randomly distributed in the initial In0.45Ga0.55As layer growth. The formation mechanism of this self-alignment was studied by changing the number of In0.45Ga0.55As/GaAs multilayers and crystallographic arrangement. Photoluminescence spectra showing clear polarization dependence indicate carrier coupling in the QD arrays. This growth technique results in spontaneously aligned InGaAs QDs without any preprocessing technique prior to growth.

In situ ellipsometric study of the formation process of metalorganic vapor-phase epitaxy-grown quantum dots

Our in situ ellipsometry study of metalorganic vapor-phase epitaxy-grown quantum dot (QD) structures showed that the Δ–Ψ trajectory of the ellipsometric signal in Stranski–Krastanow QD growth sharply differs from that in layer-by-layer growth. When QD formation starts, Δ rapidly decreases, inflecting the Δ–Ψ trajectory. This indicates increased scattering loss, deduced from increased surface roughness produced in the transition from two- to three-dimensional surface morphology. Ex situ atomic force microscopy and photoluminescence results correspond well to the ellipsometric signal at the start of QD formation. Based on these results, we discuss growth-dependent QD formation processes such as QD formation onset and growth mode transition.

Optical Properties of InP Self-Assembled Quantum Dots Studied by Imaging and Single Dot Spectroscopy

Optical properties of single InP quantum dots (QDs) were investigated by means of micro-photoluminescence (µ-PL) spectroscopy. The temperature dependence of the integrated µ-PL intensity revealed that each QD has different thermal activation energy ranging from 5 to 40 meV. The PL intensities of some QDs recovered when the temperature was raised beyond a certain value. This phenomenon was explained by competition between radiative and nonradiative processes. In this work, the mechanism of fluorescence intermittency was also investigated. It was found that the switching is triggered by a photo-induced process and that the confined excitons in the excited states play an important role. From the observation of the µ-PL spectra in the external electric field and their temperature dependence, we concluded that the large intensity change is caused by a local electric field generated by a carrier trapped at a localized center. These results indicate that the optical properties of the zero-dimensional system are strongly influenced by the local environment surrounding the QDs.

Phonon-enhanced intraband transitions in InAs self-assembled quantum dots

Phonon-mediated intraband carrier relaxation was studied in InAs/GaAs self-assembled quantum dots (QDs) by combining resonant photoluminescence and photoluminescence excitation experiments with high spectral resolution. The phonon-related resonances were found to dominate both the luminescence and excitation spectra. Spectral widths of 1LO-phonon peaks are surprisingly small and comparable with the inverse lifetime of the LO phonons in QDs, whereas, the multiphonon resonances are much broader most likely reflecting the width of n-phonon density of states.

Breakdown of the Phonon Bottleneck Effect in Self-Assembled Quantum Dots

Spectral and temporal behavior of photoluminescence of site-selectively excited InP self-assembled quantum dots were studied in external electric fields. External electric field accelerates the nonradiative relaxation and competing fast phonon mediated relaxation processes make prominent structures in the static spectra. The spectra agree with the time-resolved spectra around the time zero and reflect the phonon relaxation rate. Acoustic phonon mediated carrier relaxation is much faster than predicted theoretically. The increases of the excitation intensity, temperature, electric current and charging of quantum dots accelerate the relaxation of the carriers in quantum dots, indicating build-up of Auger-like processes. This observation demonstrates the breakdown of the predicted phonon bottleneck effect not only at the condition for the practical application of the quantum dots but also at low temperature and under weak excitation.

Photoluminescence and micro-imaging study of optically anisotropic InP self-assembled quantum dots

Abstract The optical properties of InP self-assembled quantum dots (SADs) were investigated under band-to-band excitation of the Ga 0.5 In 0.5 P matrix by means of macro- (conventional) and micro-photoluminescence (μ-PL) spectroscopy. We clearly observed that the number of bright spots in the μ-PL images of InP SADs depends on the detection energy, reflecting the size distribution of the SADs. The macro-photoluminescence spectra and μ-PL images of the InP SADs were found to exhibit a strong optical anisotropy with two-fold symmetry, which reflects the anisotropic structure of the Ga 0.5 In 0.5 P matrix due to the Cu-Pt B type long-range ordering.

Optical Study of Strain-Induced GaAs Quantum Dots

The excitation-intensity-dependent nonlinear luminescence as well as the luminescence of strain-induced GaAs quantum dots (SIQDs) was studied. The luminescence spectrum of SIQDs was composed of three well-resolved transitions and each transition had a different saturation excitation intensity. The increase and saturation of luminescence was more clearly seen in the nonlinear luminescence, where the saturation excitation intensities for each energy level were demonstrated to be proportional to the degeneracies of each level. The nonlinear luminescence spectra were simulated by rate equations, taking account of the relaxation rate, the recombination rate, and the state-filling effect caused by Pauli blocking. As a result of fitting, the relaxation rate was estimated to be about 30 ps.