H.-W. Ren

Coherent Control of Stress-Induced InGaAs Quantum Dots by Means of Phonon-Assisted Resonant Photoluminescence

We report the control of coherent status of the lowest energy electronic state of InGaAs/GaAs stress-induced quantum dots in an inhomogeneously broadened system by time-integrated detection of narrow Raman-like lines in resonant photoluminescence spectra. The dephasing time of 18.5 ps has been found at 2K.

Highly Uniform and Small InP/GaInP Self-Assembled Quantum Dots Grown by Metal-Organic Vapor Phase Epitaxy

InP self-assembled quantum dots embedded in Ga0.51In0.49P were prepared by metal-organic vapor phase epitaxy. In order to reduce the dot diameter and improve the size uniformity, InP dots were grown at a low temperature using tertiarybutyl-phosphine instead of phosphine. Growth of 4 ML InP on a 4 ML GaP interface layer at 550°C resulted in InP islands of about 30 nm in base diameter and 7 nm in height that were remarkably uniform to within 10% deviation. The insertion of another 2 ML GaP interface layer between the InP islands and the GaInP cap-layer further reduced the half-width of the photoluminescence (PL) spectra from the InP dots to 29 meV at both 2 K and 77 K. It is necessary to grow the GaInP caplayer at higher than 630°C in order to alloy the undeveloped InP clusters and wetting layer with GaP interface layers.

Size Quantization in InAs/GaAs Self-Assembled Quantum Dots Grown by Gas-Source Molecular Beam Epitaxy

The number of confined states in a self-assembled quantum dot (SAD) is determined by its size, shape and composition. By employing a gas-source molecular beam epitaxy, the average size of InAs self-assembled islands on the GaAs (001) surface was controlled by the InAs growth temperature. Meanwhile, indium segregation during the GaAs cap-layer growth was found to greatly modify the shape and actual composition of InAs/GaAs SADs resulting in a blue-shift in the ground state energy. By suppressing indium segregation, InAs/GaAs SADs 39 nm, 29 nm, 22 nm and 15 nm in their average diameters were obtained. The corresponding number of confined states observed was five, three, two and one, respectively. The energy separation between the states is about 50 meV, it does not change appreciably with decreasing the dot size but reduces with intensifying indium segregation.

Many carrier effects in self-assembled InP quantum dots

Abstract Excitation power density dependence of InP self-assembled quantum dots (SADs) was investigated under band-to-band excitation in a Ga0.5In0.5P matrix by means of macro- (conventional) and micro-spectroscopy. State filling of confined excitonic states was studied in detail by micro-photoluminescence (μ-PL) spectra and images. The efficiency of the state filling differs from dot to dot, which depends on the inflow and outflow rates of the excitons. Sharp μ-PL lines from a single confined exciton and biexciton were clearly observed. The observed biexciton binding energy of an InP SAD is a few times larger than that of the bulk InP, suggesting that the multi-exciton states are stabilized because of the confinement.

Quantum Beats in Photoluminescence of InP Quantum Dots in Electric Field

The photoluminescence (PL) kinetics of heterostructures with InP self-assembled quantum dots are studied under quasi-resonant pulse excitation in the presence of an external electric field. An oscillatory behavior of the PL kinetics is shown to arise due to quantum beats of the radiative states. A model of the coherent excitation of nonresonant PL is proposed.

Temperature Dependence of Luminescence Decay Time of InP Quantum Disks

We measured temperature dependence of luminescence decay time of self-assembled InP dots in GaInP lattice-matched to GaAs. The radiative lifetime of InP-dot luminescence is independent of temperature below ~40 K and is linear with temperature between ~40 and ~120 K. These two features in the two temperature regimes are characteristic of zero-dimensional and two-dimensional structures, respectively. This temperature behavior of the lifetime is thought to be caused by the disklike shape of the InP dots; the dot lateral widths are longer than their heights, and thus they have an intermediate character between zero-dimension and two-dimensions.

Observation of Franz-Keldysh Oscillations in InP Self-Assembled Quantum Dot Systems

Distinct Franz-Keldysh oscillations were observed in the nonlinear reflection spectra of heterostructures with InP self-assembled quantum dots measured by the pump-probe method. These oscillations showed the presence of a built-in electric field of about 30 kV/cm. The built-in electric field is considered to originate from the electric charge captured by the structural defects on the dot interface. The estimated areal density of the electric charge is about 2×1011 cm-2. Electroreflectance showed nonmonotonic changes with the change in applied bias.

Carrier relaxation dynamics in self-assembled quantum dots studied by artificial control of nonradiative losses

Abstract : A novel technique to study carrier relaxation dynamics based on the artificial control of nonradiative losses by an external electric field is proposed. A clear evidence of phonon assisted relaxation as the main relaxation mechanism of hot electron-hole pairs in InP self-assembled quantum dots is found by the proposed method. Efficient one step relaxation processes with emission of acoustic and optical phonons are observed. These findings give new and important insight into the interaction of the electron-hole pairs in quantum dots with the phono subsystem.

Heterostructure optical phonons in dynamics of quantum dot electronic excitations: new experimental evidences

Unusual coupling of heterostructure optical phonons and electronic excitations in quantum dots (QDs) was observed by photoluminescence spectroscopy in strain-induced InGaAs/GaAs QDs (SIQDs) and self-assembled InAs/GaAs QDs (SAQDs). Phonon-assisted interband transitions in SIQDs were found to be governed by zone center bulk GaAs TO and LO phonons via deformation potential interaction, whereas polar interaction was inessential. For SAQDs, a n-doped GaAs substrate was found to effect on QD intraband relaxation of carriers via coupling between them and substrate LO-phonon-plasmon modes at spacing between QDs and substrate as long as 100 nm.

Optical properties of InP self-assembled quantum dots studied by imaging and single dot spectroscopy

The temperature dependent optical properties of InP self-assembled quantum dots (QDs) were investigated by means of micro-spectroscopy. Successive thermal quenching with increasing temperature was clearly observed. A few QDs, however, showed anomalous temperature dependence with the photoluminescence (PL) intensity recovering when the temperature was raised beyond a certain value. This phenomenon is explained by competition between radiative and nonradiative processes. In this work, the mechanism of blinking behavior was also studied. Notably, the micro-PL spectra of a blinking QD were artificially reproduced by applying an electric field to a normal QD. We concluded that the blinking is due to the trapping and delocalization of carriers at localized states near the QD. These results indicate that the optical properties of the zero-dimensional system are strongly influenced by the local environment surrounding the QDs.