Families Of Quantum Dots Research Articles

Optical properties of InGaN quantum dots grown by SiNx nanomasks

InGaN quantum dots (QDs) deposited on SiNx nanomasks have been investigated by atomic force microscopy, photoluminescence (PL), and photoluminescence excitation (PLE) measurements. It was found that the size of QDs can be well controlled by SiNx nanomasks, enabling the manipulation of quantum confinement effect. The PL spectra of InGaN QDs contain several fine structures, and the main peaks can be attributed to families of QDs with different sizes. The emission arising from InGaN QDs and GaN buffer layer can be clearly distinguished based on PLE measurement, which can be used to improve the interpretation in the previous reports. Our study indicates that the quantum confined Stark effect due to piezoelectric field plays a very important role in the optical properties of InGaN QDs, which is very useful for the application of optoelectronic devices.

Effect of cap-layer growth rate on morphology and luminescence of InAs∕InP(001) quantum dots grown by metal-organic vapor phase epitaxy

This work reports on the influence of the InP cap-layer growth rate on the structural and optical properties of InAs∕InP quantum dots (QDs) grown by metal-organic vapor phase epitaxy. A careful correlation between the structural and optical properties of the QDs completed by a modeling of their interband transition energy evidences the presence of different QD families with heights varying by monolayer steps. The analysis of transmission electron microscopy images and photoluminescence spectra demonstrates a drastic decrease of the QD height during the growth of the InP cap layer, due to As∕P exchange. The efficiency of this erosion mechanism is shown to be strongly related to the QD exposure time to PH3, depending on the growth rate of the InP cap layer.

Optimizing the spacer layer thickness of vertically stacked InAs/GaAs quantum dots

Vertically stacked multilayers of self-organized InAs/GaAs quantum dots (QDs) structures with different GaAs intermediate layer thicknesses varying between 2.8 and 17 nm are grown by solid source molecular beam epitaxy (SSMBE) and investigated by photoluminescence spectroscopy (PL). For 17 nm thick GaAs spacer, the PL spectra show two well separated features attributed to the formation of two QDs family with a bimodal size distribution indicating no correlation between the dots in different layers. In the meanwhile, the structures having thinner spacer thickness demonstrate single PL peaks showing an enhancement of high energy side asymmetrical broadening when increasing the excitation power. The corresponding emission energies exhibit a red shift when the spacer layer thickness decreases and correlated with the enhancement of the vertical electronic coupling as well as the rise of the QD's size in the upper layers induced by the build up of the strain field along the columns. The spacer thickness of 8.5 nm is found to yield the best optical properties.

Long wavelength vertically stacked InAs/GaAs(001) quantum dots with a bimodal size distribution: Optical properties and electronic coupling

A molecular beam epitaxy (MBE) grown vertically stacked InAs/GaAs(001) quantum dots (QDs) structure emitting at 1.3 μm at room temperature has been investigated by photoluminescence (PL) experiment. The PL measurement has shown the coexistence of bimodal size distribution in the QD ensemble. Furthermore, the large size QDs are found to exhibit an interlayer vertical coupling in addition to a lateral coupling with neighboring small size QDs. A sigmoidal temperature dependence of the large size QD family’s PL peak as well as an anomalous decrease of the corresponding PL line width at intermediate temperature range are shown to occur. This behavior has been interpreted in terms of in-plane carrier transfer and repopulation process facilitated by the intermediate small size QDs states. These results can help to improve understanding some fundamental properties of long wavelength vertically stacked InAs/GaAs QDs.

Tuning of the electronic properties of self-assembled InAs/InP(001) quantum dots by rapid thermal annealing

We have investigated the effect of post-growth rapid thermal annealing on the low-temperature photoluminescence (PL) spectra of self-assembled InAs quantum dots (QDs) grown in InP(001) by chemical-beam epitaxy using both conventional and modified capping procedures. As-grown samples are characterized by a broad emission peak centered near 800–900 meV corresponding to distinct QD families of different sizes with no observable wetting-layer emission. Rapid thermal anneals were performed at 650 to 800 °C for 210 s, resulting in blueshifts of up to 120 meV due to intermixing. While the PL emission energies of the various QD families shift at similar rates upon annealing, the peak widths remain approximately constant. Finally, we show that the growth of a low-temperature InP cap layer containing a large number of point defects significantly enhances interdiffusion and results in PL blueshifts in excess of 300 meV.

Photoreflectance characterization of InAs/GaAs self-assembled quantum dots grown by ALMBE

We report on a photoreflectance investigation in the 0.8-1.5 eV photon energy range and at temperatures from 80 to 300 K on stacked layers of InAs/GaAs self-assembled quantum dots (QDs) grown by Atomic-Layer Molecular Beam Epitaxy. We observed clear and well-resolved structures, which we attribute to the optical response of different QD families. The dependence of the ground state transition energy on the number of stacked QD layers is investigated and discussed considering vertical coupling between dots of the same column. It is shown that Coulomb interaction can account for the observed optical response of QD families with different morphology coexisting in the same sample.

Recombination emission from InAs quantum dots grown on vicinal GaAs surfaces

Photoluminescence (PL) spectra of InAs/GaAs heteroepitaxial structures with quantum dots (QDs) have been studied. The structures were grown by submonolayer migration-enhanced epitaxy on vicinal substrates with the amount of deposited InAs close to the critical value of 1.8 monolayer (ML). The origin and evolution of the structure of PL spectra were studied in relation to the direction and angle of misorientation, temperature, and power density and spectrum of the exciting radiation. A blue shift and narrowing of the PL band with increasing misorientation angle was established experimentally. The fact that QDs become smaller and more uniform in size is explained in terms of a lateral confinement of QDs on terraces with account taken of the step bunching effect. The temperature dependences of the positions and full widths at half-maximum (FWHM) of PL bands are fundamentally different for isolated and associated QDs. The exciton ground states contribute to all low-temperature spectral components. The excited exciton state contributes to the recombination emission from QDs, as evidenced by the temperature dependence of the integrated intensity of the PL bands. A quantitative estimate is given of the electronic structure of different families of InAs QDs grown on GaAs substrates misoriented by 7° in the [001] direction.

Pressure behaviour of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots with multi-modal distribution in size

The pressure behaviour of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots (QDs) has been studied at 15 K in the pressure range of 0-1.3 GPa. The atomic force microscopy image shows that the QDs have a multi-modal distribution in size. Three emission peaks were observed in the photoluminescence (PL) spectra, corresponding to the different QD families. The measured pressure coefficients are 82, 94 and 98 meV GPa-1 for QDs with average lateral size of 26, 52 and 62 nm, respectively. The pressure coefficient of small QDs is about 17% smaller than that of bulk In0.55Al0.45As. An envelope-function calculation was used to analyse the effect of pressure-induced change of barrier height, effective mass and dot size on the pressure coefficients of QDs. The -X state mixing was also included in the evaluation of the reduction of the pressure coefficients. The results indicate that both the pressure-induced increase of effective mass and -X mixing respond to the decrease of pressure coefficients, and the -X mixing is more important for small dots. The calculated -X interaction potentials are 15 and 10 meV for QDs with lateral size of 26 and 52 nm, respectively. A type-II alignment for the X conduction band is suggested according to the pressure dependence of the PL intensities. The valence-band offset was then estimated as 0.15±0.02.

Photoreflectance Investigation of Inas/Gaas Self-Assembled Quantum Dots Grown by Almbe

AbstractPhotoreflectance measurements have been performed in the 0.8–1.5 eV photon energy range and at temperatures from 80 to 300 K on stacked layers of InAs/GaAs self-assembled quantum dots (QDs) grown by Atomic-Layer Molecular Beam Epitaxy. The spectral features due to the QD optical response were analyzed by using lineshape models characteristic of modulation spectroscopy of confined systems. The dependence of the ground state transition energy on the number of stacked QD layers is investigated and it is shown that Coulomb interaction can account for the observed different behavior of the ensemble optical response of QD families characterized by different morphologies and coexisting in the same sample.