Thermal activation and thermal transfer of localized excitons in InAs self-organized quantum dots

We have investigated the temperature dependence of photoluminescence (PL) properties of a number of InAs/GaAs heterostructures with InAs layer thickness ranging from 0.5 monolayer (ML) to 3 ML. The temperature dependence of the InAs exciton energy and linewidth was found to display a significant difference when the InAs layer thickness is smaller or larger than the critical thickness around 1.7 ML, indicating spontaneous formation of quantum dots (QDs). A model, involving exciton recombination and thermal activation and transfer, is proposed to explain the experimental data. In the PL thermal quenching study, the measured thermal activation energies of different samples demonstrate that the InAs wetting layer may act as a barrier for thermionic emission of carriers in high quality InAs multilayers, while in InAs monolayers and submonolayers the carriers are required to overcome the GaAs barrier to thermally escape from the localized states. (C) 1998 Academic Press Limited.

Electronic investigation of self-organized InAs quantum dots

Deep Level Transient Spectroscopy (DLTS) has been applied to investigate the electronic properties of self-organized InAs quantum dots. The energies of electronic ground states of 2.5ML and 1.7ML InAs quantum dots (QDs) with respect to the conduction band of bulk GaAs are about 0.21 eV and 0.09 eV, respectively. We have found that QDs capture electrons by lattice relaxation through a multi-phonon emission process. The samples are QDs embedded in superlattices with or without a 500 Angstrom GaAs spacing layer between every ten periods of a couple of GaAs and InAs layers. The result shows that the density of dislocations in the samples with spacer layers is much lower than in the samples without the spacer layers.

Material transport in self-assembled InAs/GaAs quantum dot ensemble

Introducing the growth interruption between the InAs deposition and subsequent GaAs growth in self-assembled quantum dot (QD) structures, the material transport process in the InAs layers has been investigated by photoluminescence and transmission electron microscopy measurement. InAs material in structures without misfit dislocations transfers from the wetting layer to QDs corresponding to the red-shift of PL peak energy due to interruption. On the other hand, the PL peak shifts to higher energy in the structures with dislocations. In this case, the misfit dislocations would capture the InAs material from the surrounding wetting layer and coherent islands leading to the reduction of the size of these QDs. The variations in the PL intensity and Linewidth are also discussed.

Influence of interdot electronic coupling on photoluminescence spectra of self-assembled InAs/GaAs quantum dots

The influence of interdot electronic coupling on photoluminescence (PL) spectra of self-assembled InAs/GaAs quantum dots (QDs) has been systematically investigated combining with the measurement of transmission electron microscopy. The experimentally observed fast red-shift of PL energy and an anomalous reduction of the linewidth with increasing temperature indicate that the QD ensemble can be regarded as a coupled system. The study of multilayer vertically coupled QD structures shows that a red-shift of PL peak energy and a reduction of PL linewidth are expected as the number of QD layers is increased. On the other hand, two layer QDs with different sizes have been grown according to the mechanism of a vertically correlated arrangement. However, only one PL peak related to the large QD ensemble has been observed due to the strong coupling in InAs pairs. A new possible mechanism to reduce the PL linewidth of QD ensemble is also discussed.

The enhancement of diffusion by strain of InAs quantum dots in a GaAs matrix

We investigate the annealing behavior of Photoluminescence (PL) from self-assembled InAs quantum dots (QDs) with different thicknesses GaAs cap layers. The diffusion introduced by annealing treatment results in a blue-shift of the QD PL peak, and a decrease in the integrated intensity. The strain present in QDs enhances the diffusion, and the QDs with the cap layers of different thicknesses will experience a strain of different strength. This can lend to a, better understanding of the larger blue-shift of the PL peak of the deeper buried QDs, and the different variance of the full width at half maximum of the luminescence from QDs with the cap layers of different thicknesses.

Optical anisotropy of InAs submonolayer quantum wells in a (311) GaAs matrix

The steplike density of states obtained from reflectance-difference spectroscopy demonstrates that ultrathin InAs layers should be regarded as two-dimensional quantum wells rather than isolated clusters, even for the sample with only 1/3 monolayer InAs in (311)-oriented GaAs. The degree of anisotropy is within the intrinsic anisotropy of (311)-oriented ultrathin quantum wells, indicating that there is little structural or strain anisotropy in the InAs islands. (C) 1998 Elsevier Science B.V.

Annealing behavior of InAs/GaAs quantum dot structures

We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.

Optical anisotropy of InAs monolayer in (311)-oriented GaAs matrix

In-plane optical anisotropy which comes from the heavy hole and the light hole transitions in an InAs monolayer inserted in (311)-oriented GaAs matrix is observed by reflectance difference spectroscopy. The observed steplike density of states demonstrates that the InAs layer behaves like a two-dimensional quantum well rather than isolated quantum dots. The magnitude of the anisotropy is in good agreement with the intrinsic anisotropy of (311) orientation quantum wells, indicating that there is little structural or strain anisotropy of the InAs layer grown on (311)-oriented GaAs surface.

Characterization of self-organized InAs/GaAs quantum dots under strain-induced and temperature-controlled nucleation mechanisms by atomic force microscopy and photoluminescence spectroscopy

Atomic force microscopy and photoluminescence spectroscopy (PL) has been used to study asymmetric bilayer InAs quantum dot (QD) structures grow by molecular-beam epitaxy on GaAs (001) substrates. The two InAs layers were separated by a 7-nm-thick GaAs spacer layer and were grown at different substrate temperature. We took advantage of the intrinsic nonuniformity of the molecular beams to grow the seed layer with an average InAs coverage of 2.0 ML. Then the seed layer thickness could be divided into three areas: below, around and above the critical thickness of the 2D-3D transition along the 11101 direction of the substrate. Correspondingly, the nucleation mechanisms of the upper InAs layer (UIL) could be also divided into three areas: temperature-controlled, competition between temperature-controlled and strain-induced, and strain-induced (template-controlled) nucleation. Small quantum dots (QDs) with a large density around 5 x 10(10) cm(-2) are found in the temperature-controlled nucleation area. The QD size distributions undergo a bimodal to a unimodal transition with decreasing QD densities in the strain-induced nucleation area, where the QD densities vary following that of the seed layer (templating effect). The optimum QD density with the UIL thickness fixed at 2.4 ML is shown to be around 1.5 x 10(10) cm(-2), for which the QD size distribution is unimodal and PL emission peaks at the longest wavelength. The QDs in the in-between area exhibit a broad size distribution with small QDs and strain-induced large QDs coexisting.

水平法低位错GaAs的单晶

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高压液封法生长热稳定不掺杂半绝缘GaAs

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GaAs表面和介面性质对毫米波器件性质的影响

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微重力条件下从溶体生长GaAs单晶及性质研究

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分子束外延GaAs-AlGaAs材料

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分子束外延GaAs/AlGaAs高速器件与光电子器件用料

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