Dynamics of InAs Quantum Dots Formation on AlAs and GaAs
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Dynamics of InAs Quantum Dots Formation on AlAs and GaAs M. Yakimov, V. Tokranov, and S. Oktyabrsky UAlbany Institute for Materials, University at Albany - SUNY, CESTM, 251 Fuller Rd., Albany, NY 12203, U.S.A. ABSTRACT We have studied the formation of InAs quantum dots (QDs) grown by molecular beam epitaxy on top of GaAs and 2 ML-thick AlAs layers in the temperature range from 350 to 500 0 C. In-situ reflection high energy electron diffraction (RHEED) patterns were recorded in real time during the growth and analyzed to characterize the 2D-to-3D transition on the surface, including QD formation, and ripening process. The kinetics of QD formation was studied using the InAs growth rates ranging from 0.01 to 1 ML/s and different ratios of As2/In fluxes. RHEED patterns and ex-situ atomic force microscopy images were analyzed to reveal the development of sizes and shapes of the single-layer and stacked QD ensembles. The critical InAs coverage for QD formation was shown to be consistently higher for dots grown on the AlAs overlayer than for those grown on GaAs surface. Self-assembly of multilayer QD stacks revealed the reduction of the critical thickness for dots formed in the upper layers.
INTRODUCTION InAs self-assembled quantum dots are considered as a promising active medium for optoelectronic devices [1]. Still, despite the significant efforts to reduce the QD size distribution, it remains still too wide [2, 3] for advanced device applications. To overcome this problem, an accurate control of the surface processes with enhanced reproducibility is required. One of the further challenges to utilize the QD media for the devices operating at room and elevated temperatures is to suppress the thermal emission of carriers from the quantum-confined states. One of the possible approaches employs the use of the barrier material with a wider band gap in the QD structures [3, 4]. The structural quality of AlGaAs alloy layers is not high enough, especially when grown at low temperatures, required for QD technology. Our general approach is to use a short period superlattice (SPSL) with wide enough effective band gap instead of AlGaAs alloys for the barrier material in the QD structures. The use of SPSL is known to smoothen the surface of the structure that might enhance the flatness and epitaxial quality of the upper QD sheets in particular in the multilayered stacks of QDs. Growth of SPSL matrix can be terminated either with AlAs or GaAs layers giving an additional degree of freedom to control the QD properties. In the present paper, we study the growth of the InAs QD layers on top of GaAs/AsAs SPSL with the emphasis on the differences of the QD formation process on top of AlAs and GaAs surfaces. The process of QD formation on each of these two different surfaces is different due to different rate of intermixing with underlying layer [3] and different surface diffusion properties of adatoms. The SPSL also allows to increase the effective band gap of the barrier without deteriorating the surface smoothness and preventing the form
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