Control of Self-Organized In(Ga)As/GaAs Quantum Dot Growth
Self-organized growth of InAs, InGaAs, and GaSb quantum dots in GaAs matrix is investigated to establish a basis for a targeted engineering. All studied dots form in the Stranski–Krastanow mode on a two-dimensional wetting layer in a regime which is predo
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Abstract. Self-organized growth of InAs, InGaAs, and GaSb quantum dots in GaAs matrix is investigated to establish a basis for a targeted engineering. All studied dots form in the Stranski–Krastanow mode on a two-dimensional wetting layer in a regime which is predominantly controlled by kinetics. Equilibrium-near conditions are found on a local scale. The material transfer during dot formation shows distinct differences for the three studied kind of dots. For InGaAs and GaSb dots a pronounced transfer from an intermixed wetting layer to the dots is found. Such dots have an inhomogenous composition, and dot ensembles with a bimodal size distribution may be obtained in both cases. For InAs dots material transfer is observed solely between the purely binary dots, while the wetting layer remains unaffected and is not intermixed. Dots with a multimodal size distribution are demonstrated, referring to well-defined self-similar dot shapes and size variations in steps of integral InAs monolayers.
2.1 Introduction Semiconductor research and device development has seen a progressive reduction in dimensionality, going from bulk down to quantum wells and quantum wires, finally to quantum dots (QDs). QDs represent the ultimate limit in carrier confinement with discrete atomic-like energy states. Such states are significantly different from those of systems with higher dimensionality, resulting in potential applications in novel optoelectronic devices. The most successful approach for the fabrication of coherent, dislocation-free semiconductor QDs is the self-organized (also referred to as self-assembled) technique of Stranski–Krastanow growth. This kind of growth mode may be induced by epitaxially growing a layer of, e.g., InAs on a substrate with a significantly different lattice constant like GaAs. The unstrained lattice constant of InAs is ≈ 7% larger than that of GaAs. The deposited InAs initially grows as a highly strained two-dimensional layer. However, beyond a thickness of only ≈ 1.5 InAs monolayers (MLs) growth transforms to a three-dimensional mode. The
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U.W. Pohl, A. Strittmatter
driving force for this transformation is a reduction of the elastic strain energy [1] as the material in the spontaneously formed islands is not constrained by surrounding material and can relax laterally. Some part of the InAs material does not redistribute to form islands but remains as a two-dimensional layer, because the surface free energy of InAs is lower than that of the GaAs substrate. Since this layer wets the substrate surface it is referred to as wetting layer (WL). For practical use of such islands the structure is covered by a cap layer that is typically identical to the substrate material. The size of embedded self-organized InAs/GaAs structures is in the nanometer range, i.e. of the order of the exciton Bohr radius, giving rise to fully quantized confined electron and hole states. These nanostructures are therefore generally termed quantum dots. The electronic properties of the QDs are determined by the size, shape and c
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