The nature of islanding in the InGaAs / GaAs epitaxial system
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The nature of islanding in the InGaAs / GaAs epitaxial system T. Walther 1, A.G. Cullis 2, D. J. Norris 2 and M. Hopkinson 2 1 Institut für Anorganische Chemie, Universität Bonn, Römerstraße 164, 53117 Bonn, Germany 2 Dept. Electronic and Electrical Eng., University of Sheffield, Mappin St., Sheffield S1 3JD, UK ABSTRACT The interest in the phenomenon of islanding in a range of semiconductor systems is in part due to the fundamental importance of the Stranski-Krastanow transition but also driven by potential device applications of self-organized quantum dot arrays. However, the mechanism underlying the island formation is still to a significant degree unclear. In the present work, we focus on the epitaxial InGaAs / GaAs(001) system, with layer deposition by molecular beam epitaxy. Atomic force microscopy is used to measure the surface topography of nominally 4nm thick InxGa1-xAs films. It is shown that the growth mode switches abruptly from flat layer to island growth if a critical Indium composition of x(In)§LVUHDFKHG7KHVWUXFWXUHRIVXFK layers during early stages of growth is examined using energy-filtered transmission electron microscopy. Indium gradients in the islanded layers are measured and the driving force for the islanding transition itself is considered.
INTRODUCTION Misfit accommodation in strained epitaxial layers can take place through dislocation formation or the occurrence of islanding and general surface roughness [1-4]. Often, the transition of the growth mode from layer-by-layer growth to three-dimensional islanding occurs abruptly when a certain critical strain state is reached [5]. The latter so-called StranskiKrastanow growth has been observed for many strained layer systems and can yield selforganized arrays of quantum dots suitable for advanced device applications. However, the physics behind the islanding transition needs to be better understood to enhance the reproducibility and uniformity of the dot arrays produced. Calculations have shown that compositional variations within corrugated alloy layers may be expected under certain growth conditions, either as a direct result of the strain [6] or due to different ad-atom mobilities on a rough surface [7]. Numerous experimental studies have speculated about lateral segregation, mostly on the basis of indirect methods that measure effects related to the local chemistry of the layers [8-10]. Whether corrugated thin films are still coherent or broken up into separate islands and whether the composition within the layers is homogeneous or not can only be determined by a reliable chemical study in cross-sectional geometry. Measuring the changes of lattice periodicities in high-resolution electron micrographs of cross-sectional specimens [11, 12] provides high lateral resolution, however, this technique only maps the local lattice parameter which depends on composition as well as on imaging conditions. For a wedge sample, the latter vary sensitively with the distance from the specimen edge and in particular the degree of thin foil relaxat
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