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Surface morphology dynamics in strained-layer epitaxy Pinnington, Thomas Henry

Abstract

The surface of a film grown epitaxially on a crystalline substrate is generally rough, even if the initial growth surface is smooth on the atomic scale. In the case of strained-layer epitaxy, in which the composition of the film is such that it does not share the same lattice constant as the substrate, the roughness often develops in response to strain-relief processes occurring in> the film during growth. In this thesis we show that a careful analysis of the time evolution of the surface morphology during strained-layer growth, can reveal quantitative information about both the strain-relief mechanisms acting within the film and the diffusion processes occurring at the surface. Two complementary measurement techniques, namely atomic force microscopy (AFM) and elastic light scattering, are used to acquire the surface morphology information necessary for the analysis. In this work we demonstrate quantitative agreement between roughness measurements obtained by both techniques. A major advantage of light scattering over AFM is its suitability to real-time monitoring of the surface during growth. We consider the growth of In0.18Ga0.82As and InAs, on (001)-oriented GaAs substrates, by molecular beam epitaxy (MBE). In both cases the film is compressively strained owing to the 7% lattice mismatch between InAs and GaAs. In the case of I n In0.18Ga0.82As growth, the strain is relieved plastically as the film thickness increases, through the introduction of misfit dislocations at the film/substrate interface. A characteristic Crosshatch pattern develops at the surface, consisting of ridges aligned along the (110) crystal directions. We present an analytical model to describe this roughening, in which the ridges arise from surface diffusion in response to the dislocation strain fields. Although it has only three fitting parameters, the model is able to reproduce both the time dependence and the length scale dependence of the surface morphology, as measured by light scattering and AFM. For the case of InAs growth on GaAs, the strain is relieved elastically through a morphological transition in which nearly identically-sized three-dimensional islands form, known in the literature as quantum dots. For typical growth conditions the islands are too small and closely spaced to be detected using visible wavelengths. An ultraviolet light scattering apparatus is described, which we show can detect the onset of quantum dot formation. The UV scattering signal increases linearly with time after the dots have formed, which we interpret as evidence that the dots are diffusing on the surface. During prolonged annealing we observe the emergence and growth of larger islands that initially consume material from the quantum dots and then compete with each other for material. The in situ light scattering measurements reveal that these processes are sensitively dependent on annealing temperature and arsenic overpressure.

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