Shape Transitions of Self-Assembled Ge Islands on Si(001)
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Shape Transitions of Self-Assembled Ge Islands on Si (001) Armando Rastelli1, Matthias Kummer2, Hans von Känel2 INFM - Università di Pavia, Via Bassi 6, I-27100 Pavia, Italy 2 Laboratorium für Festkörperphysik, ETH Zürich, CH-8093 Zürich, Switzerland
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ABSTRACT Coherently strained Ge islands were grown at a substrate temperature of 550°C by magnetron sputter epitaxy on Si (001) and studied by scanning tunnelling microscopy (STM). The shape changes induced by exposure to a Si-flux at 450°C were investigated as a function of the Sicoverage. During Si-capping, multifaceted domes were found to flatten and to transform into {105}-faceted pyramids and subsequently into stepped mounds through intermediate shapes. The observed sequence of morphological changes is induced by Si-Ge intermixing and is shown to be the inverse of that occurring during Ge or Si1-xGex growth on Si (001). The results are interpreted with a model in which the stable shape of an island mainly depends on its volume and composition. INTRODUCTION In a wide range of growth parameters, the epitaxial deposition of Ge on Si (001) leads to the spontaneous formation of three-dimensional islands which allow to partially relieve the strain due to the ~4.2% lattice-mismatch. By embedding these islands in a Si matrix, they can be converted into self-assembled quantum dots [1]. The knowledge of possible shape and composition changes taking place during the Si-capping is essential to produce structures with predictable properties. Several previous investigations [1-3] on Ge and Si1-xGex islands overgrown with Si pointed out that the island shape is affected during the capping process. Buried islands are generally flatter than the as-grown ones, but, particularly for dome-shaped islands, the details of the shape changes have been only recently reported [4]. We present here an STM investigation of the shape evolution of Ge/Si (001) islands during Si-capping and a comparison with that occurring during Ge and SiGe deposition on Si (001). EXPERIMENTAL DETAILS The specimens used in this study were prepared by ultra high vacuum (UHV) magnetron sputter epitaxy. p+-Si (001) substrates were outgassed in UHV and then flash-cleaned by direct current heating to remove the native oxide. A 60 nm-thick Si buffer was grown to get a reproducibly clean surface. Ge islands were obtained by depositing Ge at a substrate temperature Ts=550°C and a rate of 0.3 ML/s. Samples were cooled to 450°C before depositing the Si cap layer and eventually to room-temperature (RT) for the STM characterization. We chose the capping temperature such as to avoid appreciable island ripening and possible intermixing with the Si substrate. In fact, while samples annealed for several minutes at Ts=450°C show no significant change of island-size-distribution with respect to those cooled to RT immediately after growth, the volume of islands annealed at 550°C clearly increases. Thus, at 450°C, islands can be
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Figure 1. Shape evolution of Ge islands (obta
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