Structural properties of SiGe islands: Effect of capping
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Structural properties of SiGe islands: Effect of capping unther Bauer, Ullrich Denker2 , Oliver G. Schmidt2 , Julian Stangl, Anke Hesse, Vaclav Hol´ y1 , G¨ 3 3 utzmacher O. Kirfel , and D. Gr¨ Institut f¨ ur Halbleiterphysik, Johannes Kepler Universit¨at, A-4040 Linz, Austria 1 Department of Solid State Physics, Faculty of Science, Masaryk University, Brno, Czech Republic 2 Max-Planck Institut f¨ ur Festk¨orperforschung, Stuttgart, Germany 3 Paul-Scherrer Institut, Villingen, Switzerland ABSTRACT Self-organized nanostructures are increasingly important for novel optoelectronic devices, as high densities of quantum dots can be deposited directly during heteroepitaxial growth. Due to the elastic relaxation in 3D nanostructures, properties differing from 2D systems can be achieved, e.g., higher Ge contents in SiGe islands than in planar layers with the same total amount of Ge can be realized, enhancing the design freedom for, e.g., detectors or high frequency transistors. However, when such nanostructures are overgrown, which is inevitable for any application, they often undergo significant changes, partly eliminating their distinct advantages. We present an investigation of SiGe islands formed by deposition of pure Ge on Si (001). X-ray scattering and diffraction is employed to obtain the shape, strain and Ge distribution in uncapped as well as Si-capped islands. For the analysis, model calculations of the strain fields are required. The finite element method is used for uncapped islands, for buried nanostructures a novel analytical approach has been developed. For Tgrowth 650◦ C, optimized for island formation, the Ge content in the top part of uncapped islands reaches 100% , at an elastic relaxation of about 50% . During capping, the maximum Ge content considerably decreases to about 50%. At the same time, the islands become very flat, with a decrease in height from 13 to 6 nm accompanied by an increase in base diameter from about 100 nm to 180 nm. The elastic relaxation decreases to about 2.5% , so that the buried islands have properties not very much different from 2D layers. Lowering the growth temperature for the Si cap, this effect can be avoided, for Tgrowth as low as 460◦ C, the structure of the uncapped islands can indeed be preserved.
INTRODUCTION Semiconductor nanostructures are currently a field of intensive investigations due to their potential in optoelectronic devices. In III-V heterostructures, self-organizes islands are already being used as quantum dots: carrier confinement within the small structures result in an enhanced density of states, which is advantageous for, e.g., semiconductor laser applications. In group IV semiconductors, in particular in Si/SiGe heterostructure systems, efforts are being made in exploiting nanoscale islands in order to circumvent the indirect band gap of Si and SiGe. But also apart from the quantum confinement effects, self-organized nanostructures offer the possibility to enhance the range of growth parameters: it has been shown that, e.g., the Ge content in self-organi
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