Endotaxial Growth Mechanisms of Sn Quantum Dots in Si Matrix
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Endotaxial growth mechanisms of Sn Quantum Dots in Si matrix P. Möck1, Y. Lei2, T. Topuria2, N.D. Browning3,2, R. Ragan4*, K.S. Min4**, and H.A. Atwater4 1
Department of Physics, Portland State University, P.O. Box 751, Portland, OR 97207-0751, [email protected] Department of Physics, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, IL 60607-7059 3 Department of Chemical Engineering and Materials Science, University of California at Davis, One Shields Avenue, Davis, CA 95616; and National Center for Electron Microscopy, MS 72-150, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 4 Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, MS 128-95, Pasadena, CA 91125 * now at: Hewlett-Packard Laboratories M/S 1123, 1501 Page Mill Rd., Palo Alto, CA 94304, ** now at: Intel Corporation, California Technology and Manufacturing, MS RNB-2-35, 2200 Mission College Blvd., Santa Clara, CA 95052-8119 2
ABSTRACT Two distinct mechanisms for the endotaxial growth of quantum dots in the Sn/Si system were observed by means of analytical transmission electron microcopy. Both mechanisms operate simultaneously during temperature and growth rate modulated molecular beam epitaxy combined with ex situ thermal treatments. One of the mechanisms involves the creation of voids in Si, which are subsequently filled by Sn, resulting in quantum dots that consist of pure α-Sn. The other mechanism involves phase separation and leads to substitutional solid solution quantum dots with a higher Sn content than the predecessor quantum well structures possess. In both cases, the resultant quantum dots possess the diamond structure and the shape of a tetrakaidecahedron. (Sn,Si) precipitates that are several times larger than the typical (Sn,Si) quantum dot possess an essentially octahedral shape.
INTRODUCTION Self-assembled semiconductor quantum dots (QDs) are expected to lead to “paradigm changes in semiconductor physics” [1]. For semiconductor opto-electronic devices, the QD size must be of the order of magnitude of the exciton Bohr radius. The energy band gap of the QDs must be smaller than that of the surrounding semiconductor matrix. No structural defects such as dislocations, which lead to non-radiative recombination, are allowed to exist in the QDs [2]. As α-Sn is a direct, 0.08 eV, band gap semiconductor and substitutional SnxSi1-x solution are predicted to possess direct band gaps for 0.9 < x < 1 [3], QDs in a Si matrix consisting of pure α-Sn or substitutional (Sn,Si) solutions with a sufficiently high Sn content have potential applications as direct band-gap material for cheap and effective optoelectronic and thermo-photovoltaic devices. There are, however, at room temperature a 41.8 % bulk unit cell volume mismatch between α-Sn and Si and an equilibrium solid solubility of Sn in Si of only 0.12 %. This restricts the growth of pseudomorph SnxSi1-x layers on Si by molecular beam epitaxy (MBE) [4-7] to a Sn content of about 10 % and a thickness of the order of magnitude 10 nm. At gro
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