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P.W. DEELMAN,* L.J. SCHOWALTER,* and T. THUNDAT** *Rensselaer Polytechnic Institute, Troy, NY 12180 {pdeelman, schowalt}tunix. cie.rpi .edu **Oak Ridge National Laboratory, Oak Ridge, TN 37831 ugt;Q0RNL. G0V ABSTRACT In order to understand Ge island nucleation and evolution, we have studied strain relaxation and clustering of Ge grown on Si(111) by molecular beam epitaxy (MBE) with in situ reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), and Rutherford backscattering spectrometry (RBS). Our goal is to tailor the size and density of the nanocrystals by controlling thermodynamics and kinetics. At low temperature ('-.4500C), we observe a sharp 2D-3D growth mode transition after 2.5ML ±+.1ML (we define a thickness of 1ML to be one-third the length of the body diagonal of the Ge conventional unit cell), when transmission diffraction features appear in RHEED and the surface lattice constant begins to relax. The mechanisms of island growth and strain relaxation change with growth temperature. At - 700'C, transmission diffraction spots never appear in RHEED for Ge/Si(111) and strain relaxation occurs gradually. After 37ML of growth, the apparent in-plane lattice parameter increases only 1.5% over that of the Si substrate. This behavior is explained by the different manner in which islands initially nucleate and grow in the two temperature regimes. At low temperature, small islands nucleate and grow on a relatively rough wetting layer (which itself provides preferential sites for dislocation introduction). The areal density of the small islands is relatively high. At high temperature, a small number of islands grow very large from the outset. A general model indicates how, at low temperature, the relative difficulty of overcoming the barrier to dislocation formation actually results in an apparent larger degree of strain relief than at high temperature. INTRODUCTION The growth of Ge on Si, primarily Si(100), has been widely studied over the last few years, and the growth mode is known to be Stranski-Krastanov [1, 2, 3]. The surface free energy of Ge is less than that of Si (we will consider only the (111) and (100) surfaces), so Ge wets Si provided the energy benefit of the Ge overlayer is not offset by the energy cost associated with the epilayersubstrate interface (it is not). However, due to the 4% lattice mismatch between Ge and Si, the Ge film is subjected to a bilateral, compressive stress, and the resulting strain energy grows linearly with the thickness of the Ge film. At some "critical thickness," it is energetically favorable for the film to relieve strain by roughening and/or by creating dislocations [4, 5]. Impinging adatoms attach preferentially to these strain-relieved locations, and subsequent growth will proceed with the evolution of these islands. The spontaneous formation of these islands, requiring no artificial patterning or lithography, is termed "self-assembly." Our goal is to exploit the self-assembly of Ge islands during MBE growth to form nanocrystals exhibiting th