Heteroepitaxial Self-Assembly of Higher-Complexity Structures by Combining Growth Control with Nanopatterning
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Heteroepitaxial Self-Assembly of Higher-Complexity Structures By Combining Growth Control with Nanopatterning Jerrold A. Floro1, Jennifer L. Gray2, Surajit Atha2, Nitin Singh2, Dana Elzey2 and Robert Hull2 1 Sandia National Laboratories, Albuquerque, NM 87185-1415 2 University of Virginia, Department of Materials Science and Engineering, Charlottesville, VA 22904
ABSTRACT We provide an overview of a novel self-assembly process that occurrs during GeSi/Si(001) strain-layer heteroepitaxy under conditions of limited adatom mobility. Suppression of copious surface diffusion leads to limited three-dimensional roughening in the form of pits that partially consume a thick, metastable wetting layer. The material ejected from the pits accumulates alongside, eventually forming a symmetric quantum dot molecule consisting of four islands bound to a {105}-faceted pit. These structures, which are of interest in nanologic applications, appear to arise from an intrinsic strainrelief mechanism in a relatively narrow regime of deposition conditions. An additional degree of morphological control is obtained by annealing films containing pits, before they evolve to full quantum dot molecules. Annealing promotes a one-dimensional growth instability leading to the formation of highly anisotropic grooves, bounded by long, wire-like islands. Finally, we show that patterns created in the Si substrate using a focused ion beam can control the location of quantum dot molecules, which is an additional critical step towards being able to use these structures for computing. INTRODUCTION In a coherently strained, heteroepitaxial thin film, the stored elastic energy can do work upon the film, either in the form of shear, mediated by the passage of misfit dislocation half-loops, or in the form of three dimensional roughening, mediated by the surface diffusion of adatoms. The latter mechanism can be exploited to self-assemble quantum nanostructures, e.g., dots and wires that might be useful in high-performance optoelectronics, nanologic, or low-dimensional physics studies. Strain-induced quantum dot formation is considered to be a limited form of selfassembly – dots form “on their own”, but they typically lack many requisite characteristics to be fully useful. Ideally, complete self-assembly would result in dots with the desired size, shape, composition (or strain), isolation, and placement in order to form a functional array for a desired application. We are still far from achieving this, but steady progress is being made in controlling and understanding many important aspects of strain-induced heteroepitaxial self-assembly. In this paper, we examine how deposition parameters can be manipulated to form bound assemblages of quantum dots, i.e., quantum dot molecules (QDMs), during
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molecular beam epitaxial growth of Ge0.3Si0.7 films on Si (001) substrates. QDMs form in a growth regime where pit formation, rather than islanding, occurs as the initial means of strain relief during heteroepitaxy. While QDMs are compact, symmetric stru
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