Nano-scale Chemistry of Complex Self-Assembled Nanostructures in Epitaxial SiGe Films
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Nano-scale Chemistry of Complex Self-Assembled Nanostructures in Epitaxial SiGe Films Prabhu Balasubramanian1, a), Jerrold A. Floro2, Jennifer L. Gray3 and Robert Hull1 1 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA. Electronic Mail a): [email protected] 2 Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904-4745, USA 3 Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA
ABSTRACT Heteroepitaxy of SiGe alloys on Si (001) under certain growth conditions has previously been shown to cause self-assembly of nanostructures called Quantum Dot Molecules, QDMs, where pyramidal pits and 3D islands cooperatively form. QDMs have potential applications to nanologic device architectures such as Quantum Cellular Automata that relies on localization of charges inside islands to create bi-stable logic states. In order to determine the applicability of QDMs to such structures it is necessary to understand the nano-scale chemistry of QDMs because the chemistry affects local bandgap which in turn affects a QDM’s charge confinement property. We investigate the nanoscale chemistry of QDMs in the Si0.7Ge0.3/Si (100) system using Auger Electron Spectroscopy (AES). Our AES analysis indicates that compressively strained QDM pit bases are the most Ge rich regions in a QDM. The segregation of Ge to these locations cannot be explained by strain energy minimization. INTRODUCTION Heteroepitaxy of SixGe1-x alloy films on a silicon substrate produces strain in the system because of the lattice mismatch between Si and Ge. It is well documented that the epitaxial film growth under appropriate growth conditions leads to self-assembly of Quantum Dots (QDs) to minimize strain energy1-3. Under growth conditions where adatom mobility is limited, a complex structure consisting of four QDs bound to a central pyramidal pit, known as a Quantum Dot Molecule, QDM has been observed4, 5. In QDMs the presence of the adjoining self-assembled pit can affect the composition of bound islands. Also, QDMs, based on their dimensions and geometry, have potential nanologic applications for Quantum Cellular Automata architectures6, 7. Therefore investigation of QDM nanoscale chemistry is of significant interest. In this work we present our Auger Electron Spectroscopic measurements of composition distribution across QDMs. With a spatial resolution of better than 10 nm under optimum conditions and a compositional sensitivity of ~2% (in this AES work) it is possible to determine local chemistry within the nanostructure. EXPERIMENTAL MEASUREMENTS The morphology of QDMs was obtained through ex-situ Atomic Force Microscopy using Digital Instruments-Veeco Metrology Group Dimension 3100 (tapping mode).
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The Si0.7Ge0.3/Si(100) QDMs are fabricated by co-deposition of Si and Ge on a Shirakicleaned Si(100) wafer at a growth temperature of 550 °C and a growth rate of ~0.09 nm/second in a MBE growth chamber. The detailed growth procedure
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