Analysis of the Three-Dimensional Nanoscale Relationship of Ge Quantum Dots in a Si Matrix Using Focused Ion Beam Tomogr
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Analysis of the Three-Dimensional Nanoscale Relationship of Ge Quantum Dots in a Si Matrix Using Focused Ion Beam Tomography. Alan J. Kubis1, Thomas E. Vandervelde2, John C. Bean3, Derren N. Dunn4, Robert Hull1 1 Univ. of Virginia, Dept of Materials Science and Engineering, Charlottesville, VA 22904, U.S.A; 2Univ. of Virginia, Dept of Physics, Charlottesville, VA 22904, U.S.A; 3Univ. of Virginia, Dept of Electrical and Computer Engineering, Charlottesville, VA 22904, U.S.A; 4Now at IBM Microelectronics, Hopewell Junction, NY 12533, U.S.A. ABSTRACT It is well documented that buried layers in quantum dot (QD) superlattices influence the position of quantum dots in the subsequently grown layers through strain field interactions (e.g.1,2, 3,4). Using the Focused Ion Beam (FIB) tomographic technique we have reconstructed the 3D relationship of successive layers of coherent Ge QDs separated by epitaxial Si capping layers – a “QD superlattice”. Techniques such as Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) can only look at a single surface layer of QDs or, in the case of Transmission Electron Microscopy (TEM), look at a two-dimensional projection of a three-dimensional volume so that 3D relationships need to be inferred. Since the strain interactions are complex, an enhanced fundamental understanding of these self-organization mechanisms can more directly be obtained from full 3D reconstructions of these structures. By capping with Si at 300oC we were able to grow QD superlattices with QDs tens of nanometers in height. This places them within the resolution of the FIB tomographic technique to reconstruct. Using the FIB we performed in-situ serial sectioning of the QD superlattice and then reconstructed the QD superlattice. The reconstruction was then analyzed to investigate the ordering of the QDs. Results from a reconstruction of a superlattice matrix will be presented with analysis of the self-ordering of the QDs. Observations of a novel self-limiting (in height) morphology, the quantum mesa, associated with the capping technique used will also be discussed. INTRODUCTION Much work has been done to study the phenomenon of self-assembly in quantum dot systems (e.g.1-8). Multi-layered structures, known as superlattices, are seen to organize so that the Quantum Dots (QDs) on successive layers nucleate and grow above the buried QDs5. The selfassembly occurs such that the QDs in the later layers deviate from being centered on the lower QDs to positions that more evenly space the quantum dots. The driving force for this is believed to be due to minimization of the system strain energy driven by QD-QD interaction energies in lattice mismatched systems6,7,8. In the past, techniques such as Transmission Electron Microscopy (TEM), Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) have been used to analyze these structures. In the case of STM and AFM only the top most layer can be observed while TEM gives a two-dimensional projection of a threedimensional volume. In several pape
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