The crystallographic phase and morphology of many materials change with the crystal size so that new needs arise to determine the crystallography of nanocrystals. Direct space high-resolution phase-contrast transmission electron microscopy (HRTEM) and atomic resolution scanning TEM (STEM) when combined with tools for image-based nanocrystallography in two (2D) and three (3D) dimensions possess the capacity to meet these needs. After a concise discussion of lattice-fringe visibility spheres and maps, this paper discusses lattice-fringe fingerprinting in 2D and tilt protocol applications. On-line database developments at Portland State University (PSU) that support image-based nanocrystallography are also mentioned.
This updated review consists of three parts. The first part gives an introduction to epitaxially and endotaxially self-assembled semiconductor quantum dots. The second part of this review deals with both epitaxially grown (In,Ga)Sb compound semiconductor quantum dots in GaSb matrix and epitaxially grown In(As,Sb) compound semiconductor quantum dots in InAs matrix. These quantum dots are grown in the Stranski-Krastanow growth mode, are compressively strained to several percent and initially possess the sphalerite structure with the mixed cations and anions more or less randomly distributed over their respective sublattices. Experimental evidence for the existence of long-range atomic order within such III-V compound semiconductors quantum dots is reviewed. Employing the thermodynamics of small misfitting precipitates, a simple calculation for a model III-V compound semiconductor quantum dot system is given. This calculation demonstrates the possibility of structural transitions from ordinarily strained random semiconductor alloy quantum dots (with the sphalerite structure) to long-range atomically ordered quantum dots (i.e. crystallographic superlattices) that are negligibly strained because they possess lattice mismatch strain minimizing orientation relationships with the surrounding matrix. The third part of this review deals with endotaxially grown α-Sn (grey tin) quantum dots in Si matrix. Both the phase separation formation mechanism and the void-mediated formation mechanism of these entities are briefly discussed. The thermodynamics of small misfitting precipitates provide reasonable explanations for structural transitions and morphological transformations of such quantum dots. Morphological transformations within the diamond structure with the precipitate size are explained by an increasing contribution of the elastic mismatch strain energy to the Gibbs free energy.
New needs to determine the crystallography of nanocrystals arise with the advent of science and engineering on the nanometer scale. Direct space high-resolution phase-contrast transmission electron microscopy (HRTEM) and atomic resolution Z-contrast scanning TEM (Z-STEM), when combined with tools for image-based nanocrystallography possess the capacity to meet these needs. This paper introduces such a tool, i.e. fringe fingerprinting in two dimensions (2D), for the identification of unknown nanocrystal phases and compares this method briefly to qualitative standard powder X-ray diffractometry (i.e. spatial frequency fingerprinting). Free-access crystallographic databases are also discussed because the whole fingerprinting concept is only viable if there are comprehensive databases to support the identification of an unknown nanocrystal phase. This discussion provides the rationale for our ongoing development of a dedicated free-access Nano-Crystallography Database (NCD) that contains comprehensive information on both nanocrystal structures and morphologies. The current status of the NCD project and plans for its future developments are briefly outlined. Although feasible in contemporary HRTEMs and Z-STEMs, fringe fingerprinting in 2D (and image-based nanocrystallography in general) will become much more viable with the increased availability of aberration-corrected transmission electron microscopes. When the image acquisition and interpretation are, in addition, automated in such microscopes, fringe fingerprinting in 2D will be able to compete with powder X-ray diffraction for the identification of unknown nanocrystal phases on a routine basis. Since it possesses a range of advantages over powder X-ray diffractometry, e.g., fringe fingerprint plots contain much more information for the identification of an unknown crystal phase, fringe fingerprinting in 2D may then capture a significant part of the nanocrystal metrology market.
Typical nanometrology device standards for scanning probe microscopes that are produced by the top-down approach are described first. This is followed by a discussion of some of the atomic precision standards that nature provides. Since there is an order of magnitude feature size gap between these two classes of nanometrology standards, a novel class of nanometrology device standards that fills this gap is proposed together with its intrinsically inexpensive "bottom-up" fabrication process. This nanofabrication process is heteroepitaxy of semiconductors or ceramics that are morphologically, structurally, and chemically stable in typical laboratory environments. For special crystallographic orientations of the substrates and special material combinations, the heteroepitaxy process leads to self-assembled arrays of nano-islands with a known morphology, dimensions in the range of a few nanometers to a few tens of nanometers, and large height-to-width aspect ratios. Suitable crystallographic directions are then marked macroscopically on the nanometrology device standards in order to be able to direct the scanning probe scans in certain directions for precise calibration procedures.
Transmission electron microscopy studies in both the scanning and parallel illumination mode on samples of two generic types of self-assembled semiconductor quantum dots are reported. III-V and II-VI quantum dots as grown in the Stranski-Krastanow mode are typically alloyed and compressively strained to a few %, possess a more or less random distribution of the cations and/or anions over their respective sublattices, and have a spatially non-uniform chemical composition distribution. Sn quantum dots in Si as grown by temperature and growth rate modulated molecular beam epitaxy by means of two mechanisms possess the diamond structure and are compressively strained to the order of magnitude 10 %. These lattice mismatch strains are believed to trigger atomic rearrangements inside quantum dots of both generic types when they are stored at room temperature over time periods of a few years. The atomic rearrangements seem to result in long-range atomic order, phase separation, or phase transformations. While the results suggest that some semiconductor quantum dots may be structurally unstable and that devices based on them may fail over time, triggering and controlling structural transformations in self-assembled semiconductor quantum dots may also offer an opportunity of creating atomic arrangements that nature does not otherwise provide.
We report on electroluminescence and photoluminescence studies of arsenic rich InAs1-xSbx heterostructure LED's for the MIR region. Single-quantum- well LED's have demonstrated 300 K of approximately 24 (mu) W and approximately 50 (mu) W and approximately 8 micrometers , respectively, with corresponding internal quantum efficiencies of 0.8% and 1.6%. We also demonstrate 4.2 micrometers , 300 K emission from strained-layer superlattice (SLS) LED's with AlSb electron confining barriers with output powers > 0.1 mW. In reverse bias, these SLS devices exhibit negative luminescence efficiencies of approximately 14% at 310 K.
High resolution X-ray topography and diffraction measurements have been made at the Daresbury Synchrotron Radiation Source during in-situ molecular beam epitaxial growth of InGaAs on GaAs. Critical analysis of dislocation contrast, intensity and geometrical distortion reveals that use of the 224 reflection in the Bragg geometry at 1.48 Angstrom wavelength is optimal for such double crystal topography experiments. Examination of the beam-conditioner characteristics and performance had resulted in use of the 333 reflection from a channel-cut silicon crystal as a monochromator. We deduce and show experimentally that a 004 channel-cut monochromator is optimal, resulting in significant reduction in exposure times. We present images of misfit dislocations showing evidence for the first time of phase contrast from lattice distortions.
InAs/In(As,Sb) heterostructure LEDs are studied in forward (FB) and reverse (RB) bias where the phenomenon of 'negative luminescence' is seen for the first time in this materials system. Pseudomorphic 300K SQW LEDs, lattice matched to InAs and emitting at λ-5 micrometers and λ-8 micrometers , have internal conversion efficiencies of > 1.3 percent and > 0.83 percent respectively and maximum outputs in excess of 50 μW, in spite of an extremely low overall epilayer Sb content. Strain-relaxed InAs/In(As,Sb) SLS LEDs with AlSb barriers for electron confinement give 300K outputs in excess of 0.1mW at λ-4.2μm, approximately 3.5 times greater than control devices without the AlSb barrier. In RB the same SLS diodes exhibited efficient negative luminescence with output powers which increase with increasing device temperature to within 0.8 of the FB figures at 320 K.
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