Browsing by Author "Sands, Timothy D."

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    2017 State of the University (transcript)
    Sands, Timothy D. (Virginia Tech, 2017-09)
    ...This afternoon I would like to take a few minutes to celebrate our successes over the past year and to share my perspectives on the opportunities and challenges ahead as we strive to become a leading Global University in the spirit of the engaged land-grant institution that we are today...
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    Dislocation-pipe diffusion in nitride superlattices observed in direct atomic resolution
    Garbrecht, Magnus; Saha, Bivas; Schroeder, Jeremy L.; Hultman, Lars; Sands, Timothy D. (Springer Nature, 2017-04-06)
    Device failure from diffusion short circuits in microelectronic components occurs via thermally induced migration of atoms along high-diffusivity paths: dislocations, grain boundaries, and free surfaces. Even well-annealed single-grain metallic films contain dislocation densities of about 1014 m-2; hence dislocation-pipe diffusion (DPD) becomes a major contribution at working temperatures. While its theoretical concept was established already in the 1950s and its contribution is commonly measured using indirect tracer, spectroscopy, or electrical methods, no direct observation of DPD at the atomic level has been reported. We present atomically-resolved electron microscopy images of the onset and progression of diffusion along threading dislocations in sequentially annealed nitride metal/semiconductor superlattices, and show that this type of diffusion can be independent of concentration gradients in the system but governed by the reduction of strain fields in the lattice.
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    Open letter to Ms. Malia Obama
    Sands, Timothy D. (Virginia Tech, 2015-04)
    An open letter from President Tim Sands to Malia Obama and all high school juniors and seniors engaged in the college search.
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    Open Letter to the Virginia Tech Community, Jan. 29, 2017
    Sands, Timothy D. (Virginia Tech, 2017-01-29)
    Friday’s U.S. Presidential Executive Order on interior immigration enforcement will have an impact on members of the Virginia Tech community...
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    President's Letter to the Virginia Tech Class of 2019
    Sands, Timothy D. (Virgini aTech, 2015-07)
    An open letter from President Tim Sands welcoming the incoming class of 2019 to the Virginia Tech community.
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    Tailoring of surface plasmon resonances in TiN/(Al0.72Sc0.28)N multilayers by dielectric layer thickness variation
    Garbrecht, Magnus; Hultman, Lars; Fawey, Mohammed H.; Sands, Timothy D.; Saha, Bivas (Springer, 2017-11-28)
    Alternative designs of plasmonic metamaterials for applications in solar energy-harvesting devices are necessary due to pure noble metal-based nanostructures’ incompatibility with CMOS technology, limited thermal and chemical stability, and high losses in the visible spectrum. In the present study, we demonstrate the design of a material based on a multilayer architecture with systematically varying dielectric interlayer thicknesses that result in a continuous shift of surface plasmon energy. Plasmon resonance characteristics of metal/semiconductor TiN/(Al,Sc)N multilayer thin films with constant TiN and increasing (Al,Sc)N interlayer thicknesses were analyzed using aberration-corrected and monochromated scanning transmission electron microscopy-based electron energy loss spectroscopy (EELS). EEL spectrum images and line scans were systematically taken across layer interfaces and compared to spectra from the centers of the respective adjacent TiN layer. While a constant value for the TiN bulk plasmon resonance of about 2.50 eV was found, the surface plasmon resonance energy was detected to continuously decrease with increasing (Al,Sc)N interlayer thickness until 2.16 eV is reached. This effect can be understood to be the result of resonant coupling between the TiN bulk and surface plasmons across the dielectric interlayers at very low (Al,Sc)N thicknesses. That energy interval between bulk and decreasing surface plasmon resonances corresponds to wavelengths in the visible spectrum. This shows the potential of tailoring the material’s plasmonic response by controlling the (Al,Sc)N interlayer thickness, making TiN-based multilayers good prospects for plasmonic metamaterials in energy devices.
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    Void-mediated coherency-strain relaxation and impediment of cubic-to-hexagonal transformation in epitaxial metastable metal/semiconductor TiN/Al0.72Sc0.28N multilayers
    Garbrecht, Magnus; Hultman, Lars; Fawey, Mohammed H.; Sands, Timothy D.; Saha, Bivas (2017-08-17)
    Bulk metastable phases can be stabilized during thin-film growth by employing substrates with similar crystal structure and lattice parameter, albeit over a thickness range limited by coherency-strain relaxation. Expanding that strategy, growth of superlattices comprising one stable and another metastable compound with similar crystal structure and lattice parameters are known to yield epitaxial stabilization over a few nanometers of thickness. In this work, the high-pressure rocksalt (B1) phase of Al0.72Sc0.28N was stabilized epitaxially in a multilayer with TiN with thicknesses of up to 26 nm. In order to investigate the microstructural changes leading to the phase transformation of the metastable B1 phase to its wurtzite allomorph, we demonstrate a design based on a multilayer architecture with systematically varying thicknesses of the metastable compound within a constant-thickness lattice of stable metallic TiN with the cubic rocksalt structure. The multilayer films show an increasing hardness and elastic modulus for decreasing period thickness, in correspondence with both coherency-strain and Koehler hardening. The phase transition is accompanied by an increase of lattice strain with increasing multilayer periods, and resulting ultimately in coherency-strain relaxation upon phase transformation. Further, we show that the phase transformation is mediated by voids decorating the {130} planes that separate regions of different growth rates and act as additional growth fronts for wurtzite growth during the phase transformation. The TiN/(Al, Sc) N interfaces themselves remain atomically sharp and smooth until the interface structure roughens along with the epitaxial rocksalt to wurtzite transition of (Al, Sc) N. These results show the strong influence of the voids on controlling the target thickness of epitaxially stabilized thin-film growth to the range relevant for applications, such as coatings, plasmonic materials, and electronic device technology, where the mechanical integrity of the material is critical.