Browsing by Author "Kim, TH"

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    Adsorption and desorption kinetics of Ga on GaN(0001): Application of Wolkenstein theory
    (Physical Review B - Condensed Matter and Materials Physics, 2010-08-26) Bruno, G; Losurdo, M; Kim, TH; Brown, A
    The kinetics of Ga adsorption/desorption on GaN(0001) surfaces is investigated over the temperature range of 680-750°C using real-time spectroscopic ellipsometry. The adsorption and desorption kinetics are described in the framework of the Wolkenstein theory, which considers not only the equilibrium between Ga adsorbed on the surface and Ga in the gas phase but also the electronic equilibrium at the surface. It is shown that, because of the fixed polarization charge existing at the GaN(0001) surface, Ga adsorption and desorption processes involve neutral and charged Ga states. By considering the GaN surface charge involved in the surface processes, we demonstrate that a second-order kinetics more accurately describes Ga desorption, in comparison with conventional models, and yields an apparent activation energy of 2.85±0.02eV for Ga desorption consistent with experiments. © 2010 The American Physical Society.
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    Room temperature photoluminescence from InxAl(1-x)N films deposited by plasma-assisted molecular beam epitaxy
    (Applied Physics Letters, 2014-09-29) Kong, W; Mohanta, A; Roberts, AT; Jiao, WY; Fournelle, J; Kim, TH; Losurdo, M; Everitt, HO; Brown, AS
    © 2014 AIP Publishing LLC.InAlN films deposited by plasma-assisted molecular beam epitaxy exhibited a lateral composition modulation characterized by 10-12 nm diameter, honeycomb-shaped, columnar domains with Al-rich cores and In-rich boundaries. To ascertain the effect of this microstructure on its optical properties, room temperature absorption and photoluminescence characteristics of InxAl(1-x)N were comparatively investigated for indium compositions ranging from x = 0.092 to 0.235, including x = 0.166 lattice matched to GaN. The Stokes shift of the emission was significantly greater than reported for films grown by metalorganic chemical vapor deposition, possibly due to the phase separation in these nanocolumnar domains. The room temperature photoluminescence also provided evidence of carrier transfer from the InAlN film to the GaN template.
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    Surface-electrode point Paul trap
    (Physical Review A - Atomic, Molecular, and Optical Physics, 2010-10-12) Kim, TH; Herskind, PF; Kim, T; Kim, J; Chuang, IL
    We present a model as well as experimental results for a surface electrode radiofrequency Paul trap that has a circular electrode geometry well suited for trapping single ions and two-dimensional planar ion crystals. The trap design is compatible with microfabrication and offers a simple method by which the height of the trapped ions above the surface may be changed in situ. We demonstrate trapping of single Sr88+ ions over an ion height range of 200-1000 μm for several hours under Doppler laser cooling and use these to characterize the trap, finding good agreement with our model. © 2010 The American Physical Society.
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    Ultraviolet-Visible Plasmonic Properties of Gallium Nanoparticles Investigated by Variable-Angle Spectroscopic and Mueller Matrix Ellipsometry
    (ACS Photonics, 2014-07-16) Yang, Y; Akozbek, N; Kim, TH; Sanz, JM; Moreno, F; Losurdo, M; Brown, AS; Everitt, HO
    © 2014 American Chemical Society.Self-assembled, irregular ensembles of hemispherical Ga nanoparticles (NPs) were deposited on sapphire by molecular beam epitaxy. These samples, whose constituent unimodal or bimodal distribution of NP sizes was controlled by deposition time, exhibited localized surface plasmon resonances tunable from the ultraviolet to the visible (UV/vis) spectral range. The optical response of each sample was characterized using a variable-angle spectroscopic ellipsometer, and the dielectric response of the ensemble of NPs on each sample was parametrized using Lorentz oscillators. From this, a relationship was found between NP size and the deduced Lorentzian parameters (resonant frequency, damping, oscillator strength) for most unimodal and bimodal samples at most frequencies and angles of incidence. However, for samples with a bimodal size distribution, Mueller matrix ellipsometry revealed nonspecular scattering at particular frequencies and angles, suggesting a resonant interparticle coupling effect consistent with recently observed strong local field enhancements in the ultraviolet. (Graph presented).