Browsing by Author "Liu, W. K."

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    Infrared studies of hole-plasmon excitations in heavily-doped p-type MBE-grown GaAs : C
    Songprakob, W.; Zallen, Richard H.; Liu, W. K.; Bacher, K. L. (American Physical Society, 2000-08-15)
    Infrared reflectivity measurements (200-5000 cm(-1)) and transmittance measurements (500-5000 cm(-1)) have been carried out on heavily-doped GaAs:C films grown by molecular-beam epitaxy. With increasing carbon concentration, a broad reflectivity minimum develops in the 1000-3000 cm(-1) region and the one-phonon band near 270 cm(-1) rides on a progressively increasing high-reflectivity background, An effective; plasmon/one-phonon dielectric function with only two free parameters (plasma frequency omega(p) and damping constant gamma) gives a good description of the main features of both the reflectivity and transmittance spectra. The dependence of omega(p)(2) on hole concentration p is linear; at p = 1.4 x 10(20) cm(-3), omega(p) is 2150 cm(-1). At each doping, the damping constant gamma is large and corresponds to an infrared hole mobility that is about half the Hall mobility. Secondary-ion mass spectroscopy and localized-vibrational-mode measurements indicate that the Hall-derived p is close to the carbon concentration and that the Hall factor is dose to unity, so that the Hall mobility provides a good estimate of actual de mobility. The observed dichotomy between the de and infrared mobilities is real, not a statistical-averaging artifact. The explanation of the small infrared mobility resides in the influence of intervalence-band absorption on the effective-plasmon damping, which operationally determines that mobility. This is revealed by a comparison of the infrared absorption results to Braunstein's low-p p-GaAs spectra and to a k.p calculation extending Kane's theory to our high dopings. For n-GaAs, which lacks infrared interband absorption, the de and infrared mobilities do not differ.
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    Intervalenceband and plasmon optical absorption in heavily doped GaAs:C
    Songprakob, Wantana; Zallen, Richard H.; Tsu, D. V.; Liu, W. K. (American Institute of Physics, 2002-01-01)
    By using direct numerical-solution techniques for the reflectance (R) and transmittance (T) equations of a multilayer structure, we have analyzed infrared R and T measurements on heavily doped p-type GaAs:C films grown by molecular beam epitaxy. The optical properties, for films with hole concentrations up to 1.4x10(20) cm(-3), were determined for photon energies from 0.07 to 0.6 eV, in which region plasmon (intraband) and intervalenceband contributions are in competition. Our results for the optical absorption coefficient resolve two separate peaks located (at high doping) at about 0.1 and 0.2 eV. By carrying out calculations of the intervalenceband (IVB) absorption processes for our dopings, we identify the peak near 0.2 eV with light-hole to heavy-hole IVB transitions, and we attribute the lower-energy peak to hole-plasmon excitations. Our experimental absorption spectra are very well described by a model combining the IVB contribution to the dielectric function with a plasmon contribution. The hole-plasmon parameters we obtain for highly doped p-GaAs yield an infrared mobility which (unlike the too-small IVB-entangled infrared mobility implied by the use of the usual effective-plasmon model) is in substantial agreement with the dc mobility. (C) 2002 American Institute of Physics.
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    Role of InAs and GaAs terminated heterointerfaces at source/channel on the mixed As-Sb staggered gap tunnel field effect transistor structures grown by molecular beam epitaxy
    Zhu, Yizheng; Jain, Nikhil; Vijayaraghavan, S.; Mohata, Dheeraj K.; Datta, Suman; Lubyshev, Dmitri; Fastenau, Joel M.; Liu, W. K.; Monsegue, Niven; Hudait, Mantu K. (American Institute of Physics, 2012-07-15)
    The structural, morphological, defect properties, and OFF state leakage current mechanism of mixed As-Sb type-II staggered gap GaAs-like and InAs-like interface heterostructure tunnel field effect transistors (TFETs) grown on InP substrates using linearly graded InxAl1-xAs buffer by molecular beam epitaxy are investigated and compared. Symmetric relaxation of >90% and >75% in the two orthogonal < 110 > directions with minimal lattice tilt was observed for the terminal GaAs0.35Sb0.65 and In0.7Ga0.3As active layers of GaAs-like and InAs-like interface TFET structures, respectively, indicating that nearly equal numbers of alpha and beta dislocations were formed during the relaxation process. Atomic force microscopy reveals extremely ordered crosshatch morphology and low root mean square roughness of similar to 3.17 nm for the InAs-like interface TFET structure compared to the GaAs-like interface TFET structure of similar to 4.46 nm at the same degree of lattice mismatch with respect to the InP substrates. The GaAs-like interface exhibited higher dislocation density, as observed by cross-sectional transmission electron microscopy, resulting in the elongation of reciprocal lattice point of In0.7Ga0.3As channel and drain layers in the reciprocal space maps, while the InAs-like interface creates a defect-free interface for the pseudomorphic growth of the In0.7Ga0.3As channel and drain layers with minimal elongation along the Delta omega direction. The impact of the structural differences between the two interface types on metamorphic TFET devices was demonstrated by comparing p(+)-i-n(+) leakage current of identical TFET devices that were fabricated using GaAs-like and InAs-like interface TFET structures. Higher OFF state leakage current dominated by band-to-band tunneling process due to higher degree of defects and dislocations was observed in GaAs-like interface compared to InAs-like interface where type-II staggered band alignment was well maintained. Significantly lower OFF state leakage current dominated by the field enhanced Shockley-Read-Hall generation-recombination process at different temperatures was observed in InAs-like TFET structure. The fixed positive charge at the source/channel heterointerface influences the band lineup substantially with charge density greater than 1 x 10(12)/cm(2) and the band alignment is converted from staggered gap to broken gap at similar to 6 x 10(12)/cm(2). Clearly, InAs-like interface TFET structure exhibited 4x lower OFF state leakage current, which is attributed primarily to the impact of the layer roughness, defect properties on the carrier recombination rate, suggesting great promise for metamorphic TFET devices for high-performance, and ultra-low power applications. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737462]