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  • 1
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    Examination of the temperature dependent electronic behavior of GeTe for switching applications (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-06-23
    Description: The DC and RF electronic behaviors of GeTe-based phase change material switches as a function of temperature, from 25 K to 375 K, have been examined. In its polycrystalline (ON) state, GeTe behaved as a degenerate p -type semiconductor, exhibiting metal-like temperature dependence in the DC regime. This was consistent with the polycrystalline (ON) state RF performance of the switch, which exhibited low resistance S -parameter characteristics. In its amorphous (OFF) state, the GeTe presented significantly greater DC resistance that varied considerably with bias and temperature. At low biases (
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 2
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    First-principles calculation of the bulk photovoltaic effect in the polar compounds LiAsS2, LiAsSe2, and NaAsSe2 (2014)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2014-11-26
    Description: We calculate the shift current response, which has been identified as the dominant mechanism for the bulk photovoltaic effect, for the polar compounds LiAsS 2 , LiAsSe 2 , and NaAsSe 2 . We find that the magnitudes of the photovoltaic responses in the visible range for these compounds exceed the maximum response obtained for BiFeO 3 by 10–20 times. We correlate the high shift current response with the existence of p states at both the valence and conduction band edges, as well as the dispersion of these bands, while also showing that high polarization is not a requirement. With low experimental band gaps of less than 2 eV and high shift current response, these materials have potential for use as bulk photovoltaics.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 3
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    Coverage-dependent structural phase transformations in the adsorption of pentacene on an aperiodically modulated Cu film (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-10-22
    Description: Surface ordering of pentacene molecules adsorbed on an aperiodic Cu surface has been studied with density functional theory (DFT) and scanning tunnelling microscopy as a function of coverage. Below 0.73 ML (5.3 × 10 13 molecules cm −2 ), the adsorbate structure is row-like with the molecular axes aligned with the rows in the Cu structure. Between this coverage and 1 ML (7.3 × 10 13 molecules cm −2 ), a structural phase with a checkerboard structure is seen. At this coverage region, the molecules are very close to each other which leads to unusual bending. At higher coverages, a further phase transition to a high-density row structure is seen for most of the film. DFT with van der Waals functionals is employed to study how the molecule-molecule and molecule-surface interactions evolve as a function of coverage.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 4
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    Low-loss latching microwave switch using thermally pulsed non-volatile chalcogenide phase change materials (2014)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2014-07-08
    Description: A high performance RF (radio-frequency) switch based on the phase change effect in germanium-telluride (GeTe) is described. Thermal pulses applied to a separate independent thin film heating element for 0.1–1.5 μ s toggles the switch in a latching fashion. Being non-volatile, no power is required to hold the switch in the on- or off-state. State-of-the-art solid-state RF switches currently in use have an on-state loss of 1 dB; here, we demonstrate an inline phase change switch with a low on-state resistance showing over a frequency range of 0-40 GHz an insertion loss of just 0.1–0.24 dB.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 5
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    On the calculation of solubilities via direct coexistence simulations: Investigation of NaCl aqueous solutions and Lennard-Jones binary mixtures (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-10-20
    Description: Direct coexistence molecular dynamics simulations of NaCl solutions and Lennard-Jones binary mixtures were performed to explore the origin of reported discrepancies between solubilities obtained by direct interfacial simulations and values obtained from the chemical potentials of the crystal and solution phases. We find that the key cause of these discrepancies is the use of crystal slabs of insufficient width to eliminate finite-size effects. We observe that for NaCl crystal slabs thicker than 4 nm (in the direction perpendicular to the interface), the same solubility values are obtained from the direct coexistence and chemical potential routes, namely, 3.7 ± 0.2 molal at T = 298.15 K and p = 1 bar for the JC-SPC/E model. Such finite-size effects are absent in the Lennard-Jones system and are likely caused by surface dipoles present in the salt crystals. We confirmed that μ s-long molecular dynamics runs are required to obtain reliable solubility values from direct coexistence calculations, provided that the initial solution conditions are near the equilibrium solubility values; even longer runs are needed for equilibration of significantly different concentrations. We do not observe any effects of the exposed crystal face on the solubility values or equilibration times. For both the NaCl and Lennard-Jones systems, the use of a spherical crystallite embedded in the solution leads to significantly higher apparent solubility values relative to the flat-interface direct coexistence calculations and the chemical potential values. Our results have broad implications for the determination of solubilities of molecular models of ionic systems.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 6
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    Morphological analysis of GeTe in inline phase change switches (2015)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2015-09-04
    Description: Crystallization and amorphization phenomena in indirectly heated phase change material-based devices were investigated. Scanning transmission electron microscopy was utilized to explore GeTe phase transition processes in the context of the unique inline phase change switch (IPCS) architecture. A monolithically integrated thin film heating element successfully converted GeTe to ON and OFF states. Device cycling prompted the formation of an active area which sustains the majority of structural changes during pulsing. A transition region on both sides of the active area consisting of polycrystalline GeTe and small nuclei (
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 7
    Electronic Resource
    Electronic Resource
    Nonlinear spectroscopy near half-gap in bulk and quantum well GaAs/AlGaAs waveguides (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 1927-1935 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We study the nonlinear properties of bulk AlGaAs and GaAs/AlGaAs multiple quantum wells (MQW) below the half-band-gap energy using subpicosecond pulses between 1.65 and 1.7 μm. In the bulk material we find a value for the nonlinear index n2 = +3.6× 10−14 cm2/W and a two-photon absorption coefficient β = 0.26 × 10−4 cm/MW. In the MQW we measure an n2 up to 2.4 times larger, and we attribute this enhancement to a stronger 1S-exciton intermediate state. The β value is up to 25 times larger in the MQW. This larger value may result from midgap states that resonantly enhance the virtual intermediate state in two-photon absorption and act as a real transition in a two-step absorption process. The resulting figure of merit (2n2/βλ) for the bulk (MQW) material is 17 (1.6), which means that these semiconductors below half band gap are appropriate for all-optical switching and quantum optics applications. We confirm that n2 is instantaneous on the 300 fs time scale of our pulses from self-phase-modulation spectra as well as time-resolved pump-probe measurements. However, we find an intriguing exchange of energy between the two orthogonal axes as evidenced by the signal along the probe axis following the negative derivative of the pump intensity. This result may be explained by self-phase modulation of the pump combined with a low-frequency Raman process that couples the modes along orthogonal axes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351182
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  • 8
    Electronic Resource
    Electronic Resource
    Electrical inhomogeneity in Ga-rich undoped GaAs crystals: Dependence on melt stoichiometry and dislocation distribution (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 4140-4148 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Studies have been made of the macroscopic and microscopic electrical inhomogeneity in undoped bulk GaAs single crystals grown from As/As+Ga melt ratios of 0.48–0.45. Microscopic inhomogeneity was characterized by contact resistance line scans and whole wafer anodization, which gives high-resolution two-dimensional images of the low-resistivity p-type regions within the wafers. The resistivity was nonuniform in wafers from 0.48 As/As+Ga crystals, having a W-shaped radial dependence with minima in the 〈110〉 directions and varying by six orders of magnitude across a wafer, whereas it was uniform in low-resistivity wafers from 0.45 As/As+Ga crystals at fractions of melt solidified, g〉0.4. Precise correlation of microscopic inhomogeneity with grown-in linear and cellular arrays of dislocations was obtained in nonuniform wafers, but no correlation with slip dislocations was observed. Anodization images show that the carrier concentration is quantitatively in agreement with a uniform acceptor background compensated by EL2 concentration fluctuations of factors of 2–3 at the dislocation networks. Inhomogeneity variation with g and As/As+Ga ratio is consistent with macroscopic and microscopic decrease in EL2 with melt stoichiometry.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.344975
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  • 9
    Electronic Resource
    Electronic Resource
    Room-temperature stability of the electrical properties of metalorganic vapor phase epitaxial Hg1−xCdxTe on GaAs (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 7173-7177 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Significant changes have been observed in the electrical properties after prolonged storage at room temperature of undoped metalorganic vapor phase epitaxial layers of Hg1−xCdxTe grown on (100) GaAs. These changes occur in uniform n-type and nonuniform n-p layer structures and are due to the out-diffusion of donors from the bulk of the layers. In situ growth of a surface passivation layer of CdTe prevents these effects and produces stable electrical properties. Uniform n-type layers grown simultaneously on (111) GaAs do not show changes in electrical properties even when not in situ passivated with CdTe. The time dependent properties do not appear to be associated with impurities and are attributed to the diffusion of a combination of native donor defects and Hg acceptor vacancies, which are estimated to have diffusion coefficients, respectively, of 7×10−14 and 2–3 ×10−16 cm2 s−1 at room temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.347609
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  • 10
    Electronic Resource
    Electronic Resource
    Vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 clusters (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5268-5277 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first excited electronic state (S1) vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 van der Waals (vdW) clusters have been studied using molecular jet and time resolved emission spectroscopic techniques. The rates of intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) as functions of vibrational energy are reported for both clusters. For vibrational energy in excess of the cluster binding energy, both clusters are observed to dissociate. The dispersed emission spectra of these clusters demonstrate that aniline(Ar)1 dissociates to all energetically accessible bare molecule states and that aniline(CH4)1 dissociates selectively to only the bare molecule vibrationless state. The emission kinetics show that in the aniline(Ar)1 case, the initially excited states have nanosecond lifetimes, and intermediate cluster states have very short lifetimes. In contrast, the initially excited aniline(CH4)1 states and other intermediate vibrationally excited cluster states are very short lived (〈100 ps), and the intermediate cluster 00 state is observed. These results can be understood semiquantitatively in terms of an overall serial IVR/VP mechanism which consists of the following: (1) the rates of chromophore to vdW mode IVR are given by Fermi's golden rule, and the density of vdW vibrational states is the most important factor in determining the relative [aniline(Ar)1 vs aniline(CH4)1] rates of IVR; (2) IVR among the vdW modes is rapid; and (3) VP rates can be calculated by a restricted vdW mode phase space Rice–Ramsberger–Kassel–Marcus theory. Since the density of vdW states is three orders of magnitude greater for aniline(CH4)1 than aniline(Ar)1 at 700 cm−1, the model predicts that IVR is slow and rate limiting in aniline(Ar)1, whereas VP is slow and rate limiting in aniline(CH4)1. The agreement of these predictions with the experimental results is very good and is discussed in detail.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457572
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