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The movement of heat through amorphous solids on an atomic level remains an outstanding question. Recent studies suggest that the primary thermal carrier in amorphous materials, propagons, essentially behaves like phonons. In this work, we provide experimental evidence that shows the interaction between propagons and phonons by utilizing the phase change chalcogenide germanium telluride. A series of ultra-long time-delay time-domain thermoreflectance measurements are used to analyze the scattering of vibrational thermal carriers at the boundaries of amorphous GeTe thin films relative to scattering across a crystalline-amorphous bilayer. We find that amorphous long wavelength propagons that would otherwise scatter can instead be hosted by a crystalline underlayer and its phonon population. This experimental evidence directly demonstrates propagon–phonon interactions in a clear experimental manner. [ABSTRACT FROM AUTHOR]

Germanium telluride is a well-known phase change material (PCM) used in non-volatile memory cells and radio frequency switches. Controlling the properties of GeTe for improved PCM device performance has sometimes been achieved by doping and/or alloying with metals, often at concentrations greater than 10 at. % and using non-equilibrium methods. Since switching PCMs between the low-resistance crystalline and high-resistance amorphous states requires a heating cycle, the stability of metal-incorporated GeTe ( Ge 0.5 − x M x Te 0.5 ) films is also critical to practical implementation of these materials in electronic and optoelectronic devices. In this work, we use both density-functional theory and experimental characterization methods to probe the solubility and critical properties of Ge 0.5 − x M x Te 0.5 films. Using first-principles calculations, we determine the enthalpy of formation for GeTe with 2.08, 4.17, and 6.25 at. % of Cu, Fe, Mn, Mo, and Ti and show trends between the stability of the Ge 0.5 − x M x Te 0.5 systems and the atomic position, composition, and distribution of the metal atoms in the GeTe matrix. Out of all the studied systems, Mo was the only metal to cluster within GeTe. Analysis of the Ge–Te bond lengths and volumes of the Ge 0.5 − x M x Te 0.5 supercells shows that increasing the atomic concentration (2.08, 4.17, 6.25 at. %) of the different metals causes varied distortions of the crystal structure of GeTe that are accompanied by significant changes in the projected density of states. Computational predictions concerning metal solubility and the effect of metal incorporation on critical properties of GeTe are compared to experimental results in the literature (Cu, Mn, Mo, and Ti) and to transmission electron microscopy and transport data from newly characterized co-sputtered Ge 0.5 − x Fe x Te 0.5 films. The computational predictions of decreasing solubility (Mn > --> Cu, Fe > --> Ti, Mo) shows good agreement with experimental observations (Mn, Cu > --> Fe > --> Ti, Mo), and Ge 0.5 − x Fe x Te 0.5 films exhibited increased crystallization temperatures from pure GeTe. [ABSTRACT FROM AUTHOR]

Inelastic scattering and transmission of externally injected hot carriers across graphene layers are considered as a function of graphene carrier density, temperature, and surrounding dielectric media. A finite temperature dynamic dielectric function for graphene for an arbitrary momentum q and frequency x is found under the random phase approximation and a generalized scattering lifetime formalism is used to calculate the scattering and transmission rates. Unusual trends in scattering are found, including declining rates as graphene carrier density increases and interband transition excitations, which highlights the difference with out-of-plane as compared to in-plane transport. The results also show strong temperature dependence with a drastic increase in scattering at room temperature. The calculated scattering rate at T = 300K shows a wide variation from 0.2 to 10 fs-1 depending on graphene carrier density, incident carrier momentum, and surrounding dielectrics. The analysis suggests that a transmission rate greater than 0.9 for a carrier with kinetic energy over 1 eV is achievable by carefully controlling the graphene carrier density in conjunction with the use of high-j dielectric materials. Potential applications to electronic and electro-optical devices are also discussed. [ABSTRACT FROM AUTHOR]

The DC and RF electronic behaviors of GeTe-based phase change material switches as a function of temperature, from 25K 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 (<1V) and temperatures (<200 K), the amorphous GeTe low-field resistance dramatically increased, resulting in exceptionally high amorphous-polycrystalline (OFF-ON) resistance ratios, exceeding 109 at cryogenic temperatures. At higher biases and temperatures, the amorphous GeTe exhibited nonlinear current-voltage characteristics that were best fit by a space-charge limited conduction model that incorporates the effect of a defect band. The observed conduction behavior suggests the presence of two regions of localized traps within the bandgap of the amorphous GeTe, located at approximately 0.26-0.27 eV and 0.56-0.57 eV from the valence band. Unlike the polycrystalline state, the high resistance DC behavior of amorphous GeTe does not translate to the RF switch performance; instead, a parasitic capacitance associated with the RF switch geometry dominates OFF state RF transmission. [ABSTRACT FROM AUTHOR]

This paper presents an in-depth theoretical examination of graphene-based field effect transistors, looking at thermal statistics, electrostatics, and electrodynamics. Using a first principles approach, the unique behavior observed in graphene-based field effect transistors, such as the V-shaped transfer characteristic, limited channel pinch-off, and lack of off-state (under gate modulation), are described. Unlike previous attempts, a description of both drift and diffusion currents in the device is presented. The effect of external resistance on steady-state and high-frequency performance is examined. Comparisons of the theoretical results to experimental results are made and show good agreement. Finally, the theoretical work in this paper is used as a basis to discuss the possible source of some observed behavior in practical graphene-based field effect transistors. [ABSTRACT FROM AUTHOR]

We investigate total ionizing dose (TID) effects in graphene field effect transistors comprised of chemical vapor deposition grown graphene transferred onto trimethylsiloxy(TMS)-passivated SiO2Si substrates. TID exposure with a positive gate bias increases the concentration of positive oxide trapped charges near the SiO2/TMS/graphene interface making Coulomb-potential scatterer limited mobility more apparent. In particular, we observe asymmetric degradation in electron and hole mobility, the former degrading more rapidly. Consistent with the electron-hole puddle description, we observe an increase in intrinsic electron carrier density that varies linearly with the oxide trapped charge density, while the hole carrier density remains largely unaltered. These effects give rise to an increasing minimum conductivity. [ABSTRACT FROM AUTHOR]

Abstract: InAs-based heterostructure barrier varactor (HBV) diodes with In0.3Al0.7As0.4Sb0.6 as the barrier material are demonstrated. Current–voltage and capacitance–voltage characteristics, as well as S-parameters, of HBV diodes with varying barrier thicknesses are examined. Maximum capacitance values and maximum-to-minimum capacitance ratios greater than those predicted by traditional HBV models were measured. The HBVs’ unconventional behavior in terms of charge accumulation layers adjacent to the wide bandgap barrier is discussed. [Copyright &y& Elsevier]

Unannealed Pd/Pt/Au contacts to p-type In0.27Ga0.73Sb were fabricated and measured. Relatively high hole mobilities, with respect to similarly doped InP-lattice-matched materials, and associated low sheet resistances were measured for the p-type In0.27Ga0.73Sb material. The unannealed Pd/Pt/Au contacts were found to be Ohmic in nature; and for a hole density of 2.9×1019 cm-3 and a mobility of 160 cm2/V s, a specific contact resistance of 7.6×10-8 Ω cm2 was measured. [ABSTRACT FROM AUTHOR]