The Experts below are selected from a list of 276 Experts worldwide ranked by ideXlab platform
Rama Venkatasubramanian - One of the best experts on this subject based on the ideXlab platform.
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aspects of thin film Superlattice thermoelectric materials devices and applications
Mrs Bulletin, 2006Co-Authors: Harald Bottner, Gang Chen, Rama VenkatasubramanianAbstract:Superlattices consist of alternating thin layers of different materials stacked periodically.The lattice mismatch and electronic potential differences at the interfaces and resulting phononand electron interface scattering and band structure modifications can be exploited to reduce phonon heat conduction while maintaining or enhancing the electron transport.This article focuses on a range of materials used in Superlattice form to improve the thermoelectric figure of merit.
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Thermal characterization of Bi2Te3/Sb2Te3 Superlattices
Journal of Applied Physics, 2001Co-Authors: M. N. Touzelbaev, Rama Venkatasubramanian, P Zhou, Kenneth E. GoodsonAbstract:Superlattices offer the potential to enhance the figure of merit for thermoelectric cooling by increasing the Seebeck coefficient while decreasing the thermal conductivity compared to bulk samples. The large bulk value of ZT makes Superlattices containing Bi2Te3 attractive for demonstrating benefits of using low-dimensional materials in thermoelectric applications. The present work describes measurements of the effective thermal conductivity normal to Bi2Te3/Sb2Te3 Superlattices deposited on GaAs using noncontact pulsed laser heating and thermoreflectance thermometry. The data show a strong reduction in the effective thermal conductivity of the Bi2Te3/Sb2Te3 Superlattices compared to bulk Bi2Te3, which can further increase thermoelectric figure of merit. The dependence of thermal conductivity on Superlattice period is found to be weak, particularly at periods above 60 Angstrom. This indicates that disorder in Bi2Te3/Sb2Te3 Superlattices may limit the heat conduction process at shorter periods than in Si/Ge Superlattices, for which measurements were previously reported in the literature. (C) 2001 American Institute of Physics.
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lattice thermal conductivity reduction and phonon localizationlike behavior in Superlattice structures
Physical Review B, 2000Co-Authors: Rama VenkatasubramanianAbstract:A study of thermal conductivities perpendicular to the interfaces in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}{/\mathrm{S}\mathrm{b}}_{2}{\mathrm{Te}}_{3}$ Superlattices is presented. The lattice thermal conductivities in these short-period Superlattices are less than those in homogeneous solid-solution alloys and exhibit a minimum for a period of \ensuremath{\sim}50 \AA{}. For periods less than 50 \AA{}, the adjoining layers of the Superlattice apparently become coupled and, in effect, make their thermal conductivities approach that of an alloy. Using the mean free path from kinetic theory, a diffusive transport analysis suggests a low-frequency cutoff $({\ensuremath{\omega}}_{\mathrm{cutoff}})$ in the spectrum of heat-conducting phonons. A physical model based on the coherent backscattering of phonon waves at the Superlattice interfaces is outlined for the reduction of lattice thermal conductivity; this suggests conditions of localizationlike behavior for the low-frequency phonons. The ${\ensuremath{\omega}}_{\mathrm{cutoff}}$ from the diffusive transport model is comparable to that estimated from applying the Anderson criterion to the potential localization of phonon waves. The general behavior of localizationlike effects is not unique to ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}{/\mathrm{S}\mathrm{b}}_{2}{\mathrm{Te}}_{3}$ Superlattices; it is also apparent in the thermal conductivities of Si/Ge Superlattices. These Superlattice structures offer a scope for studying the phonon localization phenomena while the lattice thermal conductivity reduction could lead to high-performance thermoelectric materials.
Shin-ichiro Tamura - One of the best experts on this subject based on the ideXlab platform.
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Group velocities of phonons in Superlattices
Physical Review B, 2002Co-Authors: K. Imamura, Yukihiro Tanaka, Shin-ichiro TamuraAbstract:We theoretically study the magnitude of the group velocities of acoustic phonons in periodic Superlattices. The group velocities in a perfect, periodic (infinite) Superlattice are calculated from the dispersion relations and those in a finite Superlattice are defined from the motion of phonon wave packets with the help of the stationary-phase approximation. In a frequency band, the group velocities in the infinite and finite Superlattices coincide with each other. In a frequency gap, their magnitude in the finite Superlattice becomes much larger than that in the band region, and increases as the periodicity N increases. This N dependence is qualitatively different depending on whether the gap in the corresponding infinite Superlattice is due to the intramode or intermode Bragg reflection. To explain these unusual behaviors of phonon group velocities, we develop analytical calculations based on the transfer-matrix method. Numerical examples are given for a GaAs/AlAs Superlattice sandwiched between GaAs substrate layer and GaAs cap layer.
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Phonon group velocity and thermal conduction in Superlattices
Physical Review B, 1999Co-Authors: Shin-ichiro Tamura, Yukihiro Tanaka, H. MarisAbstract:With the use of a face-centered cubic model of lattice dynamics we calculate the group velocity of acoustic phonons in the growth direction of periodic Superlattices. Comparing with the case of bulk solids, this component of the phonon group velocity is reduced due to the flattening of the dispersion curves associated with Brillouin-zone folding. The results are used to estimate semiquantitatively the effects on the lattice thermal conductivity in Si/Ge and GaAs/AlAs Superlattices. For a Si/Ge Superlattice an order of magnitude reduction is predicted in the ratio of Superlattice thermal conductivity to phonon relaxation time consistent with the results of P. Hyldgaard and G. D. Mahan, Phys. Rev. B 56, 10 754 ~1997!. For a GaAs/AlAs Superlattice the corresponding reduction is rather small, i.e., a factor of 23. These effects are larger for the Superlattices with larger unit period, contrary to the recent measurements of thermal conductivity in Superlattices.
H. Maris - One of the best experts on this subject based on the ideXlab platform.
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Phonon group velocity and thermal conduction in Superlattices
Physical Review B, 1999Co-Authors: Shin-ichiro Tamura, Yukihiro Tanaka, H. MarisAbstract:With the use of a face-centered cubic model of lattice dynamics we calculate the group velocity of acoustic phonons in the growth direction of periodic Superlattices. Comparing with the case of bulk solids, this component of the phonon group velocity is reduced due to the flattening of the dispersion curves associated with Brillouin-zone folding. The results are used to estimate semiquantitatively the effects on the lattice thermal conductivity in Si/Ge and GaAs/AlAs Superlattices. For a Si/Ge Superlattice an order of magnitude reduction is predicted in the ratio of Superlattice thermal conductivity to phonon relaxation time consistent with the results of P. Hyldgaard and G. D. Mahan, Phys. Rev. B 56, 10 754 ~1997!. For a GaAs/AlAs Superlattice the corresponding reduction is rather small, i.e., a factor of 23. These effects are larger for the Superlattices with larger unit period, contrary to the recent measurements of thermal conductivity in Superlattices.
Manijeh Razeghi - One of the best experts on this subject based on the ideXlab platform.
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dark current suppression in type ii inas gasb Superlattice long wavelength infrared photodiodes with m structure barrier
Applied Physics Letters, 2007Co-Authors: Binhminh Nguyen, Darin Hoffman, Pierre Yves Delaunay, Manijeh RazeghiAbstract:We presented an alternative design of type II Superlattice photodiodes with the insertion of a mid-wavelength infrared M-structure AlSb∕GaSb∕InAs∕GaSb∕AlSb Superlattice for the reduction of dark current. The M-structure Superlattice has a larger carrier effective mass and a greater band discontinuity as compared to the standard type II Superlattices at the valence band. It acts as an effective medium that weakens the diffusion and tunneling transport at the depletion region. As a result, a 10.5μm cutoff type II Superlattice with 500nm M-Superlattice barrier exhibited a R0A of 200Ωcm2 at 77K, approximately one order of magnitude higher than the design without the barrier. The quantum efficiency of such structures does not show dependence on either barrier thickness or applied bias.
Klaus Ploog - One of the best experts on this subject based on the ideXlab platform.
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Nature of the band gap (direct versus indirect) of short-period (GaAs)n/(AlAs)nSuperlattices grown along the [111] confinement direction
Applied Physics Letters, 1990Co-Authors: Roberto Cingolani, Leander Tapfer, Klaus PloogAbstract:We report on spectroscopic experimental evidence of type II band alignment in a (GaAs)6/(AlAs)6 Superlattice grown by molecular beam epitaxy along the (111) direction. This result is in contrast to recent theoretical calculations predicting that the (GaAs)n/(AlAs) n Superlattices grown along the (111) direction should be direct for all n's.