Superlattice applications to thermoelectricity

Whitlow, L. W.; Hirano, Tohru

doi:10.1063/1.359661

Cited by 81 publications

(33 citation statements)

References 17 publications

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“…In the past decade, the use of confined structures such as quantum-well and quantum-wire superlattices have gained attention as thermoelectric materials due to a number of advantages: 1) the Seebeck coefficient can be increased by the increase in the local density of density of states per unit volume near the Fermi energy [1]; and 2) the thermal conductivity can be decreased due to phonon confinement [2], [3] and phonon scattering at the material interfaces in the superlattice [4]- [6]. Normally, the electrical conductivity is assumed not to be significantly affected due to the large semiconductor bandgap and the disparity between the electron and phonon mean free paths [7].…”

Section: Introductionmentioning

confidence: 99%

“…The most common method of predicting thermoelectric parameters is based on a semiclassical RTA model [1], [4]- [6] where the system is assumed to be only slightly perturbed from equilibrium. Some of the early models employed parabolic energy dispersion for the conduction and valance band energies, whereas later models incorporated the effect of low dimensionality through a subband energy-dispersion relation [1], [4], [5], [8].…”

Section: Introductionmentioning

confidence: 99%

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Quantum Modeling of Thermoelectric Properties of Si/Ge/Si Superlattices

Bulusu

Walker

2008

IEEE Trans. Electron Devices

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Abstract-Using a nonequilibrium Green's function approach, quantum simulations are performed to assess the device characteristics for cross-plane transport in Si/Ge/Si-superlattice thin films. The effect of quantum confinement on the Seebeck coefficient and electrical transport and its impact on the power factor of superlattices are studied. In this case, decreasing well width leads to an increased subband spacing causing the Seebeck coefficient of the superlattice to decrease. Electron confinement also causes a drastic reduction in the overall available density of states. Results show that confinement effects in the silicon barrier are responsible for a 40% decrease in the electrical conductivity of superlattices where barrier films are thinner by a factor of 1.5. In the same two devices, there is a negligible change in the Seebeck coefficient, which results in a decrease in the power factor corresponding to the decrease in conductivity. This decrease in the electrical performance for superlattices with thinner layers may offset the previously hypothesized gains of highly scaled superlattice structures resulting from reduced thermal conductivity. Simulations of the present superlattice structure at varying doping levels show a decrease in power factor with a decrease in device size parameters.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Quantum Modeling of Thermoelectric Properties of Si/Ge/Si Superlattices

Bulusu

Walker

2008

IEEE Trans. Electron Devices

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“…Since the operating currents for the device is very high (10 5 A/cm 2 ), non-ideal effects such as the Joule heating at the metal-semiconductor contact resistance, and the reverse heat conduction have limited the experimental cooling results to <1 o C [3]. There is another regime of operation in which electron transport is dominated by the multi barrier structure [10][11][12]. A superlattice is chosen so that hot electrons move easily in the materials, but the movement of cold electrons is more restricted.…”

Section: I-introductionmentioning

confidence: 99%

Conservation of Lateral Momentum in Heterostructure Integrated Thermionic Coolers

Vashaee

Shakouri

2001

MRS Proc.

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Thin film thermionic coolers use selective emission of hot electrons over a heterostructure barrier layer from emitter to collector resulting in evaporative cooling. In this paper a detailed theory of electron transport perpendicular to the multilayer superlattice structures is presented. Using Fermi-Dirac statistics, density-of-states for a finite quantum well and the quantum mechanical reflection coefficient, the currentvoltage characteristics and the cooling power density are calculated. The resulting equations are valid in a wide range of temperatures and electric fields. It is shown that conservation of lateral momentum plays an important role in the device characteristics. If the lateral momentum of the hot electrons is conserved in the thermionic emission process, only carriers with sufficiently large kinetic energy perpendicular to the barrier can pass over it and cool the emitter junction. However, if there is no conservation of lateral momentum, the number of electrons participating in thermionic emission will dramatically increase. The theoretical calculations are compared with the experimental dark current characteristics of quantum well infrared photodetectors and good agreement over a wide temperature range is obtained. Calculations for InGaAs/InGaAsP superlattice structures show that the effective thermoelectric power factor (electrical conductivity times the square of the effective Seebeck coefficient) can be improved comparing to that of bulk material. We will also discuss methods by which the conservation of lateral momentum in thermionic emission process can be altered such as by creating a controlled roughness at the interface of the superlattice barriers. The improvement in the effective power factor through thermionic emission can be combined with the other methods to reduce the phonon thermal conductivity in superlattices and thus obtain higher thermoelectric figure-of-merit ZT.

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“…While Smax(x) decreases rapidly with temperature, the electron energy or the doping level increases linearly with increasing temperature for all cases, except for x=0. 15, where it shows a nonlinear behavior. This composition is close to the crossover composition, x = 0.85, where the minimum of the conduction band switches its position from the rx: direction to the f'L direction.…”

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confidence: 99%

Electron-dependent thermoelectric properties in Si/Si1-xGex heterostructures and Si1-xGex alloys from first-principles

Hossain

Johnson

2012

Applied Physics Letters

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Unlike phononic thermal conductivity (which is shown in the literature to be reduced due to alloying and has a nearly constant value over a range of compositional variations), electron-dependent thermoelectric properties are shown here, from first-principles, to vary nonlinearly with composition. Of the Si/Si 1 _xGex systems considered, the maximum thermopower observed, which is 10% higher than that of crystalline Si, is obtained for a Si 0 . 875 (J, Ke, K 1 are the Seebeck coefficient or thermopower, electrical conductivity, electron thermal conductivity, and lattice or phonon thermal conductivity, respectively, characterizes the efficiency of a thermoelectric material and involves transport of electrons as well as phonons. To enhance zr, phononic properties must be altered to reduce the thermal conductivity, and electronic properties must be altered to maximize the thermopower and electrical conductivity. However, due to the complexity of optimizing several interdependent phononic and electronic properties simultaneously, it remains a challenge to find a combination of the properties or to find mechanisms that would provide promising values of zr.Although both experimental 6 -9 and theoretical efforts 1 0-13 have focused on finding ways to enhance zr in different material/design conditions, accurate determination of electronic and phononic contributions and their dependence on alloy composition, strain, or nanostructuring, particularly on the nanoscale, continues to be a formidable task. While experimentally it is difficult to carry out transport measurements, particularly on the nanoscale, 5 • 13 theoretical studies, 1 3-16 mostly use a finite number of bands to estimate the electronic contribution. In these methods, using parabolic or non-parabolic dispersion relations for computing the effective mass of electrons, local strain-induced charge effects on band bending at the heterointerface are disregarded. Consequently, contradictory conclusions for the same system are reported. For example, unlike single band calculations, 14 · 15 multiple-miniband calculations13·16 show oscillatory behavior for the Seebeck coefficient and the Lorenz number (which is proportional to the ratio between Ke and (J).On the other hand, first-principles based transport calculation methods (which do not have any approximations and serve as the most accurate method, though with a high computational cost) incorporate a complete description of the •)E-mail: zubaer@caltech.edu. electronic bands and have been used to find electron dependent transport properties and mechanisms accurately, for a range of electronic materials. 17 -19 Using a combination of density functional theory (DFf) and the Boltzmann transport equation, we study the Si/Si 1 _xGex system (which holds particular importance in widely used Si-based technologies and offers high zr values, particularly at high temperatures 4 ) as a model for determining the role of alloying and heterostructuring on its electron dependent transport properties. We find that neither S nor ...

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Superlattice applications to thermoelectricity

Cited by 81 publications

References 17 publications

Quantum Modeling of Thermoelectric Properties of Si/Ge/Si Superlattices

Quantum Modeling of Thermoelectric Properties of Si/Ge/Si Superlattices

Conservation of Lateral Momentum in Heterostructure Integrated Thermionic Coolers

Electron-dependent thermoelectric properties in Si/Si1-xGex heterostructures and Si1-xGex alloys from first-principles

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