Raseong Kim

Influence of dimensionality on thermoelectric device performance

The role of dimensionality on the electronic performance of thermoelectric devices is clarified using the Landauer formalism, which shows that the thermoelectric coefficients are related to the transmission, T(E), and how the conducting channels, M(E), are distributed in energy. The Landauer formalism applies from the ballistic to diffusive limits and provides a clear way to compare performance in different dimensions. It also provides a physical interpretation of the “transport distribution,” a quantity that arises in the Boltzmann transport equation approach. Quantitative comparison of thermoelectric coefficients in one, two, and three dimensions shows that the channels are utilized more effectively in lower dimensions. To realize the advantage of lower dimensionality, however, the packing density must be very high, so the thicknesses of the quantum wells or wires must be small. The potential benefits of engineering M(E) into a delta function are also investigated. When compared with a bulk semiconducto...

On Landauer versus Boltzmann and full band versus effective mass evaluation of thermoelectric transport coefficients

transport is mathematically related to the solution of the Boltzmann transport equation, and expressions for the thermoelectric parameters in both formalisms are presented. Quantum mechanical and semiclassical techniques to obtain from a full description of the bandstructure, Ek, the density of modes in the Landauer approach or the transport distribution in the Boltzmann solution are compared and thermoelectric transport coefficients are evaluated. Several example calculations for representative bulk materials are presented and the full band results are related to the more common effective mass formalism. Finally, given a full Ek for a crystal, a procedure to extract an accurate, effective mass level description is presented.

Relaxation of optically excited carriers in graphene

We explore the relaxation of photo-excited graphene by solving a transient Boltzmann transport equation with electron-phonon (e-ph) and electron-electron (e-e) scattering. Simulations show that when the excited carriers are relaxed by e-ph scattering only, a population inversion can be achieved at energies determined by the photon energy. However, e-e scattering quickly thermalizes the carrier energy distributions washing out the negative optical conductivity peaks. The relaxation rates and carrier multiplication effects are presented as a function of photon energy and dielectric constant.

Cooling of photoexcited carriers in graphene by internal and substrate phonons

We investigate the energy relaxation of hot carriers produced by photoexcitation of graphene through coupling to both intrinsic and remote (substrate) surface polar phonons using the Boltzmann equation approach. We find that the energy relaxation of hot photocarriers in graphene on commonly used polar substrates, under most conditions, is dominated by remote surface polar phonons. We also calculate key characteristics of the energy relaxation process, such as the transient cooling time and steady-state carrier temperatures and photocarrier densities, which determine the thermoelectric and photovoltaic photoresponse, respectively. Substrate engineering can be a promising route to efficient optoelectronic devices driven by hot carrier dynamics.

Physics of Carrier Backscattering in One- and Two-Dimensional Nanotransistors

The physics of carrier backscattering in 1-D and 2-D transistors is examined analytically and by numerical simulation. An analytical formula for the backscattering coefficient is derived for elastic scattering in a 1-D channel. This formula shows that the critical length for backscattering is somewhat longer than the kT length, and it depends on the shape of the channel potential profile. For inelastic scattering, Monte Carlo (MC) simulations show that the critical length is related to the phonon energy. The MC simulations also show that although the scattering physics in 1-D and 2-D transistors is very different, the overall backscattering characteristics are surprisingly similar. For an elastic process, this similarity is due to the compensating effects of the scattering rate and the fraction of scattered carriers, which contribute to the backscattering coefficient. For an inelastic process, the critical length is determined from the phonon energy for both 1-D and 2-D channels.

Effects of Surface Orientation on the Performance of Idealized III–V Thin-Body Ballistic n-MOSFETs

Ballistic on-currents of thin-body n-channel metal-oxide-semiconductor field-effect transistors (n-MOSFETs) are compared across group IV (Si, Ge) and III-V (InAs, In0.5Ga0.5As, GaAs, GaSb) materials for different body thickness values, surface orientations, and transport directions under several idealization assumptions. Previous simulation studies have shown that, as oxide capacitance increases, typical III-V channels with (100) surface perform worse than Si in the ballistic limit due to the degraded density-of-states (DOS). In this letter, simulation results based on tight-binding band structure calculations verify a recent proposal that confined III-V n-MOSFETs with small Γ-L separations overcome the DOS bottleneck and deliver high injection velocities, boosting on-current performance. By using the quantized L-valleys, GaSb with (100) or (111) surface orientations shows the best ballistic performance, outperforming all other materials. Although GaAs (100) and InAs or In0.5Ga0.5As with any surface orientation suffer from the DOS bottleneck, GaAs (111) gives higher ballistic on -currents than Si does.

Computational study of the Seebeck coefficient of one-dimensional composite nano-structures

The Seebeck coefficient (S) of composite nano-structures is theoretically explored within a self-consistent electro-thermal transport simulation framework using the non-equilibrium Green’s function method and a heat diffusion equation. Seebeck coefficients are determined using numerical techniques that mimic experimental measurements. Simulation results show that, without energy relaxing scattering, the overall S of a composite structure is determined by the highest barrier within the device. For a diffusive, composite structure with energy relaxation due to electron-phonon scattering, however, the measured S is an average of the position-dependent values with the weighting factor being the lattice temperature gradient. The results stress the importance of self-consistent solutions of phonon heat transport and the resulting lattice temperature distribution in understanding the thermoelectric properties of a composite structure. It is also clarified that the measured S of a composite structure reflects its power generation performance rather than its cooling performance. The results suggest that the lattice thermal conductivity within the composite structure might be engineered to improve the power factor over the bulk by avoiding the conventional trade-off between S and the electrical conductivity.The Seebeck coefficient (S) of composite nano-structures is theoretically explored within a self-consistent electro-thermal transport simulation framework using the non-equilibrium Green’s function method and a heat diffusion equation. Seebeck coefficients are determined using numerical techniques that mimic experimental measurements. Simulation results show that, without energy relaxing scattering, the overall S of a composite structure is determined by the highest barrier within the device. For a diffusive, composite structure with energy relaxation due to electron-phonon scattering, however, the measured S is an average of the position-dependent values with the weighting factor being the lattice temperature gradient. The results stress the importance of self-consistent solutions of phonon heat transport and the resulting lattice temperature distribution in understanding the thermoelectric properties of a composite structure. It is also clarified that the measured S of a composite structure reflects it...

Characteristic Features of 1-D Ballistic Transport in Nanowire MOSFETs

In this paper, we theoretically investigate characteristic features of 1-D, ballistic transport in nanowire (NW) MOSFETs. An analytic model at T = 0 K is first derived using the top-of-the-barrier ballistic transport model. When the drain voltage is low, this model shows that the drain current increases stepwise with increasing gate voltage, and the transconductance vs. gate voltage displays spikes. These features are the most evident signatures of 1-D transport. Next, we examine the finite-temperature performance numerically and show how I-V characteristics change as device parameters and temperature are varied. Finally, recently reported silicon NW gate-all-around MOSFETs are analyzed with our model. We show that some quantum features of these experiments can be explained with our simple, ballistic model. This approach may be a possible tool for subband spectroscopy and device performance assessment.

Computational study of energy filtering effects in one-dimensional composite nano-structures

Possibilities to improve the Seebeck coefficient S versus electrical conductance G trade-off of diffusive composite nano-structures are explored using an electro-thermal simulation framework based on the non-equilibrium Green’s function method for quantum electron transport and the lattice heat diffusion equation. We examine the role of the grain size d, potential barrier height ΦB, grain doping, and the lattice thermal conductivity κL using a one-dimensional model structure. For a uniform κL, simulation results show that the power factor of a composite structure may be improved over bulk with the optimum ΦB being about kBT, where kB and T are the Boltzmann constant and the temperature, respectively. An optimum ΦB occurs because the current flow near the Fermi level is not obstructed too much while S still improves due to barriers. The optimum grain size dopt is significantly longer than the momentum relaxation length λp so that G is not seriously degraded due to the barriers, and dopt is comparable to or...

On the best bandstructure for thermoelectric performance: A Landauer perspective

The question of what bandstructure produces the best thermoelectric device performance is revisited from a Landauer perspective. We find that a delta-function transport distribution function (TDF) results in operation at the Mahan-Sofo upper limit for the thermoelectric figure-of-merit, ZT. We show, however, the Mahan-Sofo upper limit itself depends on the bandwidth (BW) of the dispersion, and therefore, a finite BW dispersion produces a higher ZT when the lattice thermal conductivity is finite. Including a realistic model for scattering profoundly changes the results. Instead of a narrow band, we find that a broad BW is best. The prospects of increasing ZT through high valley degeneracy or by distorting the density-of-states are discussed from a Landauer perspective. We conclude that while there is no simple answer to the question of what bandstructure produces the best thermoelectric performance, the important considerations can be expressed in terms of three parameters derived from the bandstructure—th...