On the effectiveness of the thermoelectric energy filtering mechanism in low-dimensional superlattices and nano-composites

Electron energy filtering has been suggested as a promising way to improve the power factor and enhance the ZT figure of merit of thermoelectric materials. In this work, we explore the effect that reduced dimensionality has on the success of the energy-filtering mechanism for power factor enhancement. We use the quantum mechanical non-equilibrium Green's function method for electron transport including electron-phonon scattering to explore 1D and 2D superlattice/nanocomposite systems. We find that, given identical material parameters, 1D channels utilize energy filtering more effectively than 2D as they: (i) allow one to achieve the maximal power factor for smaller well sizes/smaller grains which are needed to maximize the phonon scattering, (ii) take better advantage of a lower thermal conductivity in the barrier/boundary materials compared to the well/grain materials in both: enhancing the Seebeck coefficient; and in producing a system which is robust against detrimental random deviations from the optimal barrier design. In certain cases, we find that the relative advantage can be as high as a factor of 3. We determine that energy-filtering is most effective when the average energy of carrier flow varies the most between the wells and the barriers along the channel, an event which occurs when the energy of the carrier flow in the host material is low, and when the energy relaxation mean-free-path of carriers is short. Although the ultimate reason for these aspects, which cause a 1D system to see greater relative improvement than a 2D, is the 1D system's van Hove singularity in the density-of-states, the insights obtained are general and inform energy-filtering design beyond dimensional considerations

On the Lorenz number of multiband materials

There are many exotic scenarios where the Lorenz number of the Wiedemann-Franz law is known to deviate from expected values. However, in conventional semiconductor systems, it is assumed to vary between the values of ∼1.49×10−8WΩK−2 for nondegenerate semiconductors and ∼2.45×10−8WΩK−2 for degenerate semiconductors or metals. Knowledge of the Lorenz number is important in many situations, such as in the design of thermoelectric materials and in the experimental determination of the lattice thermal conductivity. Here, we show that, even in the simple case of two- and three-band semiconductors, it is possible to obtain substantial deviations of a factor of 2 (or in the case of a bipolar system with a Fermi level near the midgap, even orders of magnitude) from expectation. In addition to identifying the sources of deviation in unipolar and bipolar two-band systems, a number of analytical expressions useful for quantifying the size of the effect are derived. As representative case studies, a three-band model of the materials of lead telluride (PbTe) and tin sellenide (SnSe), which are important thermoelectric materials, is also developed and the size of possible Lorenz number variations in these materials explored. Thus, the consequence of multiband effects on the Lorenz number of real systems is demonstrated

The influence of non-idealities on the thermoelectric power factor of nanostructured superlattices

Cross-plane superlattices composed of nanoscale layers of alternating potential wells and barriers have attracted great attention for their potential to provide thermoelectric power factor improvements and higher ZT figure of merit. Previous theoretical works have shown that the presence of optimized potential barriers could provide improvements to the Seebeck coefficient through carrier energy filtering, which improves the power factor by up to 40%. However, experimental corroboration of this prediction has been extremely scant. In this work, we employ quantum mechanical electronic transport simulations to outline the detrimental effects of random variation, imperfections, and non-optimal barrier shapes in a superlattice geometry on these predicted power factor improvements. Thus, we aim to assess either the robustness or the fragility of these theoretical gains in the face of the types of variation one would find in real material systems. We show that these power factor improvements are relatively robust against: overly thick barriers, diffusion of barriers into the body of the wells, and random fluctuations in barrier spacing and width. However, notably, we discover that extremely thin barriers and random fluctuation in barrier heights by as little as 10% is sufficient to entirely destroy any power factor benefits of the optimized geometry. Our results could provide performance optimization routes for nanostructured thermoelectrics and elucidate the reasons why significant power factor improvements are not commonly realized in superlattices, despite theoretical predictions

An event bias technique for Monte Carlo device simulation

Abstract In Monte Carlo (MC) simulations of semiconductor devices it is necessary to enhance the statistics in sparsely populated regions of interest. In this work the Monte Carlo method for stationary carrier transport, known as the Single-Particle MC method, is considered. It gives a solution to the stationary boundary value problem defined by the semi-classical Boltzmann equation (BE). Using a formal approach which employs the integral form of the problem and the Neumann series expansion of the solution, the Single-Particle MC method is derived in a formal way. The independent, identically distributed random variables of the simulated process are identified. Estimates of the stochastic error are given. Furthermore, the extension of the MC estimators to the case of biased events is derived. An event bias technique for particle transport across an energy barrier is developed and simulation results are discussed

Reduction of the Dark-Current in Carbon Nanotube Photo-Detectors

Abstract-Carbon nanotubes have been considered in recent years for future opto-electronic applications because of their direct band-gap and the tunability of the band-gap with the CNT diameter. The performance of infra-red photo-detectors based on carbon nanotube field-effect transistors is analyzed, using the non-equilibrium Green's function formalism. The relatively low ratio of the photo-current to the dark current limits the performance of such devices. We show that by employing a double gate structure this ratio can be significantly increased. Carbon nanotubes (CNTs) have been extensively studied in recent years due to their exceptional electronic, optoelectronic, and mechanical properties. CNTs can be considered as a graphene sheet which has been wrapped into a tube. The way the graphene sheet is wrapped is represented by a pair of indices (n, m) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m = 0, the CNT is called zigzag. If n = m, the CNT is called armchair. Otherwise, it is called chiral. CNTs with n−m = 3 are metals, otherwise they are semiconductors. Semiconducting CNTs can be used as channels for transistors. Depending on the work function difference between the metal contact and the CNT, carriers at the metal-CNT interface encounter different barrier heights. Fabrication of devices with positive [1] and zero Some of the interesting electronic properties of CNTs are quasi-ballistic carrier transport [2], suppression of shortchannel effects due to one-dimensional electron transport IR photo detectors based on carbon nanotube field effect transistors (CNT-FETs) have been reported i

Lessons learned and ways forward on CGIAR capacity development: A discussion paper

This paper is a contribution to the establishment of a new capacity development (CD) strategy, a process that the Consortium Office will facilitate, with external input, during 2013. The paper explores the lessons learned from CGIAR’s experience with CD and reflects the findings of a working group that was brought together in late 2012

A numerical study of partial-SOI LDMOSFETs

Abstract The high-voltage and self-heating behavior of partial-SOI (silicon-on-insulator) LDMOSFETs were studied numerically. Different locations of the silicon window were considered to investigate the electrical and thermal effects. It is found that the potential distribution of the partial-SOI LDMOSFET with the silicon window under the drain is similar to that of standard junction isolation devices. With the silicon window under the source the potential distribution is similar to that of the conventional SOI LDMOSFET. Using the two-dimensional numerical simulator MINIMOS-NT, we confirm that the breakdown voltage of partial-SOI LDMOSFETs with a silicon window under the source is higher than that of partial-SOI LDMOSFET with a silicon window under the drain

Modeling, properties, and fabrication of a micromachined thermoelectric generator

Different electrical and thermoelectric properties of a Si-based thermoelectric generator (TEG) are described based on the Kubo–Greenwood formalism. Temperature and doping dependence, phonon scattering (acoustic and optical phonons), and scattering on impurities are included. Comparisons with experimentally verified data confirm the validity of the model. Experimental studies were carried out on a micromechanically fabricated TEG. Devices were realized using a standard CMOS SOI technology in a lateral geometry. All thermopiles are located on a thin membrane to reduce the heat flow. The thickness of the membrane was adjusted between 20 and 30 µm ensuring also sufficient mechanical stability. Measurements on individual devices confirm the results of the theoretical model. The Seebeck coefficient was calculated and experimentally measured as S = 0.5 mV/K at an acceptor level of 1019 cm−3 at room temperature. The power factor is S2 · σ = 0.0073 W/mK2

Numerical study of the thermoelectric power factor in ultra-thin Si nanowires

Low dimensional structures have demonstrated improved thermoelectric (TE) performance because of a drastic reduction in their thermal conductivity, {\kappa}l. This has been observed for a variety of materials, even for traditionally poor thermoelectrics such as silicon. Other than the reduction in {\kappa}l, further improvements in the TE figure of merit ZT could potentially originate from the thermoelectric power factor. In this work, we couple the ballistic (Landauer) and diffusive linearized Boltzmann electron transport theory to the atomistic sp3d5s*-spin-orbit-coupled tight-binding (TB) electronic structure model. We calculate the room temperature electrical conductivity, Seebeck coefficient, and power factor of narrow 1D Si nanowires (NWs). We describe the numerical formulation of coupling TB to those transport formalisms, the approximations involved, and explain the differences in the conclusions obtained from each model. We investigate the effects of cross section size, transport orientation and confinement orientation, and the influence of the different scattering mechanisms. We show that such methodology can provide robust results for structures including thousands of atoms in the simulation domain and extending to length scales beyond 10nm, and point towards insightful design directions using the length scale and geometry as a design degree of freedom. We find that the effect of low dimensionality on the thermoelectric power factor of Si NWs can be observed at diameters below ~7nm, and that quantum confinement and different transport orientations offer the possibility for power factor optimization.Comment: 42 pages, 14 figures; Journal of Computational Electronics, 201