Gehong Zeng

Bio: Gehong Zeng is an academic researcher from University of California, Santa Barbara. The author has contributed to research in topics: Thermoelectric effect & Thermoelectric materials. The author has an hindex of 24, co-authored 56 publications receiving 2323 citations.

Demonstration of electron filtering to increase the Seebeck coefficient in In 0.53 Ga 0.47 As ∕ In 0.53 Ga 0.28 Al 0.19 As superlattices

Abstract: In this paper, we explore electron filtering as a technique to increase the Seebeck coefficient and the thermoelectric power factor of heterostructured materials over that of the bulk We present a theoretical model in which the Seebeck coefficient and the power factor can be increased in an ${\mathrm{In}}_{053}{\mathrm{Ga}}_{047}\mathrm{As}$-based composite material Experimental measurements of the cross-plane Seebeck coefficient are presented and confirm the importance of the electron filtering technique to decouple the electrical conductivity and Seebeck coefficient to increase the thermoelectric power factor

Thermal conductivity of Si/SiGe and SiGe/SiGe superlattices

Abstract: The cross-plane thermal conductivity of four Si/Si0.7Ge0.3 superlattices and three Si0.84Ge0.16/Si0.76Ge0.24 superlattices, with periods ranging from 45 to 300 and from 100 to 200 A, respectively, were measured over a temperature range of 50 to 320 K. For the Si/Si0.7Ge0.3 superlattices, the thermal conductivity was found to decrease with a decrease in period thickness and, at a period thickness of 45 A, it approached the alloy limit. For the Si0.84Ge0.16/Si0.76Ge0.24 samples, no dependence on period thickness was found and all the data collapsed to the alloy value, indicating the dominance of alloy scattering. This difference in thermal conductivity behavior between the two superlattices was attributed to interfacial acoustic impedance mismatch, which is much larger for Si/Si0.7Ge0.3 than for Si0.84Ge0.16/Si0.76Ge0.24. The thermal conductivity increased slightly up to about 200 K, but was relatively independent of temperature from 200 to 320 K.

SiGeC/Si superlattice microcoolers

Abstract: Monolithically integrated active cooling is an attractive way for thermal management and temperature stabilization of microelectronic and optoelectronic devices. SiGeC can be lattice matched to Si and is a promising material for integrated coolers. SiGeC/Si superlattice structures were grown on Si substrates by molecular beam epitaxy. Thermal conductivity was measured by the 3omega method. SiGeC/Si superlattice microcoolers with dimensions as small as 40×40 µm^2 were fabricated and characterized. Cooling by as much as 2.8 and 6.9 K was measured at 25 °C and 100 °C, respectively, corresponding to maximum spot cooling power densities on the order of 1000 W/cm^2.

Thermoelectric power factor in semiconductors with buried epitaxial semimetallic nanoparticles

Abstract: We have grown composite epitaxial materials that consist of semimetallic ErAs nanoparticles embedded in a semiconducting In0.53Ga0.47As matrix both as superlattices and randomly distributed throughout the matrix. The presence of these particles increases the free electron concentration in the material while providing scattering centers for phonons. We measure electron concentration, mobility, and Seebeck coefficient of these materials and discuss their potential for use in thermoelectric power generators.

Effect of nanoparticle scattering on thermoelectric power factor

Abstract: The effect of nanoparticles on the thermoelectric power factor is investigated using the relaxation time approximation. The partial-wave technique is used for calculating the nanoparticle scattering cross section exactly. We validate our model by comparing its results to the experimental data obtained for ErAs:InGaAlAs samples. We use the theory to maximize the power factor with respect to nanoparticle and electron concentrations as well as the barrier height. We found that at the optimum of the power factor, the electron concentration is usually higher in the sample with nanoparticles, implying that Seebeck is usually unchanged and conductivity is increased.

Complex thermoelectric materials.

Abstract: Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.

Thin-film thermoelectric devices with high room-temperature figures of merit

Abstract: Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.

New and Old Concepts in Thermoelectric Materials

Abstract: Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

Bulk nanostructured thermoelectric materials: current research and future prospects

Abstract: Thermoelectrics have long been recognized as a potentially transformative energy conversion technology due to their ability to convert heat directly into electricity. Despite this potential, thermoelectric devices are not in common use because of their low efficiency, and today they are only used in niche markets where reliability and simplicity are more important than performance. However, the ability to create nanostructured thermoelectric materials has led to remarkable progress in enhancing thermoelectric properties, making it plausible that thermoelectrics could start being used in new settings in the near future. Of the various types of nanostructured materials, bulk nanostructured materials have shown the most promise for commercial use because, unlike many other nanostructured materials, they can be fabricated in large quantities and in a form that is compatible with existing thermoelectric device configurations. The first generation of these materials is currently being developed for commercialization, but creating the second generation will require a fundamental understanding of carrier transport in these complex materials which is presently lacking. In this review we introduce the principles and present status of bulk nanostructured materials, then describe some of the unanswered questions about carrier transport and how current research is addressing these questions. Finally, we discuss several research directions which could lead to the next generation of bulk nanostructured materials.

Nanoscale thermal transport. II. 2003–2012

Abstract: A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of ∼1 nm, the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interface...