Rama Venkatasubramanian

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

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 materials 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 ∼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.

Thermal conductivity of Si–Ge superlattices

The thermal conductivity of Si–Ge superlattices with superlattice periods 30  2 × 109 W m−2 K−1 at 200 K. Superlattices with relatively longer periods, L>130 A, have smaller thermal conductivities than the short-period samples. This unexpected result is attributed to a strong disruption of the lattice vibrations by extended defects produced during lattice-mismatched growth.

On-chip cooling by superlattice-based thin-film thermoelectrics

There is a significant need for site-specific and on-demand cooling in electronic, optoelectronic and bioanalytical devices, where cooling is currently achieved by the use of bulky and/or over-designed system-level solutions. Thermoelectric devices can address these limitations while also enabling energy-efficient solutions, and significant progress has been made in the development of nanostructured thermoelectric materials with enhanced figures-of-merit. However, fully functional practical thermoelectric coolers have not been made from these nanomaterials due to the enormous difficulties in integrating nanoscale materials into microscale devices and packaged macroscale systems. Here, we show the integration of thermoelectric coolers fabricated from nanostructured Bi2Te3-based thin-film superlattices into state-of-the-art electronic packages. We report cooling of as much as 15 degrees C at the targeted region on a silicon chip with a high ( approximately 1,300 W cm-2) heat flux. This is the first demonstration of viable chip-scale refrigeration technology and has the potential to enable a wide range of currently thermally limited applications.

MOCVD of Bi2Te3, Sb2Te3 and their superlattice structures for thin-film thermoelectric applications

The characteristics of metalorganic chemical vapor deposition (MOCVD) of Bi2Te3, Sb2Te3 and their superlattice structures are discussed in this paper. We have grown c-oriented films on both hexagonal sapphire and fcc GaAs substrates, with specular morphology and occasional stacking faults. Single crystallinity was confirmed by X-ray diffraction and low-energy electron diffraction (LEED). The stoichiometry (Bi:Te = 2:3, Sb:Te = 2:3) of the films were confirmed by X-ray photo-emission spectroscopy (XPS) and Rutherford back-scattering. We have also attempted to grow short-period (∼ 10 to 80 A) superlattice structures in the Bi2Te3Sb2Te3 materials system. X-ray diffraction data indicating the quality of these layered structures is presented. The advantages offered by the nature of chemical bonding in these materials, along the growth direction, for obtaining abrupt interfaces is discussed. The electrical transport properties of the MOCVD-grown p-type Bi2Te3Sb2Te3 structures and other thermoelectric properties including thermal conductivity and Seebeck coefficient are discussed. The initial results on the performance parameter known as figure-of-merit of the superlattice structures, measured parallel to the plane of the superlattice interfaces, are significantly higher than in conventional bulk materials. These initial results suggest a significant potential for MOCVD-based materials technology for high-performance, thin-film, thermoelectric refrigeration.

Thermal Challenges in Next-Generation Electronic Systems

Thermal challenges in next-generation electronic systems, as identified through panel presentations and ensuing discussions at the workshop, Thermal Challenges in Next Generation Electronic Systems, held in Santa Fe, NM, January 7-10, 2007, are summarized in this paper. Diverse topics are covered, including electrothermal and multiphysics codesign of electronics, new and nanostructured materials, high heat flux thermal management, site-specific thermal management, thermal design of next-generation data centers, thermal challenges for military, automotive, and harsh environment electronic systems, progress and challenges in software tools, and advances in measurement and characterization. Barriers to further progress in each area that require the attention of the research community are identified.

Enhanced thermoelectric performance in PbTe-based superlattice structures from reduction of lattice thermal conductivity

We have fabricated two-dimensional n-type PbTe∕PbTe0.75Se0.25 structures using an evaporation process. In optimized films exhibiting a high-quality superlattice structure, a significant reduction in lattice thermal conductivity has been experimentally measured. The reduction would indicate enhanced thermoelectric device performance compared to standard PbTeSe alloys given that the electrical components, specifically, the Seebeck coefficient and electrical resistivity, were not observed to deteriorate from bulk values. The analysis of these films shows continuous layers with a true two-dimensional superlattice structure, as opposed to the PbTe∕PbSe system that exhibits zero-dimensional structures from self-assembly. The room-temperature measurement of cross-plane figure-of-merit in a n-type PbTe∕PbTe0.75Se0.25 device structure by the transient method has been combined with temperature-dependent measurements of in-plane resistivity and Seebeck coefficient to yield evidence of enhanced thermoelectric perform...

Low-temperature organometallic epitaxy and its application to superlattice structures in thermoelectrics

We describe a simple, yet phenomenologically very different, low-temperature modification to the conventional metal–organic chemical vapor deposition. It has been applied to the epitaxy of hexagonal-phased Bi2Te3/Sb2Te3 superlattices on zinc-blende GaAs substrates. The modification enables a two-dimensional, layer-by-layer, epitaxy instead of a three-dimensional islanded growth. Therefore, this approach is of generic importance to the epitaxy of many electronic and magnetic materials and their superlattices. High-resolution transmission electron microscopy studies indicate that the interface between the GaAs substrate and Bi2Te3 film is qualitatively defect free and that periodic structures are formed in the Bi2Te3/Sb2Te3 superlattices, with one of the individual layers as small as 10 A. Such ultra-short-period superlattices offer significantly higher carrier mobilities than their respective solid-solution alloys, apparently due to the elimination of alloy scattering and the minimal effects of random inte...

Topological insulator Bi2Te3 films synthesized by metal organic chemical vapor deposition

Topological insulator (TI) materials such as Bi2Te3 and Bi2Se3 have attracted strong recent interests. Large scale, high quality TI thin films are important for developing TI-based device applications. In this work, structural and electronic properties of Bi2Te3 thin films deposited by metal organic chemical vapor deposition (MOCVD) on GaAs (001) substrates were characterized via X-ray diffraction (XRD), Raman spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and electronic transport measurements. The characteristic topological surface states (SS) with a single Dirac cone have been clearly revealed in the electronic band structure measured by ARPES, confirming the TI nature of the MOCVD Bi2Te3 films. Resistivity and Hall effect measurements have demonstrated relatively high bulk carrier mobility of ~350 cm^2/Vs at 300K and ~7,400 cm^2/Vs at 15 K. We have also measured the Seebeck coefficient of the films. Our demonstration of high quality topological insulator films grown by a simple and scalable method is of interests for both fundamental research and practical applications of thermoelectric and TI materials.

Effect of nanodot areal density and period on thermal conductivity in SiGe∕Si nanodot superlattices

We report on the effect of nanodot (ND) areal density and period on cross-plane thermal conductivity κ⊥ in SiGe∕Si nanodot superlattices (NDSLs). For all ND areal densities considered, we found that κ⊥ in SiGe∕Si NDSLs decreased monotonically with decreasing period and reached values lower than those in typical SiGe alloys (∼6.5Wm−1K−1). At short periods, κ⊥ was as low as 2.0–2.7Wm−1K−1 and at a fixed period, increasing the ND areal density led to lower κ⊥. This work indicates that low κ⊥ can be attained in SiGe∕Si NDSLs either with a low SL period, a high ND areal density, or both.

Energy Harvesting for Electronics with Thermoelectric Devices using Nanoscale Materials

Significant developments have occurred in the last few years in the area of nanoscale thermoelectric materials using superlattices and self-assembled quantum-dots. Thin-film thermoelectric (TE) devices employing these materials have been developed for many applications including energy harvesting. Thin-film TE devices, for a 1 mm3 of converter volume, are available that can produce well over 775 muW/mm3 with an external DeltaT of 9 K. Such modules can be packaged within conventional chip packages, unobtrusively, and provide valuable DC electric power in the range of 100 mW without the need for any DC-DC conversion using heat produced by ~10 to 20 Watt chips. Even larger power levels are harvestable in high power electronics such as IGBTs. It appears that the advanced TE modules can provide sufficient power, over the background requirements, to directly power electronics with temperature differentials as little as 1degC. Near-term candidate applications are in bio-implants, sensors, robotics and energy-limited electronics in thermally active environments. Miniature thermoelectric power harvesters can be integrated with other energy harvesting technologies such as photovoltaics and vibration energy harvesters to provide universal energy harvesting for autonomous systems.