Nanostructured Superlattice Thin-Film Thermoelectric Devices

Chapter 47 Nanostructured Superlattice Thin-Film Thermoelectric Devices

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch047

Rama Venkatasubramanian*, Edward Siivola, Brooks O'Quinn, Kip Coonley, Thomas Colpitts, Pratima Addepalli, Mary Napier, and Michael Mantini Research Triangle Institute, Research Triangle Park, NC 27709 *Corresponding author: email: [email protected]; fax: 1-919-541-6515

Introduction Thin-film nanostructured materials offer the potential to dramatically enhance the performance of thermoelectrics, thereby offering new capabilities ranging from efficient cooling of small footprint communication lasers to eliminating hot spots in microprocessor chips in the near-term, to CFC-free refrigeration, portable electric power sources for replacing batteries, thermochemistry-on-a-chip, etc. in the long-term We demonstrated (1) a significant enhancement in the thermoelectric figure-of-merit (ZT) at 300K. We have shown a ZT of about 2.4 in 1-nm/5-nm p-type Bi Te /Sb Te superlattice structures and recently, of about 1.7 to 1.9 in 1-nm/4-nm n-type Bi Te / Bi Te Se superlattices. These improvements have been realized using the concept of phonon blocking, electron-transmitting superlattice structures. The phonon blocking arises from a complex localization-like behavior for phonons in nanostructured superlattices and the electron transmission is facilitated by optimal choice of band-offsets in these semiconductor heterostructures. This represents a significant improvement over state-of-the-art ZT of ~0.9 to 1 in bulk Bi Te -based thermoelectric materials. The concept of using superlattices to obtain enhanced ZT over alloys has been demonstrated in other material systems as well, including achieving a ZT of ~0.8 in Si/Ge superlattices at 300 K, as compared to ~0.1 in SiGe alloys (2). Note that although this represents an eightfold improvement and utilizes Si-based materials, even this magnitude of ZT is insufficiently attractive for widespread applications. More recently Harman et al. (3) have demonstrated a ZT of about 1.6 in PbTe/PbTeSe 2

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© 2005 American Chemical Society

In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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348 quantum-dot superlattices and Beyer et al. (4) have indicated a ZT of ~0.8 at 300 Κ in PbTe/Bi Te superlattices, compared to a ZT of ~0.5 in PbTe-based alloys. Most of these successful demonstrations have been based on the concept of reduction in thermal conductivity with nanostructures (5-8). The thin-film devices, resulting from microelectronic processing, allow high cooling power densities to be achieved for a variety of applications, with potential localized active-cooling power densities approaching 700 Wcm" . In addition to high performance and power densities, these thin-film microdevices are also extremely fast acting, with time constants of -10 to 20 psec, which is about a factor of 23,000 faster than bulk thermoelectric devices. These early results (1) have set the stage for a wide range of applications for the superlattice thin-film thermoelectric technology. This paper focuses on our transitioning the enhancedfigure-of-merit(ZT) in p-type Bi Te /Sb2Te3 and η-type Bi Te /Bi Te . Sex superlattices (1) to demonstrable functionality and performance at the module level with several initial device demonstrations. Although we have made significant headway, there remains much to be done to realize the full potential impact of the intrinsic ZT of superlattice materials. 2

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Experimental Methods and Results The p-type Bi Te /Sb Te and η-type Bi Te /Bi Te _xSex superlattices were grown by a low-temperature metallorganic chemical vapor deposition technique (9,10) that allows the deposition of high-quality superlattice interfaces in "lowtemperature materials," with periodic van der Waals bonding along the growth direction. This low-temperature growth process has been indicated (11) to be essential in obtaining the enhanced ZT in the Bi Te /Sb Te and n-type Bi Te /Bi Te . Se superlattice materials. The fundamental cooling or power conversion unit in an operational thermoelectric module is the o-n couple. An important next step towards device technology has been to demonstrate working p-n couples using these thin-film superlattice materials. We note that a full device implementation with thin-films in general and using these high-performance materials, in particular, offers many significant challenges. One involves the minimization of ohmic resistances to the p- and η-thermoelectric elements, which was already discussed in Réf. 1, as part of the extrinsic ZT demonstration. The next step is to minimize the interconnect resistances between the p- and η-type elements. Towards this end, we have been able to obtain a best ZT o f - 2 in a superlattice p-n couple (12). With these early p-n couples, we have looked at both cooling and power conversion device demonstrations. Shown in Figure 1 is the schematic of a p-n couple in a power-conversion test mode. The p-n couple exhibited a ZT of as much as 1.6, as obtained from a heat-to-power efficiency method (13). Single 2

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349 p-n couples have produced a power of -8.5 mW for a ΔΤ across the couple of 29 K. These devices indicate an active (as defined by the power produced by actual area of the p-n couple) power density of -24 Wcrcf . When the thermal interface resistances between the active device and the external heat source were optimized, as much as 85 % of the external ΔΤ was transferred to the device. A power level of 38 mW per couple has been obtained with a 4^m-thick element at a ΔΤ of about 107 K. This performance translates to an active power density of-54 Won" and a mini-module power density of -10.5 Wcm" (12). 2


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Figure 1. Schematic of a thin-film superlattice p-n couple in a power conversion test mode showing the p-type superlattice (P-SL) and n-type superlattice (N-SL), where TCI and TC2 are thermocouples and V, I represent the voltage and currents at the positive and negative termini.

In the cooling demonstration effort, we have been able to obtain over 40 Κ active cooling with thin-film superlattice cooling devices, useable in several laser and microprocessor cooling needs where the hot-sides run at -80 °C. This level of performance is demonstrated in the cooling data shown in Figure 2. We note that this cooling has been achieved in spite of severe thermal management issues that had to be overcome, and are not yet completely resolved. Because of to insufficient thermal management at high heat-flux levels, the "true" hot-side temperature and hence the "true" ΔΤ across the device is much higher than observed in Figure 2. Even so, these small-scale, 600 μιη χ 600 μηι superlattice thermoelectric cooling modules can address hot-spot cooling needs of current and future generations of microprocessors (14).


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Figure 2. The preliminary cooling performance in a 600 JLtm χ 600 μπι superlattice thermoelectric cooling module that can address the emerging spotcooling needs of microprocessors (reproducedfrom reference 14) and other communication laser devices.

Conclusion The transition from the high "intrinsic" ZT attainable in thin-film p-type Bi Te /Sb2Te3 and η-type Bi Te3/Bi Te3. Se superlattice materials and the high "extrinsic" ZT attainable in their respective thermoelements (1) to module-level performance is a huge engineering challenge. However, such a transition would 2

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enable a plethora of possibilities ranging from thermal management of hot spots in microprocessors to efficient thermal-to-electric power conversion devices. With the results presented here, both power and cooling thermoelectric devices with thin-film superlattice materials appear to be near-term potential realities. Some of the challenges that remain to be addressed in the full development of this nanoscale thermoelectric-materials technology include optimization of the various electrical and thermal interfaces both within the p-n couple and between the couple and the external thermal management components.

Acknowledgements The thermoelectric cooling results presented here were made possible through support by the Defense Advanced Research Projects Agency (DARPA) and the Office of Naval Research (ONR) under U. S. Navy Contract No. N00014-97-C-0211. The thermoelectric power conversion results presented here were supported by DARPA through an Army Research Office (ARO) Contract No. DAAD19-01-C-0070. These supports are gratefully acknowledged.

References 1. Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O'Quinn, B. Thin-film thermoelectric devices with high room-temperature figures of merit. Nature 2001, 413, 597-602. 2. Venkatasubramanian, R.; Siivola, E.; Colpitts, T. S. Proc. of 17 th

International Conference on

Thermoelectrics,

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98TH8365, 1998,p191. 3. Harman, T.; Taylor, P. J.; Walsh, M.P.; LaForge, B . E . Quantum dot superlattice thermoelectric materials and devices. Science 2002, 297, 22292232. 4. Beyer, H.; Nurnus, J.; Bottner, H.; Lambrecht, Α.; Wagner, E.; Bauer, G. High thermoelectric figure of merit ZT in PbTe and Bi Te -based superlattices by a reduction of the thermal conductivity. Physica E, 2002, 13, 965-968. 5. Venkatasubramanian, R.; Timmons, M. L.; Hutchby, J. Α.; Borrego, J. Proc. 2

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of 1 National Thermogenic Cooler Workshop, Horn, S., Ed.; Fort Belvoir,

VA, 1992, pp 196-231. 6. Venkatasubramanian, R.; Timmons, M. L.; Hutchby, J. A. Proc. of 12

th

International Conference on Thermoelectrics, Yokohama, Matsuura, K.,

Ed.; 1993,p322.


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12. 13. 14.

Lee, S. M.; Cahill, D. G.; Venkatasubramanian, R. Thermal conductivity of Si-Ge superlattices. Appl. Phys. Lett. 1997, 70, 2957-2959. Venkatasubramanian, R. Lattice thermal conductivity reduction and phonon localization-like behavior in superlattice structures. Phys. Rev. Β 2000, 61, 3091-3097. Venkatasubramanian, R.; Colpitts, T.; O'Quinn, B.; Liu, S.; El-Masry, N.; Lamvik, M . Low-temperature organometallic epitaxy and its application to superlattice structures in thermoelectrics. Appl. Phys. Lett. 1999, 75, 11041106. Venkatasubramanian, R. Low temperature chemical vapor deposition and etching apparatus and method. U.S. Patent no. 6,071,351 (6 June 2000). Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O'Quinn, B. Phonon -blocking, electron-transmitting low-dimensional structures. U.S. Patent Application No. 20,030,099,279, www.uspto.gov. Venkatasubramanian, R.; Siivola, E.; O'Quinn, B.C.; Coonley, K.; Addepalli, P.; Napier, M.; Colpitts, T.; Mantini, M., to be published. Goldsmid, H. J. Electronic Refrigeration, Pion Ltd., 1983. Bannerjee, K.; Mahajan, R. www.intel.com/showcase/silicon.


Nanostructured Superlattice Thin-Film Thermoelectric Devices

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