Design and thermoreflectance imaging of high-speed SiGe superlattice microrefrigerators
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Design and thermoreflectance imaging of high-speed SiGe superlattice microrefrigerators Bjorn Vermeersch*, Je-Hyeong Bahk, James Christofferson and Ali Shakouri Department of Electrical Engineering, University of California – Santa Cruz, 1156 High Street, SOE2, Santa Cruz, CA 95064, U.S.A. *E-mail: [email protected] ABSTRACT Over the past few years, thermoelectric (TE) materials have been receiving an increasing amount of attention owing to their promising potential for energy conversion and thermal management applications. Thermal characterisation techniques offer a powerful tool in investigating and optimizing the TE device performance. In addition, they can provide a better understanding of the underlying fundamental principles such as Peltier effects at the interfaces of the active medium. In this paper, we present the design and thermal characterisation of integrated highspeed microcoolers based on SiGe superlattices. The electrode metalisation is laid out as a coplanar waveguide, enabling to supply electrical pulses with short rise times to the coolers. We employ a variety of CCD-based transient thermoreflectance imaging methods to perform an extensive dynamic thermal analysis. These techniques provide 2-D temperature maps of the chip surface with ~100ns temporal and submicron spatial resolution without the need to scan the sample. Net cooling in the 2 degree range is observed, with response times well below 1µs. This is almost two orders of magnitude faster compared to the best in the literature. The obtained images also confirm the previous observations that the Peltier cooling term responds faster than the Joule heating term, in agreement with their expected locality and associated thermal mass. This provides potential to study ultrafast electron-phonon interactions during Peltier effects. INTRODUCTION The field of thermoelectrics (TEs) has recently been growing quickly owing to the promising potential of these devices for large-scale energy conversion applications [1,2] and on-chip thermal management [3]. Large amounts of attention have been devoted to increasing the performance of TE materials, as quantified by their dimensionless figure of merit ZT. Continuous advances in this field, among which nanostructuring in particular [4], have lead to vast progress and steadily paved the way in making thermoelectrics both more efficient and competitive. The fundamental basis of thermoelectricity is constituted by the Peltier and Seebeck effects. These phenomena govern the direct solid-state conversion between thermal and electrical energy, and are crucial to the working of any TE system. Few studies have been dedicated to the behaviour of TE modules in fast transient operation, and little is known about the electronphonon interactions occurring at an active Peltier junction at very short time scales. As a means to perform such a dynamic investigation, we have developed high-speed integrated microcoolers with a SiGe superlattice as active medium. In the following sections we present the design of the sample, t
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