Cross-plane Thermoreflectance Imaging of Thermoelectric Elements
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Cross-plane Thermoreflectance Imaging of Thermoelectric Elements. Peter M. Mayer and Rajeev J. Ram Research Laboratory of Electronics, Massachusetts Institute of Technology Cambridge, MA 02139, U.S.A. ABSTRACT This paper presents the first cross-plane thermoreflectance image of the temperature distribution in a thermoelectric (TE) element under bias. Using the technique of lock-in CCD thermoreflectance imaging, we can map the temperature distribution of an operational device with submicron spatial resolution and a temperature resolution of 10 mK. As such it offers a complete picture of the quasi-equilibrium transport within the device. The submicron resolution of the thermoreflectance image enables clear determination of localized heating due at interfaces - for example to due contact resistance - and thermal impedance mismatch within samples. The high spatial resolution is ideal for the characterization of thin-film thermoelectric materials where data from conventional techniques (such as the transient Harman method) are difficult to interpret. This paper also presents the first thermoreflectance data we are aware of for BiTebased material systems. Identification and separation of the Peltier and Joule components of the heating are possible, and finite difference simulations of the devices are presented for comparison with experiment. In this way it is possible to simultaneously acquire information about the Seebeck coefficient, electrical conductivity, and thermal conductivity of the thermoelectric material. The measurements demonstrate the feasibility of non-contact thermal measurements at the sub-micron scale.
INTRODUCTION High resolution thermal imaging at the micron and sub-micron scale is a useful tool for material and device characterization. One technique suitable for such studies is lock-in thermoreflectance imaging using a CCD camera [3]. A thermoreflectance measurement probes the surface of a material, monitoring the small fractional change in the reflectance of light occurring when the temperature of the surface is modulated [4]. Because the thermoreflectance measurement can be performed with visible light, sub-micron spatial resolutions greater than those attainable by traditional IR imaging techniques are possible [1]. In this work, the technique is applied to single n-type polycrystalline Bi2Te3 thermoelectric elements, 508 µm thick and 1 mm2 in area. Previous work by others has established thermoreflectance imaging as a tool for characterizing thermoelectric microrefrigerators by measuring the temperature change in a metal contact on the device [5]. We extend that work here by directly imaging an important thermoelectric material in the cross-plane direction, allowing for the direct observation of the heat transport through the TE element. By biasing the element with a sinusoidal current and locking in successively on the Peltier and Joule components of the heating, the contributions to the heating from each can be separately observed in a thermoreflectance image. Using a calibration
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