Investigation of Low Resistance Contacts to Pb-Sb-Ag-Te (LAST) Materials for Module Fabrication
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Investigation of Low Resistance Contacts to Pb-Sb-Ag-Te (LAST) Materials for Module Fabrication Jonathan D’Angelo1, Jarrod L. Short1, Adam D. Downey1, Michael A. Pajor1, Timothy P. Hogan1 Duck-Young Chung2, Mercouri G. Kanatzidis2, Ed Timm3, Harold Schock3, 1
Electrical and Computer Engineering Department, Michigan State University 2 Chemistry Department, Michigan State University 3 Mechanical Engineering Department, Michigan State University East Lansing, MI 48824-1322
ABSTRACT Low electrical contact resistance is essential for the fabrication of high efficiency thermoelectric generators. These contacts must be stable to high temperatures and through thermal cycling. Here we present the fabrication procedure and characterization of several contacts to Pb-Sb-Ag-Te (LAST) compounds. Contact materials investigated include tungsten, antimony, tin, nickel, and a bismuth antimony based solder. The contacts were typically deposited by an electron beam evaporation method after careful preparation of the sample surface. The resistances were measured by using the transmission line model (TLM), and ohmic behavior was verified through current vs. voltage measurements. The best contact resistivities of less than 20 µΩ·cm2 have been measured for annealed antimony to n-type LAST samples. We present these procedures for fabricating low resistance contacts and the use of these contact materials toward the fabrication of high efficiency thermoelectric generator modules. INTRODUCTION Contact resistance can significantly degrade the performance of thermoelectric generators [1]. In an effort to minimize this loss, we have begun investigations of contact resistance measurements and the characterization of contacts between Pb-Sb-Ag-Te (LAST) or Pb-Sb-Ag-Te-Sn (LASTT) materials and various metals and alloys. Diffusion bonding techniques and procedures are also presented. EXPERIMENTAL PROCEDURE We have investigated the e-beam deposition of various metals onto LAST and LASTT materials in an appropriate pattern for transmission line model measurements of contact resistivity [2]. By measuring the resistance between multiple contact pads with varying spacings, a plot of the resistance vs. contact spacing can be made where a linear extrapolation to the zero spacing results in two times the contact resistance, 2Rc. Figure 1 shows the typical pattern used.
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Pad D
Pad C
Pad B Pad A
Figure 1. Typical pattern for depositing contacts for the TLM measurement technique. Each contact pad is approximately 0.5mm in length, and the width of the contact extends across the width of the sample. By extending the contact across the sample width, errors due to current crowding are minimized. Each contact was e-beam deposited to a thickness ranging from 100nm to 1micron as measured by a quartz crystal monitor during deposition. The system is pumped down to a vacuum level of around 4x10-6 Torr. This method is an excellent way of depositing thin films anywhere from nanometers thick to even a micron, and was found to work well eve
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