Fabrication and Characterization of Thermoelectric Materials and Their Stable Contacts at Elevated Temperatures
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Contact resistance plays an important role in maintaining high device performance in thermoelectric (TE) generators and coolers. In order to achieve 95% device efficiency, the contact resistance needs to be < 10% of the total leg resistance. Therefore, stable contact electrodes with low contact resistivity are necessary to ensure good device efficiency. In this dissertation, various contact materials (Hf, Ni, Ti and Au) were evaluated as potential contact metals to Mg2Si used for moderate temperature applications. The data indicate that Ni is the best candidate among the four: it can be stable up to 400°C. The contact failure analysis showed that Ni diffuses into Mg2Si after annealing at 450°C for as short as two hours. The TaAlN ternary thin film system was evaluated as a contact material to SiGe to be used at high temperature. The thermal stability study indicates that TaAlN films are stable and conductive after at least 106 hr post-deposition annealing, without showing signs of cracking or delamination. The contact resistance of TaAlN-SiGe contacts remains stable at approximately 10-6 Ω-cm after annealing for 106 hr. Furthermore, experimental data showed a sharp interface between TaAlN and SiGe, indicating no inter-diffusion occurs after post-deposition annealing. The excellent thermal stability, good conductivity and low contact resistance to SiGe makes thin TaAlN films a promising candidate for contact electrodes to high-temperature SiGe based thermoelectric devices.