Improvement in Contact Resistivity to thin film Bi 2 Te 3
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1267-DD05-20
Improvement in Contact Resistivity to thin film Bi2Te3 Rahul P. Gupta1, Ka Xiong1, John B. White2, Kyeongjae Cho1, and Bruce E. Gnade 1,a) 1
Department of Material Science & Engineering, University of Texas at Dallas, Richardson, TX 75080 2 Marlow Industries Incorporated, a subsidiary of II-VI Incorporated, Dallas, TX 75238 a)
[email protected]
ABSTRACT A study of the impact of surface preparation and post-deposition annealing on contact resistivity for sputtered Ni and Co contacts to thin film Bi2Te3 is presented. The contact resistance values obtained using the transfer length method (TLM) for Ni is compared to Co as a potential contact metal to Bi2Te3. Post-deposition annealing at 100°C on samples that were sputter cleaned reduces the contact resistivity to < 10-7 Ω-cm2 for both Ni and Co contacts to Bi2Te3. Co provided similar contact resistance values as Ni, but had better adhesion and less diffusion into the thermoelectric (TE) material, making it a suitable candidate for contact metallization to Bi2Te3 based devices. INTRODUCTION Contact resistance becomes a serious limitation to efficiency of thermoelectric (TE) material based solid-state coolers with thermoelement leg lengths < 100 µm [1]. From a device point of view, although a high Z material can be achieved, the device coefficient of performance (COP) can still be low due to the degradation of Z due to the contact resistivity [2]. For thin TE materials, the losses become even more extreme and low electrical contact resistivity of 10X from 1.5 x 10-6 to 7.0 x 10-8 Ω-cm2 and from 2.1 x 10-6 to 4.0 x 10-8 Ω-cm2 for Ni and Co contacts, respectively. As the measured specific contact resistivity is reduced the systematic measurement error becomes large. Ueng et. al., [10] reported that the contribution from systematic errors can become as much as ~106 % for specific contact resistivity < 10-7 Ω-cm2 for non optimized TLM structures. The variation in the measured specific contact resistivity due to systematic error plus random error was found to be 55% and 76% for Ni and Co contacts for 100°C post annealed samples, respectively. The overall reduction in contact resistivity is much greater than the measurement error, indicating significant improvement using the proposed process. The reductions in contact resistivity at anneal temperatures as low as 100°C is attributed to the electrically favorable interfacial phase formation for both Ni and Co. Previous work on interfacial reactions has shown the formation of NiTe and CoTe2 as the preferred phases formed at temperature as low as 100°C for Ni and Co [11, 12]. The electrical behavior for Co contacts is similar to Ni contacts even after post-annealing. Results from previous work where first principle calculations were performed to study the stability of the interfaces, show the Co/Bi2Te3 interfaces to be more thermodynamically stable than Ni/Bi2Te3 interfaces [13].Co diffuses significantly less than Ni, therefore it is less likely to degrade the thermoelectric property of the TE material [14]. The
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