Fabrication and contact resistivity of W–Si3N4/TiB2–Si3N4/p–SiGe thermoelectric joints

Abstract

The design and fabrication of silicon germanium (SiGe) thermoelectric elements, typically including the selection of electrode and intermediate materials, the process of joining electrode and intermediate layer onto thermoelectric materials, are the major challenge for SiGe thermoelectric device technology. In this study, W–Si3N4 and TiB2–Si3N4 composites are designed as the electrode and intermediate layer, respectively, and the W–i3N4/TiB2–Si3N4/p–Si80Ge20B0.6 joints are fabricated by a one-step spark plasma sintering process. The influences of the composition of TiB2–Si3N4 intermediate layer on the interfacial structure, contact resistivity and interfacial thermal stability are investigated. The interfacial thermal stability is improved with increasing Si3N4 content in TiB2–Si3N4 intermediate layer due to the reduced mismatch of coefficients of thermal expansion between the intermediate layer and SiGe. On the other hand, the contact resistivity increases with the rising of Si3N4 content due to the weakened TiB2/SiGe ohmic contact, which degrades the device efficiency. As the balanced point, the intermediate layer with the composition of 80vol% TiB2+20vol% Si3N4 provides good interfacial thermal stability and moderately small contact resistivity (~75μΩcm2 after aging at 1000°C for 120h) simultaneously, which is an optimized intermediate layer composition for W–Si3N4/TiB2–Si3N4/p–Si80Ge20B0.6 thermoelectric element. The TiB2–Si3N4 intermediate layer has excellent chemical stability to both W–Si3N4 electrode and SiGe thermoelectric material at high temperatures, which contributes to the sharp interface of the joint and effectively prevents the inter-diffusion between the electrode and the SiGe.