Thermal stability and interfacial structure evolution of Bi2Te3-based micro thermoelectric devices

Bi2Te3 based thermoelectric device is the only commercialized thermoelectric module. The thermal stability of the interface between the thermoelectric materials and barrier layer or electrode plays pivotal role for the stability and reliability of micro thermoelectric devices especially for power generation. In this work, the thermal stability, bonding strength and structure evolution of the interface between Bi2Te3 based materials (both n type and p type materials) and Ni barrier layer were systematically investigated. The interface between Ni and Bi2Te3 based materials demonstrates an ohmic contact with the contact resistivity of 3.64 μΩ cm2 and 5.31 μΩ cm2 for p-type and n-type element respectively. The bonding strength of p-type and n-type element with electrode is 13.43 MPa and 17.62 MPa, respectively. Upon annealing at 423 K for 7 days, due to the mass diffusion between Ni and Bi2Te3 based materials, a thin Ni-Te layer is formed. With further extending the annealing time at 423 K to 42 days, the thickness of the diffusion layer increases to 3 µm. The growth of diffusion layer enhances the bonding strength to 19.81 MPa. However, this is accompanied with a sharp increase in the contact resistivity to 14.48 μΩ cm2 for n-type element and to 6 μΩ cm2 for p-type element with Ni barrier. The increase in the contact resistance deteriorates the output power of thermoelectric module especially for the micro-device from 2.60 mW to 2.48 mW under the temperature gradient of 20 K. This work points out that for the application of Bi2Te3 based thermoelectric device in power generation above 423 K, appropriate barrier layer is indispensable and plays significant role for the performance of the device.