High temperature stable contacts for thermoelectric sensors and devices

Thermoelectric materials and devices are used for energy conversion,, cooling and thermal sensors. Semiconductors like Si, Si_1-xGe_x, SiC, β-FeSi₂ are suitable as thermal sensor materials at elevated temperature due to their high thermopower and high temperature stability. Studying the electrical properties of semiconductors, as well as the normal operation of most semiconductor devices, requires the presence of non-rectifying metal-semiconductor contacts. High temperature stable ohmic contacts to these materials are necessary for good sensor performance and long term stability. Laser welding allows fast processing of thermally stable contacts to semiconductors and offers a convenient and reliable way to form electrical contacts for high temperature material characterization. Ohmic contacts to Si and FeSi₂ were prepared by laser welding of thin tungsten studs and foils to the semiconductor. Thermocouple wires can be welded to such a tungsten interlayer for temperature measurement immediately at the semiconductor surface. Si-W contacts and FeSi₂-W contacts show good thermal stability up to 450°C in air. For use at even higher temperatures tungsten must be protected against oxidation. Different oxidation protective layers were studied. To form ohmic contacts to moderately doped n-type SiC Ni-based alloys were used. The thermal stability of these contacts was investigated up to 750°C in air. The electrical characteristics (I-V interdependence and contact resistance) and the thermal stability of the contacts were investigated. Electron microscopy, EDX and XRD analyses were used to detect chemical reactions arising in the laser welding process and under thermal treatment