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Development of transition metal silicides as thinfilm materials for thermoelectric applications

: Nichenametla, Charan Krishna
: Wagner-Reetz, Maik; Schulz, Stefan E.

Chemnitz, 2017, 79 S.
Chemnitz, TU, Master Thesis, 2017
Master Thesis
Fraunhofer IPMS ()

The research focus on thermoelectric materials that are compatible with Complementary metal–oxide–semiconductor (CMOS) technology is motivated with Si and SiGe materials due to their considerable figure-of-merit ZT. However these materials have challenges such as metallization of contacts and oxidation at high temperatures. In general, various aspects have to be considered for metallization of TE materials such as thermal mismatch, low contact resistance, adhesion. Thus there is a demand for new materials that exhibit good thermoelectric properties and perhaps be integrated with existing CMOS technology. Silicides have few benefits such as low resistivity, chemical- and mechanical stability at higher temperatures and capability to withstand harsh conditions without passivation layer. Most importantly these materials circumvent the need for electrical contacts in TE applications. The present work is focused on development of transition metal silicides (CoSi, CoSi2, TiSi2 and TaSi2) for thermoelectric applications. Cobalt (Co), Titanium (Ti) and Tantalum (Ta) metal films are deposited on blank silicon (Si) wafer to study growth behavior. It is observed that introducing capping layers impedes the silicide growth and annealing ambient (Ar or N2) has significant influence in terms of silicidation temperature, mechanical stability, stress and microstructure at higher temperatures. Annealing in N2 gas forms a thin nitride layer upon the refractory metal silicides at higher temperatures, whereas ToF-SIMS analysis for Co-Si system shows no such layer. Co- and Ti silicides have smooth interface to a-Si substrates, on the other hand a sand dunes like morphology is observed with TaSi2 due to high residual stress. The semi-metallic cobalt monosilicide shows comparatively higher absolute power factor (S2σ) than rest of the investigated silicides. A remarkable behavior is observed with CoSi which makes the material suitable for either n- or p - type material depending upon the dopants. Doping with boron impedes the silicide growth in Co-Si whereas phosphorous atoms distribute uniformly in all silicides.