Effects of Gold Induced Crystallization on the thermoelectric properties of Si0.8Ge0.2 thin films grown by sputtering

Abstract

Si1-XGeX alloys are widely known thermoelectric materials showing a very high figure of merit at high temperatures, and have been used for long time in different applications. The control of the crystal structure of the material has been demonstrated as a key point on the thermoelectric properties. Sputtering deposition of Si1-XGeX at room temperature gives rise to amorphous material, with very low electric conductivity and hence bad thermoelectric properties. In order to produce crystalline thin films, sputter deposition at high temperature is usually performed, but the high temperatures needed can produce a reduction of the dopant concentration, hence reducing the electric conductivity. Crystallization of amorphous SiGe (a-SiGe) is performed by heating at temperatures over 625ºC during 24 h or more. However, it was shown that metal-induced crystallization (MIC) is a promising technology for reducing crystallization temperature of a-Si, a-Ge and a-SiGe. There are many metals, such as Al, Au, Ag and Ni, that can be used for performing MIC. In spite of the low crystallization temperatures in the above processes of MIC, there were serious problems of metal contamination of the SiGe films. In this paper, we report our works concerning crystallization of Si0.8Ge0.2 thin films using Au as crystallization catalyzer. We selected Au thin film since the eutectic temperatures for Au-Si (363 ºC) and Au-Ge systems (361 ºC) are very close. Post-deposition thermal treatments in inert atmosphere show that crystallization temperature is reduced to about 350 ºC, whit treatment times as short as 1 hour. Moreover, in situ crystallization during the deposition of SiGe over gold is also demonstrated at temperatures over 350 ºC. It shall also be noted that gold acts as a crystallization catalyzer, and can be easily eliminated by chemical methods, as it does not incorporate into the material structure. The crystallized thin films show high Seebeck coefficient and low electric resistance.