Structure of nanocrystalline Re–Si thin film composites and their unusual thermoelectric properties

The structure of nanocrystalline rhenium–silicon composite $ReSix$ films with $1.4<x<2.2$ has been studied as a function of the time by means of high temperature x-ray diffraction and transmission electron microscopy. Simultaneously, the thermoelectric transport properties were measured during the heat treatment. The nanocrystallization was achieved by annealing of amorphous films deposited onto oxidized Si wafers by magnetron cosputtering. The crystallization process is characterized by a decreasing average crystallite size in the range between 7 and 19 nm. An unknown mechanism limits the grain growth after reaching a maximum size, which decreases with increasing Si content. In the final state the films contain only two phases: the amorphous phase and the nanocrystalline $ReSi1.75$ phase. The electrical conductivity and the thermoelectric power of these thin film composites show nonmonotonic dependence on the volume fraction of the nanocrystalline phase and depend on different parameters, which suggests a way to optimize the thermoelectric efficiency.