In situ high-temperature X-ray diffraction analysis of Mg2Si formation kinetics via reaction of Mg films with Si single crystal substrates

Conformal compact Mg2Si films can be formed on Si substrates (for battery, photovoltaic, thermoelectric, or other applications) via the direct reaction of Mg films with such substrates at modest temperatures (≥280 °C). In this work, in situ high-temperature X-ray diffraction (HTXRD) analyses have been used to track the reactive conversion of Mg films on Si single crystal (wafer) substrates into Mg2Si at 280-400 °C in a flowing He atmosphere. Prior reported complications associated with the evaporation or oxidation of magnesium films during heating were avoided by applying a thin (15 nm) titanium capping layer on the 1 μm thick magnesium films. The reaction of Mg films with <100> oriented and <111> oriented Si single crystals proceeded at parabolic rates with time. Similar values were obtained for the parabolic rate constants and activation energies for Mg reaction with both Si substrate orientations. These observations were consistent with solid-state diffusion through the thickening Mg2Si product as the rate-limiting step. The activation energy values obtained for Mg2Si formation were consistent with a previously reported correlation between the activation energies and melting points of other M2Si-type silicides formed by solid-state-diffusion-limited reaction with Si substrates.