Thermal stability of Pd₂Si and PdSi in thin film and in bulk diffusion couples

Thermal stability of films of Pd₂Si and PdSi formed on 〈111〉 Si wafers has been studied by x-ray diffraction. We observed that Pd₂Si transformed to PdSi at temperatures around 810±10 °C and PdSi transformed back to Pd₂Si when annealed below that temperature. We conclude that Pd₂Si rather than PdSi is the thermodynamically stable phase on Si at low temperatures, contrary to the existing Pd-Si phase diagram. The conclusion has been confirmed by annealing bulk Pd-Si diffusion couples at 750 °C to form Pd₂Si, then at 850 °C to transform part of the Pd₂Si to PdSi, and then returning to 750 °C to convert the PdSi back to Pd₂Si and Si. The bulk transformations were measured by x-ray diffraction and electron microprobe.