Heteroepitaxial Ir-silicide/SiGe layers have been grown on top of p-Si(100) at a substrate temperature of 450 degrees C under ultra-high vacuum. The epitaxial Ir-silicide layer was determined to be Ir3Si4 with four types of epitaxial modes. Reflection high-energy electron diffraction was used to examine the film quality. The high-temperature stability of the epitaxial Ir3Si4/SiGe layers was investigated by rapid thermal annealing the Ir3Si4/SiGe layers at 550, 750 and 950 degrees C for 20 s. The chemical composition, microstructures and surface morphologies of the layers were characterized by using Auger electron spectroscopy, grazing angle incidence X-ray diffraction, scanning electron microscopy and cross-sectional transmission electron microscopy. A critical transition temperature existed between 550 and 750 degrees C for the Ir3Si4/SiGe layers, beyond which the Ir3Si4/SiGe layers were unstable during the rapid thermal annealing process. The Ir3Si4 layer became a mixture of dual Ir-silicide phases containing Ge atoms as the instability occurred. Because of the thermal instability, the conventional guard-ring fabrication should be performed before deposition of epitaxial films.