Epitaxial Al films have been deposited onto the clean surface of single-crystal Si by ionized cluster beam (ICB) at room temperature. Thermal stability of the film has been examined by SEM, AES depth profiling, ion backscat. tering/channeling, and electrical characterization of the Al-Si interface. It was found that the ICB Al film on Si substrate was remarkably stable up to 550°C although pure Al was used. Alloy penetration at the interface, shift of barrier height, degradation of crystalline quality and development of annealing hillocks on the surface were not observed after the heat treatment. Extremely long electromigration life time was also confirmed. Epitaxial growth on GaAs(100) substrate was attempted and preliminary results are given.