The structural and electronic properties of the epitaxial Al/Si(111)7x7 interface and their modifications upon thermal annealing were investigated by scanning tunneling microscopy, high-resolution photoelectron spectroscopy, and low-energy electron diffraction. Room-temperature deposition of 40 Angstrom Al on Si(111)7X7 is mainly characterized by epitaxial growth and a decrease of the Fermi-level position by 0.32 eV. Annealing at temperatures around 500 degrees C results in the formation of large Al clusters with a thin reacted film in between. Subsequent annealing up to 900 degrees C results in complete desorption of Al and a recovery of the original 7X7 surface structure, while a potential variation underneath the Si surface, completely different from that of the starting surface, is observed in the Si 2p photoemission spectra. These results strongly indicate the formation of a p(+)-Si layer induced by Al diffusion.