Growth of face-centred-cubic titanium on aluminium

Ultrathin films of Ti are grown on Al{001} and Al{110} substrates and studied by quantitative low-energy electron diffraction (QLEED). On Al{001}, high-coverage films (thicker than about 10 Å) exhibit a well-developed phase. QLEED finds the bulk of a 25 Å film to have a body-centred tetragonal structure with a = 2.864 Å and c = 4.28 Å. Strain analysis shows that this structure cannot be strained body-centred cubic and identifies the equilibrium (i.e., the unstrained) phase of the film grown as face-centred cubic (fcc). This result is a direct proof of the existence of an epitaxially stabilized fcc modification of Ti (which is not encountered in nature at any temperature), whose lattice constant is found to be 4.15 Å. On Al{110}, the films are found to grow to a thickness of only 5 to 6 Å, after which the LEED pattern is almost obliterated. A 2 Å film is shown to be a pseudomorphic overlayer at a distance of 1.58 Å from the substrate. On Al{001}, low-coverage Ti films exhibit a weak c( ) LEED pattern, but the associated surface structure could not be determined; partial results indicate that the Ti atoms may reside in the second layer of the substrate.