Kim Sk

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 A) exhibit a well-developed phase. QLEED finds the bulk of a 25 A film to have a body-centred tetragonal structure with a = 2.864 A and c = 4.28 A. 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 A. On Al{110}, the films are found to grow to a thickness of only 5 to 6 A, after which the LEED pattern is almost obliterated. A 2 A film is shown to be a pseudomorphic overlayer at a distance of 1.58 A 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.

Low-energy electron diffraction study of the growth of ultrathin films of face-centred cubic Co on Rh(001)

The epitaxial growth of Co on Rh(001) at room temperature is studied by means of quantitative low-energy electron diffraction and Auger electron spectroscopy. The Co films are found to grow pseudomorphic to the substrate in the layer-by-layer mode for at least two layers. The interlayer spacing between the monolayer and the substrate is 1.77 +or- 0.03 AA. The first and second interlayer spacings for a bilayer film are 1.60 +or- 0.04 and 1.75 +or- 0.04 AA, respectively. Strained pseudomorphic films of up to 10 layers could be grown with a bulk lattice spacing of 1.60 +or- 0.03 AA and a 3.1%-expanded first interlayer spacing (1.65 +or- 0.03 AA. An elastic strain analysis shows that the equilibrium (unstrained) phase of these Co films is face-centred cubic: the bulk interlayer spacing is contracted by 9.7% with respect to the equilibrium value (1.772 AA) as a consequence of the large positive planar epitaxial strain produced by the 7.3% lattice mismatch between FCC Co and Rh.

Disorder and epitaxy: Cr on Al{001}

Deposition of small amounts (about 12 of a monolayer) of Cr onto a clean Al{001} surface completely destroys the long-range order in the top 4 or 5 layers of the substrate, as shown by the absence of low-energy electron diffraction (LEED) spots. However, prolonged deposition of Cr produces films which are epitaxial and probably also pseudomorphic with the Al substrate as indicated by quantitative LEED analysis. This observation shows that long-range order of the substrate is not necessary for epitaxial growth of ultrathin metal films.