Morphology of islanded transition-metal films on graphite studied by extreme-UV reflectivity

We report a study of the growth and morphology of islanded thin films of manganese, chromium, and vanadium on graphite substrates using surface reflectivity in the extreme ultraviolet region (6-36 nn): Over a large range of coverage, the reflectivity is well described by a film consisting of a continuous slab with an effective refractive index calculated for an inhomogeneous layer of identical ellipsoids. This model gives good agreement with experimental measurements for coverages up to the percolation threshold. At very low coverages, modeling the film as an array of independent spheres, the extinction of which is calculated using Mie theory, gives better results. The combination of models allows the film thickness, coverage, and average island size and shape to be determined from the earliest stages through to almost continuous films. In all cases, the metal islands are nearly spherical at intermediate coverages, becoming flattened with increasing coverage. In the case of chromium, at a substrate temperature of 423 K, the film grows as an increasing density of particles of constant size, whereas at 453 K the growth mode changes to one consisting of a roughly,constant number. of particles whose volume increases linearly with evaporation time. At 498 K the film consists of a large number of very small (1.2 nm) islands. The growth of vanadium at 423 K is similar to that of chromium at the same temperature. Manganese growth at 423 K is more complex showing a change in growth mode as a function of coverage.