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 surf ace reflectivity in the extreme ultraviolet region (6-36 nn): Over a l arge range of coverage, the reflectivity is well described by a film c onsisting of a continuous slab with an effective refractive index calc ulated for an inhomogeneous layer of identical ellipsoids. This model gives good agreement with experimental measurements for coverages up t o the percolation threshold. At very low coverages, modeling the film as an array of independent spheres, the extinction of which is calcula ted 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 continuou s films. In all cases, the metal islands are nearly spherical at inter mediate 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 45 3 K the growth mode changes to one consisting of a roughly,constant nu mber. of particles whose volume increases linearly with evaporation ti me. 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 chrom ium at the same temperature. Manganese growth at 423 K is more complex showing a change in growth mode as a function of coverage.