Growth and atomic structure of chromium overlayers on W(110) and W(100)

The growth, structure and thermal stability of chromium overlayers vapor-deposited onto W(110) and W(100) substrates have been studied using Auger electron spectroscopy, temperature programmed desorption, work function measurements and low energy electron diffraction. Layer-by-layer growth is observed on both substrates in the 120–400 K temperature range, despite a 9% Cr : W bulk lattice mismatch. On W(110), a pseudomorphic (1 × 1) Cr monolayer (ML) forms at 100 K and remains stable up to desorption at 1290 K. A metastable surface structure of (2 × 2) symmetry is observed for ∼2 ML Cr W(110) in the 500 < T < 800 K range, for which a quad-cluster model is proposed. Cr coverages above ~2 ML are thermally unstable above 400 K and form three-dimensional clusters which co-exist with the pseudomorphic (1 × 1) monolayer and (2 × 2) bilayer. An (11 × 11) coincidence lattice is observed for multilayer films confirming the epitaxial Cr(110):W(110) relationship reported in earlier field emission microscopic studies. On W(100), the clean surface (2 × 2) structure reverts to (1 × 1) symmetry with the formation of a pseudomorphic Cr monolayer, which remains stable up to desorption at 1400 K. A 2 ML Cr W(100) overlayer exhibits no long range order but remains stable up to 1100 K, while thicker films are thermally unstable above 500 K, forming three-dimensional clusters on top of the pseudomorphic Cr W(100) monolayer.