Oriented growth of ultrathin tungsten films on sapphire substrates

Thin layers of tungsten∼30 and 300 Å thick were grown by electron-beam evaporation on (11̄02) sapphire substrates in a vacuum better than 10−8 Torr at a substrate temperature of 450 °C. After deposition, the films were characterized by x-ray diffraction in three ways: conventional Bragg diffraction, small-angle interferences, and grazing-incidence x-ray diffraction. In spite of the large lattice mismatch (∼10.5%) and the low deposition temperature, a well-oriented deposit with the orientation (001)W tilted ∼5° from (11̄02)Al2O3 towards (0001), and [110]W∥[112̄0]Al2O3 was obtained. Films with thicknesses ≳30 Å showed excellent planeness and smoothness. Films less than 30 Å thick are not continuous but consist of separate, oriented islands. In the films ≲30 Å thick, a small tensile strain of ≲0.6% was observed parallel to the interface. No strain was observed in the ∼300-Å-thick film.