HIGH-RESOLUTION X-RAY-SCATTERING STUDY OF THE STRUCTURE OF NIOBIUM THIN-FILMS ON SAPPHIRE

The structure of two thin films of Nb grown epitaxially on sapphire ha s been studied using high-resolution x-ray-scattering techniques. The films, of nominal thickness 400 and 150 angstrom, were grown using mol ecular-beam epitaxy on a (1120BAR) sapphire substrate. They were of ex ceptionally high quality, with rocking curves of almost-equal-to 0.004 -degrees for the (110)Nb Bragg peak. The x-ray scattering observed in transverse scans of the wave-vector transfer through the (110)Nb Bragg peak exhibits two other unusual features. First, weak satellite Bragg peaks are observed. These are shown to arise from a regular sinusoida l distortion of the Nb thin film. Using a frozen phonon model for this distortion, we relate the position of the satellites to the length of a terrace on the vicinal sapphire surface, and the intensity of the s atellites to the lattice mismatch between the two materials in the sur face normal direction. Second, in addition to the sharp Bragg peaks, t here is a broader, diffuse component. This is isotropic in the plane o f the film and has a Lorentzian-squared line shape. The diffuse compon ent is shown to arise from a random network of two-dimensional Nb doma ins, with the size of domain related to the in-plane lattice mismatch. By combining these results with x-ray-reflectivity measurements from the thin films, and X-ray-reflectivity and crystal-truncation-rod meas urements from the bare substrates, we develop a detailed model of the structure of Nb films on sapphire.