Ge δ layer in Si(100) characterized by X-ray reflectivity, grazing incidence diffraction and standing-wave measurements

Abstract

We report on the structural characterization of a Si-capped MBE-grown Ge δ layer of monolayer thickness on Si (100). We combined X-ray standing wave, X-ray reflectivity and grazing incidence diffraction measurements. X-ray standing wave measurements provide a precise, model-independent determination of the atomic positions of the Ge atoms in the δ layer. Results for the (400) and (220) Si reflections give coherent positions of 1.05±0.02 and 1.03±0.02, respectively. The coherent fraction is 0.89±0.02 in both cases, indicating that Ge is placed on substitutional sites with a lattice expansion perpendicular to the substrate surface, consistent with pseudomorphic growth. This measured lattice expansion has been used to estimate a Ge content of ≤64% for the δ layer. X-ray reflectivity measurements in the vicinity of the critical angle of total reflection render a cap layer thickness of 164±5 A and a surface roughness of 9±1 A.Grazing incidence diffraction measurements (GID) at the (022) in-plane reflection show intensity oscillations along the diffraction rod. Model calculations using kinematic scattering theory give layer thicknesses and roughnesses consistent with the reflectivity results. Furthermore they confirm the preservation of the lateral lattice parameter expected for pseudomorphic growth and are consistent with the δ-layer stoichiometry determined by XSW.