GE DELTA-LAYER IN SI(100) CHARACTERIZED BY X-RAY REFLECTIVITY, GRAZING-INCIDENCE DIFFRACTION AND STANDING-WAVE MEASUREMENTS

We report on the structural characterization of a Si-capped MBE-grown Ge delta layer of monolayer thickness on Si (100). We combined X-ray s tanding wave, X-ray reflectivity and grazing incidence diffraction mea surements. X-ray standing wave measurements provide a precise, model-i ndependent determination of the atomic positions of the Ge atoms in th e delta layer. Results for the (400) and (220) Si reflections give coh erent positions of 1.05+/-0.02 and 1.03+/-0.02, respectively. The cohe rent fraction is 0.89+/-0.02 in both cases, indicating that Ge is plac ed on substitutional sites with a lattice expansion perpendicular to t he substrate surface, consistent viith pseudomorphic growth. This meas ured lattice expansion has been used to estimate a Ge content of less than or equal to 64% for the delta layer. X-ray reflectivity measureme nts in the vicinity of the critical angle of total reflection Fender a cap layer thickness of 164+/-5 Angstrom and a surface roughness of 9/-1 Angstrom Grazing incidence diffraction measurements (GID) at the ( 022) in-plane reflection show intensity oscillations along the diffrac tion rod. Model calculations using kinematic scattering theory give la yer thicknesses and roughnesses consistent with the reflectivity resul ts. Furthermore they confirm the preservation of the lateral lattice p arameter expected for pseudomorphic growth and are consistent with the delta-layer stoichiometry determined by XSW.