Application of Grazing-Angle X-ray Standing Waves to the In-Plane Structure of a 100Å-Thick Epilayer Film

The grazing-angle X-ray standing-wave technique is applied to a 100 Å-thick Ca_0.39Sr_0.61F₂ epilayer film on a GaAs(111) substrate. Experimental data collected with the (520) Bragg planes are explained by calculations taking into account dynamical diffraction in the thin film, but not by a homogeneous medium, only refracting and absorbing X-rays on the substrate. In this grazing-angle geometry, both X-ray penetration and extinction length are much smaller than in the conventional X-ray standing-wave geometry where epilayer diffraction does not significantly disturb the X-ray field produced by the bulk substrate. The results show that the epilayer (Ca, Sr)-atom planes perpendicular to the interface have laterally coincident positions with the (220) atomic planes of the substrate GaAs within an experimental uncertainty of 2% of the lattice spacing. The coherent fraction of 66% observed for the epilayer suggests a disordered Sr-atom distribution in the in-plane direction, which is ascribed to combined effects of thermal vibration, interstitial atoms and interfacial dislocations. It is the first time, to the authors' knowledge, that the grazing-angle standing-wave technique has been applied to the structure determination of an epilayer film.