LOW-ANGLE X-RAY REFLECTION STUDY OF ULTRATHIN GE FILMS ON (100) SI

We report a glancing incidence x-ray reflection study of ultrathin (1-12 monolayers) Ge films grown on (100) Si by molecular beam epitaxy. Thin buried Ge layers were characterized by analyzing intensity oscillations arising from the interference of the x rays reflected at the Si/Ge interface and at the surface of a Si capping layer. Strong interference fringes were detected even for a 1-monolayer-thick Ge film. The intensity of the fringes was found to increase with the Ge thickness up to 6 monolayers. A damping of the fringes was observed for a thicker film, suggesting a breakdown of two-dimensional growth. This interpretation was corroborated by a transmission electron microscopy investigation which showed evidence of three-dimensional growth only in the 12 monolayer film.