Low angle x-ray reflection study of ultrathin Ge films on (100) Si

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Abstract
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.