Interface roughness in SiGe quantum-cascade structures from x-ray reflectivity studies.

We have investigated the structural properties of Si/SiGe electroluiminescent quantum-cascade structures, by means of x-ray reflectivity and diffraction. The cascade structures were grown at a comparatively low temperature of T = 350 °C to avoid misfit dislcoation formation. Despite an overall thickness of the cascade structures of about 9000 Å and Ge contents of up to more than 40% in some of the SiGe wells, the entire stack of layers is indeed pseudomorphic with respect to the Si substrate. The analysis of x-ray reflectivity data yields a rather small rms interface roughness, which increases only slightly from 2 to 2.9 Å from the bottom to the top of the cascade structures. From x-ray reflectivity maps we obtain in addition the vertical and lateral correlation of the interface roughness. It turns out that for long range interface fluctuations the vertical correlation length is larger than for short range fluctuations. [ABSTRACT FROM AUTHOR]