INVESTIGATION OF STRAIN-SYMMETRIZED AND PSEUDOMORPHIC SIMGEN SUPERLATTICES BY X-RAY RECIPROCAL SPACE MAPPING

Double-crystal and triple-axis x-ray diffractometry was used to characterize in detail the strain and composition of short period Si6Ge4, Si8Ge8, Si9Ge6, and Si17Ge2 strained-layer superlattices (SLSs), grown by molecular-beam epitaxy. Nominally strain-symmetrized superlattices, intended to be free standing from underlying buffer layers and the substrate, grown on rather thin (20 nm thick) SiGe alloy buffers with constant Ge content (Si6Ge4 and Si8Ge8) are compared to those grown on 1.3-mu m-thick step-graded SiGe alloy buffers (Si6Ge4 and Si9Ge6). Due to the much higher instrumental resolution offered by triple-axis diffractometry (Delta 2 theta=12 arcsec) buffer and SLS peaks are clearly separated from each other, which overlap in corresponding double-crystal-diffractometry measurements (Delta 2 theta in the range of 180 arcsec to 2 degrees). The lattice constants parallel and perpendicular to the [001] growth direction are determined independently from each other and thus precise strain data of the buffers and the SLS constituting layers were extracted from two-dimensional reciprocal space maps around (004) and (224) reciprocal lattice points (RELPs). The only fitting parameter in a dynamical simulation of conventional rocking curves is then the relative thickness ratio of the Si and Ge layers in the superlattice. The strain relaxation process and the principle of tailoring the strain status in the electronically active layers are shown to be different in structures with single-step and step-graded buffers. For these superlattice samples the RELPs originating from the zeroth-order SLS reflection show significant mosaic broadening (full width at half-maximum of 1100-1300 arcsec perpendicular to the omega/2 theta scan direction in reciprocal space). In contrast, the corresponding RELPs from a pseudomorphic SLS with a much higher average Si content (Si17Ge2), but consisting just of 10 periods, which was grown directly on a Si buffer layer deposited on (001)-oriented Si substrate, are not broadened; (full width at half-maximum of 14 arcsec). Besides the determination of strain and composition, examples for the interpretation of diffuse x-ray scattering are given, conveniently measured by reciprocal space mapping.