Micro-Raman investigations of the degree of relaxation in thin SiGe buffer layers with high Ge content

Abstract

Virtual substrates with high Ge content x of 0.25<x<0.5 for metal oxide semiconductor field effect transistor (nMOSFET) structures are grown by molecular beam epitaxy (MBE). Thin strain-relaxed SiGe buffers are of special importance for this application, therefore, sub-100 nm layer growth procedures have been developed. Micro-Raman spectroscopy has been extensively employed for measurements of Ge content and the degree of relaxation (r) in our virtual substrates. Two growth stages – a very low temperature (VLT) stage and overgrowth at 550 °C (OT stage) were used to provide the high relaxation in thin SiGe layers. Implementation of this process under careful temperature control within 50 °C during the VLT growth stage allows us to regulate precisely the degree of relaxation. The role of low temperatures and of Sb surfactants during the VLT growth stage for process-window widening is studied. The Sb pre-buildup is found to increase the degree of relaxation in a higher temperature range during the VLT stage. Optimum processing conditions are determined and high uniformity of composition and residual-strain distribution on virtual substrate wafers are demonstrated.