Thin SiGe buffers with high Ge content for n-MOSFETs

Virtual substrates for n-MOSFET structures are grown by MBE with high Ge content of 0.25<x<0.5. Since thin strain relaxed SiGe buffers are of a special importance for this application sub-100 nm layer growth procedure is developed. Extremely low temperatures and Sb surfactants are implemented during the first growth stage of the SiGe buffers. The role of these factors in crystal structure formation is in situ monitored by reflectivity measurements. ‘Process windows’ for both methods are determined. The Sb pre-build up is found to increase the degree of relaxation and to contribute to the surface smoothness. Implementation of this method under precise temperature variation within 50 °C, allows us to regulate degrees of relaxation between values of 2 and 99%. SIMS, X-ray diffraction (XRD) and μ-Raman are employed for measurements of Ge content, composition profiles and degree of relaxation. Surface morphology of the layers is characterised by atomic force microscopy.