Strain adjustment with thin virtual substrates

摘要

Virtual substrates contain standard Si substrates with overgrown SiGe strain-relaxed buffers. They are particularly suitable for the strain adjustment in Si-based heterostructures. A special interest in virtual substrates is connected with their applications in MOSFETs. For this task, thin, highly relaxed SiGe buffer layers with a high Ge content are in demand. To grow such layers, we have developed a method employing point defects during molecular beam epitaxy (MBE). Point defects further the early relaxation and can improve the layer quality. For the generation of point defects, a very low temperature (130-165℃) in the first growth stage has been used. In the second stage, at a conventional MBE temperature (550℃), point defects coalesce to dislocation loops. In situ growth monitoring allows the control of the main stages of layer formation. These observations allowed us to find a process window in which a high degree of relaxation and good surface morphology are achieved in ultra-thin (40-80 nm) layers with 25-55% Ge. It is shown that the degree of relaxation in SiGe buffer layers grown by our method is tunable by the value of the low growth temperature used for the first growth stage. Layer characterization is performed by Raman spectroscopy, X-ray diffraction, optical, atomic force, and transmission electron microscopy. Device examples demonstrate a high potential of these virtual substrates in high frequency devices.