Strain relaxation process and evolution of crystalline morphologies during the growths of SiGe on Si(110) by solid-source molecular beam epitaxy

In a previous study, we reported on a (110)-oriented strained Si pMOSFET with an effective hole mobility as high as 480 cm2/Vs. To impose the lattice strain on the Si channel layer, strain-relaxed SiGe buffer layer is utilized. Defect formation is requisite for strain relaxation in SiGe layer, and at the same time, has the adverse effects for electrical properties of the strained Si film. Therefore, it is necessary to control the formation of the crystalline defects. The aim of this study is to investigate the evolution of the crystalline morphology during the growth of SiGe layers by solid source molecular beam epitaxy, which is utilized to optimize the structural parameters of the strain-relaxed SiGe buffer layers. It is revealed in this study that the lattice strain in SiGe formed on Si(110) begin to relax at a lower film thickness than in the case of SiGe on Si(001). Orientation dependences of surface morphology and degree of strain relaxation are found to be dependent on Ge composition. The development process of the growth twin is also discussed. It is shown that the growth twin generation is effectively suppressed by the composition grading technique.