Strain effects on the thermal stability of SiGe/Si heterostructures

Abstract

Strain effects on thermal stability of SiGe/Si heterostructures were studied by theoretical and experimental method. All $Si_{1-x}Ge_{x}/Si$ heterostructures (0.2 ＞ x) in this study were deposited by reduced pressure chemical vapor deposition at 650℃, and various measurement methods, such as transmission electron microscopy, medium energy ion scattering spectroscopy, secondary ion mass spectroscopy, and high-resolution x-ray diffractometer, were used to characterize the strain effects on the thermal stability of the SiGe/Si heterostructures. As explained below, the main result in this study is related with the enhanced diffusion by strain effect, i.e., strain-induced diffusion. By high-resolution transmission electron microscopy measurement of interface migration in chapter 3, by medium energy ion scattering spectroscopy measurement of anisotropic diffusion of Ge in chapter 4, and by secondary ion mass spectroscopy measurement of post-annealing effect in oxidized SiGe layers in chapter 6, it was confirmed that strain-induced diffusion is a main strain relaxation mechanism in SiGe/Si heterostructures under thermal budget. As well as strain-induced diffusion, other strain relaxation mechanism, such as defect formation and free surface effect, was discussed in chapter 5, 7, and 8. Furthermore, a method for dry thermal oxidation of strained SiGe layer was proposed in chapter 6. Firstly, a theoretical model of the strain effect on the diffusion in SiGe/Si heterostructures was proposed, and a strain-induced diffusivity was formulated by defining a strain potential gradient. This model proposed that the strain potential gradient is also an additional driving force for the diffusion in the structure, as well as concentration gradient, and thereby the apparent diffusion in this strained heterostructure is significantly enhanced by strain-induced diffusion. The contribution of the reduced strain energy to the diffusion in the structure was defined by a diffusion enhanc...