On the formation mechanism of epitaxial Ge islands on partially relaxed SiGe buffer layers

Abstract

We describe the morphological evolution of Ge epitaxial films grown on partially relaxed SiGe buffer layers. Three different types of surface sites exist on the buffer layer surface when the spacing between the buried dislocations is significantly larger than the surface diffusion length of Ge adatoms. The three types of nucleation sites include those over dislocation intersections, those over single dislocation lines, and those far away from dislocations. Epitaxial growth of Ge on such a relaxed buffer under near equilibrium condition exhibits three-stage nucleation and growth of Ge islands, also known as self-assembled quantum dots (SAQD). Denuded zones exist next to the buried dislocation lines. The temperature dependence of the denuded zone width together with the average spacing between SAQDs provides sufficient information for determining both the activation energy and the pre-exponential factor of surface diffusion of Ge adatoms. SAQDs located at the three sites are strained differently as indicated by the distinct difference in the critical dot size for pyramid-to-dome transition. The experimental observation can be explained by assuming the energy barrier for the surface diffusion of Ge adatoms increases with decreasing misfit strain as predicted by theory [van de Walle et al., Phys Rev. B 67, R41308 (2003)]. Calculations of Ge adatom concentration profile based on Fick’s second law of diffusion does predict the presence of denuded zones with their widths being dependent on the diffusion coefficient; i.e., on the substrate temperature. The observed morphological evolution of Ge epitaxial films supports Ge island nucleation and growth as the SAQD formation mechanism as opposed to strain-induced surface instability.