Quantitative nanoscale characterization of undulated silicon-germanium/silicon(100) epitaxial thin films.

摘要

Heteroepitaxial thin films are important for the fabrication of advanced devices and as model systems for understanding fundamental materials phenomena. Depending on the growth conditions, film thickness, and extent of lattice mismatch at the film/substrate interface during growth, the epitaxial film will undergo morphological surface evolution to help release the stored elastic strain energy. For low-mismatched structures, a compressively-strained film often roughens and/or forms misfit dislocations after achieving a characteristic thickness. Using undulated Si1-xGex/Si(100) thin films, this work focuses on measurement of quantitative nanoscale variations of compositional and strain/stress fields within roughened heteroepitaxial films.; The research was initiated by calculating local and overall stresses for roughened films assuming a sinusoidal film geometry with finite element (FE) models. Chemical wet etching was then applied to experimentally determine the lateral variations of Ge composition within the undulated film. Additional FE modeling results indicated that such composition variations would contribute further overall stress reduction in the film. In-situ transmission electron microscopy (TEM) annealing experiments were performed to measure the dislocation propagation velocity in strained Si1-xGex films to explore the nanoscale correlations between stress variations and surface morphology. The local dislocation velocity data were translated into stresses using previously established stress-velocity relations developed for this system. The inferred stress variations correlated well spatially to the period of surface undulations. However, there was a large discrepancy in the stress variations between undulation peaks and troughs inferred from the local dislocation velocities compared to those calculated by FE models. A stress-dependent single-kink nucleation model was then successfully applied to reconcile these differences.; The detailed local configurations of threading segments on their glide planes were further studied via TEM observations and numerical simulations to explore whether stress variations within the film can be interpreted by studying local dislocation configurations. In TEM, the threading dislocations observed under weak beam dark field conditions had relatively complex nonlinear shapes. Possible changes of threading configuration at undulation peaks and troughs were also examined from series of weak beam images of a propagating misfit dislocation via in-situ TEM annealing. Equilibrium dislocation configurations on the glide plane in stressed thin films were studied using computer programs based on the energy equations for dislocations in isotropic crystals. However, experimental detection of changes in dislocation configurations as functions of varying stress were hampered by the rather slight changes in calculated shape.; Overall, this work establishes new methods for probing nanoscale stress and composition fields and dislocation configurations in thin films, and provides new insights into the interactions between these phenomena.