Alloy decomposition and surface instabilities in thin films

grantor: University of TorontoWe theoretically and numerically study the growth of thin alloy layers by slow deposition techniques. We propose microscopic and mesoscopic descriptions of the phenomena, from which composition modulations in the thin film emerge, coupled with surface undulations. We obtain excellent agreement between these two approaches and are able to predict analytically the wavelength of the modulation as a function of the deposition rate and the temperature, which is confirmed numerically. The important non-equilibrium nature of the growth process is shown to lead to the experimentally observed structures, the patterning of the film for device applications is discussed, and we calculate scattering functions that can be compared with experiments. We explore the effects of stress and strain on the composition modulations in lattice-matched films, when phase separation is the driving force for the composition inhomogeneities. It is found that the lamellar structures seen without elastic effects can become dynamically unstable to the formation of hexagonal structures, due to long-ranged elastic interactions. The hexagonal order is found to consist of elastically undeformed droplets of the hard component in an elastically deformed matrix of the soft component. The wavelength of the composition modulation is calculated numerically and is always smaller than the wavelength in the absence of elastic effects. We also discuss the effects of crystal anisotropy in our system. The stability of mismatched alloy layers is studied with emphasis on the non-equilibrium nature of the growth and the coupling between the surface morphology and the film composition. We consider static and growing films at temperatures and mean compositions inside and outside of the phase diagram miscibility gap. Stability diagram are presented in terms of the mismatch between the film and the substrate and the growth velocity. It is shown that the experimentally observed difference in stability between tensile and compressed films is due to composition-dependent elastic moduli.Ph.D