Microstructure and intrinsic stress evolution during epitaxial film growth of an Ag_(0.93)Al_(0.07) solid solution on Si(111); excessive planar faulting due to quantum confinement

摘要

The correlation of microstructural development and the kinetics of film growth has been investigated during the epitaxial film growth of an ultrathin binary Ago.93Alo.07 solid solution on a Si(111)-7×7 surface at 300K by the combination of high-resolution transmission electron microscopy, X-ray diffraction, scanning tunneling microscopy, low energy electron diffraction, and realtime in-situ stress measurements. Up to a film thickness of 6 ± 2 nm, epitaxial Ago.93Alo.o7 film growth is characterized by the strikingly extensive formation of planar faults parallel to the film/ substrate interface, while at larger thickness the film grows practically defect-free. As revealed by real-time in-situ stress measurements, the extensive formation of planar faults at the very initial stage of growth is not driven by the reduction of the system's elastic strain energy but is rather caused by a striking thickness-dependence of the stacking-fault energy owing to a quantum size effect of the ultrathin metal alloy film, resulting in a frequent succession of fcc and hcp stackings of close-packed layers during the initial stage of film growth. The extensive development of planar faults at the initial stage of film growth (<6 ± 2nm) is associated with the occurrence of a high density of kinks and corners at thereby atomically rough surface ledges, which strongly enhances the downward transport of adatoms from higher to lower terraces (interlayer mass transport) by a reduction of the effective diffusion barrier at the edge of surface steps and by increasing the driving force for adatoms to attach to the surface ledges. As a result, the epitaxial Ago.93Alo.o7 film initially grows in a 2D layer-by-layer type of growth and thus establishes atomically smooth film surfaces. For the practically planar-fault-free growth at thicknesses beyond 6 ± 2 nm, interlayer mass transport becomes distinctively limited, thereby inducing a transition from 2D to 3D type of film growth.