Generic role of the anisotropic surface free energy on the morphological evolution in a strained-heteroepitaxial solid droplet on a rigid substrate

摘要

A systematic study based on the self-consistent dynamical simulations ispresented for the spontaneous evolution of an isolated thin solid droplet on arigid substrate, which is driven by the surface drift diffusion induced by theanisotropic capillary forces (surface stiffness) and mismatch stresses. In thiswork, we studied the affect of surface free energy anisotropies on thedevelopment kinetics of the 'Stranski-Krastanow' island type morphology. Theanisotropic surface free energy and the surface stiffness were treated withwell accepted trigonometric functions. Although, various tilt angles andanisotropy constants were considered during simulations, the main emphasis wasgiven on the effect of rotational symmetries associated with the surfaceHelmholtz free energy topography in 2D space. Our computer simulations revealedthe formation of an extremely thin wetting layer during the development of thebell-shaped Stranski-Krastanow island through the mass accumulation at thecentral region of the droplet via surface drift-diffusion. In the strong(anomalous) anisotropy constant domain, we demonstrated the existence of twodistinct morphological modes: i) the complete stability of the initialCosine-shaped droplet just above a certain anisotropy constant threshold levelby spontaneous slight readjustments of the base and the height of the cluster;ii) the Frank-van der Merwe mode of thin film formation for very large valuesof the anisotropy constant by the spreading and coalescence of the dropletsover the substrate surface. During the course of the simulations, we havecontinuously tracked both the morphology (i.e., the peak height, the extensionof the wetting layer beyond the domain boundaries, and the triple junctioncontact angle) and energetic (the global Helmholtz free energy changesassociated with the total strain and surface energy variations) of the system.