  Square lattice gas models for heteroepitaxial growth are studied by means of
kinetic Monte Carlo simulations, in order to find a possible origin of
anisotropic island shape observed in growth experiments of long organic
molecules. When deposited molecules form clusters irreversibly at their
encounter during surface diffusion, islands grow in a ramified dendritic shape,
similar to DLA. Introduction of molecular detachment from edges makes islands
compact with smooth edges. Tilting of adsorbed long molecules or steps in a
vicinal substrate may induce orientation-dependence in the detachment rate of
edge molecules from an island. In simulations with orientation-dependent
detachment rates, a clear anisotropy in an island shape is observed. Shape
anisotropy on a vicinal substrate is enhanced as steps get dense, in agreement
to the experimental observation.
