Step-induced anisotropic growth of pentacene thin film crystals on a hydrogen-terminated Si(111) surface

We investigate thin film crystals of an organic semiconductor, pentacene (Pn), grown on hydrogen-terminated Si(111) (H-Si(111)) surfaces with various interstep distances to elucidate the effects of vicinal steps of H-Si(111) on the growth of the Pn layers. By observing the morphology of the thin films with atomic force microscopy, we conclude that the vicinal steps induce significant anisotropy in the growth of the first layers of Pn: dendritic branches evolve in a lower-terrace side (the Si[11 (2) over bar] direction), but a compact shape appears in an upper-terrace side (the Si[(1) over bar(1) over bar2] direction), although the first layers grow in an isotropic shape on a flat H-Si(111) surface. Furthermore, the growth of the first layers is much faster in the lower-terrace-side direction than in the upper-terrace-side direction. The anisotropy of the growth increases with decreasing interstep distances of H-Si(111), in particular <= 10 nm. Since such anisotropic growth was observed in a similar way irrespective of the directions of an incident Pn molecular beam, we conclude that the cause of the anisotropic growth is not the anisotropy of the surface diffusion and admolecule distribution of Pn molecules. Under the substrate temperature of 30-90 degrees C, the degree of the anisotropy remained constant within experimental error.