On flat and off-cut Si(1 1 1) substrates terminated with monohydride, thin film crystals of organic semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) were grown by molecular beam epitaxy (MBE) and were observed by atomic force microscopy (AFM). The thin film crystals on the hydrogen-terminated Si(1 1 1) surface (H-Si(1 1 1)) have a mesa-type island shape. On the flat H-Si(1 1 1) (average step distance >500 nm, off-angle <0.04°), with an increase in a thin film thickness, the size and number of the PTCDA islands were increased and decreased, respectively. These results show that the islands coalesced during the growth. After the coalescence of the islands, some islands became single crystals and the others became polycrystals. The ratio of the single crystal islands was estimated from the shapes of the islands. We found that the ratio of the single crystal islands after the coalescence was 48% and this value was much larger than a probability (1/6) estimated from an epitaxial relation. This result suggests that Ostwald ripening occurred in the PTCDA/H-Si(1 1 1) system. The PTCDA thin film crystals were also grown on the off-cut H-Si(1 1 1) (average step distance = 10 nm, off-angle = 1.8°). The number of the islands was much larger than that on the flat H-Si(1 1 1) and this result shows that the vicinal steps on the surface can work as effective nucleation sites. Coalescence of the islands was also observed on the off-cut H-Si(1 1 1). After the coalescence, the ratio of the single crystal islands on the off-cut H-Si(1 1 1) was 10% larger than that on the flat H-Si(1 1 1). This result shows that the orientation of the PTCDA islands can be controlled using the vicinal steps on the substrate surface.
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