Abstract
Surface of an off-cut Si(1 1 1) substrate with an average step distance of 100 Å was terminated with monohydride. On this substrate, thin film crystals of organic semiconductor PTCDA were grown by molecular beam epitaxy (MBE). Surface of the thin film crystals on the hydrogen-terminated Si(1 1 1) surface (H-Si(1 1 1)) were observed using ultra-high vacuum scanning tunneling microscopy (UHV-STM) and atomic force microscopy (AFM). The PTCDA thin film crystals had an island shape of Volmer–Weber type. From the direction of the vicinal steps of the substrate [1 1 ̄ 0] , we determined the size and orientation of the PTCDA thin film crystals on the H-Si(1 1 1). Long-axis of the two-dimensional (2D) unit cell of the thin film crystals matches the vector (6, 2) of the H-Si(1 1 1) surface. We proposed two conceivable epitaxial relations: one is 6 2 2/3 11/3 and the other is point-on-line coincidence. 2D unit cells of the thin film crystals have widely stretched structure (2–10%), and the island growth of Volmer–Weber type is probably due to this large lattice misfit.
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