EVOLUTION OF QUASI-EPITAXIAL GROWTH OF A CRYSTALLINE ORGANIC SEMICONDUCTOR ON GRAPHITE

We have studied the evolution of the growth of a crystalline organic semiconductor thin film using reflection high energy electron diffraction (RHEED). Our results indicate that highly ordered crystalline films of an organic compound; namely 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA), can be grown on graphite by the ultrahigh vacuum process of organic molecular beam deposition even though the crystal structures of the two materials are highly mismatched. The RHEED patterns show the evolution of planar crystal growth from 2 monolayer (approximately 6 angstrom) coverage until at least 50 angstrom as the films are deposited onto substrates cooled to 100 K. At larger film thicknesses, somewhat nonplanar but still crystalline growth occurs. Furthermore, crystalline thin films were obtained independent of growth rate, which was as high as 3.0 angstrom/s. To our knowledge, this is the first direct experimental evidence of the evolution of growth from monolayer to bulk film coverage into an ordered, quasi-epitaxial structure.