Epitaxial growth and phase transition in multilayers of the organic semiconductor PTCDA on InAs(001)

We study the initial stages of growth of the organic molecular semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on In-terminated InAs(001) using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The first 1-2 monolayers (ML) of PTCDA interact relatively strongly with the surface and form a two-dimensional (2D) overlayer with a periodicity similar to that of the original (4 x 2)/c(8 x 2) reconstruction. STM shows that molecular ordering is due to two preferential alignments of the molecules on the substrate which depend upon the molecular orbital symmetry and the atomic-scale substrate potential. A phase transition occurs for coverages greater than similar to 2 ML where bulk-like three-dimensional (3D) PTCDA clusters begin to form with a unique orientation with respect to the substrate. The film orientation results from the alignment of high-symmetry planes of the 3D PTCDA clusters to high-symmetry planes of the 2D PTCDA overlayer. This partially commensurate epitaxial growth supports previously formulated models of ''point-on-line coincidence'' and is apparently a general phenomenon of some weakly interacting molecular interfaces. The present example of PTCDA grown on InAs(001) is of interest due to the passivating nature of the 2D overlayer phase which forms a weakly interacting template for further bulk-like PTCDA growth in the Stranski-Krastanov mode.