Epitaxial growth and phase transition in multilayers of the organic semiconductor PTCDA on InAs(0 0 1)

We study the initial stages of growth of the organic molecular semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on In-terminated InAs(0 0 1) 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 × 2)/c(8 × 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 ∼ 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(0 0 1) 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.