Growth and characterization of thin PTCDA films on semiconducting surfaces

The growth of ordered thin films of organic semiconductors on inorganic semiconductor surfaces is of great interest owing the to potential for developing hybrid organic-inorganic heterostructures for novel electronic and optoelectronic device applications. One fundamental aspect is the interaction at the interface of the organic and inorganic semiconductor which may affect the molecular ordering. The planar perylene derivative 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) has been widely used as a model material in studies of growth of semiconducting organic thin films. Thus, the growth of PTCDA and its interactions with different semiconducting substrates has been investigated. Due to the fact that dangling bonds at the surface of inorganic semiconductors strongly affect the molecule-substrate interaction, two different semiconductor substrate systems were investigated: passivated Si(111) and unpassivated GaN(0001). As surface passivating agents silver and bismuth were used resulting in Si(111)sqrt(3)×sqrt(3) R-30°-Ag and Si(111)sqrt(3)×sqrt(3) R-30°-Bi. To investigate the growth of PTCDA on MOVPE-grown GaN a highly clean semiconductor surface is required. Therefore cleaning possibilities of GaN(0001) and GaN(-2110) surfaces were investigated. Two different two-step cleaning methods were developed and compared. Both processes showed an efficient removal of carbon and oxygen species as quantified by X-ray photoelectron spectroscopy (XPS). Furthermore, scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) investigations showed a drastic improvement of the morphology and atomic structure of the clean surface. Spot profile analyzing LEED (SPA-LEED) and STM results revealed that PTCDA grows in a bulk-like configuration (herringbone structure) on both passivated and unpassivated surfaces. Furthermore near-edge X-ray absorption fine-structure (NEXFAS) investigations were carried out and showed that the molecules are lying flat on both substrates, which is in good agreement with the observed herringbone structure. On the Si(111)- passivated surface a preferred ordering relative to the substrate lattice has been obser- ved. In contrast, on the unpassivated GaN(0001) surface rotational domains of arbitrary azimuthal orientations were identified. The different adsorption behaviour has been explained by different molecule-substrate interactions. XPS indicates a charge transfer from the carbonyl oxygen atoms of the anhydride group to the Si(111)sqrt(3)×sqrt(3) R-30°-Bi in the first monolayer while for PTCDA/GaN(0001) no strong interactions between the molecules and the substrate were identified. This distinct interaction also affects the growth mode of the molecules; a layer-by-layer growth mode was observed for PTCDA on Si(111)sqrt(3)×sqrt(3) R-30°-Bi and an island growth mode for the PTCDA/GaN(0001) system