Wachstum und Charakterisierung dünner PTCDA-Filme auf ...

Abstract
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) √ 3× √ 3 R-30 ◦ -Ag and Si(111) √ 3× √ 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 observed.
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) √ 3× √ 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) √ 3 × √ 3 R-30 ◦ -Bi and an island growth mode for the PTCDA/GaN(0001)
system.