The understanding of self–assembly and bonding mechanisms of organic molecules on semiconductor surfaces represents a central research aspect in the investigation of novel organic/inorganic interfaces and their technological applicability. Here, we investigated the adsorption and bond formation of cyclopentene and 1,4‐cyclohexadiene on a GaAs(001)‐ c(4 × 4) surface in order to clarify the influence of the number of intra‐molecular C=C double bonds on the respective adsorption sites. For a determination of the adsorption configuration, the interfaces were characterized electronically and optically by synchrotron based X‐ray photoelectron spectroscopy (SXPS), low energy electron diffraction (LEED) and reflectance anisotropy spectroscopy (RAS). The results reveal significantly different adsorption configurations for the two molecules. While cyclopentene bonds with a single covalent bond to the surface, 1,4‐cyclohexadiene adsorbs onto the surface by the formation of multiple covalent bonds, e.g. bridge bonds. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Up to now most of the experimental work regarding the adsorption of organic molecules has been concerned with silicon. Here we study the interface formation on a III-V-semiconductor, GaAs(001). We show that reflectance anisotropy spectroscopy (RAS) is a sensitive technique for investigating the interface formation between organic molecules and semiconductor surfaces. With RAS it is possible to determine the surface reconstruction and the structural changes at the interface during the deposition of organic molecules. These changes and the underlying adsorption process are discussed here for the adsorption of cyclopentene on GaAs(001)c(4x4), (2x4) and (4x2)

We have studied the interface formation between cyclopentene and ͑2 ϫ 4͒ reconstructed InP ͑001͒ surfaces by soft x-ray photoemission spectroscopy, reflectance anisotropy spectroscopy ͑RAS͒, and ab initio theory. After preparation of an uncontaminated ͑2 ϫ 4͒ reconstruction under ultrahigh vacuum conditions, the surface was exposed to cyclopentene as monitored by RAS. The changes in the In 4d, P 2p, and C 1s core-level emission lines upon molecule adsorption indicate a covalent bonding of cyclopentene to the topmost atoms of the surface at two different bonding sites. Based on these results, a structure model is suggested, which is supported by ab initio calculations of the total-energy, the RAS signature, and the In 4d and P 2p core-level shifts. Our results suggest that the cyclopentene adsorption is a two-step process: first cyclopentene adsorbs on the "mixed dimer" and second the changes in the surface structure enable the additional adsorption on the second layer In-In surface bond.

In the last couple of years there has been much methodological and computational progress in the modelling of optical properties from first principles. Reflectance anisotropy spectra (RAS) can now be calculated with true predictive power and can thus be used to draw conclusions directly on the surface geometry. In the present work we study two potentially very interesting applications for RAS: the oxidation of Si(001) and the functionalization of the Si surface with organic molecules. Our calculations confirm experimental indications that the polarity of the interface-induced optical anisotropy is reversed layer by layer with increasing oxide thickness. The oscillation of the RAS amplitude should thus allow for the quantitative monitoring of the vertical progression of the oxidation. Our results for Si(001) surfaces modified by cyclopentene and 9,10-phenanthrenequinone adsorption show a strong sensitivity of the RAS signal with respect to the adsorption geometry. Comparison with experimental data shows that cyclopentene most probably adsorbs via a cycloaddition reaction with the Si surface dimers, while phenanthrenequinone seems to adsorb across two Si dimers. 1. Modelling of reflectance anisotropy Optical spectroscopies are extremely valuable for in situ, non-destructive and real-time surface monitoring under challenging conditions as may be encountered, e.g., during epitaxial

We have investigated the modification of the surface electric field (SEF) of the GaAs (001)-c(4 Â 4) surface during the adsorption of cyclopentene and 1,4-cyclohexadiene molecules by reflectance anisotropy spectroscopy (RAS) in the spectral range from 1.5 to 5.0 eV. At around 3 eV the RAS line shape originates from the so-called linear electro-optic effect (LEO). The amplitude of the LEO oscillation scales linearly with the SEF within the RAS penetration depth and is thus a measure for it. For the separation of the LEO from the RAS spectra several different methods are described in the literature. Here, we present a modified method which allows the LEO separation in particular for interfaces between GaAs and organic molecules. The results obtained this way show a significant increase of the LEO effect upon molecule adsorption indicating a higher SEF. The observed changes of the surface electronic properties are probably related to a modification of surface strain upon molecule adsorption.

We have investigated the adsorption of lead phthalocyanine (PbPc) layers on GaAs(001)-c(4 × 4) and (2 × 4) reconstructed surfaces. Samples with different PbPc coverages from submonolayers up to ≈20-nm-thick layers were prepared under ultra-high vacuum conditions and investigated by scanning tunneling microscopy (STM) and reflectance anisotropy spectroscopy (RAS). The STM measurements showed different adsorption geometries of the PbPc molecules on the different reconstructions. The RAS results revealed that these different adsorption geometries in the first monolayer induce different molecular arrangements within thicker adsorbed layers on the two different substrates. These results give strong evidence for an epitaxial-like growth mode of PbPc molecules on GaAs(001) surfaces driven by the atomic arrangement of the GaAs surface. We could also demonstrate RAS as a powerful tool to analyze the growth behavior of thin organic layers.

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