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Abstract

Organic molecular semiconductors have been the subject of many studies because of their promising properties for potential optoelectronic applications such as light-emitting diodes, lasers or waveguide-coupled detectors. Another not so well known application of organic semiconductors is the modification of surfaces and interfaces of classical semiconductor like GaAs. In this study thin layers of the organic molecule perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) have been used to tune the barrier height of the Ag/n-GaAs(100) Schottky contacts. The electronic characteristics of organic modified Schottky diodes depend on the bulk properties of the organic interlayer and the properties of the interfaces between the organic layer and the metal and the semiconductor, respectively. A characterization of this properties needs a combination of complementary experimental and theoretical tools. The structural bulk properties of the organic films may be characterized by analysing the full width at half maximum (FWHM) and polarization dependence of phonons recorded by Raman spectroscopy. Passivation of the semiconductor substrate and/or growing the organic films at higher substrate temperatures increases the structural order of the organic films. Internal vibrational modes provide information about chemical interactions at the interfaces. A charge transfer due a chemical interactions results in changes of the internal vibrational mode pattern. Chemical interactions at interfaces can also be determined by analysing core-levels recorded by photoemission spectroscopy (PES). For PTCDA molecules evaporated on GaAs(100) surfaces the chemical interaction is very low and the molecules adsorb preferably at defect sites on the semiconductor surfaces. This results in a passivation of the defects and a removal of inhomogenous Fermi-level pinning. In addition PES may be used to determine the energy level alignment at the interfaces and the development of band bending in the organic and inorganic semiconductor as a function of the organic film thickness, respectively. The alignment of energy levels at the interfaces gives information about the existence of barrier heights, which influence the injection of charge carriers. The current-voltage characteristics of Ag/PTCDA/n-GaAs(100) diodes can be described in terms of a model including thermionic emission and space-charge-limited currents. At low current densities over the organic/inorganic contact barrier dominates, while at high current densities space-charge effects govern the charge transport. The effective barrier heights of this organic modified diodes decrease with increasing organic film thickness.