Perylene tetracarboxylic dianhydride (PTCDA) thin films were grown in several steps on tin disulfide (SnS2) single crystals and characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS), (UPS) in order to characterize the frontier orbital line-up and the interface dipole at their interface. Due to the large difference between the work functions of PTCDA (4.26 eV) and SnS2 (5.09 eV) this experiment represents a model system for the investigation of band bending related phenomena in organic semiconductor heterojunctions. Our results show that the equilibration between the Fermi levels of both materials in contact is achieved almost solely by band bending (bulk charge redistribution) in the PTCDA layer. No significant interface dipole was detected which means that the PTCDA molecular orbitals and the SnS2 bands align at the vacuum level corresponding to the electron affinity rule. Our experiments clearly demonstrate the importance of an additional XPS measurement which (in most cases) allow the measurement of band bending with much higher accuracy than could be achieved in experiments carried out by UPS alone. These experiments also show that, due to the different depth sensitivity of high binding energy cutoff (secondary edge) and XPS core levels (or UPS valence bands), it is very important to grow relatively thick overlayers in order to measure orbital alignment and interface dipole correctly.

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Articles you may be interested inNoncontact-mode scanning capacitance force microscopy towards quantitative two-dimensional carrier profiling on semiconductor devices Appl. Phys. Lett. 90, 083101 (2007);Tapping mode atomic force microscopy with applied bias was used to spatially resolve areas of different doping type on Si wafers patterned with photolithography and subsequent ion implantation. The application of a direct current bias between cantilever and sample during the measurement produces Coulomb ͑electrostatic͒ forces, whose magnitude depends on the spatial variation of the doping density. This effect was utilized to detect areas of different doping type by monitoring the phase angle between the driving frequency and the cantilever response while scanning areas of different doping density. In this article we present a series of measurements at various bias voltages demonstrating that the observed phase contrast between differently doped areas is directly connected to the bias induced surface potential ͑band bending͒ present on these areas. To investigate the contrast mechanism quantitatively, we also measured deflection ͑force͒, amplitude and phase versus distance curves for a typical cantilever with an applied bias on a gold thin film. This allowed correlation between phase contrast observed and the actual Coulomb force measured.

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