Scanning tunnelling microscopy and spectroscopy on organic PTCDA films deposited on sulfur passivated GaAs(001)

Deposition of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on sulfur passivated gallium arsenide S-GaAs(001) surfaces is investigated by scanning tunnelling microscopy and scanning tunnelling spectroscopy. The surface morphology and the film structure are studied for the multilayer growth of the organic molecules. Spectroscopic results for both clean substrate and ordered areas of PTCDA are shown in this work. We have measured I-V plots at different tip-sample distances, avoiding deformation of the organic layer. Under proper experimental conditions, a gap value of 2.2 eV has been measured on PTCDA crystals, in good agreement with the expected value for PTCDA (2.2-2.55 eV).