Influence of deposition temperature on the structure of 3,4,9,10-perylene tetracarboxylic dianhydride thin films on H-passivated silicon probed by Raman spectroscopy

Raman spectroscopy was used to characterize the structural order in thin organic films of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). Films of the same average thickness were grown by organic molecular beam deposition on hydrogen-passivated p-type silicon (100) substrates at different growth temperatures between 230 and 470 K. The Raman spectra of all samples exhibit four external vibrational modes, that occur as a consequence of the arrangement of the PTCDA molecules in a crystalline environment. The full width at half maximum of these phonon lines decreases with increasing temperature of the substrate during deposition. A similar tendency is also observed for the Raman-active internal molecular modes. In addition, with increasing deposition temperature the diffusely scattered light background in the Raman spectra increases, as well as the photoluminescence background in the high frequency range. We relate the observed spectral changes to an increase in the size of the crystalline domains within the films with increasing deposition temperature, an effect that is macroscopically reflected by an enhanced degree of surface roughness. The different quality of the crystalline PTCDA domains was also complementary revealed by X-ray diffraction measurements. (C) 2000 Elsevier Science B.V. All rights reserved.