Growth of organic films on passivated semiconductor surfaces: gallium arsenide versus silicon

We have investigated the growth of perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) on hydrogen-passivated Si(1 0 0) and sulfur-passivated GaAs(1 0 0) using in situ Raman spectroscopy. The Raman spectra provide information on the internal vibrational modes of the organic molecules as well as on external vibrational modes, or, in other words, phonons. PTCDA films grown on both types of substrates show vibrational modes which correspond to the phonon modes of PTCDA single crystals. The presence of phonon modes in the Raman spectra provides evidence of their crystalline structure. Their intensity, full width at half maximum (FWHM), and polarization dependence can be employed to determine the appropriate substrate for the growth of PTCDA films with high structural quality. The experimental results show that PTCDA films grown on sulfur-passivated GaAs(1 0 0) at 410 K substrate temperature are still polycrystalline but show the best structural properties. The FWHM of the phonon modes measured on these films are decreased for substrate temperatures of 350 and 410 K. Phonon as well as internal vibrational modes show a dependence on the polarization geometry, which has so far only been observed in PTCDA crystals up to now