Structural and morphological properties of 3,4,9,10-PeryleneTetraCarboxylic DiAnhydride films on passivated GaAs(100) substrates

Raman spectroscopy is employed to investigate in situ the structural properties and the morphology of 3,4,9,10-Perylene-TetraCarboxylic DiAnhydride (PTCDA) films deposited onto S-GaAs(100) substrates at various temperatures. Additional two-dimensional Raman mapping measurements are carried out ex situ to extract information at the microscopic level. Complementary atomic force microscopy studies reveal that all the films consist of islands. The crystalline nature of these islands is proven by the observation of libronic phonon-like modes in the Raman spectra characteristic to the molecular crystal. The decrease in the phonon band widths observed at elevated substrate temperatures is related to an increase in the size of the crystalline domains and improvement of crystallinity. The PTCDA films consist of a mixture of two polymorphs alpha and beta having the same symmetry and very similar lattice parameters. An estimation of the alpha- to beta- content at microscopic level is obtained from the deconvolution of an internal Raman mode with C-H deformation character following a previously introduced model.