Reflectance anisotropy spectroscopy of the growth of perylene-3,4,9,10-tetracarboxylic dianhydride on chalcogen passivated GaAs(001) surfaces

The properties of organic molecular layers grown from perylene-3,4,9,10-tet racarboxylic dianhydride (PTCDA) on GaAs(001) substrates were investigated using reflectance anisotropy spectroscopy/reflectance difference spectrosco py (RAS/RDS). In an attempt to grow ordered organic layers GaAs(001) surfac es were modified with sulfur prior to the evaporation of PTCDA under ultrah igh vacuum conditions. The chalcogen modification results in a gallium sulf ide-like surface layer terminated by S dimers, which shows a (2x1) low-ener gy electron diffraction pattern. The lines shapes of the RAS spectra of S m odified surfaces show sharp derivative-like features at the E-1 gap and bro ad spectral features at higher energies likely related to E'(0) and E-2 gap s of bulk GaAs. For low PTCDA coverages the shape of the spectra in the ene rgy range of the GaAs bulk features is unchanged which indicates a low inte raction between substrate and organic layer. Additional features appear in the spectra for PTCDA coverages even below 0.3 nm which can be attributed t o transitions between the highest occupied molecular orbital and the lowest unoccupied molecular orbital at 2.23 eV. While the sharp feature due to th e E-1 gap of GaAs is essentially unaffected, the optical anisotropy at high er energies is increasing strongly with increasing PTCDA layer thickness wh ich is due to interference effects as shown by a model calculation. (C) 200 0 American Vacuum Society. [S0734-211X(00)00804-0].