The Anisotropic Dielectric Function for Copper Phthalocyanine Thin Films

Copper phthalocyanine (CuPc) thin films were prepared by organic molecular beam deposition (OMBD) in high vacuum and ultra-high vacuum on passivated Si(111) using β-phase CuPc as source material. The substrates were kept at room temperature during the deposition. The IR peak positions indicate that the films consist mainly of α-phase CuPc, while the relative intensities suggest that the films are anisotropic. The anisotropic dielectric function for these CuPc layers was determined from ellipsometric spectra using uniaxial models in the range from 0.73 to 5 eV. Introduction. The phthalocyanines (Pcs) are a class of materials which exhibit a high thermal and chemical stability and also high optical absorption in the visible range. Due to their blue or green colour the Pcs were largely used in industry as dyes [1], and more recently due to their semiconducting properties they have proven their applicability in electronic devices such as organic photovoltaic cells [2, 3], organic field effect transistors (OFETs) [4], organic light emitting diodes (OLEDs) [5] and gas sensors [6]. The copper phthalocyanine (CuPc) is one of the most extensively studied Pcs by means of ellipsometry [7,8,9,10,11], infra-red spectroscopy [12,13,14,15] and X-ray diffraction [15,16,17,18]. Even though the CuPc molecule has an intrinsic optical anisotropy due to its planar structure, in the reported dielectric functions or optical constants derived from ellipsometry spectra the CuPc layers are considered as being isotropic [8-11]. In one previous analysis Debe [7] tried to approximate the anisotropic dielectric function of CuPc by investigating two different types of oriented films – with the b-axis (the molecular stacking axis) perpendicular and respectively parallel to the substrate surface. For each film only the inplane components of the dielectric function were determined. In other reports [19,20] the optical anisotropy of the CuPc layer was taken into account but these studies were limited to single wavelength ellipsometry at one angle of incidence. The experimental data in ref. [19] was modelled assuming an isotropic layer for thicknesses below 100 nm and an isotropic inner layer plus an anisotropic outer layer for thicknesses higher than 100 nm. However, the anisotropic dielectric function of CuPc was not reported in the literature so far. Therefore the aim of this work is to present the dielectric function for CuPc films over a wide spectral range (0.73 – 5 eV) obtained from simulating ellipsometry spectra using anisotropic model approaches. The implementation of new mathematical algorithms [21] based on a 4x4 transfer matrix formalism developed by Berreman [22] is a promising approach to tackle the problem. As reported previously for metal-free phthalocyanine [23] and for perfluorinated vanadyl phthalocyanine [24], the determination of the anisotropic dielectric function from spectroscopic ellipsometry does not only yield physically reliable values, but also allows the orientation of the molecules with respect to the substrate to be determined. Experimental. Organic thin films of CuPc were grown by OMBD in high vacuum (HV – 8*10 7 mbar) and ultra-high vacuum (UHV 6*10 mbar) on hydrogen passivated Si(111). The substrates were kept at room temperature during the deposition. The CuPc source material used for HV deposition was β-phase CuPc with 97% purity, supplied by Aldrich. For the UHV deposition higher purity (99%) β-CuPc was used. The Si(111) substrates were cleaned with