Orientation of perylene derivatives on semiconductor surfaces T.U. Kampen * , G. Salvan, A. Paraian, C. Himcinschi, A.Yu. Kobitski, M. Friedrich, D.R.T. Zahn Institut fu ¨r Physik, Technische Universita ¨t Chemnitz, D-09107 Chemnitz, Germany Abstract The orientation of the perylene derivatives 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N 0 -dimethyl- 3,4,9,10-perylenetetracarboxylic diimide (DiMe-PTCDI) on sulphur passivated GaAs(1 0 0) surfaces and its impact on the optical properties were studied by means of near-edge X-ray absorption fine structure spectroscopy (NEXAFS), Raman spectroscopy, and variable angle spectroscopic ellipsometry (VASE). NEXAFS shows that PTCDA molecules lie flat on the substrate with their molecular plane parallel to the substrate surface. DiMe-PTCDI molecules grown on the same type of substrates are tilted with respect to the substrate surface and are predominantly oriented with their long axis parallel to the [1 1 0] direction. The optical properties of these films investigated by VASE show that the DiMe-PTCDI films exhibit a much stronger optical anisotropy than the PTCDA films. # 2003 Elsevier Science B.V. All rights reserved. PACS: 78.66.Qn; 78.30.Jw; 68.55.-a Keywords: Organic films; PTCDA; Structural properties; Optical anisotropy 1. Introduction The two perylene derivatives 3,4,9,10-perylenetetra- carboxylic dianhydride (PTCDA) and N,N 0 -dime- thyl-3,4,9,10-perylenetetracarboxylic diimide (DiMe- PTCDI) form monoclinic crystalline structures with a C 2h symmetry. The unit cell contains 2 molecules almost perpendicular and parallel to each other in PTCDA and DiMe-PTCDI crystals, respectively. Due to their symmetry crystals show anisotropies in their physical properties. On substrates like Ag(1 1 1) [1], Au(1 1 1) [2], and HOPG [3] PTCDA is adsorbed in a ‘‘herring bone’’ structure with two molecules almost perpendicular to each other and parallel to the substrate surface in each unit cell. This molecular arrangement of the molecules is similar to the one in the (1 0 2) plane of a PTCDA crystal. On Ag(1 1 0) a phase transition is observed from a single domain oriented homogen- ous monolayer with a ‘‘brick wall’’ structure into a more condensed ‘‘herring bone’’ structure [4]. On InAs(0 0 1) substrates the monolayer PTCDA interacts relatively strongly with the substrate and two-dimen- sional overlayers are formed [5]. For larger coverages a phase transition occurs where bulk-like three-dimen- sional PTCDA clusters begin to form. The strong interaction between PTCDA and semiconductor sur- faces is considerably reduced by a passivation of the surface prior to the growth of the organic film [6,7]. Changing the structure of the molecule, e.g. by an imide group with different side groups instead of the Applied Surface Science 212–213 (2003) 501–507 * Corresponding author. Tel.: þ49-371-531-3079; fax: þ49-371-531-3060. E-mail address: kampen@physik.tu-chemnitz.de (T.U. Kampen). 0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00390-8