Electronic properties of interfaces between perylene derivatives and GaAs(001)surfaces

Abstract

The adsorption of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) andN,-dimethyl-3,4,9,10-perylenetetracarboxylic diimide (DiMe-PTCDI) on differently treatedn-doped GaAs(100) surfaces was investigated using high-resolution photoemissionspectroscopy. The chemical interaction between the molecules and the semiconductorsubstrate is found to be weak; core level photoemission spectra show no additionalchemically shifted peaks, indicating the absence of any covalent/ionic bond formation. Onlya sharpening of the core level spectra is observed for a coverage lower than onemonolayer and this is attributed to a reduction of inhomogeneous band bending atthe surface. This is interpreted in terms of preferential sticking of the organicmolecules to surface defects. The energy offset between the occupied states in thesubstrate and the organic film is directly derived from ultraviolet photoemissionspectroscopy measurements. Interface dipoles are found to form according to the electronaffinities of the substrates and PTCDA films at the interfaces and, consequently, thevacuum level alignment rule does not hold. For vanishing interface dipole the lowestunoccupied molecular orbital of PTCDA is found to align with the conductionband minimum of GaAs resulting in electron affinity of 4.12 eV for PTCDA. Thisprovides an energy gap in the range of 2.44–2.55 eV, which is larger than the onsetof optical absorption. The same procedure is applied to DiMe-PTCDI layers.