Molecule-metal polarization at rectifying GaAs interfaces

Metal/organic monolayer/GaAs junctions, prepared by adsorbing a set of dicarboxylic ligands, with systematic change of ligand substituents, on GaAs, are measured and characterized electrically. The molecules are chemically bound to the semiconductor surface under ambient conditions and form roughly a monolayer (MoL), with average order in the direction perpendicular to the semiconductor surface. This suffices to yield systematic changes in electron affinity and work function of the modified GaAs. Junctions are made by a soft metal deposition method, used here for Au and Al. Experimentally, we find strong molecular effects, reaching differences in current at a given voltage of up to 6 orders of magnitude, depending on the substituent on the molecules making up the monolayer. These and the changes in the effective barrier height of the metal/MoL/GaAs junctions, extracted by analyses of their current-voltage characteristics, can be explained by electrostatic effects of the molecular layer, rather than by electrodynamic ones (current flow through the molecular film). This can be understood by realizing that the samples are relatively large area devices with extremely narrow (similar to1 nm) films of organic molecules, showing only average order, which makes dominance of tunneling effects very unlikely. We show that not only the molecule's electronic and electrical properties but also the way the metals contact the molecules, as well as the doping type of the semiconductor, can determine the direction of the molecular effect. Also the type of metal governs the effect that we identify as being due to interfacial dipoles formed as a result of triple metal/organic molecule/semiconductor interaction.