Enhanced current densities in Au∕molecule∕GaAs devices

Metal–molecule–semiconductor heterostructures have been studied in a Au∕molecule∕p-type GaAs configuration. Stable monolayers of alkane and aromatic thiols were self-assembled from solution on heavily doped p-type (p+) GaAs surfaces. A low-energy, indirect path technique was used to evaporate Au on the molecular layer to minimize damage or penetration of the layer. Electrical characteristics of the devices were evaluated by current–voltage (I–V) measurements. In comparison to Au∕p+-GaAs control samples, which show rectifying behavior expected for Schottky barriers, the Au∕molecule∕p+-GaAs structures exhibit higher conductances and less rectification. The results indicate strong molecular coupling to the contacts with a significant density of molecular states near the Fermi level. A simple electrostatic model, which considers the dielectric constant and dipole charge of the molecular layer as well as the GaAs depletion region, has been developed to explain the observed characteristics. Variable temperature I–V measurements exhibit very little temperature dependence, consistent with tunneling-based transport through the molecular layer.