Nature of electrical contacts in a metal–molecule–semiconductor system

Abstract
The effects of the top electrode fabrication method on the transport properties of Au–alkanedithiol–GaAs junctions are examined. We found that diodes made by evaporating Au on 1,8-octanedithiol monolayer behave similarly to Au–GaAs junctions without the molecular layer. The direct Au–GaAs contact dominates the electrical conduction in these evaporated devices despite the presence of the molecular layer. In contrast, Au–dithiol–GaAs diodes made by nanotransfer printing (nTP) exhibit no direct contact between Au and GaAs. Electrical transport in the nTP junctions occurs through the 1,8-octanedithiol insulating layer. The low current levels and the high barrier-height values in the current–voltage results and the exponential energy dependence in the photoreponse yield that are observed in the nTP diodes support this conclusion. Since transport through the molecular layer is much less conductive, even a miniscule region of direct contact between the two electrodes will make it impossible to observe electrical transport through molecules. Fractional amounts of the different types of contacts for each kind of sample were obtained from the experimental results.