Metal-molecule-semiconductor heterostructures for nano-device applications

The key challenge for molecular devices is the nature of the microscopic contacts to the active molecules and the need to develop well-controlled interfaces in order to achieve stable low-resistance contacts. When compared to metals, semiconductor contacts can be engineered more controllably and are more practical for device applications. In addition to the molecular energy levels, the energy bands of the semiconductor contact give us an extra degree of freedom for enhanced device functionality. The focus of the present work is on metal-molecule-semiconductor heterostructures in a Au/molecule/GaAs configuration. To begin with, stable monolayers of alkane-monothiols, alkane-dithiols and aromatic-dithiols on heavily doped p-type GaAs were self-assembled. A chemical etching procedure was used to overcome the oxidation of the GaAs surface during SAM formation. FTIR spectra indicate the formation of uniform and crystalline monolayers. Ellipsometry measurements give layer thicknesses that are consistent with calculations. A low-energy indirect path technique is used to evaporate Au on the molecular layer without damaging or penetrating it. I-V measurements on the Au/molecule/GaAs devices indicate a substantial increase in conductivity due to the presence of the molecular layer, which can be treated as a leaky dielectric. The results are consistent with the presence of molecular dipole moments at the interface.