High-aspect-ratio nanogap electrodes for averaging molecular conductance measurements

A fabrication technique to prepare smooth, and coplanar metal contacts of a predetermined separation in the range of few nanometers, and their applicability for molecular-conductance measurement, are presented. These high-aspect ratio nanogap electrodes were prepared by growing an AlGaAs/GaAs heterostructure through molecular beam epitaxy (MBE) with monolayer precision onto a GaAs substrate. The electrodes were fabricated using an intermediate GaAs layer embedded between two AlGaAs layers, and after cleaving this plane, the thin intermediate GaAs film was selectively recess-etched into this plane. A thick metal film was also deposited using high-vacuum electron-beam evaporation. It was observed that the actual nanogap-electrode distance is affected by the roughness of the thin metal layer and by etching process. This device structure can be used for averaging measurements over a large number of molecules in parallel, assembled across the electrode gap at the desired surface density.