INCORPORATING MOLECULAR MONOLAYERS IN STANDARD SI TECHNOLOGY: A STRANGE 2D/3D CHARGE BALANCE

Technological realization of molecular electronics would probably pass through a silicon-hybrid stage. Technology also requires reliable fabrication and reproducible electronic behavior. Ensemble molecular junctions are made by chemical adsorption of a molecular array on an (semi-)conducting substrate which is then covered by a top contact. The top and bottom contacts are separated by 1-2 nm thick molecular monolayer and charge transport is measured perpendicular to the interface, in a two-terminals configuration. We have identified and optimized the major parameters that ensure robust molecular-controlled electronic: strong covalent bonding, strict elimination of surface oxides and minimal substrate roughness. Failure in depositing a contact on top of the delicate monolayers is commonly attributed to mechanical damage. In contrast, we show that it is the chemical affinity between the top and bottom contacts that drives diffusion and creation of shorts across the monolayer. Based on this, we used standard lithography processing to parallel manufacture dozens of Si –monolayer / Lead junctions, with an almost perfect yield. Reproducibility is verified by varying junctions` diameter from 3 to 100 μm, i.e. ×1000 in absolute current. Nevertheless, the standard deviation in log(current-density) was 0.2 over all tested junction diameters. A prominent exception is Pb/molecule-Si junctions made with methyl-styrene. These junctions showed the expected area-scaling of the current under reverse bias which turned into diameter-scaling under high-forward bias. This effect originates in the ultra-thin capacitor formed by the molecules, which has 2D area charge on its metal side, but a 3D space charge on its silicon electrode. This is only one example for the fascinating, unique physical effects that can be achieved by hybrid silicon-molecular electronics.