Molecular junctions of ∼1 nm device length on self-assembled monolayer modified n- vs. p-GaAs

Abstract

Fabrication of high-fidelity molecular junctions of 1 nm or less device length is a significant challenge due to penetration of the top contact through the molecular layer. Successively smaller device lengths allow for molecules to more strongly modulate semiconductor surface charge and conductivity for device gating applications; hence, it is of great interest to better understand electron transport over device length scales of 1 nm. Molecular devices on GaAs are of interest, since their integration to band-gap engineered nanostructures on GaAs can enable high-sensitivity molecular signal transduction schemes. In this study, the formation of molecular junctions of 1 nm device length was investigated. Complexation and electroless deposition (ELD) of copper on self-assembled monolayer (SAM) modified n- and p-GaAs surfaces was used to correlate the effects of variations in SAM length, terminal functional group, and substrate dopant type (n vs. p) on the morphology of ELD Cu and its penetration through the underlying SAM. Electrochemical, electron microscopy and electrical transport characterization of Cu ELD on SAM-modified n-GaAs demonstrates highly selective deposition only on carboxylic acid terminated surfaces, the strong dependence of ELD morphology on SAM order, and absence of Cu penetration through the SAM, for device lengths down to sub-nanometer scales. Cu ELD on SAM-modified p-GaAs surfaces was similar to that of the respective SAM-modified n-GaAs surface, only in those cases where the SAM layer can passivate p-GaAs from corrosion. In other cases such as the shortest SAMs studied herein, corrosion of p-GaAs resulted in the disruption of sub-nanometer molecular junctions.