The Drive Force of Electrical Breakdown of Large-Area Molecular Tunnel Junctions

The origin of electrical breakdown of molecular tunnel junctions is systematically studied by determination of the breakdown voltages as a function of six types of bottom-electrodes (Au, Ag, Pt, Pd, Ni, and Cu) and different thickness of self-assembled monolayers of n-alkanethiolates, S(CH2)(n-1)CH3 with n = 2, 4, ..., 18, with GaOx/EGaIn top contacts. It is found that at positive bias, the migration of metallic atoms is dominated by the wind force, but, at negative bias, both the wind force and direct force are involved in the mechanism of filament formation. Remarkably, the breakdown voltage is independent of the molecular length for short molecules (n < 10), and the breakdown field could be improved by a factor of approximate to 2 from 0.80 to 1.5 GV m(-1) by replacing the Ag with Pt (or Ni) bottom electrodes. These findings give insights into the design of stable molecular junctions.