Origin of the Humidity Sensitivity of Al/AlOx/MHA/Au Molecular Tunnel Junctions

Electron tunneling spectroscopies have been performed on tunnel junctions incorporating mercaptohexadecanoic acid (MHA) between gold and surface-oxidized aluminum electrodes. Low- temperature superconducting conductance spectroscopy provides direct evidence for elastic tunneling across the junctions. At room temperature the electron transport of these junctions exhibits a high sensitivity to ambient humidity; the resistance of these devices drops by more than 50% when they are placed into a dry atmosphere or vacuum and recovers after they are returned to ambient air. By comparing these results to those obtained for similar junctions incorporating different molecular monolayers, it is determined that the interaction of water molecules with the AlOx/carboxylate interface is the origin of the observed behavior. The tunneling spectra and the current-voltage characteristics indicate significant modifications of the barrier height of the AlOx upon MHA binding and in the hydration of the molecular interface.