The Noise Characteristics of Charge Transport via Localized States in Nanoscale Metal-Molecule-Metal Tunnel Junctions

Abstract

In the present study, we report on charge transport and noise properties of alkyl self-assembled monolayer (SAM) tunnel junction. As the density of localized states (e.g., impurity or defect) in the tunnel barrier is sparse, it is very difficult to detect a signature coming from the localized states by only measuring standard current-voltage characteristics. Thus, in order to explore the influence of the localized states on charge transport through the molecular tunnel junctions, we measured a shot noise signal as well as 1/f low-frequency noise and random telegraph noise (RTN) signals using metal-molecule-metal junction structures at room temperature. We could observe a shot noise enhancement at high bias region, which becomes an evidence of tunneling transport via the localized defect states arising from the associated potential barrier modulation. From the 1/f low-frequency noise signal, a local increase of the normalized noise density was observed as a bump shape over a certain bias range, which results from the trapping/de-trapping processes of charge carriers via the localized states existing in the alkanethiol tunnel barrier. Furthermore, we showed clear two-level current fluctuations from the RTN signals. This observation is also related to a generation-recombination noise arising from the trapping/de-trapping processes in the localized states.