Transport properties of Cu-AlF3-W and Cu-AlF3-Cu heterojunctions using STS measurements and a DFT-NEGF approach

Abstract

To understand and to analyze the transport properties of different metal-insulator systems, we developed an experimental study of the electronic transport properties of AlF3 thin films deposited over a Cu(1 0 0) substrate, and a theoretical study to model systems composed by an AlF3 molecule between two metallic Cu(1 0 0)-W(1 0 0) and Cu(1 0 0)-Cu(1 0 0) electrodes with different geometries. The left common electrode is always a Cu(1 0 0) layer, meanwhile the right changing electrode, W(1 0 0) or Cu(1 0 0), in some cases is represented as a layer and in others as having a tip ending. Tunnelling current against voltage (I-V) characteristic curves have been obtained by Scanning Tunneling Spectroscopy (STS) measurements and computed using density functional theory (DFT) with the non equilibrium Green function method (NEGF) within a bias voltage range from −2.5 to 5.0 V. The theoretical curves show low current values, in the order of 10-9 to 10-12, in good agreement with the I-V experimental curves in the same range. This reveals that breakdown response currents begin at higher voltages than 5.0 V. The transmission spectrum, total (DOS) and partial (PDOS) density of states are also presented being the transmission variations addressed in terms of the DOS. Keywords: Electronic transport, Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), Non Equilibrium Green Functions (NEGF), AlF3 thin films