The influence of internal and external electric fields on the transport of energetic electrons in nanostructures

Sandwich like metal-insulator-metal (MIM) nanostructures consisting of a 50 nm silver film and a 30 nm aluminum film separated by a few nanometer aluminum oxide layer were irradiated with a focused e-beam (diameter 0.5 mm) at kinetic impact energies in the range of 100 eV to 1000 eV. To distinguish between internal transport of hot charge carriers across the buried insulator (tunnel junction) and parasitic electron transport mediated by externally emitted electrons re-entering the sample, an additional “dome” electrode was implemented which was biased to positive or negative potential in order to establish an external accelerating or retarding field above the nanostructure's surface. Different device currents induced by the primary electron irradiation were measured either by metering the irradiated or non-irradiated electrode, respectively. The dependence of the detected device currents on impact parameters such as the irradiated position on the MIM surface, the kinetic energy and impact angle of the primary electrons was studied. These experiments were accomplished while changing the internal electric field by an internal bias voltage between the top and the bottom electrode of the MIM and while changing the external electric field by applying a voltage to the dome electrode. The measured currents are interpreted in terms of external and internal emission yields. It is shown that the external electric field allows a clear discrimination between true internal electron transport and external electron transport leaving the MIM nanostructure on one site and re-entering at another site. The results demonstrate that “internal” currents measured without an external dome electrode may be strongly influenced or even falsified by such cross-absorption effects.