Domocos Kovacs

Charge Transport Through Thin Amorphous Titanium and Tantalum Oxide Layers

Heterosystems of metal/insulator/gold type with titanium oxide and tantalum oxide as internal barriers are studied using internal photoemission (IPE), field induced current transport (current transients after voltage steps) and chemical reaction induced current transport (chemicurrent). IPE investigations over a broad energy range from 0.8 to 4.5 eV allow a determination of the interstitial layers band gap and the maximum height of the internal tunnel barrier. The built-in field of the heterosystem is derived by the evaluation of the slope in the photoyield versus photon energy plot. Current transients recorded after voltage steps allow the determination of the heterosystems time constants which generally have a value of some milli seconds. In titanium oxide systems additional time constants with values of several 100 s appear for bias voltages >0.5 V. These time constants are assigned to slow processes altering the height of the titanium oxide barriers.

Hot electrons induced by slow multiply charged ions

The dissipation of energy following the impact of multiply charged ions on a polycrystalline metal surface was studied using thin film metal–insulator–metal junctions as targets. The ions hit the top Ag layer of a Ag–AlOx–Al junction, where they excite electrons and holes. A substantial fraction of these charge carriers is transported across the insulating barrier and can be detected as an internal current in the bottom Al layer. The effects of potential and kinetic energies on this tunneling yield are investigated separately by varying the charge state of the Ar projectile ions from 2+ to 8+ for kinetic energies in the range from 1 to 12 keV. Per impinging ion yields of typically 0.1–1 electrons are measured within the thin film tunnel junction. The tunneling yield is found to scale linearly with the potential energy of the projectile. In addition, the tunneling yield shows a strong dependence on the internal barrier height which can be modified by an external bias voltage.

Electronic excitation in metals through hyperthermal atoms

A hyperthermal hydrogen/deuterium atom beam source with a defined energy distribution has been employed to investigate the kinetically induced electron emission from noble metal surfaces. A monotonous increase in the emission yield was found for energies between 15 and 200 eV. This, along with an observed isotope effect, is described in terms of a model based on Boltzmann type electron energy distributions.

Low-energy ion-induced electron emission in metal-insulator-metal sandwich structures

An Ag-AlO{sub x}-Al sandwich structure is used to investigate the electronic excitation induced by Ar{sup +} ions at the surface of the top 15 nm Ag film. The internal electron emission yield, i.e., the number of electrons emitted per impinging ion into the bottom Al film, is determined as a function of the kinetic energy of the ions in the range of 300-6000 eV. A comparison to the external electron emission yield, i.e., the number of electrons per projectile ejected into the vacuum, reveals two interesting aspects. First, unlike in the external emission, no significant contribution of the potential energy to the internal electron emission yield is observed. Second, the kinetic part of the electron emission yield exceeds the external one over the entire energy range. Another interesting result is that the internal emission yield shows a power-law dependence on ion kinetic energy. A Monte Carlo simulation, based on a simple theoretical treatment of the kinetically induced electron emission, supports the experimental findings. Finally, we discuss the influence of excitation properties (e.g., anisotropy) as well as of device properties (e.g., film thickness, barrier height) on the computed electron emission yields.