Photo and particle induced charge carrier ͑electrons and holes͒ transport is studied in metal-insulator-metal tunnel junctions of Ag-AlO x -Al type. The electronic excitation induced by photo irradiation with h = 4.67 eV is compared with that induced by the impact of argon ions with a kinetic energy of 12 keV. The common feature of the two experiments is that only charge carriers with energies above ͑electrons͒ or below ͑holes͒ the tunnel barrier are detected. The electron to hole induced current ratio is adjustable by applying a bias voltage to the sample. A similar bias dependence of the induced current was found in both experiments. While the bias dependence of the photo induced current cannot be unambiguously explained due to the possibility of simultaneous excitation of both metal films, one can discuss the bias dependence of particle induced currents clearly in terms of electrons and holes excited in the top electrode only. Within a computer simulation based on a three dimensional model, we show that the photo induced conduction in Ag-AlO x -Al junctions is mainly determined by the simultaneous excitation of electrons and holes in the top Ag film. Moreover, the model indicates that the dominant contribution to the induced current is given by the charge carriers excited near the silver-oxide interface. Finally, we show that the particle induced current can be modeled by carrier transport between two free-electron gases, excited to an elevated electron temperature and to room temperature, respectively. Hereby, the influence of the tunnel barrier parameters and the elevated electron temperature on the bias dependence is discussed.

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. and electron sources in spin polarized tunneling. 5 In the present work we present a comparative investigation of charge transport induced by different methods through thin oxide films. Charges are driven through the oxide by: i) application of a device voltage ii) illumination of the samples with monochromatic light at variable photon energies, leading to spectra of internal photoemission, iii) non adiabatic chemical surface reactions. All three methods are combined since it is possible to derive the nature of excited carriers transport through heterosystems.6 For aluminum and tantalum oxide heterosystems it was found, that they form a high pass filter for excited electrons and holes (defect electrons). 4,6 Since the height of the internal barriers for electrons and holes can be modified by an applied bias voltage, it became possible to characterize the spectra of excited charge carriers with energies below the vacuum barrier. Bias voltages of up to 1 V could be applied to the system which had an internal barrier height of 3 eV.To increase the detection efficiency for excited electrons travelling from the surface of a metal towards the bulk, one can either decrease the thickness of the top metal film of a heterosystem or one can reduce the height of the internal barrier. But with a decrease of the internal barrier the method of applying a bias voltage for tuning this barrier might become problematic, since tunnel currents become larger with lower barrier heights. Additionally, the influence of midgap states, impurities and remanent changes of the internal barrier might become more relevant for lower barriers.8,9 These problems are adressed in the present work.A comparison of metal/insulator/metal heterojunctions is presented with potentiostatically formed titanium and tantalum oxide as interjacent insulating layer. Titanium and tantalum oxide were chosen since both have bandgaps smaller than 4.5 eV.10 For amorphous tantalum oxide values of 4.2 eV are typical 11,12 whereas crystalline samples show values of 3.9-4.5 eV.12,13 Bulk titanium oxide values are for rut...

Thin film metal-insulator-metal tunnel junctions are used to investigate the electronic excitation process induced by the impact of multiply charged ions onto a metallic surface. Hot charge carriers ͑electrons and holes͒ generated by the dissipation of the kinetic and potential energies of the projectiles are detected as an ion induced internal emission current from the bombarded "top" metal film into the "bottom" substrate electrode. Results are presented for Ar q+ ions with a kinetic impact energy of 1 keV and charge states q =1-8 impinging onto an Ag-AlO x-Al junction. It is shown that the internal emission yield exhibits an approximately linear dependence on the potential energy of the projectile. At low potential energy, a bias voltage applied between the two metal films is found to strongly influence the internal emission current, whereas this influence becomes much weaker with increasing projectile charge state. The results are shown to be qualitatively well described in the framework of a thermodynamical free-electron model.

Thin film metal-insulator-metal junctions are used in a novel approach to investigate the dissipation of potential energy of multiply charged ions impinging on a polycrystalline metal surface.The ion-metal interaction leads to excited electrons and holes within the top layer. A substantial fraction of these charge carriers is transported inwards and can be measured as an internal current in the thin film tunnel junction. In Ag-AlO x -Al junctions, yields of typically 0.1-1 electrons per impinging ion are detected in the bottom Al layer. The separate effects of potential and kinetic energy on the tunneling yield are investigated by varying the charges state of the Ar projectile ions from 2+ to 9+ for kinetic energies in the range from 1 to 12 keV. The tunneling yield is found to scale linearly with the potential energy of the projectile.

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.