Charge Transport Through Thin Amorphous Titanium and Tantalum Oxide Layers

Abstract

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...

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