Preparation and optical properties of metal-insulator-metal devices based on Ti and Ta for photocurrent and chemocurrent applications

The optical and electrical properties of amorphous titanium and tantalum oxide films with thicknesses of 2.5-5 nm are investigated. As characterisation methods we used experiments with bias and photoinduced currents in the respective metal-metal oxide-gold junctions. The photoyield recorded for wavelengths between 200 and 1600 nm shows significant increases for higher energies than 3.5 eV and 4.4 eV for the titanium respectively tantalum samples. These values coincide with the band gaps found in literature for bulk samples of the oxides. Deviations from the ideal behaviour can be observed in bias induced currents. Voltage pulse experiments and the subsequent recording of the current transient let us assign a trap density of several 1015 cm-2. Since the photoyield with energies below 2 eV shows a broadband like behaviour, one can discuss the role of the traps as midgap states in the oxide film. The bias dependence of the photocurrent is explained within a two flux model of photoelectrons and photoholes.