Detlef Diesing

Aluminium oxide tunnel junctions: influence of preparation technique, sample geometry and oxide thickness

Tunnel junctions of the type Al/Al oxide/Ag can emit hot electrons of about 2 eV to an adjacent electrolyte and cause redox reactions. We developed a production technique of such tunnel systems, which are stable at room temperature in atmosphere and in electrolyte. Two types of Al base electrodes (wires and evaporated films on glass), three types of oxide films (gas phase oxidation, anodic oxidation and physical vapour deposition) were combined with an Ag top electrode film of 15 nm thickness. PVD oxide films are porous with a large thickness distribution. This causes a rough silver top electrode and therefore a weak corrosion resistance. Stable oxide films were only obtained for the pairs Al wire/anodic oxide and Al film/gas phase oxide. The oxides are almost equal in their properties and form dense homogenous films independent of the sample geometry. Anodic films can be formed with various thicknesses, but ionic currents may exceed the tunnel currents. A successful layer sequence was Al film/gas phase oxide (2.5 nm)/Ag (15 nm) with an emission current of hot electrons of about 1 mA/cm 2 .

Breakdown of ultrathin anodic valve metal oxide films in metal-insulator-metal-contacts compared with metal-insulator-electrolyte contacts

The anodic breakdown of thin valve metal oxide films on aluminium, hafnium, niobium, titanium, tantalum and zirconium in the system valve-metal/valve-metal-oxide/silver was investigated. For all systems valve metal wires covered by an anodically formed oxide with an evaporated silver film were used. For comparison three different types of oxide were used in the case of aluminium: anodic oxide, gas phase oxide and physical vapour deposited oxide. Due to an improved technique for the preparation a high reproducibility and reliability could be achieved. In the case of anodic oxide it is shown, that the formation field strength or reciprocal film formation factor and the breakdown field strength are equal. The initial step of anodic breakdown is clearly an ionic one. This was concluded from the strong correlation between film thickness and breakdown potential. An equation for the absolute quantitative calculation of tunnel currents is derived that takes the deformation of the barrier due to the image potential into account. The simulations are compared with the experimental results and the breakdown process is discussed in terms of ions which move into the tunnel barrier and deform the tunnel barrier.

Combining scanning tunneling microscopy and synchrotron radiation for high-resolution imaging and spectroscopy with chemical, electronic, and magnetic contrast

The combination of high-brilliance synchrotron radiation with scanning tunneling microscopy opens the path to high-resolution imaging with chemical, electronic, and magnetic contrast. Here, the design and experimental results of an in-situ synchrotron enhanced x-ray scanning tunneling microscope (SXSTM) system are presented. The system is designed to allow monochromatic synchrotron radiation to enter the chamber, illuminating the sample with x-ray radiation, while an insulator-coated tip (metallic tip apex open for tunneling, electron collection) is scanned over the surface. A unique feature of the SXSTM is the STM mount assembly, designed with a two free-flex pivot, providing an angular degree of freedom for the alignment of the tip and sample with respect to the incoming x-ray beam. The system designed successfully demonstrates the ability to resolve atomic-scale corrugations. In addition, experiments with synchrotron x-ray radiation validate the SXSTM system as an accurate analysis technique for the study of local magnetic and chemical properties on sample surfaces. The SXSTM systems capabilities have the potential to broaden and deepen the general understanding of surface phenomena by adding elemental contrast to the high-resolution of STM.

Electronic excitations induced by hydrogen surface chemical reactions on gold

Associated with chemical reactions at surfaces energy may be dissipated exciting surface electronic degrees of freedom. These excitations are detected using metal-insulator-metal (MIM) heterostructures (Ta-TaOx-Au) and the reactions of H with and on a Au surface are probed. A current corresponding to 5×10(-5) electrons per adsorbing H atom and a marked isotope effect are observed under steady-state conditions. Analysis of the current trace when the H atom flux is intermitted suggests that predominantly the recombination reaction creates electronic excitations. Biasing the front versus the back electrode of the MIM structure provides insights into the spectrum of electronic excitations. The observed spectra differ for the two isotopes H and D and are asymmetric when comparing negative and positive bias voltages. Modeling indicates that the excited electrons and the concurrently created holes differ in their energy distributions.

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.

Adsorbed state of pyridine, uracil and water on gold electrode surfaces

Abstract Attenuated total reflection (ATR) IR spectroscopy was applied to thin film metal electrodes deposited onto Si and ZnSe half-spherical prisms. Optimum conditions for the largest enhancement were quite similar for these prism materials, while their surface morphology is substantially different. In addition to a large enhancement factor of ca. 100, surface selectivity of the ATR-IR spectroscopy was confirmed by spacer experiments that give the decay length of ca. 4 nm even for smooth Ag films on ZnSe prism. Adsorbed pyridine (Py) and uracil on Au electrodes were studied with respect to their hydrophilicity. Two adsorbed states with different vibrational frequencies were observed for uracil in 0.1 M LiClO 4 solution, which have pronounced interaction with water molecules and electrolyte ions. In contrast, no evidences were obtained for distinct adsorbed states for Py, while water molecules were eliminated from the electrode surface upon Py adsorption. Furthermore, three distinct water species were observed on the Au electrode with different hydrogen bonding properties in 1 mM alkali metal halide solution.

Optical response of metal-insulator-metal heterostructures and their application for the detection of chemicurrents

The optical response of thin-film metal-insulator-metal (MIM) systems of tantalum-tantalum oxide-Au type is studied by recording the macroscopic current across the device resulting from the low-energy electron-hole pairs excited in the metals by red and near-infrared (NIR) light (h < 2eV). It is observed that current flows from the top Au to the back Ta electrode, although a larger number of photons is absorbed in the latter. This directional preference is attributed to the built-in electric field across the oxide layer. The yield per photon increases strongly as photon energy becomes comparable to the barrier height. Current exhibits a strong dependence on bias voltages applied across the oxide layer. Photoyields induced by NIR light (h 1.5eV) were found to be comparable to recently observed chemicurrents arising from exposure of a MIM sensor to atomic hydrogen, when compared on a current per photon to current per impinging hydrogen atom basis.

Preparation and properties of thin amorphous tantalum films formed by small e-beam evaporators

Large area (A = 6 cm2), thin tantalum films (5 nm < d < 100 nm) are accomplished by evaporation from tantalum rods using small pocket e-beam evaporators. Using a sample to source distance of ≈20 cm, homogeneous amorphous films with a small surface roughness (<1 nm) can be prepared on glass. Films are characterized by scanning electron microscope images, atomic force microscopy, electrochemical oxidation and resistivity measurements as a function of film thickness. The samples show high resistivities of 200–2000 µΩ cm. The temperature coefficient of the resistivity (TCR) is negative, as characteristic for highly disordered metals. A theoretical description of the thickness distribution (evaporation from plane and hemispherical sources on plane targets) is given in the appendix.

Surface reactions with hot electrons and hot holes in metals

Electrochemical reactions (oxidation or reduction) start when the electronic levels of the redoxsystem within the electrolyte cross the Fermi level E F of the electrode - from below to above E F in the case of the oxidation and from above to below in the case of reduction. This energetic shift of the redox level is effected by changing the electrochemical potential of the electrode. Hot electrons above E F or hot holes below E F incident at the metal-electrolyte interface from the metal side may induce reduction and oxidation, respectively, at electrochemical potentials where no reactions occur with the electrode in its electronic ground state. This is related to surface photochemistry, believed to be induced by intermediate hot electrons. In the present work hot electrons and hot holes are injected into a thin silver film electrode by tunnelling within a metal-insulator-silver contact. The results presented here confirm the new unusual reactions, provided there is either atomic scale roughness or a suitable redox level at the silver-electrolyte interface.

Trapping of transient processes in aluminium oxide thin films in a voltage pulse experiment

Abstract An experimental setup is presented that allows the trapping of transient states in potentiostatic and potentiodynamic experiments. The setup is suitable for electrochemical experiments as well as for dielectric investigations. The system stops an experiment by triggering at a predefined current level after a minimum time of the voltage pulse. The advantage of this device is demonstrated by means of a voltage pulse annealing procedure for a metal–insulator–metal (MIM) contact with an anodically prepared aluminium oxide film as insulator. The setup significantly increases the stability against a breakdown of the anodic oxide film.