D. Schmeisser

Structure and reactivity of the system Si/SiO2/Pd: a combined XPS, UPS and HREELS study

Pd thin films (0 d 2 layer and occupies oxygen vacancy positions in a tetrahedral bond configuration. At temperatures near 400 K, Pd 2 Si is formed at the interface between Si0 2 and Si and the metallic Pd overlayer starts to disappear. CO vibrational losses are used to monitor the amount of overlaying metallic Pd as a function of sample pretreatment. At temperatures above 700 K, these losses disappear completely indicating that no Pd is left on top of the Si0 2 . The kinetics of Pd diffusion in Si0 2 layers is discussed as a function of preparation conditions and structure of the oxide.

Ta2O5-gates of pH-sensitive devices: Comparative spectroscopic and electrical studies

Abstract Thin films of tantalum pentoxide (Ta 2 O 5 ) were prepared on Si/SiO 2 substrates by thermal oxidation of tantalum. In systematic oxidation studies we followed the growth of the Ta 2 O 5 /SiO 2 interface. The oxide layers and their interfaces were characterized by SIMS, SAM, XPS, by comparative C V measurements and by pH-(ISFET) sensitivities. Depending on the oxidation procedure, we find non-ideal stoichiometries of the Ta 2 O 5 /SiO 2 interface, whose widths vary as a function of the oxidation time of the previously evaporated metallic tantalum. Specific annealing procedures lead to unexpectedly high leakage currents, which correlate with the formation of voids in the oxide layers. Even in the absence of voids, non-ideal interfaces provide high concentrations of electrically-active states in gate oxides of ISFETS, which in turn determine the results of CV measurements and ISFET characteristics. For ideal stoichiometric and atomically abrupt interfaces, we observe long-term stability and ideal Nernstian behaviour in the pH-(ISFET) sensitivities.

Spectroscopic and electrical studies of yttria-stabilized zirconia for oxygen sensors

Surface and interface properties have been investigated for the system Pt/YSZ (yttria-stabilized zirconia). Emphasis is put on an atomistic understanding of the interface properties related to the performance of oxygen sensors based on YSZ. Elemental compositions, electronic structures and work functions are studied on pure and platinum-coated surfaces of single-crystal zirconia doped with 10 mol% yttria and of polycrystalline sintered material doped with 12 mol% yttria. Spectroscopic methods are X-ray (XPS) and u.v. (UPS) photoemission spectroscopy, ion backscattering spectroscopy (ISS) and secondary ion mass spectrometry (SIMS). Large, reversible changes of surface concentrations of yttrium are found between 300 and 1300 K on single-crystal as well as on polycrystalline YSZ. Y-enrichment by a factor between 1.5 and 2.3 is observed at T > 500 K. The electronic work function shows a large reversible change of about +1.4 eV with increasing temperature between 870 and 1170 K, with a weak dependence on the oxygen partial pressure in the range 10−3 to 5 × 10−6 Pa. The main contribution to work function changes results from the shift of the Fermi level relative to the band edges of YSZ. Evaporated platinum contacts are investigated on YSZ surfaces after heat treatment at different oxygen partial pressures. A pronounced tendency is observed for the platinum to form small clusters upon heating in oxygen, already starting at temperatures above 300 °C. Spectroscopic studies show significant compositional changes at the platinum surface and at the contact between platinum and YSZ. Pt—Zr alloys are formed at low oxygen partial pressures. A layer of adsorbed oxygen on platinum is formed at high oxygen partial pressures. For comparison, current—voltage charateristics have also been measured with Pt point-contacts on YSZ single crystals. Cathodic limiting currents proportional to pO2 are observed in the entire partial pressure range investigated (2.1 × 104 to 1.0 × 102 Pa).

Electronic and geometric structure of clean InP(001) and of the CaF2/InP(001) interface

In low‐energy electron diffraction (LEED), we find two different reconstructions of the technologically important InP(001) surface, the well known (4×2) structure and a high temperature (4×2) structure with streaks at the half order spot positions which is interpreted as a disordered c(8×2) structure. The corresponding electronic structures were investigated by ultraviolet photoelectron spectroscopy (UPS) and x‐ray photoelectron spectroscopy (XPS). For both types of reconstructions, we observe a surface state emission at the valence‐band maximum and a strong fermi‐level pinning. CaF2 was deposited onto these surfaces, and the interface formation was studied by LEED, UPS, and XPS. The overlayer grows epitaxially with (001) orientation of its bulk fluorite structure inspite of the large lattice mismatch of 7.5%. The electronic interface scheme with its valence‐band discontinuity was deduced from photoemission measurements as a function of overlayer coverage. Band bending at the interface results from p‐type...

Prototype chemical sensors for the selective detection of O2 and NO2 in gases

Thin-film structures of inorganic and organic electron-, hole- and ion-conducting materials were optimized for their use as chemical sensors to detect selectively O2 and NO2 in the gas phase. Comparative electrical and spectroscopic studies make possible an atomistic interpretation of the sensing mechanisms of our optimized sensor structures. Particular emphasis is given to the controlled mixed (i.e., electron as well as ion) conduction during the preparation and/or operation of long-term stable devices.

The electronic structure of (2-X-5-M-DCNQI)2Cu

Abstract Using XPS and UPS photoelectron spectroscopy, we have studied the electronic structure of single-crystalline needles of 2-Cl-5-M-, 2-Br-5-M- and 2,5-DM-substituted (DCNQI) 2 Cu charge-transfer salts, prepared in situ. The results indicate that the oxidation state of the counterion is Cu + . The electronic structure of the valence band contains contributions from C 2p and N 2p states, but also from Cu 3d atomic levels. There is a metallic density of states near the Fermi energy for (DM-DCNQI) 2 Cu. This is the first experimental evidence for the existence of an organic metal with an admixture of Cu 3d states. The admixture from Cu states near the Fermi energy is reduced by substitution of Cl and disappears in amorphous films, demonstrating the strong influence of local structure. The implication of these spectroscopic results on the conductivity mechanism is discussed.

Mass sensitive detection of carbon dioxide by amino group-functionalized polymers

A series of amino group-functionalized polymers was investigated as sensitive coatings for the detection of carbon dioxide in the gas phase by using quartz microbalance transducers. The sensor signals are reversible with response and decay times in the order of minutes at operating temperatures between 25°C and 70°C. The long term stability and the cross sensitivities to other gases are also discussed.

Characterization of gas-sensitive lead phthalocyanine film surfaces by X-ray photoelectron spectroscopy

Abstract XPS studies of the surface composition of lead phthalocyanine (PbPc) films prepared in various conditions from different starting materials using a range of sublimation temperatures are reported. Film stoichiometry is constant for sublimation at temperatures below 670 K, but loss of carbon and particularly nitrogen occurs above this temperature. The NO 2 -sensing properties of the films are only slightly changed by these effects. No chemical changes are detectable by XPS on heating PbPc films to 420 K in air or in 1 ppm of NO 2 in air for prolonged periods. The principal strongly adsorbed species on PbPc films exposed to air are O 2 − and H 2 O. Water is more strongly adsorbed, and the implications of this observation for observed loss of NO 2 -sensitivity of PbPc sensors in humid conditions are discussed.

Surface defects on Si(001)

Abstract The existence of defects on Si(001) surfaces and their formation as a function of the substrate temperature has been studied with a high-resolution spot profile analysis LEED system. Oscillations are measured in the half widths of the 00, 01, and 11 integral order beams at different annealing temperatures. The experimental results were simulated in a kinematic model by including the diffraction of non-primitive lattices showing that the influence of multiple scattering can be neglected. We observe the coexistence of monoatomic, diatomic, and up to six-atomic multiple steps. The formation of (113) facets is observed which are stabilized by a (3×1) reconstruction at temperatures above 1200 K. Their formation can be understood by coagulation of hitherto separated monoatomic and diatomic steps. These facets disappear again for annealing temperatures above 1500 K. Ideal (001) surfaces with terraces wider than 100 nm are obtained when the samples are flashed initially to above 1500 K. In this high temperature range exclusively monoatomic steps were observed.

Surface morphology of epitaxial CaF2 and SrF2 layers grown onto InP(001) studied by atomic force microscopy and low-energy electron diffraction

Epitaxial CaF2 and SrF2 layers were grown by molecular beam epitoxy onto clean, (4 × 2) reconstructed InP(001) surfaces. Their orientations and surface morphologies were studied with low-energy electron diffraction (LEED) and atomic force microscopy (AFM). Both fluorides grow in (001) orientation in their cubic bulk structures onto the (001) substrate and form atomically rough surfaces consisting mainly of (1 × 1) ordered (111) facets. These facet planes occur in four symmetrically equivalent directions rotated by 90° around the (001) surface normal each thus fourfold pyramids on the surfaces. The AFM pictures also show very rough surfaces with pyramid-like structures. The roughness was determined quantitatively and defects in the SrF2 layers were observed. The AFM results are in good agreement with the LEED results.