Josef Gerblinger

Electrical conductivity of sputtered films of strontium titanate

Among the most critical parameters in the fabrication of rf‐sputtered SrTiO3 films is the argon/oxygen ratio in the sputter gas and the substrate temperature, both during sputtering and during postannealing. After sputtering at 500 °C, SrTiO3 layers on Al2O3 substrates are partially crystalline. In this case the SrTiO3 lattice is enlarged compared to the bulk material. With increasing temperatures of postannealing, the crystallites grow and the lattice shrinks. In particular, films that are sputtered with a sputter gas containing oxygen show an additional phase of TiO2 when they are annealed at temperatures of more than 1000 °C. This paper shows for the first time results of the investigation of sputtered SrTiO3 films on Al2 O3 substrates as a function of oxygen partial pressure P(O2 ). The measurements performed on the sputtered films indicate semiconducting properties in agreement with the bulk material: At temperatures between 700 and 1100 °C the electrical conductivity of sputtered layers changes from...

Carbon-monoxide sensors based on thin films of BaSnO3

Abstract BaSnO 3 is a very promising sensor material for detecting carbon monoxide. The mechanism of CO sensitivity of this metal oxide is supposed to be a surface reaction process. Thus to get a high gas sensitivity, the CO elementary sensor is realized as a thin film. The sensitivities of the BaSnO 3 thin films to CO are measured as a function of the temperature and the oxygen concentration. As the most important cross-sensitivities, the influence of humidity, methane and hydrogen on the sensor signal has been examined. The CO sensitivity shows a maximum in the range 600–700 °C under dry conditions. Humidity has a different influence on the CO sensitivity, which depends on the oxygen concentration. The cross-sensitivity to methane can be described by the sum of the sensitivities to CO and methane.

Fast oxygen sensors based on sputtered strontium titanate

Abstract The preparation of oxygen sensors based on sputtered strontium titanate films is described. Like bulk materials, the thin films show p-type and n-type conductivity which depends on the oxygen partial pressure. Using mixtures of argon and oxygen at 95 °C, response times lower than 5 ms can be detected due to a change in the oxygen partial pressure. Because the temperature in an exhaust gas reaches its maximum value at lambda = 1, fast gas and lower temperature effects on the sensor signal of La-doped SrTiO 3 can be separated.

Nitrogen oxide sensors based on thin films of BaSnO3

Abstract A very promising sensor material to detect nitrogen oxide is BaSnO3. The mechanism of NO sensitivity of this metal oxide is supposed to be a surface reaction process. Thus to get a high gas sensitivity the NO elementary sensor is realized as a thin film. The sensitivity of the BaSnO3 thin films to NO is measured as a function of the temperature and the oxygen concentration. As the most important cross-sensitivities, the influence of humidty, methane, ammonia and CO on the sensor signal was examined. The NO sensitivity shows under dry conditions a maximum in the range 450–550 °C.

Comparison of transient response of exhaust-gas sensors based on thin films of selected metal oxides

New concepts of engine control are required to meet the increasingly stringent standards for the reduction of pollutants in the exhaust gas. These new concepts can be realized by new fast sensors for lambda measurement by means of which the transient operating condition of a combustion engine and the fluctuation between the individual cylinders of the engine can be recorded. The most suitable materials for a fast exhaust-gas sensor are thin films made from different metal oxides (e.g., SrTiO3, BaTiO3 or CeO2). As bench tests show, use of these materials allows a lambda measurement to be made in a wide range (0.9 < λ < 1.1). In order to permit a cylinder-selective and stroke-selective measurement, the sensors are placed directly behind the outlet valve of the engine (position I), at the point of junction of the exhaust manifold (position II) and several centimetres towards the exhaust cycles of the engine. Nevertheless, the signals indicate that apart from position I directly behind the outlet valve, the exhaust gases of the different cylinders are mixed. This is caused by the valve operation times and the different running times of the gas from the outlet to the sensor.

Effect of grain size of sputtered cerium‐oxide films on their electrical and kinetic behavior at high temperatures

Following an annealing process of several hours duration at a temperature of at least 1100 °C, reactively sputtered cerium‐oxide films with film thicknesses ranging from 0.5 to 3 μm show a dependence of electric conductivity on oxygen partial pressure similar to that of polycrystalline bulk material within the temperature range studied (700 to 1000 °C). But films with comparatively small grain structures have specific electrical conductivities that are as much as an order of magnitude higher than those of large‐grained structures, let alone bulk materials. This outcome justifies the supposition that the carrier transport in CeO2−x thin films occurs in a grain barrier layer within which electrons are enriched. This negative carrier enrichment layer may be due to a positive surface charge. An investigation of the interaction between oxygen vacancies of CeO2−x thin films and the oxygen of the environment showed that for layer thickness of from 1 to 3 μm and temperatures of 700–1000 °C, the reaction of the ox...

Sensors for monitoring environmental pollution

Abstract Increasing interest in an unpolluted environment leads to the demand for the reduction of pollutant gases emitted by combustion processes. This requires novel sensor systems which are capable of in situ and real time measurement of the exhaust gas concentrations and thus provide the input signals for an emission-regulated process control. The sensors are manufactured in microsystems technology, using thick-film and thin-film techniques. The operating principles of such sensors are separated in bulk and surface effects. Based on SrTiO3 thin films, oxygen sensors with response times of about 5 ms at operating temperatures of 1000 °C have been implemented (bulk effect). Using these sensors, a cylinder selective λ measurement in the exhaust gas of an Otto engine is possible, which is fundamental for a novel motor management concept to reduce the emissions. The use of Ga2O3 thin films yields catalytically not active oxygen sensors at high temperatures (bulk effect) and on the other hand sensors for reducing gases (surface effect) at lower temperatures.

Influence of dopants on the response time and the signals of lambda sensors based on thin films of strontium titanate

Abstract New sensors produced on the basis of oxygen-sensitive thin films of strontium titanate appear to allow cylinder-selective monitoring of the combustion of an Otto engine. Selective incorporation of dopants into the basic material permits the properties to be optimized with respect to special application effects. The incorporation of acceptors (e.g. Fe or Ga) into the sensitive material leads to a reduction of the sensor response time. If the basic material is doped with donors, the measurement sensitivity increases with respect to the lambda value.

The effect of inhomogeneous substrate-temperature distribution on the signal response of oxygen sensors

Abstract Temperature has a major influence on the electrical conductivity and response times of high-speed oxygen sensors comprising thin, sputtered films of semiconductive metal oxides such as SrTiO 3 . As it is undesirable in the majority of applications, the temperature sensitivity is usually eliminated by incorporating a heating element in the sensor in order to maintain a constant temperature. A homogeneous temperature distribution across the sensitive film is crucial for the reproducibility of the sensor signals and can be achieved by calculating the geometry of the heating element with the aid of the finite element method. The temperature distributions calculated in this way can be verified by thermographic and IR photography.

Emission and Absorption Experiments on Strontium Titanate in Reducing Atmospheres

We performed two types of experiments on polycrystalline SrTiO3 thin films at temperatures between 573 and 1273 K in reducing gas mixtures containing CO or CH4. Using an FT-IR spectrometer with a long-path gas cell, we analysed the composition of the gas mixture by means of absorption measurements before and after its exposure to the sample, and in a second step we measured the emission from the surface of the metal oxide. The absorption experiments revealed two different catalytically activated processes of CO oxidation, at 950 and 1100 K respectively. In the temperature region where catalytic conversion predominates, the oxidation of CO has an Eley-Rideal type characteristic, whereas the oxidation of CH4 obeys a Langmuir-Hinshelwood mechanism. The experiments with CH4 showed a pronounced dependence on the oxygen partial pressure above 1040 K, which could originate from reduction of the SrTiO3 surface. We found emission from physisorbed CO2 in CO-containing atmospheres, but no emission from adsorbed molecules was found in the experiments with CH4.