Hans Meixner

Gallium oxide thin films: A new material for high-temperature oxygen sensors

Abstract Semiconducting thin films from Ga2O3 sensitive to oxygen at temperatures of around 850–1000 °C were produced by a sputtering technique. Their stable oxygen sensitivity is based on a bulk effect independent of the film thickness: the electrical conductivity of the films depends on the oxygen partial pressure according to a law of the form σ ∼ pO2− 1 4 . The response times are in the range of seconds. With suitable temperature compensation, Ga2O3 thin films can be used as catalytically inactive oxygen sensors.

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...

Electron mobility in single‐ and polycrystalline Ga2O3

Ga2O3 is a purely n‐conducting metal oxide at high temperatures. Increasing interest is being shown in this material as the basic material for gas sensors since its electrical conductivity is gas dependent. To investigate its electrical conductivity mechanism in the temperature range between 800 and 1000 °C, conductivity and Hall measurements were performed on single crystals and on polycrystalline ceramics. In the conductivity measurements, identical results were obtained with dc and low‐frequency ac, thus confirming the notion of purely electron current transport. A thermally activated specific conductivity is observed in both cases, that of the single crystals lying around three and a half orders of magnitude above that of the ceramic. The carrier densities are determined from the Hall measurements, a thermally activated behavior always being observed. The carrier density of the ceramic is only 2×1013 cm−3 at 1000 °C, that of the single crystals lying somewhat more than three orders of magnitude above ...

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 gas sensors based on metal oxides which are stable at high temperatures

Abstract Recently, research into the characteristics of semiconductor metal oxides that are stable at high temperatures with a view to providing reproducible detection of oxygen and reducing gases has intensified. First of all, there is a discussion of the specifics relating to these materials, for example the reduction of the effects of grain-boundary barriers on the conduction mechanism, the reduction of the humidity cross-sensitivities and also on the various reaction mechanisms. Then, the technology for constructinggas sensors of this kind will be described. Examples that have already been implemented, for example, l detection, O2 measurements in the exhaust gases from incinerators, methane alarms, and air quality control, as well as certain trends in development, are discussed

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.

Oxygen sensing with long-term stable Ga2O3 thin films

Abstract Oxygen sensors can be implemented for temperatures around 1000 °C with thin films of n-semiconducting Ga2O3. Films with thicknesses in the μm region are manufactured by a sputtering technique using a target made of Ga2O3 ceramics. They are deposited on non-conducting BeO ceramic substrates, which yields a stable grain size of about 100 nm. The electrical conductivity of these films depends reversibly according to σ ∼ pO2− 1 4 on the prevailing oxygen partial pressure. The temperature dependence of the electrical conductivity obeys the law σ ∼ exp(2.0eV/kT). Long-term tests over 150 h determine the stability of the sensor characteristic.

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.

Preparation of stoichiometric barium stannate thin films: Hall measurements and gas sensitivities

Barium stannate (BaSnO 3 ) is a typical compound with cubic perovskite lattice. For the first time thin films with a Ba/Sn ratio of 1 were prepared by radio frequency (r.f.) sputtering. Hall measurements were performed on the thin films in a temperature range between 600°C and 1050°C. The variation of Hall mobility and charge-carrier density was investigated in the oxygen partial pressure range between 10 2 Pa and 10 5 Pa. The gas sensitivities of the material in wet air were investigated by resistance measurements of sensor chips with sputtered BaSnO 3 thin films. The material showed interesting responses to the reducing gases isobutene, H 2 , NO, CH 4 and CO in application-relevant concentrations. The best sensitivities were obtained at temperatures below 700°C. At temperatures of 600°C the thin films showed response times of typically 3 min. The material was found to be insensitive to CO 2 and NH 3 .