Maximilian Fleischer

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.

Selective gas detection with high-temperature operated metal oxides using catalytic filters ☆

Metal oxide gas sensors in general, show high sensitivity but poor selectivity if pure sensor materials are used. The performance in terms of reproducibility may be enhanced by using very stable materials, which may be operated at quite high temperature but this does not help in the problem of selectivity. This paper discusses the use of gas pretreatment using filters operated at elevated temperatures to overcome this problem for special applications. Three different types of gas filters are discussed: a physical filter which is permeated by hydrogen only, a catalyst filter which removes disturbing solvent vapours by oxidation, and a gas conversion filter which ensures a defined NO/NO2 equilibrium. These filters relate to the applications of selective indoor CH4, H2, and automotive exhaust gas nitrogen oxide detection.

High temperature Ga2O3-gas sensors and SnO2-gas sensors: a comparison

Abstract High temperature Ga 2 O 3 -gas sensors show some differences to other sensors based on other metal oxides like SnO 2 . Among the advantages of Ga 2 O 3 -based sensors good long-term stability, fast response and recovery times, good reproducibility, low cross sensitivity to humidity and short pre-ageing times have to be mentioned. The good stability in sulphur-containing atmospheres makes them suitable for use in domestic burner controls. On the other hand, is the high operation temperature of about 600–800°C which means that power consumption is comparably high ( 2 O 3 -sensors to CO and NO 2 is lower compared to other metal oxide-based sensors. In this article measurements are presented in order to compare the performances of high temperature Ga 2 O 3 -gas sensors and SnO 2 -sensors. Electronic conductivity models are proposed explaining the above mentioned differences and recommendations for the different application areas are given.

Optical properties of gallium oxide thin films

The optical functions of β-Ga2O3 thin films have been determined by ellipsometry from 0.74–5 eV. Several electron-beam evaporated and rf magnetron sputtered films of different thicknesses were investigated using a multisample technique. Refractive index values comparable to those of bulk material are found. Cauchy dispersion model fits yield a high-frequency dielectric constant e∞ of 3.57. Above 4.7 eV a direct absorption edge is observed.

Sensing reducing gases at high temperatures using long-term stable Ga2O3 thin films

Abstract Polycrystalline semiconducting Ga2O3 thin films represent a promising new basic material for sensors used to detect reducing gases at operating temperatures up to 650 °C. The electrical conductance of the Ga2O3 films provides the sensor signal. Fundamental investigations with two-component gas mixtures of a reducing gas in an inert gas are reported. The results show a long-term stable dependency of the conductance of the Ga2O3 thin films on the partial pressure of the reducing gas according to G ∼ pr.g. 1 3 at constant sensor temperature for the case of H2 and CO. In contrast to other metal oxides, Ga2O3 exhibits no disturbing bulk O2 sensitivity below 700 °C.

Solid State Gas Sensor Research in Germany – a Status Report

This status report overviews activities of the German gas sensor research community. It highlights recent progress in the field of potentiometric, amperometric, conductometric, impedimetric, and field effect-based gas sensors. It is shown that besides step-by-step improvements of conventional principles, e.g. by the application of novel materials, novel principles turned out to enable new markets. In the field of mixed potential gas sensors, novel materials allow for selective detection of combustion exhaust components. The same goal can be reached by using zeolites for impedimetric gas sensors. Operando spectroscopy is a powerful tool to learn about the mechanisms in n-type and in p-type conductometric sensors and to design knowledge-based improved sensor devices. Novel deposition methods are applied to gain direct access to the material morphology as well as to obtain dense thick metal oxide films without high temperature steps. Since conductometric and impedimetric sensors have the disadvantage that a current has to pass the gas sensitive film, film morphology, electrode materials, and geometrical issues affect the sensor signal. Therefore, one tries to measure directly the Fermi level position either by measuring the gas-dependent Seebeck coefficient at high temperatures or at room temperature by applying a modified miniaturized Kelvin probe method, where surface adsorption-based work function changes drive the drain-source current of a field effect transistor.

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

Advances in application potential of adsorptive-type solid state gas sensors: high-temperature semiconducting oxides and ambient temperature GasFET devices

This paper reviews some scientific considerations about the underlying material properties and the detailed functional principle of two important types of solid state gas sensors. This is used to predict the application potential of these sensor technologies. The classical methodology to use resistance readout of heated semiconducting metal oxides is a straightforward approach to get robust sensors that are simple in design. The usage of materials that are operated at higher temperatures opens up the way for improvements; due to a change in the mechanism of electrical conductivity, better reproducibility and stability of the electrical properties can be attained. The high operation temperatures also lead to changes in the surface reactions with the gases that allow for quicker equilibration times. The next step is taken when devices are used that are based on the readout of the work function of the sensing materials. Suspended gate FETs here serve as the transducer structures. Surface properties are directly used here, which facilitates the preparation of sensing materials. The sensors can be used with a wide range of sensing materials, allowing the development of receptor materials that optimally fit the target gases. Functional improvements include enhanced selectivity and detection of a wider range of gases. These devices may work at room temperature with little energy required for running them and additionally allow direct access to the structure of the analyte molecule without thermal decomposition.

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.

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 .