M. Fleischer

Thin-film gas sensors based on semiconducting metal oxides

In recent years, there has been a gradual realization that an intact environment and properly functioning ecosystems are essential to the continuance of human life; this has led to the tightening up of environmental legislation. As far as air pollution is concerned, gases like sulfur dioxide, the oxides of nitrogen (NO x ), carbon monoxide and carbon dioxide are considered to be the main culprits. Most sulfur dioxide is produced by the combustion of fossil fuels that contain sulfur (coal, oil and natural gas), the sulfur being oxidized to produce sulfur dioxide. This is why SO 2 emissions can only be prevented by chemical binding. Oxides of nitrogen are also produced when fossil fuels are burnt. These oxides are largely the result of reactions between oxygen and nitrogen from the air. The main source of NO 2 emissions is automobiles. In 1986 alone, more than half (2 million tonnes in the former FRG) of all NO x emissions could be directly attributed to this source. In the same year, somewhat less NO x (1 million tonnes in the former FRG) was produced by power stations, distant heating plants and other conversion areas. Air pollution by CO emissions from automobiles is an even more clear-cut case. As CO is produced by the incomplete combustion of fossil fuels, it is not surprising that automobiles alone produced 6.5 million tonnes of this gas in 1986 (former FRG), while the three other user groups (industry/households, trade and business/power stations and distance heating plant, other conversion areas) are only responsible for approximately 2.5 million tonnes (former FRG). The presence of hydrocarbons (CH x ) in exhaust gases is also due to incomplete combustion. Hydrocarbons along with carbon dioxide and water vapour are also considered to be the main causes of the greenhouse effect. With hydrocarbons too, the main source of emissions is the automobile. The emission of CO and CH x from automobiles is particularly abundant when there is an excess of fuel (rich mixture, air coefficient λ<1). The introduction of catalytic converters only provides sufficient recombustion of these components in a very narrow range around λ = 1. Therefore, to obtain the most efficient combustion and the least emission of pollutants, it would seem necessary to monitor the combustion process in each cylinder of a vehicle and to control it so that the exhaust gas expelled from each cylinder has a λ value corresponding to the maximum degree of conversion for the catalytic converter. Fast λ measurements are, therefore, of crucial importance, as they make regulation possible during non-stationary phases of engine operations (starting, braking, acceleration) that account for 80% of the total operating time. Attempts are also being made to introduce combustion regulation in incinerators by means of coefficients. This type of emission would also require sensors that are capable of continuously monitoring the concentration of pollutant components. Sensor elements based on semiconducting metal oxides seem to be promising both for individual cylinder control in automobiles and for monitoring pollutant components

Stability of semiconducting gallium oxide thin films

Abstract In bulk, gallium oxide is a semiconducting oxygen-sensitive material at temperatures of over 500 °C. With the aid of a high frequency sputter process, films could be produced with thicknesses in the micrometre range using a Ga2O3 ceramics target. The stoichiometry and purity of these films could be demonstrated with Rutherford backscattering spectroscopy and X-ray fluorescence. Scanning electron microscopy investigations showed that, at temperatures of over 1100 °C for 50 h, crystallites approximately 200 nm in size are formed. The d.c. conductivity of these films was measured for the first time. At high temperatures, semiconducting properties similar to those of the bulk material were demonstrated. The conductivity of the Ga2O3 films is dependent on the oxygen content of the surrounding atmosphere. The exponent of the σ vs. PO2−n curve was n⩽ 1 4 . Auger spectroscopy was used to show that, at temperatures around 1000 °C, interdiffusion of the Al3+ and the Ga3+ ions takes place on substrates containing aluminium, thus destroying the conductivity of the film. Therefore Al2O3 ceramic substrates which are standard in thick film technology are unsuitable for stable oxygen-sensitive Ga2O3 thin film devices. Alternatives are shown.

Selectivity in high-temperature operated semiconductor gas-sensors

High-temperature operated metal oxides like Ga2O3 developed in the last years show certain advantages like reproducibility and robustness resulting from a conduction mechanism which is independent on grain boundary effects. But similar to the lower temperature operated oxides like SnO2 and ZnO they also possess broad band sensitivity characteristics, i.e. they respond to all gases with similar chemical properties. This paper shows to which extent strategies to achieve selective gas detection with one single sensor are applicable with these metal oxides. Temperature variations, surface modifications and the use of physical and chemical filters directly attached to the sensor surface are discussed in this paper.

In situ infrared emission spectroscopic study of the adsorption of H2O and hydrogen-containing gases on Ga2O3 gas sensors

Abstract Infrared emission spectroscopy (IRES) was used for studying in situ gas-surface interactions and adsorbed species on Ga 2 O 3 at elevated temperatures used for gas sensors. Regarding the permanent presence of humidity in most gas sensor applications, the interaction of H 2 O with the sensor surface was observed under working conditions in order to emphasize the benefits of IRES for the understanding of gas sensing mechanisms. The coadsorption of water and H 2 ,C 2 H 4 , acetone and ethanol was investigated to point out the role of humidity in the sensitive reactions to organic gases. Screen printed Ga 2 O 3 films were studied in the temperature range from 250°C up to 650°C. These results were correlated with simultaneous conductivity measurements. The evident influence of the addition of water on the surface chemistry of Ga 2 O 3 during the adsorption of hydrogen-containing gases is confirmed by this work.

CO-Sensor for domestic use based on high temperature stable Ga2O3 thin films

Abstract Gas sensors based on high temperature operated metal oxides, like Ga2O3 thin films show promising properties in terms of reproducibility, long-term stability against interfering gases and low cross sensitivity to humidity. In this paper a surface modification of Ga2O3 is presented which allows the set up of a sensor suitable for indoor CO monitoring. The modification based on Au-clusters on the Ga2O3 surface yields high sensitivity to CO and a distinct reduction of the cross sensitivity towards organic solvents. With the specimens, a resistance change of approx. factor 4 to 100 ppm CO in wet air is attained. By employing catalytic filters of ceramic material, cross sensitivities to organic solvents are virtually completely eliminated.

Low-power gas sensors based on work-function measurement in low-cost hybrid flip–chip technology ☆

To implement low-power gas sensors with low component costs, the principle of work-function read out via a hybrid suspended gate FET (SGFET) is being pursued, whereby a freely selectable sensor film undergoes a reversible work-function change corresponding to the build-up of a potential difference on the surface in response to gas adsorption/reaction. This is read out via an ISFET structure. An innovative design which allows cheap manufacturing will be described for the principle that has already been successfully demonstrated. The starting point of the design is a ceramic Al2O3 substrate coated with conductor patterns and sensitive materials onto which the FET is mounted in flip–chip technology. By means of the freely selectable sensor film and its preparation method, a wide range of applications can be opened up.

Sensitive, selective and stable CH4 detection using semiconducting Ga2O3 thin films

Abstract Due to the large operating temperature range of Ga2O3 thin films a temperature programmed setting of the gas sensitivity is possible. In the temperature range between 500 to 650 °C the sensory may be operated as sensitive sensors for reducing gases (e.g., CO, H2) based on chemisorption mechanisms. Increasing the temperature causes a strong decrease in the sensitivity to these gas and a strong increase in the sensitivity to CH4. At an operating temperature of 740–780 °C no significant sensitivity of the sensor to H2 or CO in wet air is observed but there is a sensitivity conductance increase by about a factor of 80 as a response to 0.5 vol.% CH4. In the case of a further temperature increase, the effect of H2 CO is inveretd to a conductivity decrease, whereas the CH4 sensitivity remains almost unchanged. A similar behaviour is observed in the case of binary gas mixtures. At a tuned operating temperature, there is no significant effect resulting from the coadsorption of other reducing gases or humidity on the CH4 sensor characteristics. Resistance values at this temperature have proven to be stable over several hundreds of hours even in the case of continuous exposure to CH4.

Gas-sensitive electrical properties of pure and doped semiconducting Ga2O3 thick films

Abstract The gas-sensitive electrical properties of pure Ga 2 O 3 thick films are investigated. The influence of doping on these properties has also been studied. SnO 2 was used as donator type dopand. It was found that there is an increase in the overall conductivity due to the doping up to two orders of magnitude. Despite this high conductivity, the gas sensitivity remains almost unchanged in all cases. There is no effect of the dopands on the bulk controlled oxygen sensitivity. Pure and doped Ga 2 O 3 thick films have been proved to be reproducible and stable sensor base materials. The result forms the base for the use of screen-printed electrodes or a further reduction of the chip size for a decreased heating power consumption.

Enhancement of sensitivity and conductivity of semiconducting Ga2O3 gas sensors by doping with SnO2

Abstract The use of high temperature operated metal oxides, e.g. Ga 2 O 3 thin films, for gas sensors shows promising properties in terms of reproducibility, long term stability against interfering gases and low cross sensitivity to humidity. It is shown that by employing SnO 2 (0.1–3% At ) as doping material, a very effective donor for sputter deposited polycrystalline Ga 2 O 3 thin films has been found which allows an increase in conductivity of up to two orders of magnitude, as well as an enhancement of the gas-sensitivity. This is the basis for a significant reduction of the sensor chip size to obtain a reduction in the heating power.

A selective, temperature compensated O2 sensor based on Ga2O3 thin films

Abstract Pure semiconducting Ga2O3 thin films show a reaction to reducing gases as well to oxygen variations at operation temperatures between 600°C–900°C. By applying surface modifications with catalytically active oxides like La2O3 or CeO2, a complete suppression of the reaction to reducing gases in oxygen-rich atmospheres could be achieved, yielding devices that only respond to the oxygen content. By an analysis of the desorbing gases with NIR-Spectroscopy, varying production rates of carbon oxides and unsaturated carbohydrates were observed. A modification with manganese oxide yielded complete gas-insensitive devices, which still show a thermal-activated conductivity. This effect can be used for temperature compensation purposes.