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New Materials for Gas Sensitive Field-Effect Device Studies
Linköping University, Department of Physics, Chemistry and Biology, Applied Physics. Linköping University, The Institute of Technology.
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Gas sensor control is potentially one of the most important techniques of tomorrow for the environment. All over the world cars are preferred for transportation, and accordingly the number of cars increases, unfortunately, together with pollutants. Boilers and powerplants are other sources of pollutants to the environment. Metal-Insulator-Silicon Carbide (MISiC) Field-effect sensors in car applications and boilers have the potential to reduce the amount of pollutants. These devices are sensitive to several gases in exhaust and flues gases, such as hydrogen, hydrocarbons, and ammonia (for the selective catalytic reduction (SCR) application). These applications require specific and long term stable sensors. The car industry for instance wants sensors that will stand at least 240 000 km.

This thesis presents studies of the active layers in MIS Fieldeffect gas sensors. Fundamental studies of the sensor mechanism has been performed in ultra high vacuum, UHV, to understand the gas response mechanism in more detail, and to find out how the sensing mechanism is affected by the catalytic active gate material. The influence of four different insulating layers was studied at atmospheric pressure. The catalytic layer has also been altered to metal oxide nanoparticles with or without impregnation of catalytic metals.

Nanoparticles are potential candidates to be used as the gate material for high temperature, long-term stable FET sensor devices. The combination of catalytic metals and metal oxides may prevent reconstruction of the metal. The use of nanoparticles will increase the number of triple points (catalytic material and insulator in contact with gas), which are crucial e.g. for the ammonia sensitivity. Another challenging aspect of nanoparticles is the possibility to get selectivity to different gases based on the particle size.

The goal is to find new sensitive, selective and more long term stable materials, which meet the requirements above.

From the UHV studies we learned that the two catalytic active metals Pt and Pd, do behave in a similar way, although there are some quantitative differences. Values for the heat of adsorption on both the Pd and Pt surfaces are estimated as well as the dipole moments for the adsorbates on the insulator surface.

The insulators play an important role in the sensing mechanism, since the adsorption of hydrogen atoms (or protons) that are detected by the sensor occur on the insulator surface. By changing the insulator material the saturation response of the sensors is affected. It was shown that Al2O3 gave a higher saturated response to hydrogen in Pt-MIS capacitors at 140°C as compared to Ta2O5, SiO2, and Si3N4.

We have tested wet synthesized ruthenium dioxide and ruthenium nanoparticles, which are electrically conducting and catalytically active sensing material. RuO2 is especially interesting as a high temperature material since it is already oxidized. Both materials show a sensitivity pattern comparable to porous platinum. The temperature dependence of the gas response indicates a higher catalytic activity of the RuO2 as compared to Ru nanoparticles.

Nanoparticles synthesized by aerosol technology provide several advantages like a good adhesion of the particles to the substrate, many possible material combinations and efficient methods for particle separation according to size. The methods to use this technology for sensing materials in MISiC sensors are now under development and some preliminary results are obtained.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2005. , 64 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 957
Keyword [en]
ruthenium dioxide, nanoparticles, MISiCFET, sensors
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-4243ISBN: 91-8529-974-X (print)OAI: oai:DiVA.org:liu-4243DiVA: diva2:20569
Public defence
2005-10-07, Hörsal Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Note
On the day of the public defence of the doctoral thesis, the status of articles I and II was: accepted for publication.Available from: 2005-10-06 Created: 2005-10-06 Last updated: 2015-03-24Bibliographically approved
List of papers
1. Hydrogen Interaction with Platinum and Palladium Metal Insulator Semiconductor devices
Open this publication in new window or tab >>Hydrogen Interaction with Platinum and Palladium Metal Insulator Semiconductor devices
2005 (English)In: Journal of Applied Physics, ISSN 0021-8979, Vol. 98, no 1, 14505-14514 p.Article in journal (Refereed) Published
Abstract [en]

Hydrogen-sensitivePd–SiO2–Si and Pt–SiO2–Si metal–insulator–semiconductor (MIS) devices have been studied inultrahigh vacuum in the temperature range of 223–523  K. Adsorption/absorption ofhydrogen occurs at the metal surface, in the metal bulk,and at the metal–insulator interface. The sensor signal, caused byhydrogen adsorption at the interface, shows a logarithmic dependence onthe applied hydrogen pressure. The Pt-MIS device, which is fullyfunctional at atmospheric pressures, is sensitive to changes in hydrogenpressure down to the 10–12-Torr scale. We propose that theinterface adsorption follows a so-called Temkin isotherm with an interfaceheat of adsorption that varies with hydrogen coverage as Hi0(1–a).The initial heat of adsorption Hi0 is determined to 0.78  eV/hydrogenatom. The adsorption potential at the external Pt surface isfound to be 0.45  eV/hydrogen atom. These values were obtained bymodeling the hydrogen interaction with the MIS devices and fittingthe model to a number of experimental results. Also studiesof Pd-based devices were performed and compared with Pt. Thehydrogen adsorption on the metal surface, previously treated as afirst-order process on Pd, is shown to follow a second-orderprocess. Qualitatively the results from the Pd- and Pt-MIS devicesagree. Quantitatively there are differences. The hydrogen sensitivity of thePt-MIS device is only approximately one-third compared to that ofthe Pd-MIS structure. This agrees with the result that theconcentration of available hydrogen adsorption sites at the Pt–SiO2 interfaceis approximately 7×1017 m–2 whereas the concentrations of sites at thePd–SiO2 interface is roughly three times larger (2×1018 m–2). An estimateof the size of the dipole moments (0.6–0.7  D) implies thatthe interface hydrogen atoms are strongly polarized. Differences are alsoobserved in the microstructure of the metal films. Atomic forcemicroscopy results show that the Pd surface reconstructs during H2–O2exposures, while the Pt surface shows no such change atthese temperatures.

Keyword
palladium, platinum, silicon compounds, silicon, hydrogen, elemental semiconductors, MIS devices, gas sensors, atomic force microscopy, adsorption, surface reconstruction, heat of adsorption, crystal microstructure
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13383 (URN)10.1063/1.1953866 (DOI)
Available from: 2005-10-06 Created: 2005-10-06 Last updated: 2016-07-08
2. The influence of the insulator surface properties on the hydrogen response of field-effect gas sensors
Open this publication in new window or tab >>The influence of the insulator surface properties on the hydrogen response of field-effect gas sensors
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2005 (English)In: Journal of Applied Physics, ISSN 0021-8979, Vol. 98, no 3, 34903-34908 p.Article in journal (Refereed) Published
Abstract [en]

The hydrogen response of gas-sensitive field-effect devices is mainly due to trapping of atomic hydrogen on the insulator side of the metal-insulator interface of the metal-insulator-semiconductor (MIS) structure. Therefore an influence of the choice of insulator on the hydrogen response properties is expected. We have investigated this influence by producing MIS capacitors with four different insulators; SiO2, Al2O3, Si3N4, and Ta2O5. The results show that the choice of insulator influences the detection limit, the saturation concentration, and the saturation response. Furthermore, there is a strong correlation between the observed saturation response and the oxygen concentration of the insulator surface, as measured by Auger electron spectroscopy, which indicates that the trapping of hydrogen at the interface occurs at the oxygen atoms of the insulator surface. Finally, if the metal film is porous a catalytic oxidation of the insulator surface appears to be facilitated, which can increase the hydrogen response.

Keyword
silicon compounds, alumina, tantalum compounds, hydrogen, dielectric materials, gas sensors, MIS capacitors, Auger electron spectra, catalysis, oxidation, interface states
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13384 (URN)10.1063/1.1994941 (DOI)
Available from: 2005-10-06 Created: 2005-10-06 Last updated: 2015-03-24
3. Nanoparticles for long-term stable, more selective MISiCFET gas sensors
Open this publication in new window or tab >>Nanoparticles for long-term stable, more selective MISiCFET gas sensors
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2005 (English)In: Sensors and Actuators B, ISSN 0925-4005, Vol. 107, no 2, 831-838 p.Article in journal (Refereed) Published
Abstract [en]

Synthesis of metal-oxide nanoparticles and utilization of these particles as gate materials for field-effect sensor devices is reported. Improved selectivity to specific gases is expected by modulating the size of the oxide nanoparticles or impregnating them with catalytic metals. Another objective is to improve the long-term thermal stability of the sensors, since the metal loaded nanoparticles may prevent thermally induced restructuring of the gate layer, which is often a problematic issue for the catalytic metal layers. Because of its reasonably high electrical conductivity, which is especially important for the capacitive gas sensors, ruthenium dioxide has been identified to be one of the potential candidates as gate material for the field-effect sensor devices. Interestingly, this material has been found to change its resistivity in different gaseous ambients. When used as a gate material, sensitivity to reducing gases has been observed for the RuO2/SiO2/4H-SiC capacitors. Changes in the resistivity of the films due to various gas exposures have also been recorded. Morphological studies of nanoparticles (SiO2 and Al2O3), loaded or impregnated with catalytic metals (e.g. Pt), have been performed.

Keyword
Sensors; Catalytic material; MISiCFET; Ruthenium dioxide
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13385 (URN)10.1016/j.snb.2004.12.024 (DOI)
Available from: 2005-10-06 Created: 2005-10-06 Last updated: 2015-03-09
4. Ruthenium dioxide & Ru Nanoparticles for MISiCFET gas sensors
Open this publication in new window or tab >>Ruthenium dioxide & Ru Nanoparticles for MISiCFET gas sensors
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2005 (English)In: Nanotech 2005 (NSTI) Anaheim, USA, 8-12 May, 2005, Vol. 2, no Chapter 4, 269–272- p.Conference paper, Published paper (Refereed)
Abstract [en]

Catalytically active nanoparticles used as gate material on SiC-FET gas sensors. The goal is to improve the selsectivity and senstitivty.The sensors are sensitive towards oxidising and reducing gases (H2, NH3, C3H6).

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13386 (URN)
Available from: 2005-10-06 Created: 2005-10-06 Last updated: 2015-03-09
5. Nanocrystalline Ruthenium oxide and Ruthenium in sensing applications -an experimental and theoretical study
Open this publication in new window or tab >>Nanocrystalline Ruthenium oxide and Ruthenium in sensing applications -an experimental and theoretical study
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2006 (English)In: Journal of Nanoparticle Research, ISSN 1388-0764, Vol. 8, no 6, 899-910 p.Article in journal (Refereed) Published
Abstract [en]

In this project, we have explored RuO2 and Ru nanoparticles (∼ ∼10 and ∼ ∼5 nm, respectively, estimated from XRD data) to be used as gate material in field effect sensor devices. The particles were synthesized by wet chemical procedure. The capacitance versus voltage characteristics of the studied capacitance shifts to a lower voltage while exposed to reducing gases. The main objectives are to improve the selectivity of the FET sensors by tailoring the dimension and surface chemistry of the nanoparticles and to improve the high temperature stability. The sensors were characterized using capacitance versus voltage measurements, at different frequencies, 500 Hz to 1 MHz, and temperatures at 100–400°C. The sensor response patterns have been found to depend on operating temperature. X-ray photoelectron spectroscopy (XPS) analyses were performed to investigate the oxidation state due to gas exposure. Quantum-chemical computations suggest that heterolytic dissociative adsorption is favored and preliminary computations regarding water formation from adsorbed hydrogen and oxygen was also performed.

Keyword
nanoparticles, gas sensors, RuO2, Ru, FET devices
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13387 (URN)10.1007/s11051-005-9058-1 (DOI)
Available from: 2005-10-06 Created: 2005-10-06 Last updated: 2015-03-09
6. Aerosol deposited particles as gate material for MISiC-capacitor sensors
Open this publication in new window or tab >>Aerosol deposited particles as gate material for MISiC-capacitor sensors
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Manuscript (Other academic)
Identifiers
urn:nbn:se:liu:diva-13388 (URN)
Available from: 2005-10-06 Created: 2005-10-06 Last updated: 2010-01-13

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