Metal Oxide Nanoparticles as Novel Gate Materials for Field-Effect Gas Sensors

Materials and Manufacturing Processes, 21: 275–278, 2006 Copyright © Taylor & Francis Group, LLC ISSN: 1042-6914 print/1532-2475 online DOI: 10.1080/10426910500464651

Metal Oxide Nanoparticles as Novel Gate Materials for Field-Effect Gas Sensors S. Roy1 , A. Salomonsson1 , A. Lloyd Spetz1 , C. Aulin2 , P.-O. Käll2 , L. Ojamäe2 , M. Strand3 , and M. Sanati3 1

S-SENCE and Division of Applied Physics, Linköping University, Linköping, Sweden 2 Physical and Inorganic Chemistry, Linköping University, Linköping, Sweden 3 Division of Chemistry, Växjö University, Växjö, Sweden

Oxide nanoparticle layers have shown interesting behavior as gate materials for high temperature (typically at 300–400
C) metal-insulator-silicon carbide (MISiC) capacitive sensors. Distinct shifts in the depletion region of the C-V (capacitance-voltage) characteristics could be observed while switching between different oxidizing and reducing gas ambients (air, O2 , H2 , NH3 , CO, NOx , C3 H6 ). Shifts were also noticed in the accumulation region of the C-V curves, which can be attributed to the change in resistivity of the gate material. Sensor response patterns have been found to depend on operating temperature. Keywords Accumulation region; Adsorption; Capacitance-voltage (C-V); Depletion region; Field-effect; Gas sensors; Gate material; High frequency; High temperature; Interface; Metal-insulator-semiconductor (MISIC); Nano particles; Ruthenium oxide; Silicon carbide; Transient response.

1. Introduction Metal oxide nanoparticles, owing to their very high surface specific area and versatile catalytic properties, are exceedingly promising as gas-sensitive materials. When used as resistive sensing element, oxide nanoparticle layers exhibit high sensitivity (down to ppb level) and enhanced selectivity to the target gas [1]. In this project, we explored the potential of catalytically active oxide nanoparticles to be used as gate material in field-effect sensor devices. Two major objectives are: a) to improve the selectivity of the SiC-based FET sensors by tailoring the dimension and surface chemistry of the nanoparticles and b) to improve the high temperature stability, which is often a tricky issue for the field-effect sensors because of restructuring of the metal gate [2]. Both semiconducting (e.g., RuO2 , Co3 O4
and insulating (e.g., -Al2 O3 and SiO2
oxides are being investigated. In the case of insulating oxides, the nanoparticles are loaded with catalytically active metals (e.g., Pt). The semiconducting oxide nanoparticles are being synthesized by wet chemical procedure while aerosol technology is adopted to deposit the layers of insulating oxides. This article reports on the gas-sensing behavior of RuO2 nanoparticle layer, acting as the gate of MISiC capacitors. Both static and transient response characteristics have been obtained at different temperatures in various gas ambients. Temperature-dependent response patterns have been analyzed.

2. Experimental 2.1. Methods for Synthesizing RuO2 RuO2 nanoparticles were synthesized by wet chemical procedure. A couple of methods were adopted in order to obtain particles of various sizes. 2.1.1. Synthesis of rutile-phase RuO2 nanocrystals using a strong organic alkali in an alcoholic solution. Ruthenium trichloride hydrate, RuCl3 · H2 O
x , (0.2036 g) was dissolved into an alcoholic solvent, containing ethanol (10 mL) and 2-propanol (10 mL). Tetrabuthylammonium hydroxide, [CH3 (CH2 )3 ]4 NOH, TBAH, (2 mL) was added to the solution. The initially formed precipitate was dissolved during stirring. The precipitate system was left for 4 h at 90
C and later separated from the solvent by centrifuging. The precipitates were washed several times with distilled water, treated with an oxidizing agent by dropwise adding H2 O2 (30%), and then dried. After drying, the precipitate was heated in an oven at 400
C for 4 h, yielding a black powder [3]. 2.1.2. Synthesis of rutile-phase RuO2 nanocrystals by an aqueous solution gel route. RuCl3 (0.2074 g) was mixed with an aqueous solution of citric acid, C6 H8 O7 · H2 O, (0.6304 g) (molar ratio Cit : RuCl3 3 : 1) under stirring. The suspension was heated to 40
C and, subsequently, H2 O2 (30%) was added. Only small portions of H2 O2 were added each time because the reaction that takes place is highly exothermic. Upon the addition of 120 equivalents H2 O2 (∼11 mL), a clear dark-red solution was obtained with a pH of 1.9. Refluxing at 90
C for 3 h dropped pH to 1.5 and the color changed from dark red to dark green. The solution was poured into a vessel and the water was evaporated in a furnace at 60
C. A brown gel was formed, which was heat-treated in an oven at 300
C for 4 h [4].

Received June 22, 2005; Accepted November 9, 2005 Address correspondence to S. Roy, Department of Mechanical and Materials Engineering, Florida International University, 10555 West Flagler Street, Miami, FL 33174, USA; Fax: 305-348-1932; E-mail: roys@fiu.edu

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Figure 1.—Mounting of the MISiC capacitor sensors: 1) 16-pin holder, 2) sensor chip, 3) ceramic heater, and 4) Pt-100 temperature detector.

2.2. Fabrication of the Sensor Devices The sensors used in this study were MISiC capacitors based on 4H-SiC with a catalytically active layer as the gate material. The insulator on top of SiC was a multilayer of SiO2 /Si3 N4 /SiOx (total thickness 80 nm). The ohmic contact on the backside of the chip consisted of alloyed Ni with 50 nm TaSix and 400 nm Pt deposited on top (as corrosion protective layer). The bonding pad on top of the sensor was formed by depositing Ti/Pt layers. The active gate region was formed by drop deposition of the particles in a suspension of methanol and deionized water using a micropipette. A constant volume (3 L) of the suspension was taken to define a gate area of ∼1 mm diameter for each capacitor. Post-deposition annealing (400
C for 30 min) was performed to enhance the stability of the particles on the SiO2 surface. This thermal treatment was found also effective for consistent electrical behavior of the gate material. 2.3. Mounting and Measurements The sensor chip was glued onto a ceramic micro-heater, which was placed on a 16-pin holder, together with a Pt-100 element as the temperature detector (Fig. 1). The contacts of the sensors, the heater, and the Pt-100 element were connected by gold bonding to the pins of the holder. The holders were mounted in aluminum blocks, which are connected in a gas flow line. The gas was carried through the device across the sensor surfaces using a computercontrolled gas mixing system. A Boonton 7200 capacitor meter was used with 1 MHz frequency for measuring the capacitance of the sensor devices under investigation. 3. Results and discussion MISiC capacitors with ruthenium oxide nanoparticles as gate material were exposed to different gases. The corresponding capacitance vs. voltage characteristics were recorded. Figure 2 shows the representative high frequency (1 MHz) C-V curves of the capacitors in synthetic air (as the reference) and 1% H2 in air (as the test gas) at four different temperatures. It can be noticed that as the operating temperature goes up, characteristic shifts are observed in the depletion region as well as in the accumulation region

Figure 2.—Characteristics C-V curves (1 MHz) for RuO2 /insulator/SiC capacitors in oxidizing and reducing ambient.

of the C-V curves. It is apparent that the voltage shift in the depletion region, at a constant capacitance, increases as the temperature rises from 100
to 200
C. It could be related to the heat of adsorption of the analyte on the RuO2 gate layer and at the RuO2 /insulator interface. Hydrogen molecules (or other hydrogen-containing gas molecules) adsorb and dissociate onto the gate layer followed by diffusion of hydrogen atoms into the gate/insulator interface to form a layer of polarized complexes. Larger number of polarized complexes (e.g., OH− groups [5]) at the interface gives rise to the higher shift in the depletion region. Further increasing the operating temperature resulted in a reduced shift in the depletion region, although a shift in the accumulation region is gradually noticed, which is pronounced at 400
C. This observation may be attributed to the reasonable change in resistivity of the RuO2 film at higher temperatures. As more adsorbed oxygen species (O− , O2− or O2 − ) are depleted from the gate layer (intrinsically n-type conducting RuO2−x ) in a reducing ambient, by water formation reaction, more and more electrons (majority careers) are liberated into RuO2 matrix, making it electrically more conducting. Therefore, as the gate voltage sweeps toward higher positive values, the capacitance in the accumulation region becomes less sensitive to the voltage. On the other hand, in an oxidizing ambient (air), there is a potential drop across the less conducting gate layer (RuO2 ), and the resultant capacitance is a function of applied voltage even in the accumulation region. Lesser voltage shifts in the depletion region at higher temperatures (300
through 400
C) are due to the relative dearth of polarized hydrogen atoms at the RuO2 /insulator interface. Higher rate of desorption of hydrogen molecules and water formation reaction on the gate layer are the influencing parameters to determine hydrogen coverage at the interface under equilibrium condition [6]. Transient response characteristics of the capacitive sensors were performed in the constant capacitance mode. Working points for each set of experiments were chosen from the C-V characteristics curves. A closed feedback loop maintained the depletion capacitance constant while

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Figure 3.—Transient response pattern of the MISiC sensors at 200
C.

measuring the voltage shift. A pulse train of different gases of different concentrations was applied to the sensor in the constant capacitance mode. For each temperature, the corresponding output signal was recorded. Figure 3 represents the transient response behavior of the capacitors at 200
C. The noisy baseline is probably due to the nonuniformity in the gate material (drip-deposited RuO2 nano particles). Distinct sensitivity to the hydrogencontaining gas molecules can be observed from the response pattern. A shift in baseline is also observed, especially after exposure to propene. It could be due to the lack of sufficient desorption of the hydrocarbon molecules at this relatively lower temperature. It is interesting to notice that at 200
C the NH3 sensitivity is higher as compared to the identical concentration (e.g., 1000 ppm) of H2 or C3 H6 diluted in air. The detection of NH3 is related to the number of triple phase boundaries [7], where the gas (analyte), insulator, and gate material coexist, and this is likely to be high in the porous RuO2 gate layer. The response pattern at 300
C (Fig. 4) is distinct from that at 200
C in the way that the magnitude of hydrogen

Figure 5.—Transient response pattern of the MISiC sensors at 400
C. Increased baseline noise and decreased output signal for all reducing gases as referred to the sensor response obtained at 300
C.

response is somewhat lower. This behavior is similar to that observed for the Pt-gate MISiCFET sensor system [8]. Most importantly, the propene response is higher as referred to the other gases of the same concentration. Liberation of a higher number of hydrogen atoms, which are responsible for forming a dipole layer at the interface, upon dissociation of each C3 H6 molecule on the sensor surface probably explains this phenomenon. Although there is an indication of NOx sensitivity, the low signal-to-noise ratio makes it difficult to quantify. In Fig. 5, the response pattern of the sensors at 400
C is presented. The signal-to-noise ratio seems to be very high at this temperature. The output signal corresponding to each gas pulse is reduced considerably. Combustion of gas molecules at high temperature and higher rate of desorption from the sensor surface, under equilibrium condition, are the possible reasons for this observation. 4. Conclusions Metal oxide nanoparticles are potential candidates to be used as gate material for field-effect devices. MISiC capacitors with chemically synthesized ruthenium oxide nanoparticles in the gate region exhibit sensitivity to hydrogen-containing gas molecules. The sensitivity pattern depends on operating temperature. The optimum values of the sensor output signals were obtained at 200
C, similar to Pt-gate field-effect sensors. While the hydrogen and ammonia response exceeds the propene response at 200
C, the hydrogen and propene responses are highest at 300
C. Novel response behaviors are expected from the other oxide nanoparticles (Co3 O4 , Pt/ Al2 O3 , Pt/ SiO2 , etc.) under study. References

Figure 4.—Transient response pattern of the MISiC sensors at 300
C. C3 H6 response is apparently higher in comparison to identical concentration of other gas molecules in air.

1. Baraton, M.; Merhari, L. Nanoparticles-based chemical gas sensors for outdoor air quality monitoring microstations. Materials Science and Engineering B 2004, 112, 206–213. 2. Wingbrant, H.; Lloyd Spetz, A. Dependence of Pt gate restructuring on the linearity of SiC field effect transistor lambda sensors. Sensor Letters 2003, 1, 37–41.

278 3. Music, S.; Popovic, S.; Maljkovic, M.; Furic, K.; Gajovic, A. Influence of procedure on the formation of RuO2 . Material Letters 2002, 56 (5), 806. 4. Pagnaer, J.; Nelis, D.; Mondelaers, D.; Vanhoyland, G.; D’Haen, J.; Van Bael, M.K.; van den Rul, H.; Mullens, J.; Van Poucke, L.C. Synthesis of RuO2 and SrRuO3 powders by means of aqueous solution gel chemistry. Journal of the European Ceramic Society 2004, 24 (6), 919. 5. Wallin, M.; Grönbeck, H.; Lloyd Spetz, A.; Skoglundh, M. Vibrational study of ammonia adsorption on Pt/SiO2 . Applied Surface Science 2004, 235, 487–500.

S. ROY ET AL. 6. Dannetun, H.; Lundström, I.; Petersson, L.-G. Hydrocarbon dissociation on palladium studied with a hydrogen sensitive Pdmetal-oxide-semiconductor structure. Journal of Applied Physics 1988, 63 (1), 207–215. 7. Eriksson, M.; Utaiwasin, C.; Carlsson, A.; Löfdahl, M. On the ammonia response of field-effect devices. In Proceedings of Eurosensors XIII, The Hague: The Netherlands, 1999. 8. Andersson, M.; Ljung, P.; Mattsson, M.; Löfdahl, M.; Lloyd Spetz, A. Investigations on the possibilities of a MISiCFET sensor system for OBD and combustion control utilizing different catalytic gate materials. Topics in Catalysis 2004, 30/31, 365–368.