A. Principle of the method
Gas sensing is based on the interaction of the gas with the sensor surface. Charge transfer from and to the semi-conducting sensor material leads to conductivity change of the latter. The sensor is part of an electric circuit with a stabilized bias voltage and any current change is measured by a pA-meter. The current values are digitized and plotted in real time on the pC screen.
B. Measuring setup
The setup consists of the following subsystems:
1) The aluminum vacuum chamber in which the sensor reacts with the gas mixture (toxic gas in dry air).
2) The vacuum system (rotary and diffusion pump) and vacuum gauges.
3) The gas handing system to control and measure the gas flow (mass flow meters connected to a pC, a Baratron gauge, tubing and valves).
4) The electric circuit to measure the sensor response (constant voltage source and pA-meter Keithley Mo. 485).
5) The data acquisition system (A/D converter controled by a pC) with real time recording of the current changes.
6) The sensor heater (400 oC max.) with power supply, stabilized current source, and temperature controller.
Fig. 1: On the right is the sample heater/holder, in the middle the digital temperature display and on the left the constant current power supply.
Fig. 2. Measuring setup for toxic gas sensors developed at the LATA group.
C. Examples
In the following, both n-type and p-type semiconducting thin films have been tested: their conductivity response against reducing hydrogen is opposite, a fact that has been verified in all cases. Hydrogen has been used as a test gas for a variety of thin film gas sensors, grown by different procedures: magnetron sputtered, PLD grown and spin-coated films have been investigated. Finally a micro-sensor (lab prototype) produced by a special photo-lithographic process has been also investigated.
C1. n-type ZnO sensor testing
The ZnO thin film was deposited by Reactive Pulsed Laser Deposition (R-PLD). The growth was performed at 200 οC in 20 Pa oxygen pressure. The 2% hydrogen in dry air was investigated at 200 oC operating temperature.
Fig. 3 Response of a n-type ZnO semiconducting thin film against 2% Η2 in real time.
C2. Sputtered p-type NiO sensor testing
The NiO thin film was deposited by magnetron sputtering in Bratislava, Slovak Republic. The 0.2% hydrogen in dry air was investigated at 236 oC operating temperature.
Fig. 4: Response of a p-type semiconducting thin film ΝιOin 0.5% Η2 in real time.
The sensor response is opposite, compared to the one for ZnO. The response time for hydrogen sensor is 11 min.
As a further result of the above measurements, the response against a reducing gas like hydrogen can give information about the conductivity type (that is, electrons of holes) of a semiconducting thin film. A further verification can be achieved by Hall Effect measurements.
C3. PLD grown p-type NiO sensor testing
A p-type NiO thin film, deposited by R-PLD has been tested against hydrogen. The film was deposited at 10 Pa oxygen pressure on a oxidized Si substrate.
Details about the growth conditions and sensing are given in:
“Nickel oxide thin films synthesized by reactive pulsed laser deposition: characterization and application to hydrogen sensing” ,I. Fasaki, A. Giannoudakos, M. Stamataki, M. Kompitsas, E. György, I. N. Mihailescu, F. Roubani-Kalantzopoulou, A. Lagoyannis, S. Harissopulos,
Fig. 5: Response of a p-type semiconducting thin film ΝιOagainst hydrogen from 1% to 0.05% concentration range in dry air in real time.
C4. Pt-nanoparticles sensitized, magnetron sputtered p-type NiO sensor testing
NiO thin films sputtered on various substrates were tested against hydrogen. Other parameters were annealing temperature and partial coverage with Pt nanoparticles during a subsequent sputtering process. The resulting films had a thickness of ca 100 nm. The operating temperature of the sensor was investigated in the (80 – 200) oC range. Sensors sensitized with Pt show a very good response at as low as 80 oC.
Fig. 6: Response of a magnetron sputtered NiO thin film against hydrogen in the 1% - 0.25% concentration range. The sensor surface was partially covered with Pt naoparticles and annealed at 200 oC (the sensor was produced is Bratislava, Slovak Republik).
C5. Reactive PLD grown p-type NiO sensor testing
Au-doped NiO thin films using the 2-laser, 2-target PLD method developed in the LATA group have been tested against hydrogen for 1% to 0.05% concentration range. The films were deposited on non-conductive substrates (quartz or oxidized Si). The operating temperature of the sensor has been varied in a broad range.
Fig. 7: Response of a p-type NiO thin film, doped with Au by the 2-laser, 2-target PLD technique in the LATA group. The operating temperature was as low as 96 oC and the concentration range 1% - 0.1% hydrogen in dry air.
C6. Nanoparticles-doped SnO2 thin films.
n-type SnO2 thin films, doped with noble metal nanoparticles (Pt, Au) have been tested against hydrogen in dry air. The samples were deposited by spin-coating in TPCI. Doping of such films has resulted in reduced operating temperature and shorter sensor response. Hydrogen concentrations as low as 0.05% have been recorded.
Fig. 8: Response of a SnO2 thin film doped with Au nanoparticles against H2 at various concentrations in dry air at 148 OC operating temperature. The thin film was deposited by sol-gel (spin coating).
C7. Microsensor testing
The micro-sensor (lab prototype) was produced by a special photo-lithographic process in combination with magnetron sputtering in the Microelectronics Department of the Technical University of Bratislava, Slovak Republic. Both the Pt micro-heater and the NiO thin film sensor were successively deposited on a DO-8 mount. The sensor temperature was set by the current through the Pt heater. Microsensors without and with surface sensitizing with Pt nanoparticles have been tested. Pt-sensitized microsensors show a better response at lower operating temperatures.
Fig. 9: Repeatability of the response of a thin film NiO microsensor against 0.5% hydrogen in dry air. At an operating temperature 200 oC (the sensor was produced in Bratislava, Slovak Republic).
Fig. 10: Response of a magnetron sputtered NiO thin film against hydrogen in the 0.5% to 0.05% concentration range. The surface sensor was sensitized by partially covering with Pt nanoparticles and operated at 100 oC(the sensor was produced in Bratislava, Slovak Republic)
D. Sensing characteristics of Metal Oxide thin films, grown by various physico-chemical processes.
The setup presented in Fig. 2 above has been used to study the sensing characteristics of films in respect to hydrogen gas and correlate their behavious with their post-deposition treatment.
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