Semiconductor Film Sensors

The emphasis on the thickness of the selective layer is linked to the mode of interaction between the analyte and the selective layer. If this interaction takes place exclusively at the surface of the selective layer, then the bulk conductivity does not contribute and represents only a shunt which decreases the signal-to-noise ratio. This is a typical case of chemiresistors based on inorganic materials (Fig. 8.6). On the other hand in chemiresistors based on organic semiconductors, the signal usually originates in the bulk of the selective layer. In that case, the response time of the sensor is affected by its thickness. 

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FfCURE 13.54 Semiconductor gas sensors (o) tubular, (b) thick film, (e) bulk-type one-electrode sensor where a thin Pt wire spiral is embedded Inside a sintered oxide semiconductor button. ... 

Golovanov, V., J. L. Solis, V. [.antto, and S. Leppavnori. Different Thick-Film Methods in Prinr-mg ot one-Electrode. Semiconductor Gas Sensors. Sensors Actuators B34 (1996), pp. 410-416. Lantto, V., and V. Golovanov. A Comparison of Conductance Behaviour between SnO> and CdS Gas-Sensitive Films, Sensors Actuators B24-25 (1995), pp. 614-618. 

The first the methods mentioned is based on the monitoring of initial changes in electric parameters (mainly the dope conductivity o) of semiconductor film of sensor caused by adsorption of active particles. In the limiting case this value may be estimated as... 

Above reasoning can be confirmed by a number of experimental results which showed that although with some peculiarities irrelevant to the properties of semiconductor sensors the correlation between the amount of the atoms in the flux incident on the target, or their surface concentration, and the variation (increase, if we are dealing with semiconductor of n-type) of the target conductivity takes place [28]. Based on the relations cited in Chapter 1, one can estimate concentrations (i. e., flow intensities) of these particles in vacuum or in gaseous medium if these values are quite small, using the values of conductivity variation of the semiconductor film. 

Fig.4.3. Experimental arrangement for investigation of pyrolysis of molecules by the method of semiconductor sensors 1 - reaction vessel, 2 - quartz slab with a ZnO film (sensor), 3 - filter, 4 - contacts, 5 - incandescent filament, 6 - thermocouple, 7 - cell with a substance, 8 - lamp - manometer, 9 - pin, 10 - flask, 11 - sealing bulkhead, 12 - trap, 13 - thermostat.

The scheme of the element is shown in Fig. 4.16. In order to increase variation of electric conductivity semiconductor film was deposited in the center of the plate, whereas activator was deposited at the plate edges at above specified distances through a mask. All stages of preparation were conducted in high vacuum ( 10" Torr). Sensitivity of such sensors to adsorption of hydrogen atoms at room at lower temperatures was about 10 - 10 at/cm, which corresponds to surface coverage of only 10-8 10-7% ( )... 

The method of semiconductor sensors allows one to determine the flux of atoms, to which the sensor was exposed, from electric conductivity measurements (provided coefficients of ionization and reflection of oxygen atoms from zinc oxide films are known). In other words, the sensor technique can be used in this case as an absolute method [21]. Indeed, variation of electric conductivity of a semiconductor film Acrpi due to adsorption is known to be caused by variation of carrier concentration An in the film, rather than by variation of their mobility / [21] ... 

It should be noted that in a vapour phase the liquid layer on the surface of a sensitive element of the sensor (zinc oxide) must be sufficiently thin, so that it would not produce any influence on the diffusion flux of oxygen through this layer. Possible lack of the film continuity (the presence of voids) does not prevent determination of concentration of oxygen in the bulk of the cell by the vapour - gas method. In this case, one deals with a semi-dry method. On the contrary, the presence of a thick liquid layer causes considerable errors in measuring t, because of different distribution of oxygen in a system gas - liquid layer -semiconductor film (this distribution is close to that in the system semiconductor film - liquid), in addition to substantial slowing down of oxygen diffusion in such systems. 

Investigation of interaction of electrons of different energies with a solid material in plasma processes may be even more intriguing and important, especially in the case of an adsorbed layer of materials contained in the reaction vessel. Provided thin semiconductor films deposited on the walls of the reaction vessel are used as solid targets, these films can be simultaneously used as targets and semiconductor sensors. This is also the case when such films are deposited on the specially manufactured quartz plates with electrodes accessible from the outside of the vessel. These sensors can be placed in any point of the vessel. 

The most obvious way to raise the sensitivity of sensors to RGMAs is by activating their surface with additives that actively interact with metastable atoms and have some electron coupling with semiconductor. These additives can be microcrystals of metals. As previously shown, the de-excitation of RGMAs on a metallic surface truly proceeds at high efficiency and is accompanied by electron emission. Microcrystals of the metal being applied to a semiconductor surface have some electron coupling with the carrier [159]. These two circumstances allow one to suppose that the activation of metals by microcrystals adds to the sensitivity of semiconductor films to metastable atoms. 

Kim YS, Ha SC, Kim K et al (2005) Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film. Appl Phys Lett 86(21) 213105-1-213105-3... 

M. (1997) A novel electronic nose based on semiconductor films gas sensor to distinguish different types of milk. In Authenticity and Adulteration of Food the Analytical Approach, Proceedings of Euro-FoodChem IX, Interlaken, Switzerland, 24-26 September 1997, pp 89-94. 

The conductivity of a generic semiconductor film is likely to be modulated by absorption of a polar species on the film surface. For example, a group at the Weiz-mann Institute has examined GaAs surfaces and found that porphyrin receptors linked to it will attract nitrogen oxide NO, and the binding of NO caused a change in resistance [5]. Other embodiments of GaAs sensors were found to be sensitive to ions in solution [25]. 

Lantto, V. (1992) Semiconductor gas sensors based on Sn02 thick films. In Gas Sensors, G. Sberveglieri (ed.), Kluwer Academic Publishers, Dordrecht, pp. 117-67. 

Design of Selective Gas Sensors Using Combinatorial Solution Deposition of Oxide Semiconductor Films... 

Thin films of nanostructured metals and semiconductors (e.g., Pt, Sn, CdTe) can be prepared by electrodeposition of the metal ions doped into the Hi LLC phase [40,47,48]. Similar to the precipitation of CdS, these films can retain the symmetry of the LLC template during the deposition. These materials allow one to combine well-defined porous nanostructures, high specific surface areas, electrical connectivity, fast electrolyte diffusion, and good mechanical and electrochemical stability. With this approach, hexago-nally structured semiconductor films of uniform thickness can be prepared. Nanostructured thin films of this type are proposed to have relevance in catalysis, batteries, fuel cells, capacitors, and sensors. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

In contrast to equilibrium-based sensing such as described above, it is also possible to use the zeolite film as a membrane controlling molecular access to an appropriate transduction mechanism. In this case, Pd-doped semiconductor gas sensors were used as a fairly non-selective sensor platform. After coating these sensors with a thin film of MFI-type or LTA-type zeolites, they were examined with respect to gas phase sensing of different analytes such as methane, propane and ethanol, at different humidity levels (Fig. 14).[121] The response of a zeolite-coated sensor towards the paraffins was strongly reduced compared to the non-coatcd sensor device, thus resulting in an increase of the sensor selectivity towards ethanol. 

More recently inks specifically developed for sensor applications have become available, for example Sn02 pastes incorporating Pt, Pd, and Sb dopants for the construction of semiconductor gas sensors. For biosensor applications, thick-film technology based on polymer films is extremely important, and special grades of polymer pastes (carbon, Ag, and Ag/AgCl) are becoming available... 

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