- Open Access
Gas sensing properties of nanocrystalline silicon carbide films
© The Author(s) 2019
- Received: 26 February 2019
- Accepted: 23 May 2019
- Published: 28 May 2019
The influence of the air atmosphere on the electrical conductivity of nanocrystalline thin SiC films obtained by method of direct ion deposition on sapphire was studied. Measurements were performed on two series of nc-SiC films with different structure: one series contained mainly 3C-SiC polytype nanocrystals and were denoted as monopolytypic, the second series were a nanoheterostructures based on a mixture of 3C-SiC and 21R-SiC nanocrystals. Gas sensitivity of the films appeared after annealing in vacuum at temperatures above 500 K. Measurements of the gas sensitivity of the films to the air atmosphere at a temperature of 700 K showed that the resistance increased for 12 times for the monopolytype film, while the heteropolytype film showed an increase of resistance for almost 16 times.
- Silicon carbide based gas sensors
- Nanocrystalline silicon carbide film
- Air effect on electrical resistance
- Structure effect on air sensitivity
In modern developments of semiconductor sensors for detecting and determining of the concentration of gases and gas impurities in the air, layers of metal oxides with an electronic type of conductivity (SnO2, In2O3, ZnO, Fe2O3, CuO, TiO2, etc.) are widely used as gas-sensitive sensors [1, 2]. The principle of operation of such sensors is based on the fact that reversible chemisorption of reducing (e.g. H2, CH4, NH3, CO) or oxidizing gases (e.g. O2, NO, NO2) on their surface is accompanied by a reversible change in the conductivity of the functional layer.
Due to the increased requirements for the stability of sensors under the influence of intense radiation and electromagnetic fields, there is a great problem of search for new functional semiconductor materials for the creation of highly sensitive gas sensors that slightly change their properties over time under hard external influences. One of the promising materials with chemical inertness, resistance to radiation impacts and time stability of properties are materials based on SiC , in particular, nanocrystalline SiC (nc-SiC) films obtained by method of direct deposition of carbon and silicon ions . The technology of growth of nanocrystalline SiC films is much cheaper than the technology of producing monocrystalline SiC films, whose gas-sensitive properties have previously been studied [5–7].
Previously, it was found that nc-SiC films containing a single predominant polytype (monopolytype) show the thermoactivation conductivity mechanism, and the films containing a mixture of polytypes (heteropolytype) demonstrate two-channel conductivity mechanism, one of which is based on the tunneling of electrons through the barrier between nanocrystals of different polytypes . Radiation resistance of such films was studied by us earlier . The stability of the electrophysical properties of nc-SiC films in strong magnetic fields at low temperatures was shown in . At the same time, the gas sensitivity of thin films of nanocrystalline SiC has not been studied.
Therefore, the purpose of this paper was to investigate the dependence of electrical conductivity of thin nanocrystalline SiC films with different polytype structure on the air atmosphere at certain temperatures.
It can be seen that the films not annealed in vacuum had almost identical conductivity characteristics in comparison with atmospheric measurements. This is probably due to the stable layer of chemisorbed gases that make up the atmospheric air formed on the surface of the films. It is known that the main contribution to atmospheric air belongs to N2, O2, as well as water vapor H2O and hydroxyl groups OH. To restore a clean surface, the film must be annealed in vacuum at the desorption temperature of the corresponding gas. It is a fact of experiment that molecular gases and hydroxyl groups contained in the air are desorbed at temperatures up to 500 K . The highest desorption temperature is for atomic oxygen forming the strongest bonds with the SiC surface.
The figure shows that the resistance of the films of both series MP (Fig. 4a) and series HP (Fig. 4b) did not change substantially with filling pure nitrogen into the chamber to atmospheric pressure. This indicates that O2 plays the main role in the processes of the adsorption effect of atmospheric air on the conductivity of thin films. The increase in the resistance of films at the reaction to the oxidant gas is typical for films of n-type of electrical conductivity and is explained by the depletion of the surface layer of the film conductivity by charge carriers . In this case, the film with hole conductivity should show the opposite reaction. The degree of increase of the resistance for films with mono- and heteropolytype structure was different. For monopolytype film, resistance increased for 12 times (S = 91.6%), while heteropolytype film showed a resistance increase almost for 16 times (S = 93.7%). The thermoactivation mechanism of conductivity is observed in the monopolytype SiC layers, and the increase of resistance can be associated with the depletion of the conduction band by electrons, described by the linear function of the electron concentration. Multichannel conductivity is implemented in heteropolytype films, and the increase in resistance may be due to the increase in the height of the potential barrier between nanocrystals nc-3C-SiC/nc-21R-SiC. This dependence is expressed by an exponential function. Therefore, the heteropolytype structure of nc-SiC films is more sensitive to the influence of chemisorbed gases with oxidative properties.. We believe that the gas that determines the change of film conductivity on the action of air is oxygen. A practically insignificant reaction to exposure to pure nitrogen (Fig. 4) confirms our assumption about the activity of oxygen on the surface of nc-SiC films in an air atmosphere.
The influence of the air atmosphere on the electrical conductivity of nanocrystalline thin SiC films obtained by method of direct ion deposition on sapphire was studied. Measurements were performed on two series of nc-SiC films with different structure: one series contained mainly 3C-SiC polytype nanocrystals and were denoted as monopolytype, the second series were a nanoheterostructures based on a mixture of 3C-SiC and 21R-SiC nanocrystals. It is shown that the initial films of monopolytype series had greater resistance than heteropolype ones, and both series showed no reaction to changes in atmospheric pressure. Gas sensitivity of the films appeared after annealing in vacuum at temperatures above 500 K. Measurements of the gas sensitivity of the films to the air atmosphere at a temperature of 700 K showed that the resistance increased for 12 times (S = 91.6%) for the monopolytype film, while the heteropolytype film showed an increase of resistance for almost 16 times (S = 93.7%). We believe that it is possible to increase the efficiency of gas sensitivity of nc-SiC films by optimizing their structure. From a comparison of the effects of nitrogen and air on the conductivity of nc-SiC films, we attribute the gas sensitivity of films to the adsorption of atmospheric oxygen.
Thus, the values of gas sensitivity of thin nc-SiC films were measured for the first time. The obtained results allowed to speak about the prospects of development of highly sensitive gas sensors on thin films of nanocrystalline SiC.
Thanks to Sergey Krivonosov of the Institute for Single Crystals NAS of Ukraine.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
AS developed a method for producing nc-SiC films and proposed a concept for using them for sensors, and wrote the overall manuscript. AK performed measurements of the electrophysical characteristics of films and assisted in writing the manuscript. SS and DL performed the experiments for obtaining films and device fabrication. All authors read and approved the final manuscript.
Ethics approval and consent to participate
The authors declare that they have no competing interests.
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