Vladimir Filippov

Room-Temperature Hydrogen Sensitivity of a MIS-Structure Based on the

An LaF3 layer was shown to improve the characteristics of field-effect gas sensors for room-temperature hydrogen monitoring. The Pt/LaF3 interface leads to a Nernst-type response and a detection limit of 10-ppm hydrogen in atmospheric air. The response time was shown to be about 110 s and was independent of hydrogen concentration. A method for the stabilization of a long-term behavior of the sensor was successfully demonstrated. The mechanism of the sensors response to hydrogen was shown to be different from that of the metal/insulator/semiconductor (MIS)-type sensors

hboxPt/LaF_3

The sensitivity of metal‐insulator‐semiconductor structure gas sensors based on silicon or silicon carbide to different fluorinecontaining gases was studied in the temperature range 20‐5308C. Silicon based gas sensors could be used for the determination of fluorine and hydrogen fluoride at room temperature. The sensitivity to fluorine is 28.00.5 mV/lg(p(F2)), the sensitivity to HF is 44.41.6 mV/(p(HF)), and the detection is about 10 ppb in both instances. High temperature silicon carbide sensors can be applied for the determination of fluorine and fluorocarbons (CF3CH2F, CF3CCl3 ,C F 3CH2Cl, CHClF2, CCl2F2, CCl3F) up to 5308C. The sensor signal for fluorocarbon concentration measurements demonstrates a Nernstian concentration dependence. The detection limit for these gases is ca. 10 ppm. # 1999 Elsevier Science B.V. All rights reserved.

Interface

An increase in the number of gases detectable by sensors based on Pd-SiO₂-Si (MIS) and Pt-LaF₃-Si₃N₄-SiO₂-Si (MEIS) structures was achieved by the application of an external catalyst element (CE). It was shown that as a result of the decomposition of hydrocarbon and fluorocarbon molecules on a Ni coil (CE), the products detectable by metal-insulator-semiconductor (MIS) and metal-electrolyte-insulator-semiconductor (MEIS) sensors are formed. The simultaneous catalytic oxidation of hydrocarbons and their thermal decomposition result in an optimum CE temperature of about 1050 K for propane. The kinetics of the thermal decomposition of gases on Ni were investigated. The activation energy of the reaction for C₃H₈ and the enthalpy in the case of CF₃-CCl were estimated

Semiconductor sensors for the detection of fluorocarbons, fluorine and hydrogen fluoride

The effect of the catalytic metal gate structure on the selectivity of MOS gas sensors has been described. It has been shown that hydrogen cyclic treatment (HCT) modifies the structure of the palladium film evaporated on the silicon dioxide. Pore formation in the gate gives rise to the ammonia and carbon monoxide sensitivity of the field-effect sensor. Ammonia adsorption and desorption processes on the sensor surface have been investigated. These processes are of a unimolecular nature with an adsorption activation energy of 0.08±0.02 eV/molecule in an inert atmosphere.

Hydrocarbon and Fluorocarbon Monitoring by MIS Sensors Using an Ni Catalytic Thermodestructor

Abstract Physicochemical properties of a gas sensor with the Pd-SiO2-Si-based sensitive element with an external catalyst element (CE) have been investigated. It is shown that as a result of the homolytical splitting of carbon-carbon bonds in alkane molecules, MOS sensors showed sensitivity to propane and butane both in inert and oxygen containing atmospheres. The dependencies of the sensors signals on the temperature of the CE was obtained for different gas mixtures containing H2, CH4, C3H8 and C4H10. The temperature of the CE (platinum coil) located above the sensor could be varied within a wide range (up to 1100 K in the air and 700 K in helium), while the temperature of the sensitive element was kept constant (463 K). A semi-empiric model describing the temperature dependence of the sensors signal was suggested. The magnitudes of the activation energy of the dissociation methane (3.5 eV) and propane (1.1 eV) on platinum was determined. When the initial rate of change of the flat-band voltage of the MOS capacitor ∂ΔU|∂t∣t=0t = 0 was taken as the sensors signal, a linear dependence of the signal versus the propane and butane concentration in air from 100 to 2000 ppm was obtained. A possibility of using pulse heating of the CE for the formation of the sensors signal was demonstrated.

Electrode structure effect on the selectivity of gas sensors

Abstract An asymptotic Bragg diffraction method is applied for the first time to the investigation of a hydrogenated Si-SiO 2 interface. It appears that the method may be useful for physical and chemical characterization of Pd-SiO 2 -Si type sensor systems.

MOS structure (Pd-SiO2-Si) based gas sensor with an external catalyst element

Abstract In this paper we present a system to control hydrogen in the heat-transport medium (water) designed to operate in nuclear power stations. Its main elements are a MOS sensor with a Pd-electrode and hydrogen-permeable membrane, which is produced from Pd-Ru alloy. The hydrogen is given off from the analysed medium by means of this membrane and is transferred to the sensor by helium. We report results of investigations using this system. An effect of water on the interaction between hydrogen and the membrane surface is found.

An investigation of the hydrogenated Si-SiO2 interface by X-ray diffraction

Abstract The influence of oxygen on the operation of hydrogen sensors based on MOS devices (capacitor) with palladium and platinum electrodes has been investigated. An effect of oxygen on the effective hydrogen sensitivity energy and the flat-band voltage-change kinetics Δ U(t) of the sensor with a Pt gate has been detected. The dependence of the initial rate of change of the flat-band voltage upon introduction of a gas mixture versus oxygen concentration has been obtained. A difference between them for sensors based on PtSiO 2 Si and PdSiO 2 Si is observed at a working temperature above 130 °C.

System to detect hydrogen in water

Abstract An MOS capacitor with a palladium electrode is the main element of a device to measure a small concentration (0.1–100 ppm) of hydrogen in helium. In this range of concentrations even small humidity sensitivity of this structure is important. The effect of water vapour on sensor yield is studied. It is found that the sensor selectivity to H 2 with respect to H 2 O is about 3 × 10 3 . The processes on the palladium gate surface are analysed.

Oxygen effect on the operation of the MOS-structure-based hydrogen sensor

The growing interest in new under-water pipe lines for the delivery of natural gas (in Baltic Sea, Mediterranean region, Caspian Sea, Black Sea, etc.) needs a new instruments for the monitoring of leakage and/or possible destruction of the pipes, which could not only disturb fragile ecological systems of Baltic and other seas, but also can lead to the technological catastrophes. We plan to develop a prototype of the system consisting of pervaporation membrane and gas sensors. Overall system will be immersed into water and will be fabricated in two versions: as a stationary instrument dipped into sea water and as an instrument towed along the pipe line. Micromachined metal oxide semiconductor gas sensors used for the detection of methane concentrations is designed and fabricated using “nano-on-micro” approach. Overall system is optimized from the point of view of minimum power consumption, which is necessary to assure its long term operation under autonomous conditions.