Alexander Terentjev

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

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.

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

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

Silicon carbide based semiconductor sensor for the detection of fluorocarbons

Abstract The new large band gap semiconductor material SiC was used to develop a high temperature field-effect structure SiC/epi-SiC/SiO 2 /LaF 3 /Pt. The response to different fluorocarbons as CF 3 CH 2 F, CF 3 CCl 3 , CHClF 2 , CF 3 CH 2 Cl and CCl 3 F was investigated. A complex behaviour was found for the temperature range 200–300°C. The selective detection of fluorine containing molecules was shown for a temperature of 380°C. The fluoride ion conducting material LaF 3 was proven to have the substantial role for the sensor detection principal.

FIELD-EFFECT SENSOR FOR THE SELECTIVE DETECTION OF FLUOROCARBONS

Abstract A chemical semiconductor sensor for the temperature range up to 400°C was developed using silicon carbide with an epitaxial layer of SiC as the substrate. Thin layers of LaF 3 and Pt were deposited on the semiconductor/insulator structure to form a three-phase boundary with the gas under investigation. The sensor was shown to be sensitive to fluorine, hydrogen fluoride and different fluorocarbons. The influence of the operation temperature on the sensor response signal was investigated in the range from room temperature up to 400°C. For the fluorocarbons CF 3 CH 2 F, CF 3 CCl 3 , CHClF 2, CF 3 CH 2 Cl and CCl 3 F a selective detection was achieved at temperatures near to 400°C. The substantial role of the fluoride ion conducting material LaF 3 for the sensor detection principle was proven. A mechanism including the chemisorption of the fluorocarbon at the Pt surface and an insertion of fluorine into LaF 3 was discussed.

Electrode structure effect on the selectivity of gas sensors

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.

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

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.

High temperature semiconductor sensor for the detection of fluorine

The possibility of high temperature measurements up to 350/spl deg/C with a chemical semiconductor sensor for fluorine was proven using silicon carbide as the substrate. A structure SiC/SiO/sub 2//LaF/sub 3//Pt leads to results comparable to the silicon based sensor. The influence of temperature on the sensor behaviour is smaller than expected but the increased desorption rate improves the limit of detection. An impulse method using the initial slope of the response curve was shown to be advantageous.

System to detect hydrogen in water

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.

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

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.

Sensitivity of Pd/SiO2/Si sensor to humidity

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.