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Room-Temperature Hydrogen Sensitivity of a MIS-Structure Based on the Pt/LaF3 Interface Vladimir I. Filippov, Alexey A. Vasiliev, Werner Moritz, and Jan Szeponik

Abstract—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 sensor’s response to hydrogen was shown to be different from that of the metal/insulator/semiconductor (MIS)type sensors. Index Terms—Hydrogen sensor, metal/insulator/semiconductor (MIS) structure, room-temperature measurement, solid electrolyte.

I. I NTRODUCTION

S

INCE the first paper by Lundström et al. [1], which was published in 1975, it is well know that metal/ insulator/semiconductor (MIS) structures can be used as gas sensors, especially for hydrogen detection. The fundamentals of these devices and the mechanism of hydrogen detection at the Pd/oxide interface are summarized in [2]. For this sensor type, the dissociation of hydrogen molecules at the Pd gate electrode, diffusion of the atoms, and formation of a dipole layer at the metal/oxide interface lead to a change of the threshold voltage in field-effect transistors, the flatband voltage in MIS capacitors, or the photo current in the sensors, which is based on the photo effect in the semiconductor. The Pd/SiO2 /Si-based sensor was used for hydrogen detection at room temperature, but the behavior was rather poor [3]. Therefore, usually the sensor is used at temperatures near to 150 ◦ C. Silicon-based field-effect devices are restricted to a temperature range below 200 ◦ C, but other semiconductors like SiC [4] or GaN/AlGaN heterostructures [5] are applied at temperatures up to

Manuscript received May 11, 2005; accepted Febuary 3, 2006. The associate editor coordinating the review of this paper and approving it for publication was Dr. Andre Bossche. V. I. Filippov is with the Russian Research Center Kurchatov Institute, Institute of Molecular Physics, 123182 Moscow, Russia (e-mail: filippov@ imp.kiae.ru). A. A. Vasiliev is with the Russian Research Center Kurchatov Institute, Institute of Molecular Physics, 123182 Moscow, Russia, and also with the University Rovira i Virgili, DEEEA, 43007, Tarragona, Spain (e-mail: [email protected]; [email protected]; [email protected]). W. Moritz is with the Institut fur Chemie, Humboldt-Universitt zu Berlin, 12489 Berlin, Germany (e-mail: [email protected]; http://www. chemie.hu-berlin.de/wmoritz/index.html). J. Szeponik is with the BST Bio Sensor Technologie GmbH, 13156 Berlin, Germany (e-mail: [email protected]). Digital Object Identifier 10.1109/JSEN.2006.881088

400 ◦ C–600 ◦ C. Beside the sensors based on the detection at the metal/insulator interfaces, various other materials have been investigated for hydrogen measurement in semiconductor sensors, metal-oxide resistive-type gas sensors, or electrode configurations [6]–[10]. In this paper, we report the results concerning hydrogen detection at room temperature in ambient air using a Pt/LaF3 /Si3 N4 /SiO2 /Si semiconductor field-effect structure. Similar devices were already used for the detection of oxygen, fluorine, hydrogen fluoride at room temperature, and fluorocarbons at higher temperatures [11]–[15]. Therefore, some properties of this structure, especially as an oxygen sensor, will be summarized in the following paragraphs. LaF3 is known to be an excellent fluoride-ion conductor, and the exchange of fluoride ions at its interface with an aqueous solution leads to one of the best ion selective electrodes. Not only F− ions but also OH− ions can be incorporated in some 100-nm-thick surface layers of LaF3 by fast exchange with solutions [16]. The detection of gaseous fluorine or hydrogen fluoride at the interface Pt/LaF3 can be explained by rather simple mechanisms, while for the detection of oxygen, the stabilization of the hydroxide ions in the LaF3 lattice is of great importance. Therefore, the following mechanism was suggested for the oxygen sensor [17]: O2 + A∗ ∗




O2 (A∗ ) ∗

(1) −

O2 (A ) + H2 O + e−




HO2 (A ) + OH

(2)

OH− + (F∗ )




OH− (F∗ ).

(3)

In the first step, oxygen is adsorbed at surface sites A∗ at the three-phase boundary LaF3 /Pt/gas (1). This process is followed by the rate-determining step, which is a one-electron reduction of oxygen in the presence of water (2). The mobility of OH− on fluoride sites in the LaF3 lattice (3) stabilizes the OH− produced in reaction (2). The Nernst equation describes the sensitivity of the sensor. The one-electron rate determining process leads to a sensitivity of 58 mV per decade of oxygen partial pressure. Impedance spectroscopy [18] provided additional arguments for an electrochemical charge transfer process at the Pt/LaF3 interface determining the sensor behavior. Exchange current density and response time of the oxygen sensor have been shown to be independent of oxygen partial pressure. A decrease in the response rate was observed some days after the preparation of the Pt layer, but the response time was independent of the age of the LaF3 layer. A simple thermal

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treatment was proven to reactivate the sensor and to lead to a long lifetime using multiple activation [17]. Very low power consumption can be achieved by ultrafast (few microseconds or nanoseconds) heating of only the thin superficial LaF3 /Pt layer [19]. The Pt gate is additionally used as a resistive heater and temperature control for this proposed method. The mechanism of aging and reactivation was investigated in [18]. It was shown that the adsorption and the desorption of very small amounts of CO2 determine the dynamic behavior of the sensor. It is the aim of this paper to investigate the properties of a Pt/LaF3 /Si3 N4 /SiO2 /Si MIS structure as a room-temperature sensor for hydrogen concentration in air. II. E XPERIMENTAL Sensor structures were prepared using an n-Si/SiO2 (20 nm)/Si3 N4 (80 nm) substrate with donor concentration of 2.5 · 1014 cm−3 (IMS Dresden, Germany). The chip size was 5×5 mm. The backside ohmic aluminum contact was fabricated by vacuum evaporation of Al. LaF3 layers were deposited on the insulator by thermal evaporation in a high-vacuum system. The deposition rate was adjusted to 0.3 nm/s to grow a layer with a final thickness of 150 nm. DC cathode sputtering in argon plasma through a metal shadow mask was used to deposit 5–20-nm-thick Pt gate films with an area of 3 mm2 . Therefore, the final sensor structure used in our experiments was Pt/LaF3 /Si3 N4 /SiO2 /Si. The sensors were characterized by high-frequency capacitance–voltage (C–V ) method using Hewlett Packard 4284 A type LCR meter. A computer-controlled system for the determination of the bias voltage shift at a constant capacitance was used. A gas mixture of 1000 ppm of hydrogen in synthetic air or argon was diluted with synthetic air or argon, respectively, using computer-operated mass-flow controllers to obtain different concentrations of hydrogen in a range of 10–200 ppm. The flow rate in the measurement cell was adjusted to 5 mL/s. For all measurements, relative humidity of gases was adjusted to 33%. This humidity was fixed by bubbling the gases through saturated aqueous solution of MgCl2 . Gas pressure was close to the atmospheric pressure. Thermal treatments of the sensor structure before measurements were applied in some parts of the experiments using a hot plate with fixed temperature. III. R ESULTS A. Sensor Response to Hydrogen in Air It was our main interest to contribute to the development of room-temperature sensors for hydrogen detection. Therefore, results are given mainly for these conditions, and only a test at higher temperature will be shown. As it was discussed above, the sensor behavior of the Pt/LaF3 /Si3 N4 /SiO2 /Si MIS structure depends on time that passed after the preparation of the Pt layer. Consequently, initial measurements of the response to hydrogen were performed with freshly prepared sensors, approximately 1 h after platinum deposition. Sensor responses to 20, 50, and 100 ppm of hydrogen in synthetic air are shown

Fig. 1. Response of Pt/LaF3 /Si3 N4 /SiO2 /Si field-effect structure (solid line and left scale) to different concentrations of hydrogen (dotted line and right scale) in synthetic air. Room-temperature measurement 1 h after the preparation of the Pt layer.

Fig. 2. Response of Pt/LaF3 /Si3 N4 /SiO2 /Si field-effect structure (solid line and left scale) to different concentrations of hydrogen (dotted line and right scale) in synthetic air. Room-temperature measurement five days after the preparation of the Pt layer.

in Fig. 1. The sensor behavior obeys Nernst equation, and the sensitivity was found to be equal to 60 mV per decade of hydrogen concentration. The response time (t90 ) is of the order of a minute, and the recovery time is also rather fast. For comparison, we prepared similar sensor structures without LaF3 layer that are Pt/Si3 N4 /SiO2 /Si field-effect structures. Pt layers were prepared exactly in the same way as for the Pt/LaF3 /Si3 N4 /SiO2 /Si structures. Measurements of the hydrogen response did result in signals smaller than 10 mV at 100 ppm of hydrogen for the structures without LaF3 . This is the first argument that the mechanism of the hydrogen response at the Pt/LaF3 interface is different from Pt/insulator interfaces. Because of our knowledge about the time-dependent deterioration of the sensor-response kinetics for the oxygen sensor, we decided to investigate the sensor behavior after different aging time, which is just at the very initial state of the study of hydrogen response. A typical result for the sensor behavior five days after platinum deposition is given in Fig. 2. It is clearly shown that the response to hydrogen becomes much slower as well. A very unusual but reproducible observation was that besides the slower response, an increase in sensor signal compared to the fresh sensor was observed. Because of

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Fig. 3. Response of a reactivated sensor structure (solid line and left scale) to different concentrations of hydrogen (dotted line and right scale) in synthetic air. The measurements were performed at room-temperature measurement after 1-min reactivation at 350 ◦ C.

the slow response, this behavior was not investigated in detail. Furthermore, no additional investigation of freshly prepared sensors was carried out because sensor lifetime was too small for practical applications. Next, we concentrated our efforts on the investigation of the thermal treatments of the sensor, leading to the reactivation of the MIS structure. Short heating followed by hydrogensensitivity measurements at room temperature should give a much lower power consumption compared to measurements at high temperature. This is of high importance for autonomous battery-powered systems. For the reason of simplicity and reproducibility, we always use 1-min thermal treatments of the sensor in air with relative humidity of 33%. The influence of a decrease in heating time was not investigated in this part of this paper because it is assumed that the effect is comparable to the results obtained for the oxygen sensor discussed above. Thermal treatments were investigated in a temperature range from 150 ◦ C to 380 ◦ C. An improvement of the dynamic behavior of the sensor was found for temperatures higher than 200 ◦ C. The best results were achieved in a temperature range of 300 ◦ C–350 ◦ C. This result is comparable with the reactivation behavior of the oxygen sensor with the same thin-layer structure. A typical sensor response after thermal reactivation is given in Fig. 3. It is necessary to note that zero hydrogen concentration was applied only in the beginning of the measurements. For most parts of the experiments, we used baseline hydrogen concentration equal to C0H2 = 20 ppm. Even several years after the preparation, the sensor can be activated in this way. A multiple thermal treatments can be applied to achieve a long service time of the sensor, but this was not investigated for more than one month until now. A signal of the same order of magnitude as for a freshly prepared sensor was obtained, and the dynamic behavior of the response and recovery was found to be fast again. However, the sensitivity was smaller now. The sensor signal ∆U = U(CH2 ) − U(C0H2 ) as a function of the ratio of the hydrogen concentrations CH2 /C0H2 is shown in Fig. 4 on a logarithmic scale. The slope of this line, which obeys the Nernst equation,

IEEE SENSORS JOURNAL VOL. 6, NO. 5, OCTOBER 2006

Fig. 4. Sensor signal in synthetic air as a function of the hydrogen concentration. Baseline hydrogen concentration C0H2 is equal to 20 ppm. The measurements were performed at room-temperature measurement after 1-min reactivation at 350 ◦ C.

Fig. 5. Response time t90 (time necessary for a change of the signal from the initial value to 90% of the final value) as a function of the hydrogen concentration in air. Hydrogen concentration changed from 20 ppm to the final value given in the concentration axis.

was determined to be 27 mV per decade of hydrogen partial pressure. The detection limit was found to be of about 10 ppm. In Fig. 5, the response time t90 (time necessary for a change of the signal from the initial value to 90% of the final value) is given as a function of the hydrogen concentration. The dynamic behavior of the hydrogen sensor was found to be independent of the hydrogen concentration. This is the same behavior that is known for the oxygen sensor [17]. The influence of the thickness of the Pt layer was investigated in a range from 5 to 20 nm. No influence was found neither for the aging, reactivation, and sensitivity nor for the dynamic behavior. For the oxygen sensor, the influence of the thickness on the response rate was observed only for Pt thicker than 80 nm [20]. The main goal of this paper was the study of roomtemperature sensitivity; however, we tested the possibility of a high-temperature measurement of hydrogen concentration with Pt/LaF3 /Si3 N4 /SiO2 /Si structures. In Fig. 6, it is shown that the sensor is still working at a temperature of 160 ◦ C, which is typical of the Pd-MOS hydrogen sensors. The dynamic

FILIPPOV et al.: HYDROGEN SENSITIVITY OF MIS-STRUCTURE BASED ON Pt/LaF3 INTERFACE

Fig. 6.

Sensor response at 160 ◦ C after 1-min reactivation at 300 ◦ C.

behavior and the detection limit were even improved. However, as the high temperature range was not our intention, we did not qualify the experiment to reduce noise and drift. For the same reason, the exact value of the response time (t90 ) was not determined.

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Fig. 7. Response of the reactivated sensor structure (solid line and left scale) to different concentrations of hydrogen (dotted line and right scale) in argon. The measurements were performed at room temperature after 1-min reactivation at 350 ◦ C.

B. Hydrogen Response of the Sensor in Argon Atmosphere The sensor structure is sensitive not only to hydrogen but also to oxygen. Therefore, the influence of oxygen onto the hydrogen response was investigated to clarify our knowledge about the sensing mechanism. We used pure argon as a carrier gas with very low oxygen concentration. No special efforts were made to avoid a low concentration of oxygen in the measurement cell. The shift of the C−V curves, when air was substituted by argon, was equal to 240 mV. This enables the estimation of oxygen concentration in argon in the sensor measuring cell. This concentration is equal to about 10 ppm. The sensor was exposed to hydrogen concentrations in a range from 10 to 150 ppm in argon. The gate-voltage shift from pure argon to 20 ppm of hydrogen was found to be about 300 mV. This concentration was then used as baseline value for other measurements. A typical sensor-response curve is given in Fig. 7. Fast and stable response to hydrogen was found even at this very low oxygen concentrations. As shown in Fig. 8, a Nernst-type sensitivity of 103 mV per decade of hydrogen partial pressure was found. IV. D ISCUSSION It is rather clear that the response mechanism for Pt/LaF3 /Si3 N4 /SiO2 /Si structures is different from the MOS structures. This was proven first by the small response in our experiments, where a Pt/Si3 N4 /SiO2 /Si field-effect structure was used. In contrast to the dipole layer formed at the metal/insulator interface, the Pt/LaF3 interface properties seem to be determined by electrochemical exchange of charge carrier, which leads to a behavior described by Nernst equation. This gives the next argument for different mechanisms. A much larger shift of the sensor signal in argon compared to air for the change from the carrier gas to 10 ppm of hydrogen was observed (Figs. 3 and 7). And for the same hydrogen concentrations, the signal is shifted by more than 500 mV using

Fig. 8. Concentration dependence of the sensor signal in argon. Baseline hydrogen concentration C0H2 is equal to 20 ppm. The measurements were performed after 1-min reactivation at 350 ◦ C.

air or argon as the carrier gas. This gives a strong argument for a mechanism of hydrogen detection, which is connected with oxygen sensitivity. XPS measurements [17] proved the existence of several oxygen containing species like O2− , OH− , − − O2− 2 , HO2 , O2 , and HO2 in the LaF3 surface layer. In argon, the surface concentration of oxygen at the LaF3 /Pt interface will be much lower, and therefore, the influence of the hydrogen would be stronger. The signal shift of 500 mV corresponds to a change in surface concentrations by several orders of magnitude. Some differences in the mechanism of freshly prepared and reactivated sensors are obvious from the difference in the Nernst slope found. Further investigations of the gas response mechanism are necessary, but the similarities to the oxygen sensor shown in this paper give us additional arguments for the assumption that the hydrogen detection is connected with the oxygen sensing mechanism, as described in (1)–(3). The catalytic activity of Pt for the room-temperature reaction of hydrogen with oxygen, leading to the formation of water, is well known. Therefore, a reasonable hypothesis can be that hydrogen response is related

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to a decrease in surface oxygen concentration resulting from the reaction with hydrogen O2 (A∗ ) + H2




H2 O(A∗ ).

(4)

Different hydrogen sensitivities found in air and in argon have not been completely understandable until now. Lower sensitivity in air compared to argon atmosphere could be related to a competition of the electrochemical process with the direct oxidation of hydrogen on a catalytic platinum surface. However, further investigation of these processes is necessary for a better understanding of the sensitivity mechanism. The competition between different surface reactions may also explain the unusual peak observed in Fig. 7 when hydrogen concentration decreases. The influence of the oxygen concentration on the sensor signal should not be a serious problem for the measurements in air where oxygen concentration is sufficiently constant. The mechanism of the sensor activation, by heating the gate layer at certain temperature, was discussed in detail in [20]. It was shown that one of the possible reasons for sensor aging is the poisoning of the gate layer by adsorbed carbon dioxide. In this case, the activation of the sensor is due to the desorption of this gas. Therefore, the difference in the sensor sensitivity (Figs. 1 and 2) can be explained by the catalyst poisoning. The desorption of catalytic poison leads to the acceleration of the potential-forming reaction and, on the other hand, to the acceleration of nondesirable oxidation of hydrogen on platinum surface. The last process decreases the sensor sensitivity to hydrogen. This result can be compared with the previously mentioned increase in sensor sensitivity in an argon atmosphere. In the future, it will be necessary to investigate if a short heating time with a duration in the microsecond or nanosecond range can be applied, as was shown for the oxygen sensor [19]. This decrease in heating time will additionally reduce the average power consumption of the sensor. The period of time when the sensor can be used after thermal treatment depends of course on the requirements concerning the response kinetics necessary for different applications, but the use of a short electrical heating impulse, which is supplied once a day followed by a measurement period, should be acceptable for autonomous instruments.

[2] I. Lundstroem et al., “Field effect chemical sensors,” in Sensor—A Comprehensive Survey, vol. 2, W. Goepel, J. Hesse, and J. N. Zemel, Eds. Weinheim, Germany: VCH, 1991, p. 469. [3] I. Lundström, M. S. Shivaraman, L. Stiblert, and C. Svensson, “Hydrogen in smoke detected by the Pd-gate field-effect transistor,” Rev. Sci. Instrum., vol. 47, no. 6, pp. 738–740, Jun. 1976. [4] C. K. Kim, J. H. Lee, Y. H. Lee, N. I. Cho, and D. J. Kim, “A study on a platinum-silicon carbide Schottky diode as a hydrogen gas sensor,” Sens. Actuators B, Chem., vol. 66, no. 1–3, pp. 116–118, Jul. 2000. [5] J. Schalwig, G. Müller, M. Eickhoff, O. Ambacher, and M. Stutzmann, “Gas sensitive GaN/AlGaN-heterostructures,” Sens. Actuators B, Chem., vol. 87, no. 3, pp. 425–430, Dec. 2002. [6] N. Miura, T. Harada, Y. Shimizu, N. Yoshida, and N. Yamazoe, “Sensing properties of ISFET-based hydrogen sensor using proton-conducting thick film,” Sens. Actuators B, Chem., vol. 25, no. 1–3, pp. 499–503, Apr. 1995. [7] F. Opekar and K. Shtulik, “Electrochemical sensors with polymer electrolyte,” Anal. Chim. Acta, vol. 385, no. 1–3, pp. 151–162, 1999. [8] M. Fleischer, L. Höllbauer, and H. Meixner, “Effect of the sensor structure on the stability of Ga2 O3 sensors for reducing gases,” Sens. Actuators B, Chem., vol. 18, no. 1–3, pp. 119–124, Mar. 1994. [9] S. F. Chehab, J. D. Canaday, A. K. Kuriakose, T. A. Wheat, and A. Ahmad, “A hydrogen sensor based on bonded hydronium NASICON,” Solid State Ion., vol. 45, no. 3/4, pp. 299–310, Apr. 1991. [10] A. A. Vasiliev, A. V. Eryshkin, D. Y. Godovski, E. A. Koltypin, V. V. Malyshev, N. K. Kotovschikova, I. M. Olikhov, A. V. Pisliakov, and S. S. Yakimov, “Thick film semiconductor combustible gas sensors with minimum power consumption,” in Proc. 10th Eur. Conf. SolidState Transducers EUROSENSORS-X, Leuven, Belgium, Sep. 8–11, 1996, p. 537. [11] N. Miura, J. Hisamoto, and N. Yamazoe, “Solid state oxygen sensor using sputtered LaF3 films,” Sens. Actuators, vol. 16, no. 4, pp. 301–310, 1989. [12] W. Moritz, I. Meierhöfer, and L. Müller, “Fluoride-sensitive membrane for ISFETs,” Sens. Actuators, vol. 15, no. 3, pp. 211–219, 1988. [13] S. Krause, W. Moritz, and I. Grohmann, “A low-temperature oxygen sensor based on the Si/LaF3 /Pt capacitive structure,” Sens. Actuators B, Chem., vol. 9, no. 3, pp. 191–196, Oct. 1992. [14] W. Moritz, V. Fillipov, A. Vasiliev, L. Bartholomäus, and A. Terentjev, “Field effect sensor for the selective detection of fluorocarbons,” J. Fluorine Chem., vol. 93, no. 1, pp. 61–67, Jan. 1999. [15] W. Moritz, L. Bartholomäus, U. Roth, V. Filippov, A. Vasiliev, and A. Terentjev, “Semiconductor sensors for the detection of fluorocarbons, fluorine and hydrogen fluoride,” Anal. Chim. Acta, vol. 393, no. 1–3, pp. 49–57, Jun. 1999. [16] W. Moritz and L. Müller, “Mechanistic study of fluoride ion sensors,” Analyst, vol. 116, no. 6, p. 589, 1991. [17] W. Moritz, S. Krause, and I. Grohmann, “Improved long-term stability for an LaF3 based oxygen sensor,” Sens. Actuators B, Chem., vol. 18, no. 1–3, pp. 148–154, Mar. 1994. [18] W. Moritz, S. Krause, U. Roth, D. Klimm, and A. Lippitz, “Re-activation of an all solid state oxygen sensor,” Anal. Chim. Acta, vol. 437, no. 2, pp. 183–190, Jun. 2001. [19] W. Moritz, U. Roth, M. Heyde, K. Rademann, M. Reichling, and J. Hartmann, “Submicrosecond range surface heating and temperature measurement for efficient sensor reactivation,” Thin Solid Films, vol. 391, no. 1, pp. 143–148, Jul. 2001. [20] S. Krause, “Physical and chemical characterization of the system Ionic conductor/Metal/Gas and the development of field-effect sensors of oxygen for room temperature range,” Ph.D. dissertation, Humboldt-Univ., Berlin, Germany, 1994.

V. C ONCLUSION It was shown that the Pt/LaF3 /Si3 N4 /SiO2 /Si field-effect structure can be used for the detection of hydrogen in a part-permillion concentration range in atmospheric air at room temperature. Thermal treatment necessary for sensor reactivation can be performed in a simple and fast way using the gate metal as a resistive heater. Using this reactivation procedure, a stable longterm behavior of the sensor can be achieved. R EFERENCES [1] I. Lundström, S. Shivaraman, C. Svensson, and L. Lundkvist, “A hydrogen-sensitive MOS field-effect transistor,” Appl. Phys. Lett., vol. 26, no. 2, pp. 55–57, Jan. 1975.

Vladimir I. Filippov received the degree from Moscow Physical Engineering Institute, Moscow, Russia, in 1972, and the doctor’s degree in 1994 from Russian Reseach Center (RRC) Kurchatov Institute, Moscow. He joined the Igor Vasilievich Kurchatov Institute of Atomic Energy, Moscow, where he investigated the influence of radiation on the electrical properties of the semiconductors. Since 1986, he has been working in the field of semiconductor sensors with the Institute of Molecular Physics of RRC Kurchatov Institute. His current research interest includes the experimental and theoretical research of MIS chemical sensors and heterogeneous catalysis.

FILIPPOV et al.: HYDROGEN SENSITIVITY OF MIS-STRUCTURE BASED ON Pt/LaF3 INTERFACE

Alexey A. Vasiliev received the diploma degree from Moscow Institute of Physics and Technology, Moscow, Russia, in 1980, the Ph.D. degree for the “Study of the kinetics of low-temperature reactions of atomic fluorine by ESR method” from RRC Kurchatov Institute, Moscow, in 1986, and the Dr. of Science degree (habilitation) in solid-state microelectronics, for the investigation of “Physical and chemical principles of design of gas sensors based on metal oxide semiconductors and MIS structures with a solid electrolyte layer” in 2004 from Moscow Institute of Power Engineering. He has recently been working with the sensor group of the University Rovira i Virgili, Tarragona, Spain, and with the Russian Research Center Kurchatov Institute, Moscow. His research interests are related to the study of the kinetics and mechanisms of heterogeneous processes related to chemical sensing with design and prototyping of gas sensors and instruments based on these sensors.

Werner Moritz was born in Berlin, Germany, in 1952. He received the Ph.D. degree from HumboldtUniversity of Berlin in 1981, working on the investigation of noble metal electrodes. In 1989, he received the habilitation degree for his study on chemical semiconductor sensors. He is currently working as the Leader of the Chemical Sensors Group with the HumboldtUniversity of Berlin. His research interest is in chemical sensors, thin films, and impedance imaging.

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Jan Szeponik was born in 1960. He received the diploma degree in chemistry from HumboldtUniversity, Berlin, Germany, in 1985. After 1985, he focused his research on semiconductor-based chemical sensors at the Walter-Nernst-Institute, Berlin, and he defended his thesis in 1992. Since 1992, he has been working with the R&D Department of the BST Bio Sensor Technology, Berlin, which is active in the field of development and production of biosensors for medical diagnostics.