21 research outputs found
Enhanced sensing performance of MISiC schottky-diode hydrogen sensor by using HfON as gate insulator
MISiC Schottky-diode hydrogen sensor with HfON gate insulator fabricated by NO nitridation is investigated. The hydrogen-sensing characteristics of this novel sensor are studied by doing steady-state and transient measurements at different temperatures and hydrogen concentrations using a computer-controlled measurement system. Experimental results show that this novel sensor can rapidly respond to hydrogen variation and can give a significant response even at a low H 2 concentration of 48-ppm, e.g., a sensitivity of 81% is achieved at 450°C and 2.5 V, which is two times higher than its HfO 2 counterpart. The enhanced sensitivity of the device should be attributed to a remarkable reduction of the current of the sensor before hydrogen exposure by the NO nitridation because the NO nitridation can passivate the O vacancies in the insulator and facilitate the formation of a SiO 2 interlayer to suppress the leakage current associated with high-k materials. © 2006 IEEE.published_or_final_versio
Improved Sensing Characteristics of a Novel Pt/HfTiO2/SiC Schottky-Diode Hydrogen Sensor
published_or_final_versio
A comparison of MISiC Schottky-diode hydrogen sensors made by NO, N 2O, or NH 3 nitridations
MISiC Schottky-diode hydrogen sensors with gate insulator grown in three different nitridation gases (nitric oxide (NO), N 2O, and NH 3) are fabricated. Steady-state and transien-t-response measurements are carried out at different temperatures and hydrogen concentrations using a computer-controlled measurement system. Experimental results show that these nitrided sensors have high sensitivity and can give a rapid and stable response over a wide range of temperature. This paper also finds that N 2O provides the fastest insulator growth with good insulator quality and hence the highest sensitivity among the three nitrided samples. The N 2O- nitrided sensor can give a significant response even at a low H 2 concentration of 48-ppm H 2 in N 2, indicating a potential application for detecting hydrogen leakage at high temperature. Moreover, the three nitrided samples respond faster than the control sample. At 300°C, the response times of the N 2O, NO, and NH 3-nitrided sample to the 48-ppm H 2 in N 2 are 11, 11, and 37 s, respectively, as compared to 65 s for the control sample without the gate insulator. © 2006 IEEE.published_or_final_versio
Effects of annealing temperature on sensing properties of Pt/HfO2/SiC Schottky-diode hydrogen sensor
Hafnium oxide (HfO 2) is successfully used as gate insulator for fabricating Metal-Insulator-SiC (MISiC) Schottky-diode hydrogen sensor. Sensors undergone N 2 annealing at different temperatures are fabricated for investigation. The hydrogen-sensing properties of these samples are compared with each other by taking the measurements at high temperature under various hydrogen concentrations using a computer-controlled measurement system. Experimental results show that sensitivity increases with the annealing temperature. Higher annealing temperature can enhance the densification of the HfO 2 film; improve the oxide stoichiometry; and facilitate the growth of a SiO 2 interfacial layer to give better interface quality, thus causing a remarkable reduction of the current of the sensor under air ambient. The effects of hydrogen adsorption on the barrier height and hydrogen-reaction kinetics are also investigated. © 2008 IEEE.published_or_final_versio
Wide band gap materials and devices for NOx, H2 and O2 gas sensing applications
Im Rahmen dieser Arbeit sind Feldeffektgassensoren (Schottky Dioden, MOS
Kapazitäten, und MOSFET Transistoren) auf der Basis von Halbleitern mit
großer Bandlücke (Siliziumkarbid (SiC) und Gallium Nitrid (GaN), sowie
resistive Gassensoren, die auf aktiven Indiumoxid-Schichten (In2O3)
basieren, für die Detektion von reduzierenden Gasen (H2, D2) und
oxidierenden Gasen (NOx, O2), entwickelt worden. Die Entwicklung der
Sensoren ist am Institut für Mikro- und Nanoelektronik der Technischen
Universität Ilmenau in Zusammenarbeit mit General Electric (GE) Global
Research (USA) und der Umwelt- und Sensortechnik GmbH (Geschwenda)
durchgeführt worden. Kapitel 1: dient als eine Einführung in das mit
dieser Arbeit verbundene wissenschaftliche Feld. Die theoretischen
Grundlagen der Festkörper-Gassensoren werden dargestellt. Zusätzlich werden
in diesem Kapitel die relevanten Eigenschaften der Materialien mit großer
Bandlücke (SiC und GaN) präsentiert. Kapitel 2: Pt/GaN Schottky Dioden
mit verschiedener Dicke des katalytischen Metalls werden als
Wasserstoffgasdetektoren vorgestellt. Die Fläche sowie die Dicke von
Pt-gates wurden zwischen 250 × 250 µm2 und 1000 × 1000 µm2, 8 und 40 nm,
systematisch variiert. Die Sensorantwort (Sensorsreaktion) auf 1 vol.%
Wasserstoff in synthetischer Luft wurde in Abhängigkeit von der aktiven
Fläche, der Pt-Dicke, und der Betriebstemperatur untersucht. Durch Anheben
der Betriebstemperatur auf ca. 350°C und durch Reduzierung der Dicke des Pt
auf 8 nm beobachteten wir eine beträchtliche Erhöhung der Empfindlichkeit
sowie eine Verkürzung der Ansprech- und Erholzeiten. Untersuchungen am
Elektronenmikroskop zeigten, dass das dünnere Platin eine höhere
Korngrenzendichte aufwies. Die Erhöhung der Empfindlichkeit gemeinsam mit
der Reduzierung der Dicke des Pt deuten auf die Dissoziierung von
molekularem Wasserstoff an der Oberfläche, die Diffusion atomaren
Wasserstoffs entlang der Korngrenzen des Platins und die Adsorption von
Wasserstoff an der Pt/GaN Grenzfläche als ein möglicher Mechanismus der
Detektion von Wasserstoff durch Schottky Dioden hin. Die Reaktion auf
D2, NOx, and O2 von Metall-Oxid-Halbleiter (MOS) Strukturen mit Rhodium
Schottky-Kontakten mit einer Dicke von 30 nm in Abhängigkeit von der
Betriebstemperatur und der Gaspartialdrücke wurde in Kapitel 3 untersucht.
Die Reaktion dieses Gates wurde als Verschiebung entlang der Spannungsachse
in der Kapazität-Spannungs Kurve (C-V) nachgewiesen. Positive und negative
Flachband-Verschiebungen jeweils bis zu 1 V wurden für oxidierende und
reduzierende Gase beobachtet. Abhängig vom gewählten Typ des Isolators
wurden Unterschiede in den Empfindlichkeiten beobachtet. In Kapitel 4:
SiC-basierten FETs mit verschiedenen Materialien für das Gate (Gemisch aus
Metalloxiden: Indiumoxide und Zinnoxid (InxSnyOz), Indiumoxid und
Vanadiumoxid (InxVyOz) sowie ein Gemisch aus Metalloxiden mit Zugabe einer
entsprechenden Menge Metallzusätzen) wurden als NOx, O2, und D2
Gasdetektoren untersucht. Die Reaktion auf diese Gase wurde in Abhängigkeit
von der Betriebstemperatur und der Gaspartialdrücke untersucht. Die
Zusammensetzung der aktiven Metalloxid-Schicht und die Mikrostruktur der
sensitiven Gateelektrode sind die entscheidenden Parameter mit Einfluss auf
den Messmechanismus und somit die entscheidenden Leistungsparameter des
Sensors: Empfindlichkeit, Selektivität und Reaktionszeit. Durch die
Optimierung der Temperatur und des richtigen Materials des Katalysators
können Sensoren mit sehr hoher Empfindlichkeit gegenüber relevanten Gasen
realisiert werden. Wird auch der Katalysator sorgfältig ausgewählt, können
diese Erkenntnisse für eine Erhöhung der Selektivität des Sensors genutzt
werden. In Kapitel 5: Polykristalle von 200 nm Dicke und 10 nm
nanostrukturierten Dünnschichten aus In2O3, die durch MOCVD
(metallorganische Gasphasenabscheidung) gewachsen sind, wurden untersucht,
um Informationen über ihre Eigenschaften hinsichtlich der Detektion von
NOx- and O2-Gasen zu erhalten. Die Reaktion auf diese Gase wurde in
Abhängigkeit von der Betriebstemperatur und der Gaspartialdrücke
untersucht. Die Experimente in Anwesenheit verschiedener Partialdrücke des
NOx haben gezeigt, das beide Dünnschichten in der Lage sind, Stickoxide zu
detektieren. Es wurde festgestellt, dass besonders die nanostrukturierte
In2O3-Dünnschicht stärker auf NOx reagiert. Dieser Effekt wird durch das
höhe Oberflächen-zu-Volumenverhältnis infolge der niedrigen Korngröße
verbessert, so dass der relative interaktive Oberflächenbereich größer und
die Dichte der Ladungsträger höher ist. Wir haben ermittelt, dass die
Reduzierung der Korngröße des messenden Materials auf unter 10 nm
erhebliche Auswirkung auf die Empfindlichkeit des Sensors hat. Die
hinsichtlich der Empfindlichkeit und Reaktion optimalen Temperaturen des
nanostrukturierten In2O3 für den Nachweis von NOx treten im Bereich von
100-175°C auf. In diesem Temperaturbereich ist die Reaktion auf O2 sehr
schwach, was darauf hinweist, das der Sensor für die selektive Erkennung
von NOx bei niedrigen Temperaturen sehr gut geeignet ist. Zudem wurde
festgestellt, dass die nanostrukturierte In2O3-Dünnschicht für den Einsatz
in der Erkennung niedriger Partialdrücke die optimale ist. Kapitel 6
enthält Schlussfolgerungen aus den gegenwärtigen Arbeiten. In diesem
Kapitel vergleichen wir alle untersuchten Gassensoren in Bezug auf deren
Empfindlichkeit, Selektivität und Reaktionszeit und stellen diese
anschließend den entsprechenden Ergebnissen anderer, in der
wissenschaftlichen Literatur zu findenden Autoren gegenüber.In this thesis, field effect gas sensors (Schottky diodes, MOS capacitors,
and MOSFET transistors) based on wide band gap semiconductors like silicon
carbide (SiC) and gallium nitride (GaN), as well as resistive gas sensors
based on indium oxide (In2O3), have been developed for the detection of
reducing gases (H2, D2) and oxidising gases (NOx, O2). The development of
the sensors has been performed at the Institute for Micro- and
Nanoelectronic, Technical University Ilmenau in co-operation with (GE)
General Electric Global Research (USA) and Umwelt-Sensor-Technik GmbH
(Geschwenda). Chapter 1: serves as an introduction into the scientific
fields related to this work. The theoretical fundamentals of solid-state
gas sensors are provided and the relevant properties of wide band gap
materials (SiC and GaN) are summarized. In chapter 2: The performance
of Pt/GaN Schottky diodes with different thickness of the catalytic metal
were investigated as hydrogen gas detectors. The area as well as the
thickness of the Pt were varied between 250 × 250 µm2 and 1000 × 1000 µm2,
8 and 40 nm, respectively. The response to hydrogen gas was investigated in
dependence on the active area, the Pt thickness and the operating
temperature for 1 vol.% hydrogen in synthetic air. We observed a
significant increase of the sensitivity and a decrease of the response and
recovery times by increasing the temperature of operation to about 350°C
and by decreasing the Pt thickness down to 8 nm. Electron microscopy of the
microstructure showed that the thinner platinum had a higher grain boundary
density. The increase in sensitivity with decreasing Pt thickness points to
the dissociation of molecular hydrogen on the surface, the diffusion of
atomic hydrogen along the platinum grain boundaries and the adsorption of
hydrogen at the Pt/GaN interface as a possible mechanism of sensing
hydrogen by Schottky diodes. The response to deuterium D2, NOx, and O2
of metal-oxide-semiconductor (MOS) and metal-metal
oxide-oxide-semiconductor (MMOOS) structures with rhodium (Rh) gate were
investigated in dependence on the operating temperature and gas partial
pressures was investigated in chapter 3. The response of the sensor was
measured as a shift in the capacitance-voltge (C-V) curve along the voltage
axis. Positive and negative flat-band voltage shifts up to 1 V were
observed for oxidizing and reducing gases, respectively. Depending on the
type of insulator that is chosen, differences in the sensitivity of the
sensor were observed. In chapter 4: The performance of SiC-based field
effect transistors (FETs) with different gate materials (mixture of metal
oxides: indium oxide and tin oxide (InxSnyOz), indium oxide and vanadium
oxide (InxVyOz), as well as mixtures of metal oxides with metal additives)
were investigated as NOx, O2, and D2 gas detectors. The response to these
gases was investigated in dependence on the operating temperature and gas
partial pressures. The composition and microstructure of the sensing gate
electrode are the key parameters that influence the sensing mechanism, and
hence key performance parameters: sensitivity, selectivity, and response
time. By choosing the appropriate temperature and catalyst material (gate
material), devices that are significantly sensitive to certain gases may be
realized. In addition, the temperature of maximum response varies dependent
on the gas species being measured. This information, along with a careful
choice of catalyst (gate material) can be used to enhance device
selectivity. In chapter 5: Polycrystalline and nano-structured In2O3
thin films were investigated with the aim to obtain information about their
NOx and O2 gas sensing properties. The response to these gases was
investigated in dependence on the operating temperature and gas partial
pressures. The analysis in the presence of different partial pressures of
NOx has shown that both thin films are able to detect nitrogen oxide, but
their responses exhibit different characteristics. In particular,
nano-structured In2O3 thin films were found to have the higher response to
NOx. This is most probably due to the enlarged overall active surface area
of the sensing layer as a consequence of the small grain size (higher
surface to volume ratio) so that the relative interactive surface area is
larger, and the density of charged carriers per volume is higher. We have
found that reducing the grain size of the sensing material to the ~10 nm
regime can have a substantial effect on performance. The optimum detection
temperatures of the nano-structured In2O3 occur in the range of 100-175°C
for NOx considering the sensitivity as well as the response time. In this
range of temperatures the response to O2 is very low indicating that the
sensor is very suitable for selective detection of NOx at low temperatures
In addition, nano-structured In2O3 thin films were found to be more
suitable to be used in the field of application for detecting low partial
pressures. Chapter 6: offers conclusions of the current work. In this
chapter we compare also all studied gas sensors according to their
sensitivity, selectivity, and response time and then we compare them with
the related works by other authors available in the scientific literature
Design And Fabrication Of Chemiresistor Typemicro/nano Hydrogen Gas Sensors Usinginterdigitated Electrodes
Hydrogen sensors have obtained increased interest with the widened application of hydrogen energy in recent years. Among them, various chemiresistor based hydrogen sensors have been studied due to their relatively simple structure and well-established detection mechanism. The recent progress in micro/nanotechnology has accelerated the development of small-scale chemical sensors. In this work, MEMS (Micro-Electro-Mechanical Systems) sensor platforms with interdigitated electrodes have been designed and fabricated. Integrating indium doped tin dioxide nanoparticles, these hydrogen sensors showed improved sensor characteristics such as sensitivity, response and selectivity at room temperature. Design parameters of interdigitated electrodes have been studied in association with sensor characteristics. It was observed that these parameters (gap between the electrodes, width and length of the fingers, and the number of the fingers) imposed different impacts on the sensor performance. In order to achieve small, robust, low cost and fast hydrogen micro/nano sensors with high sensitivity and selectivity, the modeling and process optimization was performed. The effect of humidity and the influence of the applied voltage were also studied. The sensor could be tuned to have high sensitivity (105), fast response time (10 seconds) and low energy consumption (19 nW). Finally, a portable hydrogen instrument integrated with a micro sensor, display, sound warning system, and measurement circuitry was fabricated based on the calibration data of the sensor
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SILICON CARBIDE MICRO-DEVICES FOR COMBUSTION GAS SENSING UNDER HARSH CONDITIONS
A sensor based on the wide bandgap semiconductor, silicon carbide (SiC), has been developed for the detection of combustion products in power plant environments. The sensor is a catalytic gate field effect device that can detect hydrogen containing species in chemically reactive, high temperature environments. For these capacitive sensors we have determined that the optimum sensor operating point in terms of sensor lifetime and response time is at midgap. Detailed measurements of the oxide leakage current as a function of temperature were performed to investigate the high temperature reliability of the devices. In addition, robust metallization and electrical contacting techniques have been developed for device operation at elevated temperatures. To characterize the time response of the sensor responses in the millisecond range, a conceptually new apparatus has been built. Using laser induced fluorescence imaging techniques we have shown that the gas underneath the sensor can be completely exchanged with a time constant under 1 millisecond. Ultrahigh vacuum studies of the surface chemistry of the platinum gate have shown that sensor deactivation by adsorbed sulfur is a possible problem. Investigations on the chemical removal of sulfur by catalytic oxidation or reduction are continuing