Wide band gap materials and devices for NO_1tnx, H_1tn2 and O_1tn2 gas sensing applications [Elektronische Ressource] / von Majdeddin Ali

Majd Sayegh

Informations

• Publié par: technische_universitat_ilmenau
• Publié le: 01 janvier 2008
• Nombre de lectures: 47
• Langue: English
• Poids de l'ouvrage: 7 Mo

Extrait

Wide band gap materials and devices for NO , H and O x 2 2
gas sensing applications



Dissertation zur Erlangung des
akademischen Grades Doktor-Ingenieur (Dr.-Ing.)



vorgelegt der Fakultät Elektrotechnik und
Informationstechnik der Technischen Universität Ilmenau


von Dipl.-Ing Majdeddin Ali
geboren am 08.09.1976 in Homs, Syrien




1. Gutachter: Univ.-Prof. Dr. rer. nat. habil. Oliver Ambacher, Fraunhofer-Institut
für Angewandte Festkörperphysik, Freiburg
2. Gutachter: PD Dr.-Ing. habil. Frank Schwierz, TU Ilmenau
3. Gutachter: Dr. Martin Eickhoff, Walter Schottky Institut, TU München




Tag der Einreichung: 28.06.2007
Tag der wissenschaftlichen Aussprache: 22.01.2008


urn:nbn:de:gbv:ilm1-2007000323

Abstract III

Abstract

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 (In O ), have been developed for the detection of reducing gases (H , D ) and 2 3 2 2
oxidising gases (NO , O ). The development of the sensors has been performed at x 2
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
2area as well as the thickness of the Pt were varied between 250 × 250 µm and 1000
2× 1000 µm , 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 D , NO , and O of metal-oxide-semiconductor 2 x 2
(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. IV Abstract

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
(In Sn O ), indium oxide and vanadium oxide (In V O ), as well as mixtures of metal x y z x y z
oxides with metal additives) were investigated as NO , O , and D gas detectors. The x 2 2
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 In O thin films were 2 3
investigated with the aim to obtain information about their NO and O gas sensing x 2
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 NO has shown that both thin films are able to detect x
nitrogen oxide, but their responses exhibit different characteristics. In particular,
nano-structured In O thin films were found to have the higher response to NO . This 2 3 x
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 In O occur in the range of 100-175°C 2 3
for NO considering the sensitivity as well as the response time. In this range of x
temperatures the response to O is very low indicating that the sensor is very suitable 2
for selective detection of NO at low temperatures In addition, nano-structured In O x 2 3
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 Abstract V

time and then we compare them with the related works by other authors available in
the scientific literature.































VI Abstract

Zusammenfassung VII

Zusammenfassung

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 (In O) basieren, für die 2 3
Detektion von reduzierenden Gasen (H , D ) und oxidierenden Gasen (NO , O ), 2 2 x 2
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
2 2von Pt-gates wurden zwischen 250 × 250 µm und 1000 × 1000 µm , 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 D , NO , and O von Metall-Oxid-Halbleiter (MOS) Strukturen 2 x 2
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-VIII Zusammenfas