Metal organic chemical vapor deposition of indium oxide for ozone sensing [Elektronische Ressource] / von Chunyu Wang
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Metal organic chemical vapor deposition of indium oxide for ozone sensing [Elektronische Ressource] / von Chunyu Wang

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Metal organic chemical vapor deposition of indium oxide for ozone sensing Der Fakultät für Angewandete Wissenschaft der Albert-Ludwigs-Universität Freiburg i. Br. zur Erlangung des akademischen Grades eines Doktor-Ingenieurs vorgelegt und genehmigte Dissertation von Chunyu Wang Dekan: Prof. Dr. Hans Zappe Referent: Prof. Dr. Oliver Ambacher Korreferent: Prof. Dr. Gerald Urban Tag der mündlichen Prüfung: February 06, 2009 1Abstract In this work, a novel type of ozone sensor based on the photoreduction and oxidation principles operating at room temperature was developed. First, MOCVD techniques were used to grow high-quality Indium oxide thin films and nanostructures, to be used as ozone sensing materials. On sapphire substrates, highly textured bcc-In O films were grown using a low-temperature indium oxide film 2 3as the buffer layer. Furthermore, for the first time, meta-stable rh-In O films were 2 3epitaxially grown on sapphire substrate by means of MOCVD, which usually can only be obtained under high-temperature and high-pressure conditions. In O nanoparticles 2 3were also obtained at low substrate temperatures using TMIn and water vapour as the precursors, leading to a low-cost fabricating process. In addition, In O thin films were 2 3successfully deposited on III-N (GaN, AlN, and InN) substrates.

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Published 01 January 2009
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Metal organic chemical vapor deposition of indium
oxide for ozone sensing


Der Fakultät für Angewandete Wissenschaft
der Albert-Ludwigs-Universität Freiburg i. Br.
zur Erlangung des akademischen Grades eines

Doktor-Ingenieurs



vorgelegt und genehmigte Dissertation von

Chunyu Wang




Dekan: Prof. Dr. Hans Zappe
Referent: Prof. Dr. Oliver Ambacher
Korreferent: Prof. Dr. Gerald Urban
Tag der mündlichen Prüfung: February 06, 2009






































1Abstract
In this work, a novel type of ozone sensor based on the photoreduction and
oxidation principles operating at room temperature was developed.
First, MOCVD techniques were used to grow high-quality Indium oxide thin films
and nanostructures, to be used as ozone sensing materials. On sapphire substrates,
highly textured bcc-In O films were grown using a low-temperature indium oxide film 2 3
as the buffer layer. Furthermore, for the first time, meta-stable rh-In O films were 2 3
epitaxially grown on sapphire substrate by means of MOCVD, which usually can only be
obtained under high-temperature and high-pressure conditions. In O nanoparticles 2 3
were also obtained at low substrate temperatures using TMIn and water vapour as the
precursors, leading to a low-cost fabricating process. In addition, In O thin films were 2 3
successfully deposited on III-N (GaN, AlN, and InN) substrates. By using a thin h-InN film
as a buffer layer, epitaxial single crystalline bcc-In O (111) was grown. By comparing 2 3
the electrical properties of different In O thin films, it was found that the In O 2 3 2 3
nanostructures and In O thin layers on AlN substrates are most suitable for the 2 3
application of ozone sensing.
To characterize ozone sensors, an automated ozone measuring station was
established which allows gas sensing measurements under different conditions (in
vacuum, in different gases, in humid atmospheres). The integrated ozone sensor
structure and its electronic control unit were developed. Different operational modes of
ozone sensors were studied, and it was found that sensor operation in pulse mode
results in stable and fast sensing. The ideal operation temperature of ozone sensors
based on the photoreduction and oxidation principle was determined to be room
temperature. The required photon energy and light intensity were also determined for
the reactivation of the active In O layer, which enables the integration of In O 2 3 2 3
nanoparticle based ozone sensors with UV-LEDs, leading to compact, low-energy
consumption, low-cost ozone sensors. The smallest integrated ozone sensor developed
5was 300 μm * 300 μm. The ozone sensitivity was determined to be greater than 10 in
vacuum. Not only in vacuum but also in synthesized air, the sensor showed good ozone
sensing results. The lowest detectable ozone concentration was found to be
~ 13 ppb. In addition, this type of ozone sensor showed a good long-term stability, and
the cross-sensitivity against other oxidizing gases, such as NO , CO , O was very low. x 2 2
Tanking into account that the integrated LED operates at a current of 10 mA at 3 V, the
2total energy consumption including the electronic control unit was determined to be less
than 50 mW. Furthermore, measurements under real conditions were carried out by an
ozone sensor controlled by the developed electronic unit in the center of the Freiburg
city, Germany. Ozone concentrations as low as 12 ppb were measured. Thus, compact,
portable, low-cost, low-energy consumption, environmental ozone sensors were
developed, which can operate at room temperature and are suitable for integration in
plastic packages, in cellular phones and PDAs.
To understand the mechanisms of photoreduction and oxidation effects, electrical
and structural characterization of ozone sensing layer after various treatments were
performed. It was found that the contamination on the nanoparticle surface was
reduced after the first cycle of photoreduction and oxidation. The adsorbed oxygen
- -species, which was analyzed to be O compared to the widely accepted O in the 3
literature , plays a major role during ozone sensing, and the adsorption and desorption
process takes place mainly at the nanoparticle surfaces. The adsorbed O-species form a
dipole layer with the positive-charged vacancies, leading to an upward shift of the work
function on nanoparticle surfaces. Furthermore, the photoreduction process occurs
throughout the sensing layer at the same time, while gas diffusion dominates in the
oxidation process. Due to the high oxygen deficiency on indium oxide nanoparticle
surfaces, physical models were proposed.
3Deutsche Zusammenfassung
Im Rahmen der vorliegenden Arbeit wurde ein neuartiger, kostengünstiger und
miniaturisierter Ozonsensor, der auf dem Photoreduktions- und Oxidationsprinzip
basiert, entwickelt.
In den ersten vier Kapiteln werden die Grundlagen für die Sensorentwicklung
vorgestellt. Unterschiedliche Typen von Ozonsensoren, die auf verschiedenen
physikalischen und chemischen Prinzipien basieren, wurden verglichen. Dabei zeigte
sich, dass das Photoreduktions- und Oxidationsprinzip am besten geeignet ist, um einen
tragbaren, energieeffizienten und kompakten Ozonsensor zu entwickeln. Zwei
Möglichkeiten ozonempfindlicher Materialien wurden präsentiert: einkristalline, hoch-
resistive In O -Schichten mit glatter Oberfläche sowie In O -Nanostrukturen. 2 3 2 3
Im fünften Kapitel wird die Herstellung des ozonempfindlichen Materials
beschrieben, das mittels MOCVD-Technik auf verschiedenen Substraten deponiert sowie
strukturell, elektrisch und optisch charakterisiert wurde. Auf Saphirsubstraten wurden
sowohl hochtexturierte bcc-In O-Schichten mit Hilfe einer Niedertemperatur-In O -2 3 2 3
Schicht gewachsen als auch einkristallines bcc-In O auf einer h-InN-Bufferschicht 2 3
aufgebracht. Weiterhin wurden einkristalline rh-In O -Schichten, die normalerweise nur 2 3
unter Hochtemperatur- und Hochdruck-Bedingungen abgeschieden werden, mittels
MOCVD hergestellt. Die elektrischen und optischen Eigenschaften von rh-In O wurden 2 3
zum ersten Mal bestimmt. Nicht nur hochqualitative In O -Schichten sondern auch 2 3
In O -Nanostrukturen wurden mittels MOCVD bei sehr niedrigen Prozesstemperaturen 2 3
und hoher TMIn-Flussrate erfolgreich gefertigt. Die strukturelle und elektrische
Charakterisierung belegt, dass In O-Nanopartikelschichten wegen des hohen 2 3
Oberfläche-zu-Volumen-Verhältnisses am besten für die Ozonmessung geeignet sind.
Die In O -Nanostrukturen wurden direkt mit einer UV-LED integriert und ein Sensor 2 3
mit Abmessungen von 300 μm * 300 μm realisiert. Damit konnten kompakte,
energiesparende Ozonsensoren hergestellt werden. Für die Charakterisierung des
Ozonsensors im Labor wurde auch ein automatischer Ozonmessplatz aufgebaut, mit
dem die Ozonmessungen unter verschiedenen Bedingungen (im Vakuum, in
synthetischer Luft, in feuchter Luft) durchgeführt wurden. Um den Ozonsensor zur
Produktreife zu bringen, wurde auch ein elektronischer Schaltkreis für automatische
Ozonmessungen unter realen Bedingungen entwickelt.
In einem weiteren Schritt wurden die optimalen Arbeitsbedingungen des Sensors,
4unter dem er betrieben werden kann, untersucht. Dabei zeigte sich, dass bei
Raumtemperatur der Ozonsensor die höchste Empfindlichkeit besitzt. Im Pulsmodus
arbeitet der Sensor am stabilsten und hat höhere Empfindlichkeit als im kontinuierlichen
Modus. Die Photonenenergie und die Lichtintensität für die Photoreduktion wurden
ebenfalls bestimmt. Im Vakuum zeigt der Ozonsensor eine Empfindlichkeit von mehr als
510. In synthetischer Luft ist die Ozonempfindlichkeit kleiner. Aber die untere
detektierbare Grenze der Ozonkonzentration ist konstant und liegt bei 13 ppb. Diese
untere Grenze ist viel kleiner als der Richtwert, der zum Schutz der menschlichen
Gesundheit und Umwelt von der EU vorgegeben wird. Der Ozonsensor ist Langzeit-
stabil und zeigt sehr niedrige Querempfindlichkeiten gegen andere oxidierende Gase, z.
B. NO , CO , und O . Dieser Ozonsensor wurde auch unter realen Bedingungen getestet. x 2 2
Der Ozonsensor, der von der neu entwickelten elektronischen Platine gesteuert wird,
wurde für die Überwachung des Ozongases in der Stadtmitte von Freiburg eingesetzt.
Ozon mit einer Konzentration von 12 ppb wurde mit dem Sensor detektiert. Die vom
Sensor benötigte elektrische Leistung inklusive der elektronischen Steuerung war im
Bereich von 50 mW, ein Wert, der viel kleiner ist als bei anderen Ozonsensoren.
Um das Sensorprinzip zu verstehen, wurden die ozonempfindlichen Schichten
sowohl nach der Photoreduktion als auch nach der Oxidation strukturell und elektrisch
charakterisiert. Die Oberfläche der In O-Nanopartikelschicht hat viele O-Vakanzen 2 3
(33%). Der Photoreduktionsprozess zeigt keine Schichtdickenabhängigkeit, der
Oxidationsprozess ist jedoch schichtdickenabhängig, da hier die Gasdiffusion dominiert.
Die größte Rolle bei der elektrischen Widerstandsänderung der Nanopartikel spielt die
- -absorbierte O-Spezies. O -Ionen (33%) werden an der vakanzreichen
Nanopartikeloberfläche absorbiert und beeinflussen deren Stöchiometrie.
Wahrscheinlich wird dadurch eine neue Indiumoxidphase an der Oberfläche gebildet,
-die einen größeren Bandabstand besitzt. Andererseits könnten die O -Ionen auch mit
den positiv geladenen Vakanzen eine Dipolschicht bilden, die die Austrittsarbeit der
Oberfläche vergrößert. Um diesen Effekt besser zu verstehen, werden noch weitere
Messungen benötigt.
Insgesamt konnte im Rahmen der vorliegenden Arbeit das geeignete
ozonempfindliche Material mittels MOCVD hergestellt und ein kompakter, robuster,
energiesparender und Langzeit-stabiler Ozonsensor aufgebaut werden, der seine
Funktionsfähigkeit auch unter realen Umgebungsbedingungen bewiesen hat.
5Contents
Abstract ..................................................................................................................... 2
Deutsche Zusammenfassung.................................................................................... 4
1. Introduction........................................................................................................... 8
2. Fundamentals of ozone sensing ........................................................................ 11
2.1 Traditional ozone sensing methods................................................................... 11
2.1.1 UV absorption method............................................................................... 11
2.1.2 Electrochemical method............................................................................. 14
2.1.3 Work function method .............................................................................. 16
2.1.4 HSGFET method......................................................................................... 20
2.1.5 Resistive method based on semiconductor material .................................... 21
2.1.6 A comparison of traditional methods ......................................................... 27
2.2 Room-temperature ozone sensor based on the photoreduction and oxidation
principle................................................................................................................. 29
2.2.1 Sensor principle 30
2.2.2 Sensor setup.............................................................................................. 31
2.2.3 Sensor parameter....................................................................................... 32
3. The material In O ............................................................................................... 34 2 3
4. Characterization methods .................................................................................. 41
4.1 HRXRD method ................................................................................................ 41
4.2 TEM method .................................................................................................... 43
4.3 XPS/UPS method .............................................................................................. 45
5. Growth and characterization of In O thin films by means of MOCVD........... 47 2 3
5.1 Growth of In O thin films on sapphire substrates ............................................. 47 2 3
5.1.1 Highly textured cubic In O films grown by MOCVD on sapphire substrates 48 2 3
5.1.2 Rhombohedral In O films grown by MOCVD on sapphire substrates.......... 51 2 3
5.1.3 Cubic In O nanoparticles grown by MO .......... 56 2 3
5.2 Growth of In O thin films on III-N substrates .................................................... 63 2 3
5.2.1 Growth of In O thin films on GaN and AlN substrates ............................... 63 2 3
5.2.2 Growth of In O thin films on InN substrates .............................................. 65 2 3
5.3 Phase diagram for In O growth by MOCVD on sapphire substrates .................. 71 2 3
5.4 Electrical properties of In O films ..................................................................... 74 2 3
5.4.1 Electrical properties of polycrystalline bcc-In O films .................................. 74 2 3
65.4.2 Electrical properties of rh-In O ................................................................... 76 2 3
5.4.3 Electrical properties of In O nanoparticles.................................................. 76 2 3
5.4.4 Electri O thin films grown on III-N substrates................. 79 2 3
5.5 Optical properties of In O films........................................................................ 80 2 3
5.5.1 Optical properties of bcc- and rh-In O films............................................... 80 2 3
5.5.2 Optical properties of In O nanoparticles .................................................... 81 2 3
6. Ozone sensors based on In O layers................................................................. 85 2 3
6.1 Installation of ozone measuring station............................................................. 85
6.2 Fabrication of ozone sensors integrated with UV-LEDs ...................................... 88
6.3 Preliminary experiments for ozone sensors ........................................................ 92
6.3.1 Operation mode and operation temperature of ozone sensors ................... 92
6.3.2 Photon energy and light intensity of UV-LEDs for oz ................. 95
6.4 Characterization of ozone sensors based on In O nanoparticle containing layers2 3
.............................................................................................................................. 98
6.5 Mechanisms of In O based ozone sensors.......................................................108 2 3
6.5.1 HRXRD measurements ..............................................................................109
6.5.2 Electrical measurements............................................................................110
6.5.3 Photoelectron spectroscopy measurements ...............................................117
7. Conclusions and outlook ...................................................................................124
Symbols & Abbreviations ......................................................................................127
References: .............................................................................................................128
Appendix ................................................................................................................143
A1: Raman properties of bcc- and rh-In O ............................................................143 2 3
A2: XRD-Data of bcc-In O (JCP94). .......................................................................146 2 3
A3: XRD-Data of rh-In O (JCP94). .........................................................................147 2 3
A4: Growth of UV-LEDs by MOCVD.......................................................................148
Curriculum Vitae ....................................................................................................151
List of Publications.................................................................................................152
Acknowledgements ...............................................................................................155
Erklärung ................................................................................................................156

71. Introduction

Ozone was discovered in 1839 by Prof. Schönbein, a professor of chemistry at the
University of Basel. He noticed a smell coming from the positive electrode during
1,2electrolysis of water , and named it after the Greek word ozein. The formula for
2ozone, O , was determined by Jacques-Louis Soret in 1865 . Since then, ozone has 3
been intensively studied. Nowadays, ozone is very widely used in the industry and in
everyday life.
As is well known, there is an ozone layer in the earth´s atmosphere, and is mainly
located in the lower portion of the stratosphere at approximately 15 to 35 km above
earth’s surface. Ozone absorbs in the ultraviolet (UV) spectral region of the sun between
200 and 320 nm, with an absorption peak located a 254 nm. The suns radiation from
3, 4200 to 290 nm is most damaging to DNA molecules . Stratospheric ozone prevents
highly energetic radiation from reaching the earth’s surface, providing protection of all
terrestrial life. Furthermore, ozone is a powerful oxidizing agent. Due to its highly
oxidizing potential, ozone is applied for sterilization in fish farms and in hospital
operating rooms, disinfection of industrial and chemical freshwater, process water, and
cooling water as well as disinfection, detoxification, deodorization of industrial
5wastewater . For example, 95% of potable water in western Europe is treated by
ozone.
However, as a negative effect of the powerful oxidizing potential, high
concentration ozone gas can harm people’s health. Ozone is often undesirably
generated in everyday life, e.g. by older generation photocopiers and laser printers.
Exposure of 0.1 to 1 ppm (parts per million) causes headaches, burning eyes, and
irritation to the respiratory passages. An individual remaining in a 0.1 ppm O 3
environment for two hours will sustain a loss of 20% in breathing capacity, and after
remaining in 1 ppm O for six hours, will be suffered by an attack of bronchitis. A mouse 3
6kept in 10 ppm O will not survive . To protect people against ozone, threshold values 3
7of ozone concentration are defined by European Guidelines (2002/3/EG) . The maximal
3permissible concentration of ozone is 240 mg/m (~ 120 ppb, parts per billion). A
3warning should be given at an ozone concentration of 180 μg/m (~ 90 ppb). The
permissible highest 8-hour mean value of ozone concentration on any day will be
3reduced to 120 μg/m (~ 60 ppb) by 2010. Furthermore, the maximum ozone
83concentration at the workplace should not exceed 200 μg/m (~ 100 ppb). According to
the air quality standard established by the U. S. environment protection agency, the
standard for maximum ozone concentration for hourly exposure is set to be lower than
8~ 80 ppb . Thus, it is of great importance and interest to monitor ozone concentration
in the low ppb range, i. e. lower than 60 ppb. That means the developed environmental
9ozone sensor must be able to detect less than 60 ozone molecules in 10 other
molecules in its environment.
It is worth noting that ozone is unstable and has a half-life of about one hour at
9room temperature, reverting to oxygen . Furthermore, it can react easily with other
atoms and donate a free oxygen atom to nitrogen, hydrogen, or chlorine. Thus, ozone
concentration varies strongly with time and location. It is therefore very important to
control environmental O concentration both in work places, preventing O leaks from 3 3
O generators, and in our everyday life, monitoring O concentration for people’s health. 3 3
As a result, the monitoring of O concentration becomes more and more popular. Alone 3
in Baden-Württemberg, there are over 40 pollution measuring stations, monitoring
3exhaust gases such as O and NO . An ozone concentration of over 200 μg/m (~ 100 3 2
3ppb) or even higher than the maximal permissible concentration 240 μg/m (~ 120 ppb)
has often been measured in previous summers. The maximum value of ozone
3concentration was determined to be 282 μg/m (141 ppb) in Schwartenberg in Germany
10last summer, which was much higher than the limit values . In Fig. 1.1, an ozone
measuring station in the center of Freiburg city is shown, indicating an 1-hour mean
3value of ozone concentration of 110 μg/m (~ 55 ppb). However, such ozone measuring
stations are very large and expensive. It would be impossible to install hundreds or even
thousands of such measuring stations in one city to monitor the ozone concentration
anytime and anywhere. On the other hand, an alarm should be given at a high ozone
concentration by portable devices for sportsman training outdoors, or for children
playing outside the nursery. Thus, portable low-cost and energy-saving ozone sensors
are of great interest.

9