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Growth of SnO2 and TiO2 thin films by PE-ALD : their structural characteristics and H2 gas sensing properties

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Authors

김대홍

Advisor
홍성현
Major
공과대학 재료공학부
Issue Date
2014-08
Publisher
서울대학교 대학원
Keywords
gas sensorsemiconductor type gas sensormetal oxideH2 gas sensorthin filmtin dioxidetitanium dioxideatomic layer depositionepitaxybrookitedual-layergas selectivity
Description
학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2014. 8. 홍성현.
Abstract
One of the potential alternatives for the fossil fuels is hydrogen (H2) gas because it is
renewable and clean. However, a high flammability and explosiveness of H2 in the gas
storage are great problems. Therefore, the detection of the H2 gas from leakage is
indispensable for safety. Gas sensors for H2 gas have been studied and commercially
available for many years. In order to meet the demands of future H2 gas applications,
however, many researches are conducted to improve low cost, low working temperature
(below 100 oC), selectivity (NOx, EtOH, H2O, etc) and reliability in addition to reducing
sensor size.
In this research, semiconductor type metal oxide gas sensor is attracted due to low cost
and high sensitivity toward H2 gas and plasma enhanced atomic layer deposition (PEALD)
is used to control variables such as thickness and morphology. As sensing
materials, SnO2 and TiO2 which are most widely studied materials for H2 gas sensor are
ii
chosen. The three main topics will be discussed to investigate gas sensing mechanism and
enhance gas sensing performance: 1) Oriented SnO2 thin films grown on TiO2 single
crystals, 2) Brookite TiO2 thin film epitaxially grown on YSZ substrates, 3) SnO2-TiO2
dual layer gas sensors.
The first and second topics are focused on investigating gas sensing mechanism.
Theoretical studies of gas adsorption and desorption on specific crystallographic planes or
phases have been conducted. Although gas sensing performance is strongly depended on
gas adsorption on a specific crystallographic plane or phases, the study of relationships
between gas sensing performance and crystallographic planes or phases is very limited
due to morphology change caused by crystallographic change. Therefore, in this study, I
tried to separate of variables using epitaxial deposition methods to control the
crystallographic planes or phases without morphology change.
First, epitaxial SnO2 films were deposited on TiO2 single crystals with various
orientations by PE-ALD, and their structural characteristics and gas sensing properties
were investigated, particularly focusing on the crystallographic orientation dependence of
H2 gas response. Dibutyltindiacetate (DBTDA) was used as Sn source, and (100), (001),
(110), and (101) TiO2 were employed as substrates for SnO2 deposition. All the
SnO2 films were ∼90 nm thick after 1000 ALD cycles and epitaxially grown on
TiO2 substrates, which were confirmed by X-ray pole figure and high resolution
transmission electron microscopy (HRTEM). Differently oriented epitaxial SnO2 films
iii
showed the different H2 gas response and different temperature dependence of gas
response. The (101) SnO2 films grown on (101) TiO2 exhibited the highest H2 gas
response of ∼380 toward 1000 ppm H2/air at 400 °C, which was associated with the
different temperature dependence of resistance in (101) film rather than the
microstructural characteristics and chemical composition compared to the other films.
Next, epitaxial brookite TiO2 (B-TiO2) film was deposited on (110) YSZ substrate using
PE-ALD and its structural, optical, and gas sensing properties were investigated.
Chemical states and morphology of the TiO2 film were investigated by XPS and AFM. Xray
diffraction, X-ray pole figure, and high resolution TEM analyses revealed that
deposited TiO2 film was pure B-TiO2 and highly oriented to (120) plane. The determined
in-plane orientation relationships were
B-TiO2 YSZ [210] [110] and
B-TiO2 YSZ [001] [001] and lattice mismatches were -1.91 and 0.06 %. Phase of BTiO2
film was unchanged at 700 °C heat treatment and the sensor showed stable and high
sensing properties for H2 gas. The highest magnitude of the gas response (Rair/Rgas) was
determined to be ~150 toward 1000 ppm H2/air at 150 °C. In addition, B-TiO2 sensor
showed a high selectivity for H2 against CO, EtOH, and NH3.
The last topic is concentrated to enhance high sensing ability and selectivity in addition to
low working temperature and low cost using silicon substrate. Although SnO2 thin film is
extensively studied in H2 gas sensor owing to high gas sensing performance, most SnO2
iv
thin film sensors have great problems to be commercialized because of high working
temperature and low selectivity. First, to reduce working temperature, the thickness effect
of SnO2 gas senor is studied. When the thickness of SnO2 thin film is ~ 4 nm which is
near debye length, the maximum gas response was exhibited at low temperature (below
100 oC) but the sensors have still poor selectivity. To enhance the gas selectivity, SnO2-
TiO2 dual layer gas sensor was suggested. When TiO2 was deposited on SnO2, the gas
response was increased and the optimum thickness was ~ 4 nm of SnO2 and ~ 4 nm of
TiO2. Dual layer sensor showed the excellent gas selectivity against NOx gas
compared with SnO2 single layer sensor. Although the dual layer sensors
exhibited poor selectivity against humidity, the problem is solved with adjusting
of operating temperature up to 100 oC.
Language
English
URI
https://hdl.handle.net/10371/117969
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