new materials and structures for semiconductor gas sensors

TULE Research Programme Progress Report 2005

1
New materials and structures for semiconductor gas sensors
(NEWGAS)




Pekka Kuivalainen
1
and Vilho Lantto
2
.




Abstract

The aim of the present research project is to develop new advanced gas sensors with enhanced performance
based on new materials such as epitaxial tin dioxide, porous silicon and layered tungsten trioxide together with
new micromechanical air bridge structures. The research work utilizes the state of the art measurement systems
for gas sensor materials based, e.g., on the Kelvin probe techniques, which allow in situ studies of the gas
reactions at the sensor surfaces. The new epitaxial and layered semiconductor materials have been chosen to
maximize the sensitivity and the long-term stability. On the other hand, the new micromechanical air bridge
structures allow a significant reduction in power consumption, which is vitally important in portable
applications. Both the growth of the new materials and the demanding measurement techniques are based on
the previous experience of the present research consortium on the epitaxy of semiconductor thin films and the
extensive studies of the polycrystalline tin dioxide as a gas sensor material.

1 Partners and Funding

1.1 Electron Physics Laboratory, Helsinki University of Technology

The research group consists of the subproject leader professor Pekka Kuivalainen,
senior researcher Dr. Sergey Novikov, and a postgraduate student Mikael Kroneld.

1.2 Microelectronics and Materials Physics Laboratories, University of Oulu

The research group consists of subproject leader professor Vilho Lantto and
postgraduate student Sami Saukko. The research has been done also in a close co-
operation with the research group of Professor C.G. Granqvist at the Uppsala University
(postgraduate students Luis Reyes and Peter Heszler and Dr. Anders Hoel), and with
Professor Janos Mizsei from the Budapest University of Techology. Dr. Johannes
Frantti from the Tokyo Institute of Technology has also participated in the research.




1
Electron Physics Laboratory, Helsinki University of Technology, P.O.Box 3500, FIN-02015, HUT-
Finland
2
Microelectronics and Materials Physics Laboratories, University of Oulu, P.O.Box 4500, FIN-90014,
Universtity of Oulu, Finland
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1.3 Funding

Table 1. Funding of the project in 1000 EUR in 2003-2006

Partner Funding
Organisation
2003 2004 2005
(planned)
2006
(planned)
Total
HUT Academy 14.860 29.720 28.840 16.580
90
OU Academy 22.238 31.870 21.000 14.892
90


2 Research Work

2.1 Objectives and Work Plan

The aim of the present research consortium is to develop and utilize new materials and
structures to fabricate semiconductor gas sensors showing enhanced stability and
reduced power consumption. They can be realized by using an epitaxial growth of
semiconductor thin films and layered materials and by a proper choice of doping and
catalyst elements as well as by integrating several sensing elements on one chip, and by
reducing the size and layout of a heating element with micromechanical bridge
structures allowing a decrease in power consumption. Also the gas sensor
characterization techniques based on a Kelvin probe method will be developed and

utilized in in situ measurements of the gas reactions at the semiconductor surfaces.


2.2 Progress Report: Common Themes and Collaboration

The present research project has been going on since the beginning of July 2003. The
main goal, which was to grow high quality gas sensing materials, has been achieved in
the case of tin dioxide. By adjusting the growth parameters we are able to grow either
polycrystalline nano-particle thin films or monocrystalline thin films. Extensive
characterization of the grown thin films has been carried out including, e.g., reflection
high energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force
microscope (AFM), Hall-effect, and conductance measurements in close collaboration
between the consortium partners, which has produced two joint publications [1 and 7].
Oxygen adsorption-desorption kinetics at the tin oxide surface has been studied
experimentally using temperature-programmed desorption method together with the
conductance measurements. Also simultaneous Kelvin probe and conductance
measurements during heating and cooling in different ambient atmospheres were used
to characterize the monocrystalline SnO
2
(101) surface after various surface treatments.
Due to a cut in the planned budget the fabrication of the heated substrates was
transferred to a TEKES project. However, these substrates are available also for the
present project.

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Our research results so far are very promising. X-ray diffraction results confirmed that
all the sample films grown are highly oriented along the surface as only one peak in
addition to the substrate could be seen in the scan. A comparison between samples of

different crystalline quality is resented in Fig. 1.




(a) (b)

100nm

200nm

(c) (d)

0,95
1,15
1,35
1,55
1,75
1,95
2,15
2,35
2,55
2,75
200 300 400 500 600 700 800 900
Time / s
G
gas
/G
air
0,99

1,01
1,03
1,05
1,07
1,09
1,11
1,13
1000 1200 1400 1600 1800 2000 2200 2400
Time / s
G
gas
/G
air

(e) (f)


Fig. 1. A comparison between two samples grown at different temperatures; a)
RHEED image of a clearly polycrystalline sample, b) RHEED image of a
monocrystalline sample, c) and d) AFM images of the corresponding samples, e) and
f) conductivity responses towards an exposure of 1000 ppm hydrogen at 400 °C.


Three-dimensional diffraction spots and rings on the RHEED image obtained from
sample in Fig. 1. a) indicate a rough surface and a polycrystalline nature of the film.
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Sample grown at higher temperature (Fig. 1. b)) gives an image with elongated spots,
which indicates that the electron beam hits a monocrystalline region. The AFM

inspection confirms that the crystalline quality in the case of higher growth temperature
is clearly better, see Fig. 1 c) and Fig. 1 d).

As is evident from Fig. 1. e) and Fig. 1. f), the film of better crystalline quality shows
lower response towards the test gas, which may be expected, due to the reduced
number of grain boundaries. The rise time for the film of smaller grains is shorter, but
the decrease in the response during the gas exposure is evident in the case of smaller
grains. Results allow and encourage us to further optimize the growth parameters and
thus the structural characteristics of the sensing films.

So far the sample films have been grown on an r-cut sapphire substrate, with very
encouraging results. To further optimise the growth, we have started growth
experiments with sapphire substrates with a particular off-cut from the r-plane, which
are to provide us even better film qualities due to the optimisation of three-dimensional
growth.



2.3 Progress Report: Progress by the Electron Physics Laboratory, HUT

We have succeeded in the growth of the single crystal tin dioxide thin films on sapphire
substrates. This was one of the main goals in the project plan. The samples have been
grown by using molecular beam epitaxy, MBE. For comparison also polycrystalline tin
dioxide thin films have been grown. Altogether the number of succesful sample growths
is 16 at the present. For these samples the following measurements have been carried
out: Resistivity vs. temperature, Hall-effect (carrier concentration) vs. temperature,
AFM-measurements to determine the grain size in the thin films, and X-ray diffraction
measurements to check the crystallinity of the samples. At Oulu University we have
carried out gas response measurements for hydrogen, sulphur hydrogen and carbon
monoxide. Based on the results of these measurements we can state that we have

succeeded in the growth of the single crystal tin dioxide, and the samples’ resistivity
reacts strongly to the gas exposure. The measured sensitivity is higher in polycrystalline
samples, but the stability seems to be better in single crystal thin films. This remains to
be verified during the project. Recently we have fabricated sensor material including
catalyst such as Pt.
A new gas measurement system has been built at HUT, which includes gas sources for
over ten different gases and a new scanning Kelvin probe to measure the gas responses
of the sensor materials. This system will be utilized during the latter part of the project.



2.4 Progress Report: Progress by the Microelectronics Laboratory, OU

Traditionally, semiconductor gas sensors, and in particular gas sensors using SnO
2
as a
sensing material, are based on polycrystalline thin or thick films. Sensing mechanisms
of these types of sensors have been widely studied and are quite well understood. The
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novel approach is to use monocrystalline SnO
2
thin film as the sensing layer. The
intergrain potential barriers that are mostly responsible for the sensing mechanism in
polycrystalline films are missing in monocrystalline films. With monocrystalline
structure, however, it may be possible to overcome some of the problems that are
typical for polycrystalline sensors such as long term stability, response and recovery
rates and variation in electrical properties from sensor to sensor, especially in the case
of thick film sensors.

We have studied different surface phenomena on SnO
2
surfaces using both
monocrystalline and polycrystalline samples using various experimental techniques,
such as TPD, surface potential (Kelvin probe) and conductance measurements in
different ambient conditions in order to understand better the sensing mechanism in
monocrystalline films. So far, the results show that the monocrystalline film behaves
very differently compared to the polycrystalline samples. Stability with both time and
temperature is better and overall characteristics of the different sensors are more alike.
Even though the premilinary testing shows that the sensitivity is lower, as expected, in
the case of monocrystalline films as compared to the polycrystalline films, the
increased overall stability of the new structure makes it, as a whole, a promising
structure for gas sensors. In the last part of the project we will do more
characterizations for the films and try to improve the sensitivity and selectivity using
different catalyst materials and film structures. Monocrystalline film makes it also
easier to study the possible influence of electrodes on the sensitivity and selectivity.
In addition, we have been able to improve the excellent sensitivity of WO
3
-based
sensors towards H
2
S at room-temperature operation by modifying the microstructure of
the sensing layer using different growth parameters. Further, by adding small amounts
of noble metal nanocatalysts into the sensing layer we were able to improve the
sensitivity down to the sub ppm level at H
2
S exposure [3]. Research with porous silicon
has also continued in the Microelectronics Laboratory at OU and some test
measurements on the gas response of these samples have been also done.

3 International Aspects

The new scanning Kelvin probe applicable in a gas atmosphere was planned together
with a Hungarian group headed by Dr. Tibor Pavelka at Semilab Ltd. During the
project Professor Janos Mizsei from the Budapest University of Technology has visited
both the Microelectronics Laboratory at OU and the Electron Physics Laboratory at
HUT. As a result a joint publication has been written ([1]; see the list below). There has
been also a close research co-operation in the research with tetragonal nanocrystalline
WO
3
samples between researchers in the Microelectronics Laboratory at OU and in the
Ångström Laboratory at the Uppsala University (research group of Professor
Granqvist). Dr. Johannes Frantti from the Tokyo Institute of Technology has
participated also in the experimental characterization of the structure of the WO
3

sensing films using Raman spectroscopy.

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4 Publications and Academic Degrees

Table 2. Publications and academic degrees produced in the project.
Partner Type of publication 2003 2004 2005
Total
Publication numbers
HUT Ref. journal art. - - 1
1
1
Ref. conf. papers - - 1
1
7
Monographs - - -
-
-
Doctoral dissert. - - -
-
-
Licentiate degrees - - 1
1
9
Master degrees - - - -
OU Ref. journal art. - 4 2
6
1,2,3,4,5,6
Ref. conf. papers - 1 1
2
7,8

Monographs - - - -
Doctoral dissert. - 1 -
1
10
Licentiate degrees - - - -
Master degrees - - - -


5 Other Activities

The results of the project have been presented to Dr. Mikko Utriainen at Environics
Ltd. in several meetings.

6 Publications

6.1 Refereed Journal Articles

[1] S. Saukko, U. Lassi, V. Lantto, M. Kroneld, S. Novikov, P. Kuivalainen, T.T. Rantala, and J.
Mizsei, Experimental studies of O
2
-SnO
2
surface interaction using powder, thick film and
monocrystalline thin films, Thin Solid Films 2005. In press.

[2] L.F. Reyes, S. Saukko, H. Hoel, V. Lantto, and C.G. Granqvist. Structure engineering of WO
3

nanoparticles for porous film applications by advanced reactive gas deposition. Journal of
European Ceramic Society, 24, Issue 6:1415–1419, 2004.

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[3] A. Hoel, L. F. Reyes, S. Saukko, P. Heszler, V. Lantto, and C. G. Granqvist. Gas sensing with
films of nanocrystalline WO
3
and Pd made by advanced reactive gas deposition. Sensors and
Actuators B: Chemical, 105(2):283–289, 2005.

[4] V. Lantto, S. Saukko, N.N. Toan, L.F. Reyes, and C.G. Granqvist. Gas sensing with
perovskitelike oxides having ABO
3
and BO
3
structures. Journal of Electroceramics, 13:721–
726, 2004.

[5] A. Hoel, L. F. Reyes, P. Heszler, V. Lantto, and C. G. Granqvist. Nanomaterials for
environmental applications: novel WO
3
-based gas sensors made by advanced gas deposition.
Current Applied Physics, 4(5):547–553, 2004.

[6] L. Reyes, S. Saukko, A. Hoel, V. Lantto, and C.G. Granqvist. Improved gas response at room
temperature of activated nanocrystalline WO
3
films. Physica Scripta, T114:240–243, 2004.

6.2 Refereed Conference Papers

[7] M. Kroneld, S. Novikov, S. Saukko, P. Kuivalainen and V. Lantto: “Gas sensing properties of
SnO
2
thin films grown by MBE”. Abstract submitted to Eurosensors'05 (a full length manuscript
also ready).

[8] L.F. Reyes, S. Saukko, A. Hoel, V. Lantto and C.G. Granqvist, H
2
S sensing with nano-
structured WO
3
films in different oxygen atmospheres, Proc. (CD-ROM) 17
th
European
Conference on Solid-State Transducers (Eurosensors XVII), Sept. 21-24, 2003, Guimaraes,
Portugal, pp 859-860.


6.3 Monographs


6.4 Doctoral, Licentiate, and Master Theses

[9] M.Kroneld, Gas sensing properties of mono- and polycrystalline tin dioxide thin films grown by
MBE, HUT 2005, 90 pages. (Licentiate thesis)

[10] A. Hoel, Electrical Properties of Nanocrystalline WO
3

for Gas Sensing Applications, Acta
Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the
Faculty of Science and Technology 948. Uppsala (2004) 86 pp. (Doctoral thesis)


7 Other Outputs

 An invited lecture with the title “On some structural effects of transition metal oxides with
relevance to gas sensing: WO
3
as an example oxide” was given by V. Lantto 12.2.2004 in the
Yamazoe Laboratory at the Kyushu University, Fukuoka, Japan.
 An oral lecture with the title “Gas sensing with nanocrystalline tetragonal WO
3
films” was
given by V. Lantto in the 9
th
International Conference on Electroceramics & their Applications,
31.5 3.6.2004, Cherbourg, France.
 An invited plenary lecture with the title “Some structural and electronic aspects on gas sensing
with transition metal oxides” was given by V. Lantto in the Opening Session of the
Semiconductor Gas Sensors (SGS´2004) Seminar, 19 22.9.2004, Ustron, Poland.