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.



1.3 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

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

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

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.

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


TULE Research Programme Progress Report 2005

7
[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.
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.