Sensors and Actuators B 139 (2009) 322–328
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Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Transport and gas sensing properties of In
2
O
3
nanocrystalline thick films:
A Hall effect based approach
A. Oprea
a,∗
, A. Gurlo
b
, N. Bârsan
a
, U. Weimar
a
a
Institute of Physical and Theoretical Chemistry, University of Tuebingen, Auf der Morgenstelle 8, 72076 Tuebingen, Germany
b
Fachbereich Material- und Geowissenschaften, Technische Universitaet Darmstadt, Petersenstr. 23, 64287 Darmstadt, Germany
article info
Article history:
Received 14 August 2008
Received in revised form 15 December 2008
Accepted 5 March 2009
Available online 17 March 2009
Keywords:
In2O3
Mobility
Gas sensitivity
abstract
Undoped nanosized In
2
O
3
with n-type conduction was produced in both polymorphic forms (cubic and
rhombohedral) and deposited by screen-printing as thick films. These films show high sensitivity to low
O
3
concentration levels. They have been investigated by four point conductance and Hall effect mea-
surements under sensor operating conditions (elevated temperature and ozone exposure). The effective
values of the charge carrier concentration and mobility have been calculated from the experimental
records using the recipe for the single crystals. The response to O
3
is discussed in the frame of the stan-
dard models for gas sensors. The observed deviations from the model are explained in connection with
the film crystalline structure and microscopic parameters spread.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
In
2
O
3
is investigated since decades. At the beginning the sci-
entific interest was, more or less, the reason and the stimulus
of investigations. However, remarkable optical transmission and
the metallic like conduction (when suitably impurified or having
stoichiometric deficiencies) of indium oxide led soon to practi-
cal application. Either alone or, most frequently, in combination
with other oxides of transition metals, it became the most utilised
transparent conducting oxide (TCO) in optoelectronics and related
fields. For such purposes the material is produced as thin and com-
pact layers with low structural defect concentration. The literature
abounds in studies performed on films of this type, deposited by dif-
ferent means on a large variety of substrates. Some comprehensive
overviews are also available [1]. After observing the high sensitivity
of In
2
O
3
towards ozone [2–4] (see also survey in [5]) or other oxidis-
ing gases [6–13] and in direct connection with the rising concern
about the ozone negative effects on ambient quality and human
health the interest for gas sensors based on this compound begun
to increase. Since then several pertinent investigations [5,14] on
the O
3
-sensing properties of In
2
O
3
have been performed resulting
in laboratory versions of chemoresistive gas sensors with detection
limits in few parts per billion (ppb) range.
Paper presented at the International Meeting of Chemical Sensors 2008 (IMCS-
12), July 13–16, 2008, Columbus, OH, USA.
∗
Corresponding author. Tel.: +49 70712977633; fax: +49 7071295960.
E-mail address: alexandru.oprea@ipc.uni-tuebingen.de (A. Oprea).
The material utilised in gas sensing should have a very large spe-
cific area and therefore it is typically prepared as porous thin/thick
layer. The most employed manufacturing paths are making use of
powder technologies, at least in their last step. Morphologies with
grain/crystallite dimensions spread over many order of magnitude
(few nanometer to micrometer) [6,10,15,16] have been reported.
A good understanding of electrical conduction in films with such
structures is indispensable for the optimal design of the sensing
devices. In the same time, there is a scientific interest on this
topic as well, due to the strong connection between the combined
electrical transport mechanisms, taking place in and across the
material grains and the surface interactions with the gaseous ambi-
ent, mainly at elevated temperatures (250–450
◦
C) where the films
have to operate as gas sensing elements.
In spite of this principle scientific interest we did not find any
articles dealing with the electrical transport in granular In
2
O
3
layers
for gas sensing and, to the best of our knowledge, there are no refer-
ences addressing the concentration and the mobility of the charge
carriers in such layers. The electron concentration and mobility in
polycrystalline / granular In
2
O
3
layers / tablets have been reported
in a few older publications (see the data and the overview in Ref.
[17]). However, due to the quite different synthesis conditions and
to the high – temperature / pressure sintering of the samples (at
1300
◦
C; 0.3GP) the direct comparison with the sol-gel prepared
materials – which are usually used for gas sensing studies – is rather
difficult.
The present paper deal with the electrical conduction of O
3
sens-
ing films deposited by screen printing from undoped nanosized
In
2
O
3
powders. Both In
2
O
3
polymorphs, i.e. bixbyite-type c-In
2
O
3
(cubic, C-type structure of rare-earth oxides, space group Ia
¯
3, No.
0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2009.03.002