Sensors and Actuators B 118 (2006) 142–148
Cr–Sn oxide thin films: Electrical and spectroscopic characterisation
with CO, NO
2
, NH
3
and ethanol
S. Morandi
a
, E. Comini
b
, G. Faglia
b
, G. Ghiotti
a,∗
a
Dipartimento di Chimica IFM and NIS Centre of Excellence, University of Torino, Via P. Giuria 7, 10125 Torino, Italy
b
SENSOR Lab CNR-INFM, University of Brescia, Via Valotti 9, 25133 Brescia, Italy
Available online 30 May 2006
Abstract
We have studied the electrical responses of Cr–Sn oxide thin films towards CO, NO
2
, NH
3
and ethanol vapours at different temperatures and
for different concentrations. Moreover, we employed FT-IR absorbance and UV–vis–NIR diffuse reflectance spectroscopies to study the electronic
transitions caused by reducing treatments with ethanol and CO. Conductance measurements, performed with CO, NO
2
, NH
3
and ethanol, show low
responses towards CO, NO
2
and NH
3
but high responses towards ethanol. FT-IR spectroscopy, in qualitative agreement with electrical measurements,
gives indications of a lower effect of CO with respect to ethanol in producing electronic absorptions related to surface reduction. Ethanol induces
the formation of strong electronic absorptions in the medium IR (MIR) and near IR (NIR) regions of the sample spectra. Furthermore, from the
study of vibration modes of the species formed in the gas phase, it was possible to propose surface reactions involved in the sensing mechanism
with ethanol.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Cr–Sn oxides; Thin films; Conductance measurements; FT-IR; UV–vis–NIR spectroscopy
1. Introduction
In the last few decades the research interest in the gas-sensing
field via semiconductor-based devices has increased with a par-
ticular interest in new materials. A big effort is being made in
the search for sensitive and selective materials. Indeed, novel
materials that are seemingly not very attractive for sensitivity or
selectivity can play a significant role since they could exhibit a
response that is different than other known materials. These can
be profitably employed as an input element for electronic noses
to simplify the deconvolution process of sensor outputs [1].
Even if, among metal-oxide-based gas sensors, those based
on tin dioxide are the most investigated materials for detection
of a wide spectrum of gases [2–6], no great attention has been
put on Cr–Sn oxides. Cr can have different oxidation states (VI,
III, II) and it can affect electrical and in turn sensing properties
of tin oxide. Furthermore it segregation for high doping level
may cause catalytic effects on the thin film surface. We have
studied the electrical response of Cr–Sn oxide thin films towards
∗
Corresponding author. Tel.: +39 011 670 7539; fax: +390 011 670 7855.
E-mail addresses: sara.morandi@unito.it (S. Morandi),
giovanna.ghiotti@unito.it (G. Ghiotti).
CO and NO
2
(of importance for environmental monitoring) and
to ethanol vapours and NH
3
(of importance for breath analysers
and food quality control). CO and ethanol interactions were also
investigated by FT-IR and UV–vis–NIR spectroscopies to study
the changes in the type and amount of donor levels induced by
the presence of different atmospheres.
2. Experimental
Film deposition was performed by magnetron sputtering in
an argon atmosphere using a pure tin target (4 in. in diameter
supplied by CERAC, purity of 99.999%) equipped with seven
holes that can be filled either with Cr or Sn insets. We prepared
films with four and seven insets filled with Cr, named Sn4Cr and
Sn7Cr, respectively. The sputtering plant was operated at 50W
power and during film deposition the substrate was maintained
at 400
◦
C, according to the RGTO technique [7]. The depo-
sition was carried out at an argon pressure of 7 × 10
-3
mbar.
In such conditions, the deposition rate was 1.4 nm/s for tin, as
checked by a profilometer and the thickness of the films obtained
was 300 nm. The films were deposited onto 3 mm × 3 mm wide
250 m thick Al
2
O
3
substrates equipped with a Pt meander
heater on the backside; Pt interdigitated contacts were sputtered
on the thin film side for electrical measurements. Annealing was
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2006.04.052