current status of micro and nano structured optical fiber sensors

Accepted Manuscript
Title: Transport and gas sensing properties of In
2
O
3
nanocrystalline thick films: a Hall effect based approach
Authors: A. Oprea, A. Gurlo, N. B
ˆ
arsan, U. Weimar
PII: S0925-4005(09)00204-4
DOI: doi:10.1016/j.snb.2009.03.002
Reference: SNB 11382
To appear in: Sensors and Actuators B
Received date: 14-8-2008
Revised date: 15-12-2008
Accepted date: 5-3-2009
Please cite this article as: A. Oprea, A. Gurlo, N. B
ˆ
arsan, U. Weimar, Transport and gas
sensing properties of In
2
O
3
nanocrystalline thick films: a Hall effect based approach,
Sensors and Actuators B: Chemical (2008), doi:10.1016/j.snb.2009.03.002
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Page 1 of 27
Accepted Manuscript
1
Transport and gas sensing properties of In
2
O
3
nanocrystalline thick films: a Hall effect
based approach
A.Oprea*
1
, A. Gurlo
2
, N. Bârsan
1
, U. Weimar
1
1
Institute of Physical and Theoretical Chemistry, University of Tuebingen, Auf der
Morgenstelle 8, 72076 Tuebingen, Germany
2
Fachbereich Material- und Geowissenschaften, Technische Universitaet Darmstadt,
Petersenstr. 23, 64287 Darmstadt, Germany
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 measurements 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 standard models for gas sensors.
The observed deviations from the model are explained in connection with the film crystalline
structure and microscopic parameters spread.
Keywords: In
2
O
3
, mobility; gas sensitivity
1. Introduction
In
2
O
3
is investigated since decades. At the beginning the scientific 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
* Corresponding author:
Paper presented at the International Meeting of Chemical Sensors 2008 (IMCS-12),
July 13-16, 2008, Columbus, OH, USA
revised Manuscript
Page 2 of 27
Accepted Manuscript

2
stoichiometric deficiencies) of indium oxide led soon to practical 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 compact layers with low structural defect
concentration. The literature abounds in studies performed on films of this type, deposited by
different 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 oxidising 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.
The material utilised in gas sensing should have a very large specific 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] or nanostructures [17] 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 ambient, 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,
Page 3 of 27
Accepted Manuscript
3
there are no references addressing the concentration and the mobility of the charge carriers in
such layers.
The present paper deal with the electrical conduction of O
3
sensing 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
3Ia
, No. 204) and
corundum-type rh-In
2
O
3
(rhombohedral, space group cR3 , No. 167) were studied. To the best
of our knowledge the O
3
sensing properties of the undoped corundum-type rh-In
2
O
3
and the
electrical parameters under operation conditions of O
3
sensing layers from both polymorphs
have not been evaluated until now. Only two articles report the gas sensing investigations of
corundum-type ITO [18] and rh-In
2
O
3
[19].
Therefore the investigations, based on Hall Effect [20-22] and four point conductance
measurements, have been performed in synthetic atmospheres with controlled composition
and temperature. They aim to provide the missing information concerning the charge carrier
concentration and mobility in In
2

O
3
thick porous layers for gas sensing and, by using it, to
sketch some features of the interplay between the electrical transport and sensing properties in
such films.
2. Experimental
As just stated above, the experimental basis of the investigations consists of Hall
Effect and four point conductance measurements on nano-granular screen-printed In
2
O
3
thick
films. The samples, heated at temperatures appropriate for gas sensing, have been exposed
during the electrical tests to synthetic atmospheres containing nitrogen, oxygen, ozone and
water vapour (humidity) prepared with a dynamic gas mixing station [23]. In this way it is
possible to either reproduce the condition in which a sensor made from the same material is
usually operated (that is, to make operando investigations) [24], or to create a completely
unusual atmosphere, relevant for gas sensing mechanisms and their relation with the electrical
transport [25].
Page 4 of 27
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4
2.1. Synthesis, structural characterisation of the materials and sample preparation.
Synthesis was performed by the sol-gel method based on the ammonia-induced
hydrolysis of indium nitrate in methanol with acetylacetone as a complexing agent
(acetylacetone route hereafter) and without acetylacetone (hydrolysis route hereafter) [26].
The processed powders (calcined in air at 500°C for 1 h) were used for the structural
characterization and for the screen-printing. For the present approach it is important to point
out that both In
2

O
3
polymorphs, have been obtained through the above referred sol-gel routes.
The acetylacetone route resulted in the bixbyite-type c-In
2
O
3
, while the hydrolysis route leads
to the corundum-type rh-In
2
O
3
. The rh-In
2
O
3
reveals nanorhombohedra terminated by planes
with a size ranging between 50 – 100 nm; the c-In
2
O
3
possess much smaller, highly
agglomerated, crystallites with a size below 20 nm.
The structural and morphological characterisation of c-In
2
O
3
is reported in Ref. [14],
those of rh-In
2

O
3
, in Ref. [27, 28]; the detailed characterisation of the sensing properties of
the cordundum-type rh-In
2
O
3
will be presented in detail elsewhere [29].
In the last technological step thick (~ 20 µm) sensing films have been screen printed
[14, 30] on substrates suitable for Hall Effect and electrical measurements. They are provided
with platinum electrodes on the layer side and with a platinum heating meander on the
opposite one. The description of the sample geometry and electrode functionality can be
found in [31, 32].
2.2. Measuring system
The main part of the measuring system has already been described elsewhere [31, 32].
It consists in a gas mixing station which supplies with gaseous test mixture a flat measuring
chamber placed between the polar pieces of a Brucker electromagnet. A computer driven
power electronics ensure magnetic field with the strength up to 1 T and required time
dependencies. For the present study some supplementary facilities have been added to the
above referred experimental setup. They are related to the ozone generation and humidity
Page 5 of 27
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5
level control. O
3
, the main target gas in the investigations was produced with an Anseros
ozone test system SIM 6000 with an integrated ozone generator and MP UV ozone analyzer;
its concentration in the carrier gas was determined before and after sample exposure by using
the Anseros MP UV ozone analyzer and Environics Series 300 computerized UV ozone
analyzers, respectively. Because the wide pulse modulation (WPM) regime of the O

3
generator was factory-set in the low frequency range, it was necessary to smear off the O
3
concentration variations with a 1 l buffer. The time constants of the buffer itself, of about
10 min at 100 sccm carrier gas flow, are strongly reflected by the sample response, but did not
affect the present investigations, intended for near equilibrium conditions.
For reasons not clarified yet, in the O
3
delivered by the generator a non negligible
level of humidity was present, mainly at high O
3
generation rates. The exact values could not
be determined because of the incompatibility between the existing psychometers and the O
3
containing atmosphere. In many experiments this parasitic humidity and the humidity
background accidentally present in the gas circuitry, with important consequences in the
sample response towards strong oxidising gases, was reduced below 30 ppm (parts per
million) with a cryogenic N
2
vapour trap (to avoid the oxygen condensation). However,
controlled amounts of humidity have been provided by a dedicated channel of the gas
manifold, when required.
The O
3
loss by adsorption and reaction in the pipe lines and measuring chambers,
which would randomly modify the test mixture composition, was prevented by using adequate
materials as perfluoroalkoxy (PFA) and polytetrafluoroethylene (PTFE, Teflon
®
).
2.3. Measurement procedure and acquired data

In order to gather data relevant for the influence of the surrounding atmosphere on the
conduction mechanisms in the investigated material three main types of operando
measurements have been carried out: I-V characteristics, four point conductance and dc Hall
voltage. In all of them four point sample geometry and electrical set-ups have been utilised.
Page 6 of 27
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6
During the investigations the samples have been exposed to N
2
/ O
2
mixtures with a
mixing ratio varying from 100 ppm O
2
/ N
2
to 100% O
2
and, additionally, to O
3
dosed in
concentrations from 10 ppb to 2 ppm in some O
2
/ N
2
selected combinations. Due to set-up
functional limitations the mixtures containing 50% r.h. (relative humidity) contained up to
1ppm O
3
only. The experimental data have been usually recorded very near to the

thermodynamic equilibrium of the gaseous and solid interacting phases. Waiting times of 12 –
24 h between the gas exposure steps have been typically used. In the routine tests, intended to
check the material suitability for low level O
3
sensing, significantly shorter intervals of 2 – 4 h
appeared to be sufficient. The parasitic thermoelectric and thermomagnetic effects occurring
together with the Hall Effect, especially at elevated operation temperature, have been
eliminated by making use of reversing magnetic fields and polarisation voltages. The time
dependency of the magnetic field, a trapezoidal one, avoids, on one hand, large transient Eddy
currents in the electromagnet coils, and, on the other hand, allows the direct visualisation of
the sample response linearity. Therefore the electrical and magneto-electrical measurements
extended over both stationary and transitory regions.
3. Results and discussions
The outputs of the performed measurements are three types of raw data: the I-V
characteristics, four point conductance and trapezoidally shaped Hall voltages, all of them
depending on the ambient composition and working temperature. Once the linearity of the
responses confirmed by the transitory regions of the records, only the steady state values of
the electrical parameters and the specific sensitivity curves have been considered in the
further analysis of the experimental results.
Thus, in the first step of data evaluation, the effective charge carrier concentration and
mobility have been obtained by using the recipe for single crystals [21, 22, 33-35]. They give
an intuitive picture of the material behaviour and provide the basis for subsequent analysis
and discussions.
Page 7 of 27
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7
The discussions on the results are starting from the general accepted models for gas
sensing and conduction in MOX semiconductors (as presented and commented in [32, 36,
37]).
3.1. I-V characteristics

For each material and measuring condition an I-V characteristic was determined. All
of them are very linear over more than 4 orders of magnitude, proving, by that, the pure
ohmic character of the samples, at least under all particular condition occurring during the
investigations. Fig. 1 shows typical I-V plots in both standard linear scales (left panel) and
logarithmic scales (right panel) for rh-In
2
O
3
samples heated at 200°C. One has to briefly
remark the wide range of slopes encountered in the graph with linear scales reflecting the
strong dependence of the material resistance on the O
3
concentration. In the graph with
logarithmic scales the slope is always the same, and expresses the linearity of the response;
the intercept on a vertical axis is decreasing with the resistance increase. The linear behaviour
results from the low voltage drop on each grain, less than 3 mV, which, at the working
temperature of 150°C - 270°C, is well below the thermal voltage
5035 
e
Tk
V
B
T
mV
(where:
T
,
e
,
B

k
denote respectively the absolute temperature, elementary charge and
Boltzmann constant). The evaluation has been done in the most unfavourable conditions, that
is, maximal applied voltage of 100 V, and maximal crystallite diameter of 100 nm, by using
the actual electrode spacing of the samples of 3 mm. At the above specified polarisation the
double barriers associated to each grain to grain contact never reach the Schottky diode
operation region (due to polarisation) and the ohmic response is the “normal” one. Therefore,
for each measuring conditions the sample resistance / conductance is the only one significant
parameter resulting from an I-V characteristic (actually due to its high linearity). The
experimental values of the conductance determined during the investigation will be given
later on in the §3.4.
Page 8 of 27
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8
3.2. Four point resistance
Self-standing four point resistance measurements have been performed only to prove
the sensitivity of the sensing layers and to provide a “sensor fingerprint” familiar in the field
of gas sensing. Fig. 2 is well stressing the general trend of the studied materials, namely a
high response to reduced concentrations of O
3
. The saturation trend of the curves is enhanced
to some extent by the presence of the residual humidity, not removed for these routine tests.
We have to point out that we have recently reported unusual O
3
-sensing properties for c-
In
2
O
3
, i.e. we observed that the screen-printed c-In

2
O
3
sensors showed saturation in the two-
point conductance measurements even at low O
3
concentrations [14]. This effect was
explained by possible influence of adsorbed oxygen, similar effects were also observed for -
Fe
2
O
3
[38, 39].
An exhaustive discussion concerning the different sensor parameter (gas response,
sensitivity, selectivity, reproducibility, time constants) will be provided in a dedicated paper
(including also film preparation and morphology), currently under preparation [29].
The results from four point resistance / conductance measurements acquired together
with the Hall voltages during the Hall Effect measurements are included in common graphs
(Fig. 3 and Fig. 4)
3.3. Hall voltage
The steady state values of the Hall voltage have themselves no direct meaning if taken
alone, but they can provide a rough estimate of the effective concentration of the majority
charge carrier, when using the standard single crystal recipe [21, 22, 33-35] in the first data
evaluation step. In the same frame one can also calculate the effective Hall mobility of the
majority charge carriers from the conductance records in a subsequent step. The mobility
extraction procedure is susceptible of some errors as long as one utilises the ratio of two
quantities extending over many order of magnitude (with a roughly exponential dependency
on the O
3
concentration) and therefore the rapid variations of the obtained values have to be

Page 9 of 27
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9
considered with care. Independent of the mobility value, the Hall measurements confirmed
the negative sign of the free charge carriers participating to the electrical transport in our
In
2
O
3
samples; this feature was already known from the decrease of the material resistivity
under reducing gases exposure [40].
The effective parameters give a more “friendly” description of the transport properties
presented by the investigated materials at macroscopic scale, close to the classical Drude
model [41-44]. To, however, relate them to the processes taking place at microscopic level
and in relation with the sample structure / morphology is a difficult - if still solvable - task
[31, 32, 35].
3.4. The influence of the ambient atmosphere on the effective electrical parameters
In the following some significant experimental dependencies of the effective electrical
parameters (single crystal recipe) are presented (Fig. 3 and Fig. 4) and shortly commented.
The trends in the behaviour of the samples are better visible on Fig. 5, where the normalised
(relative) values of the considered parameter are included.
Before addressing the original results of the investigation it is important to shortly
comment on the experimental dependency of the conductance (and implicitly electron
concentration) on temperature. As Fig. 4 shows the material under consideration follows some
general trends of n type MOX. In a first stage, with the increasing of the temperature over the
room value, the shallow donor bulk levels ionise more and more towards complete ionisation,
if possible. Deeper donor levels will follow at higher temperatures. In parallel with these
electronic processes other physical and chemical processes are activated by the increase of the
temperature. So, the ambient oxygen adsorbs at the semiconductor surface, at the beginning
(below 150°C), as molecular ions and then (above 150°C), when O

2
molecule dissociate, as
atomic ions by trapping conduction electrons. In this way occupied surface levels appear that
are not available in the absence of the adsorbat. The release of the electrons and the
desorption processes are necessarily occurring together (for a thorough description in relation
Page 10 of 27
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10
with sensing mechanisms [36, 37] and in relation with Hall Effect see [32]). Capturing free
charge carriers at the surface / interface of the grains results also in surface / interface
barriers / double barriers. They significantly obstruct the electron drift in the material,
between the grains, being the reason for the observed conductance decrease. The temperature
range addressed in Fig. 4, that is the one interesting for In
2
O
3
as O
3
sensing material, displays
exactly the functional region where the trapping processes are prevailing over the donor
ionisation ones, with the total effect of increasing the MOX resistance. (see also the
comments in §3.5 in connection with Fig.6)
At this point one comes back to the original results. The first important output of the
experimental data is itself the value of effective electron mobility in undoped In
2
O
3
gas
sensing films under operation conditions, not reported until now in the literature for none of
the known In

2
O
3
polymorphs (cubic and rhombohedral). Here, one has to additionally remark
that unexpected high values have been obtained. For porous and granular layers deposited
through powder techniques such large values are not encountered in the literature [32]. The
electron mobility of rh - In
2
O
3
is 2 - 5 times that of c - In
2
O
3
but one can not assign this to the
structural difference between both polymorphs. Though the samples have been prepared and
deposited in similar conditions the few differences in the layer morphology underlined in §2.1
could also influence the ratio referred above. The slowly increased porosity of the c - In
2
O
3
,
due to the formation of nano / micro agglomerates together with the reduced grain size are
strong reasons for decreasing the mobility of the charge carriers (see the discussions in the
next paragraph, §3.5).
Another significant result is the reduced spread (less than a factor 2) of the effective
mobility values in one gas exposure sequence (better seen on Fig. 5). This means that the
investigated samples are well fitting the standard modelling procedures for gas sensors, which
ascribe the target gas effects to the charge carrier concentration only, considering the mobility
as a constant material parameter. In these conditions one expects power law dependencies

Page 11 of 27
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11
[36] of the conductance on the analyte concentration. They are evident in Fig. 4 for the 150°C
and 200°C plots. The results obtained with both In
2
O
3
polymorphs operated at high
temperatures (270°C and 300°C) are deviating from this tendency showing some limiting
trends, to be explained below.
As the experimental data are suggesting, there is a strong dependence of the recorded
baseline resistance / conductance on the oxygen content (obvious if comparing the
conductance plots in Fig. 3). An enhanced oxygen concentration, like that existing in the
natural atmosphere, could drastically reduce the concentration of the free charge in the
conduction band of the metal oxide semiconductors limiting their sensitivity to oxidising
gases (the case in Fig. 3, left panel). If the sensing material enters into accentuated depletion
then the chemisorption of oxidising gases at the semiconductor surface, requiring conduction
electrons, is hindered. Indeed, the charge transfer from conduction band to the surface states
of the analyte is restricted and, in consequence, the sensitivity is either decreasing or
saturating. This behaviour is obvious in all electron concentration and conductance graphs for
high temperature (270°C, 300°C) and O
3
concentration (see Fig. 3 and Fig. 4.).
The reversed mechanism holds for the influence of the humidity. According to the
experimental evidence the water vapour induces the enhancement of the sample conductance
and charge carrier concentration, acting against the oxygen effect. This could simply be a
competition with the oxygen for the same adsorption sites resulting in a decrease in the O
2
coverage degree of the sensing layer surface or a more complicated process (spectroscopic

information, not available yet, is required to discern the nature of the adsorbed species [24,
25]). Independent of the interaction details the result is the same: fewer trapped electrons at
the surface and lower depletion level of the grains. In consequence the power law dependence
is recovered (conductance panel in Fig. 4, plot for 50% r.h.).
The dependency of the electro-kinetic parameters (normalised or not) on the O
3
concentration follows the trends for the oxygen, both O
2
and O
3
being oxidising gases. One
Page 12 of 27
Accepted Manuscript
12
easily observes that there is a strong charge carrier concentration decrease with the increase of
the ozone content in the test mixture. This is due to the acceptor character of the surface state
induced by the O
3
chemisorption, states which are able to trap significant amounts of
electrons from the conduction band [37] increasing , in this way, the heights of the
intergranular barriers. The process is similar to the oxygen adsorption referred before. As
Fig. 5 indicates, the normalised conductance and normalised electron concentration (differing
up to a factor 1.5 introduced by the normalised mobility), are evolving over 2 order of
magnitude in dry air and over one order in humid air (for the O
3
concentration range of 0 –
1000 ppb). The mobility however increases with a factor of 1.5 or less. Such a small variation
does not allow deciding the cause without doubt. Two processes could be at the origin of this
behaviour: the decrease in the rates and strengths of the electron scattering at the grain
interfaces, which are less crowded on the relatively small contact areas when the electron

concentration diminishes and / or the reduction of the bulk electron - electron scattering due to
the same decrease in the electron number participating at the conduction.
In any case, the deconvolution procedure of the conductivity in two disproportioned
terms, as effective electron concentration and effective electron mobility are, can result itself
in slightly altered results as already accounted for in § 3.3.
The humidity influence on the mobility seems to stem, through the mechanisms
addressed above, from its influence on the electron concentration (increase with mobility)
and, in turn, from the influence of the electron concentration on mobility leading to the
observed decrease.
3.5. Consequences of the microscopic peculiarities on the electrical parameter
values / dependencies and gas sensing properties
The use of nanosized materials in the sample fabrication has positive influence on its
sensitivity because of increased free surface / accessible interface areas. The nano-granular
morphology, however, brings complications in the understanding and modelling of the
Page 13 of 27
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13
sensing layer response towards the target gases. As already shown in literature [31, 32, 35,
36], the interplay of the grain size (
l
grain
), mean free path (l), and Debye length (
D
L ) controls
the amount of the free charge still available in each grain, the depletion level of the grains and
the height of the associated Schottky barriers. For the studied films these parameter have been
estimated by using the relations (2), (3) of Ref. [31] supposing no initial (room temperature)
trapped charge at the grain surface and associated band banding [32]. In the case of c - In
2
O

3
one has:
grain
l : 20 – 50 nm;

: 0.5 – 1 nm;
D
L : 10 – 20 nm while for rh - In
2
O
3
: l
grain
: 50 –
100 nm;

: 1-5 nm;
D
L : 20-40 nm.
From this data it results that there always are more than 10 collisions of the charge
carriers inside each grain (
grain
l

) and therefore a drift mobility, directly proportional to
the collision relaxation time, can be defined for both materials. In such conditions, with strong
electron scattering in the bulk, the surface influence on the mobility should be relatively
reduced, as actually observed. Not equally simple and likely in all investigated cases is the
situation of the Debye length, where values exceeding the half of the grain size are possible
for both polymorphs (small grains in combination with large Debye lengths). This means that

some grains almost reach the full depletion (flat band condition). Indeed, in the Schottky
approximation [45, 46] of full ionised donors, the width (
w
) of the depleted region is given
by [46]:




DBB
LTkVew  /2 ,
where the notation are the before explained ones and
B
V , the barrier height.
The considered approximation actually means that the shallow donors are already all
ionised at the lowest working temperature and that the further increase of the temperature will
only favour the O
2
dissociation and Oxygen adsorption at the grain surface / interface with the
consequences described above: electron trapping on surface states, enlargement of the
Page 14 of 27
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14
depletion region, intergrain barrier increase, and finally the decrease of the conductivity.
Fig. 6 successively sketches the possible grain states due to these processes.
Taking the plain example of a barrier with about 50 meV one already obtains
D
Lw 2
at a
working temperature of ~ 300°C. A depletion region width in this range results in the strong

saturation of the O
3
sensitivity analysed in § 3.4. The information gained from the analysis of
the three microscopic parameters,
grain
l
,

and
D
L
, shows that is almost impossible to model
the sensing layer response under increased analyte concentrations, but gives a fair explanation
of the observed trends.
Conclusions
The preparation and operando investigation of the two polymorphic forms of the
nanosized - In
2
O
3
brought novel and interesting results: the high sensitivity to ozone of the
rh - In
2
O
3
films, the values of the effective concentration and mobility of the charge carriers
for both materials and their dependencies on the measuring conditions, that is, the O
3
exposure level and temperature. On this basis it was possible to show that the nanosized -
In

2
O
3
layers at relative low operation temperature and under low ozone partial pressure are
almost ideal systems for the standard modelling procedures of the gas sensors. Moreover the
analysis of the interplay of the microscopic parameters allowed to additionally explain the
trends of the samples when operated in regions situated outside of the standard model validity.
Page 15 of 27
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15
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2
O
3
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Biographies
Alexandru Oprea
received the diploma in physics from the University of Bucharest in 1976
and the PhD in solid state physics from the Central Institute of Physics, Bucharest, Romania
in 1996. He is senior scientist in the Gas Sensor Group of the University of Tübingen,
Germany. The research fields: thin films solar cells, high field electroluminescent devices,
polymer and metal oxide gas sensors.
Aleksander Gurlo obtained a Ph.D. in Chemistry from the Belarusian State University
(Minsk, Belarus) in 1998. He is a researcher at the Institute of Materials Science of the
Technische Universitaet Darmstadt (Germany) working in the field of synthesis, gas sensing
properties, and spectroscopic characterisation of nanoscaled metal oxides.
Nicolae Bârsan received in 1982 his diploma in Physics from the Faculty of Physics of the
Bucharest University and in 1993 his Ph.D. in Solid State Physics from the Institute of
Atomic Physics, Bucharest, Romania. Since 1995 he is a researcher at the Institute of Physical
Chemistry of the University of Tübingen and actually is in charge with the developments in
the field of metal oxides based gas sensors.
UdoWeimar received his diploma in physics 1989, his PhD in chemistry 1993 and his
Habilitation 2002 from the University of Tübingen. He is currently the head of Gas Sensors
Group at the University of Tübingen. His research interest focuses on chemical sensors as
well as on multicomponent analysis and pattern recognition.
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21
Captions
Fig. 1 Example of I-V characteristics for rh-In
2
O
3
under different O
3

exposure levels:
0% r.h. background (upper panel) and 50% r.h. background (down panel). Measuring
conditions: 200°C, 0 / 50% r.h., 1% O
2
in N
2
as carrier gas (100 sccm). Humidity trap
has been used.
Fig. 2 Four point resistance measurement proving the O
3
sensitivity of the
investigated materials. Measuring conditions: 270°C, 0% r.h., 1% O
2
in N
2
as carrier
gas (100 sccm). Measurement performed without humidity trap.
Fig. 3 Effective electrical parameters of c – In
2
O
3
in synthetic air (upper panel and
1% O
2
in N
2
(down panel) carrier gas. Measuring conditions: 300 / 270°C, 0% r.h.,
100 sccm flow, performed without humidity trap.
Fig. 4 Effective electrical parameters of rh – In
2

O
3
in synthetic air as carrier gas.
Measuring conditions: 150 / 200 / 270°C, 0 / 50% r.h., 100 sccm flow, performed with
humidity trap.
Fig. 5 Relative values of the effective electrical parameters for rh – In
2
O
3
in synthetic
air as carrier gas. Measuring conditions: 200°C, 0 / 50% r.h., 100 sccm flow,
performed with humidity trap.
Fig. 6 Evolution of the conduction band edge with the concentration and temperature
in a nanometric In
2
O
3
grain. Further comments in text.
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Figure 1
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Figure 2
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Figure 3