Analytica Chimica Acta 437 (2001) 183–190
Re-activation of an all solid state oxygen sensor
W. Moritz a,∗ , S. Krause b , U. Roth a , D. Klimm c , A. Lippitz d
a
Walther Nernst Institute of Physical and Theoretical Chemistry, Humboldt-University Berlin, Bunsenstrasse 1, 10117 Berlin, Germany
b Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK
c Institute of Crystal Growth, Berlin, Germany
d Federal Institute for Materials Research and Testing, Berlin, Germany
Received 28 November 2000; received in revised form 8 March 2001; accepted 14 March 2001
Abstract
The silicon based semiconductor structure Si/SiO2 /Si3 N4 /LaF3 /Pt can be used as a potentiometric oxygen sensor working
at room temperature. A thermal re-activation can be applied to overcome the earlier disadvantage of an increase in response
time with continuous use. Using the Pt gate electrode as a resistive heater, very short electrical high-power pulses can be
applied. A heating time as short as 300 ns was sufficient for the re-activation of the sensor. This way, only the sensitive
thin layer system LaF3 /Pt was heated, and the whole sensor was at room temperature immediately after heating. Impedance
spectroscopy, X-ray photoelectron spectroscopy (XPS) and quadruple mass spectrometric (QMS)–thermogravimetry (TG)
were used to investigate the mechanism of deterioration in dynamic sensor behaviour and re-activation. The formation of
hydrated carbonate and the desorption of CO2 and H2 O have been shown to be the causes. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Chemical sensor; Thermal treatment; Activation; LaF3 ; Pt
1. Introduction
There are several oxygen sensors using different
principles on the market. However, they either need
high temperature operation or liquid components
[1–3]. An all-solid state sensor operated at room temperature was suggested by Miura et al. [4] using the
reaction at the LaF3 /Pt interface. A semiconductor
sensor using the field effect in silicon was developed
based on the same sensitive interface in a structure
Si/LaF3 /Pt or Si/SiO2 /Si3 N4 /LaF3 /Pt [5]. A sensitivity
according to the Nernst equation of 58 mV per decade
of oxygen partial pressure p(O2 ) was determined. A
∗
Corresponding author. Tel.: +49-302-093-5566;
fax: +49-302-093-5559.
E-mail address: [email protected] (W. Moritz).
detailed investigation of the sensor behaviour is given
in [6]. X-ray photoelectron spectroscopy (XPS) measurements, thermodynamic data and kinetic investigations lead to a suggestion for the sensor mechanism
[7]. In a first step, oxygen adsorbs at surface sites A∗
at the three phase boundary LaF3 /Pt/gas (1). This is
followed by the rate-determining step, a one-electron
reduction of oxygen in the presence of water (2). The
well-known mobility of OH− on fluoride sites in the
LaF3 lattice (3) stabilises the OH− produced in reaction (2) shifting the equilibrium of reaction (2) to the
right.
O2 + A∗ O2 (A∗ )
(1)
O2 (A∗ ) + H2 O + e− HO2 • (A∗ ) + OH−
(2)
OH− + (F∗ ) OH− (F∗ )
(3)
0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 3 - 2 6 7 0 ( 0 1 ) 0 0 9 9 3 - X
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W. Moritz et al. / Analytica Chimica Acta 437 (2001) 183–190
sensor surface to a constant temperature on this time
scale [8]. In this paper, nanosecond range heating is
used for sensor re-activation. The activation energy of
the process was determined and several methods such
as impedance spectroscopy, thermogravimetry (TG)
and XPS were used to investigate the mechanism of
sensor ageing and re-activation in more detail.
Fig. 1. Schematic of the sensor structure.
Although, the oxygen sensor showed several promising properties such as Nernst sensitivity and a fast response, a considerable decrease in response rate was
observed within a few days. A solution to this problem was achieved by a simple thermal treatment. Typically the sensor structures were heated on a hot plate
for 10 s at 300◦ C and cooled own for 5 min [7]. This
re-activation procedure can be repeated without limitations, and therefore, a lifetime of the sensors of at
least 8 months was shown. The influence of the ambient gas was investigated in [7] showing that the ageing
of the sensor was faster with increasing humidity and
oxygen concentration while in vacuum no ageing was
observed [6]. An advantageous solution to the thermal
treatment was realised by using the Pt gate electrode
of the structure shown in Fig. 1 as a heating resistance. The re-activation was shown to be a fast process, and therefore, electrical heating pulses as short
as 100 ␮s enabled the sensor re-activation. It is an advantage of short heating pulses applied to the Pt layer
as a part of the oxygen sensitive LaF3 /Pt thin layer
system, that mainly this region of the sensor is at high
temperature while the much thicker Si substrate remains nearly at room temperature. As a consequence
the cooling of the sensor is very fast and measurements can be continued immediately. XPS and AFM
investigations have been carried out to explain the
mechanism of the ageing and re-activation, but only
the Pt/La surface atomic ratio showed a correlation to
the alternating ageing and activation processes. This
was interpreted by reversible changes in the geometry
of the three-phase boundary LaF3 /Pt/gas due to surface diffusion of Pt caused by the strong adsorption
of species from the gas phase [7]. Recently, we developed a method for the measurement of the surface
temperature in the nanosecond range and heating the
2. Experimental
The sensor structures were prepared by thermal evaporation of 240 nm LaF3 on a n-Si/SiO2
(60 nm)/Si3 N4 (80 nm) substrate (5 mm × 10 mm)
(donor concentration 2.5 × 1014 cm−3 ; IMS Dresden,
Germany). The dc cathode sputtering in an argon
plasma through a metal mask was used to deposit
60 nm thick Pt films in the geometry shown in Fig. 1.
The small path in the centre was heated and had dimensions of 0.5 mm × 1.5 mm. The large areas on
both sides were used as contact pads. The high temperature contact adhesive EPO-TEK P-1011 (Polytec)
was used to provide a stable contact between the Pt
in the enlarged area and a copper strip. A contact
resistance of less than 0.1 was achieved. High current POGO contacts of Everett Charles technologies
were used for the current input via the copper strips.
A four-point resistance measurement was achieved
by the measurement of the voltage drop in the path
region at two needle shaped POGO contacts with a
distance of 1 mm. Therefore, the resistance was determined directly using the heating current and the
effective voltage.
High power pulses were generated using a combination of the computer controlled high power supply BERTAN 606C-15P, the pulse generator PHILIPS
PM 5712 and the fast high voltage transistor switch
HTS 31 of BEHLKE. The voltage and current were
recorded using the digital oscilloscope Le Croy 9450
A. This Equipment reaches pulse times down to 10 ns
and a heating power up to 100 kW (␮s range).
Photocurrent/voltage measurements were used for
determining the oxygen sensitivity. Only the area
of the small path of Pt film shown in Fig. 1 was
illuminated ensuring sensitivity measurements only
resulting from the area heated before. An IR-laser
diode with λ = 850 nm was modulated at 10 kHz. The
photocurrent was detected using a lock-in amplifier
W. Moritz et al. / Analytica Chimica Acta 437 (2001) 183–190
SR 830 DSP from Stanford Research Systems. The
oxygen sensitivity was determined in oxygen/nitrogen
mixtures in the concentration range from 0.3 to 100%
oxygen with a relative humidity ranging from 5 to
80%.
A Zahner IM5d system was used for impedance
spectroscopy.
The XPS-spectra were obtained by employing a VG
ESCALAB 200X electron spectrometer using Mg K␣
X-ray radiation (15 kV, 300 W). The electron analyser
was used in the constant analyser energy mode (CAE
10). The pressure in the analysis chamber was in the
10−7 Pa range. For the analyser calibration, the method
proposed by Anthony and Seah [9] was applied. Binding energy data were referenced to the aliphatic C 1s
peak at 285.0 eV [10].
Simultaneous differential thermoanalytic (DTA),
TG and quadruple mass spectrometric (QMS) measurements were performed using a NETZSCH STA
409C thermoanalytic device with sample carrier
HIGH RT 2 (Pt-Pt90/Rh10 thermocouples). The samples (LaF3 powder or LaF3 /Pt layers on Al foil) were
placed in corundum crucibles without lid. The heating with rates of typically 5 K/min in the temperature
range from 20◦ C ≤ T ≤ 350◦ C in argon was performed using a graphite furnace. A vitreous carbon
skimmer coupling with intermediate vacuum managed the gas flow from the sample room to the QMS
to maintain a constant pressure of 10−5 mbar within
the QMS.
3. Results and discussion
3.1. Sensor re-activation
For the re-activation of the oxygen sensor, thermal
treatments using an external heater and in situ heating by electrical high power pulses applied to the gate
electrode of the sensor were performed. The electrical
heating and the surface temperature measurement at
the sensor structure shown in Fig. 1 were described in
the experimental part and in more detail in [8]. It was
proven that the resistance change of the Pt film could
be used for a fast measurement of the surface temperature down to the nanosecond range. For fundamental
investigations, e.g. the determination of activation energy of the re-activation process, electrical heating at
a constant voltage is disadvantageous in that there is
185
Fig. 2. Surface temperature measurement at the sensor structure
while electrical heating is applied, maximum voltage 293 V, maximum current 8.4 A.
no constant temperature but a continuous increase in
temperature. Therefore, it was necessary to develop a
specific heating power profile, which lead to a constant temperature at a defined point in time during the
heating period. An example for sensor heating and
temperature measurement is given in Fig. 2. The temperature increased for 700 ns in this experiment and
then remained constant in the narrow range between
703 and 708 K for a period of 310 ns. Even for this
very short time a re-activation of the sensor was successfully proven. Energy of only 1.8×10−3 J was sufficient for the thermal reactivation, because only the
thin sensitive layers, LaF3 and Pt, were heated while
the thick Si-substrate remained at room temperature
[8]. This low energy consumption is especially advantageous for battery powered sensor systems.
The dynamic response of a sensor after reactivation
for 320 ns is compared to the behaviour before activation in Fig. 3. The change of the equilibrium potential
with the oxygen concentration is given in Fig. 4 for a
reactivated sensor. The sensitivity was determined to
be 58.1 ± 0.9 mV per decade of oxygen partial pressure p(O2 ). The sensor response time was proven to
be independent of the oxygen concentration.
For the control and the understanding of the
mechanism of the re-activation process in the thin
layer system, knowledge of the activation energy is
important. However, the activation energy cannot be
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Fig. 3. Response of the oxygen sensor to changes in the oxygen
concentration (as indicated in the graph): (a) after electrical heating
(320 ns constant temperature range of 520 K); (b) 25 days after
last reactivation.
determined at the final state of activation but should
be determined during the process of re-activation.
Therefore, the process was divided into several steps,
each leading only to a partial improvement in the
sensor response dynamics. An example for such a set
of measurements is given in Fig. 5 (only three curves
are given for simplification). Sensors aged for a long
period were used for this experiment. Before activation, there was nearly no response to a change in gas
concentration. After heating the sensor at 120◦ C for
1 min, a response to the change in gas concentration
was observed, but it was rather slow. A prolongation
Fig. 4. Sensitivity of the oxygen sensor after reactivation (30%
relative humidity, oxygen/nitrogen mixtures).
Fig. 5. Response of the oxygen sensor to changes in the oxygen
concentration (as indicated in the graph) after thermal treatment
at 120◦ C for 1, 3 and 9 min.
of heating time lead to the improvements shown. From
the response curves a response time t50 (the time necessary for 50% of potential change) was determined.
In a next step these t50 values were plotted against
activation time. From this plot we interpolated to an
activation time that would lead to t50 = 20 s, which
was defined as the reference sensor state. For activation on an external heater, the temperature was kept
constant and the heating time varied. However, for
thermal treatment with electrical heating pulses, the
total time of the single pulses was kept constant and
the heating voltage (and hence, the temperature) was
increased from pulse to pulse in order to simplify the
experiment. In this case, the response time–activation
temperature relationships were interpolated to the
temperature required to achieve t50 = 20 s in order to
obtain the activation times at different temperatures.
The resulting activation time and temperature values
are given in Fig. 6. Since the initial and the final states
of the sensor were identical for all experiments, the
reciprocal of the activation time corresponded to a
rate constant and the slope in Fig. 6 leads to an activation energy according to the Arrhenius equation. The
activation time varied by nearly 10 orders of magnitude in our experiments, while the temperature was
in the relatively narrow range of 120–250◦ C only. As
a consequence a very high value of the activation energy E A = 262 ± 28 kJ/mol was determined. Due to
the high value of the activation energy, re-activation
can be performed at times even shorter than 300 ns
W. Moritz et al. / Analytica Chimica Acta 437 (2001) 183–190
Fig. 6. Dependence of the logarithm of the reciprocal re-activation
time (method of determination see text) on the reciprocal of the
re-activation temperature.
at only marginally higher temperatures. Since no further improvement of the reactivation procedure was
expected, this time range was not investigated.
3.2. Impedance spectroscopy
Impedance spectroscopy is a valuable tool for
characterisation of electrochemical processes. A detailed investigation of the impedance behaviour of
the LaF3 /Pt interface used as an oxygen sensor was
given in [6]. A single crystal of LaF3 was used in
order to eliminate the dominating capacitance of the
insulator in the semiconductor sensor. It was shown
that the results obtained with a LaF3 single crystal
were transferable to those obtained with a polycrystalline layer of LaF3 . The interface LaF3 /Pt could be
described by a typical combination of a capacitance
and a resistance in parallel. The capacitance was determined to be 86 ± 21 ␮F/cm2 for a polished crystal,
which was a typical value for a double layer capacitance. Therefore, the resistance in parallel could be
understood as the charge transfer resistance of the
potential forming process. The response rate of the
oxygen sensor was not influenced by the oxygen
concentration [7]. In agreement with this, the charge
transfer resistance was also found to be independent
of the oxygen concentration.
The results of the investigation into the long-term
behaviour and the re-activation are presented in Fig. 7.
187
Fig. 7. Response time t90 (upper part) and exchange resistance
R (lower part) determined at a single crystal LaF3 /Pt interface at
different times after preparation (䊏) and after re-activation (䊐).
The response time t90 increased with time and decreased again after re-activation. A similar behaviour
was observed for the charge transfer resistance R.
It can, therefore, be concluded that re-activation and
ageing phenomena of the sensor can be attributed to
changes in the rate of an electrochemical charge transfer process.
3.3. Differential TG
The re-activation of the oxygen sensor might be due
to desorption of gases from the surface or from the
bulk of the thin layer system. TG combined with QMS
proved to be a suitable tool to examine this hypothesis. Investigations at Si/SiO2 /Si3 N4 /LaF3 /Pt structures
did not show any reasonable result. This might be due
to the unfavourable mass ratio Si (500 ␮m) to LaF3
(240 nm) and Pt (60 nm). Therefore, in a next set of experiments, the LaF3 and Pt layers were deposited onto
an Al foil of 0.8 ␮m thickness. An area of 84.3 cm2
of this multi-layer system was investigated. Small but
unambiguous signals were observed for mass numbers 1, 18 and 44 corresponding to H, H2 O and CO2 ,
respectively. In Fig. 8 the temperature dependence for
M = 44 is given. A maximum was found at 128◦ C.
For M = 1 and 18, the maximum is in the same
temperature range. For a heating rate of 5 K/min as
used in this experiment, the thin layer system under investigation was in the temperature range 120–
140◦ C for several minutes. This concurred with the
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W. Moritz et al. / Analytica Chimica Acta 437 (2001) 183–190
Fig. 8. QMS signals for M = 44 (CO2 , dashed lines) and M = 18 (H2 O, solid lines): (1) LaF3 (240 nm)/Pt (60 nm) thin layer system
prepared on Al foil (0.8 ␮m) (outer scales and curves with dots); (2) LaF3 powder (inner scales and curves without dots).
observation that sensor re-activation took several
minutes in this temperature range. Therefore, the
gas desorption observed could be correlated to the
re-activation process.
Gas adsorption (or a chemical reaction) can occur
at either the LaF3 or the Pt. Therefore, LaF3 powder
was also investigated. The result is shown in Fig. 8
for M = 44 and 18. The width of the signal was
greater here, but it appeared at the same temperature
as for the thin layer system. The QMS peaks shown
in Fig. 8 were accompanied by a drop of the TG signal of about 0.1%. Hence, for the thin layer system,
the source of desorption is the LaF3 . A difference between the powder and the thin layer system was, that
for the powder the ratio of desorbed H2 O to desorbed
CO2 was about 6:1 while it was 1:1 for the thin layer
system. This variation seemed to be caused by the
presence of a Pt layer, which was shown to change
the surface chemistry, e.g. by production of OH−
(reaction (2)).
Using the thermogravimetric data, a mass loss of 6×
10−8 g/cm2 at 128◦ C was calculated for the thin layer
system. This represents the sum of the H2 O and the
CO2 desorption signal. With respect to the LaF3 layer
this mass loss represents 0.05 mass% of the 240 nm
thick layer, i.e. it probably originates from a very thin
surface film.
3.4. Photoelectron spectroscopy
XPS of the LaF3 /Pt thin layer system was used
for investigating the re-activation process. Apart
from the main elements (La, F and Pt), oxygen and
carbon were found. In the La 4d spectrum (Fig. 9a),
the peak visible at 104.0 eV could be interpreted
as a mixed-phase of LaF3 and lanthanum bound to
oxygen. This oxygen-containing phase was located
near the surface, because with increasing take-off-angle
Fig. 9. Photoelectron spectra of the sensor surface; La 4d signal: (a)
take-off-angel θ = 15◦ ; (b) take-off-angel θ = 60◦ ; (c) difference
between (a) and (b).
W. Moritz et al. / Analytica Chimica Acta 437 (2001) 183–190
Fig. 10. Photoelectron spectra of the sensor surface; C 1s signal: (a)
surface exposed to air for several days; (b) measurement directly
after thermal re-activation at 300◦ C.
an increase of the intensity was observed. (Fig. 9b).
This effect was expressed better in the difference
spectrum (Fig. 9c). However, changes in this signal
could not be related to the change in sensor kinetics.
Therefore, the C 1s signal was studied in more detail.
For sensors aged for several days, a signal at a binding energy of 289 eV was found which disappeared
during the course of measurement (Fig. 10a). This
signal was not observed for sensors measured directly
after re-activation (Fig. 10b). The binding energy
measured can be related to carbonate ions, which decomposed during the measurement [11]. The heating
of the sample during the measurement (estimated to
be near 70◦ C) and the additional influence of vacuum
and X-rays can explain the fact that the carbonate
signal was only observed in the beginning of the
measurement.
4. Conclusions
It was shown that the re-activation of an oxygen
sensor based on the LaF3 /Pt interface reaction can be
accomplished in very short times down to the nanosecond range. The advantage of very short electrical
heating pulses is that the sensor can be used directly
after activation because only the sensitive surface region is heated. Furthermore, energy consumption is
reduced, which is advantageous for battery powered
applications.
189
The very high value of 262 kJ/mol for the activation energy determined for the re-activation process is
the reason for the possibility of ultra-fast activation at
moderate temperatures. Furthermore, it improves the
understanding of the mechanism of the reactivation
process.
The results of impedance spectroscopy reveal that
an electrochemical charge transfer process at the
LaF3 /Pt interface determines the sensor behaviour.
This process slows down with continuous use of the
sensor and is accelerated again after thermal treatment. A change in the geometry of the three-phase
boundary as suggested in [7] can influence the rate of
an electrochemical process. This hypothesis was connected with a diffusion of Pt atoms on the surface. But
activation energies determined for surface diffusion of
Pt are near to 25 kJ/mol [12,13] and even for clusters
such as Pt7 the maximum value given is 113 kJ/mol
[14]. These much lower activation energies indicate
that the surface diffusion of Pt and the changes in
geometry of the three-phase boundary are not responsible for ageing and re-activation of the sensor.
TG and XPS data indicate that CO2 and H2 O desorption are connected with the re-activation process.
Although, the oxygen concentration was shown to influence the sensor ageing, no change in the surface
oxygen concentration related to the re-activation was
observed. Hence, the influence of oxygen may be due
to an increase in surface pH according to reaction (2),
which is promoting the production of surface carbonate. For Lanthanum carbonates, different hydrated carbonates as well as oxide and hydroxide carbonates are
known [15]. The decomposition of the carbonates was
observed at temperatures higher than 500◦ C. On the
other hand, a loss of CO2 was observed simultaneously
with the dehydration of hydrated carbonates. This process was observed at temperatures between 127 and
160◦ C [16]. For the activation energy of the decomposition of the lanthanum carbonate, values between 209
and 345 kJ/mol were found [17]. At the surface of the
hygroscopic LaF3 , the formation and decomposition
of a carbonate may differ from the behaviour of pure
phases, but nevertheless it is noteworthy that the very
high value of 262 kJ/mol for the activation energy of
the re-activation process corresponds to the values for
the decomposition of the carbonate.
Therefore, hydrated carbonate incorporation in the
surface of the LaF3 is assumed to be the reason for a
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W. Moritz et al. / Analytica Chimica Acta 437 (2001) 183–190
decrease in the rate of the potential determining reaction and the increase in response time of the sensor.
The poisoning of the surface as it is known for several
catalysts was only avoided in vacuum while even in
very pure gases a small amount of CO2 was sufficient
to cause the sensor to age. Because of the electrical
re-activation procedure developed this poisoning is not
a relevant disadvantage of the sensor anymore.
Acknowledgements
The financial support of DFG is gratefully acknowledged.
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