- Email: [email protected]
High temperature transport kinetics in heteroepitaxial LaFeO3 thin films
Solid-State Electronics 47 (2003) 2279–2282 www.elsevier.com/locate/sse
High temperature transport kinetics in heteroepitaxial LaFeO3 thin films I. Hole
a,*
, T. Tybell a, J.K. Grepstad a, I. Wærnhus b, T. Grande b, K. Wiik
b
a b
Department of Physical Electronics, Norwegian University of Science and Technology, 7491 Trondheim, Norway Department of Materials Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway Received 28 September 2002; received in revised form 17 November 2002; accepted 14 December 2002
Abstract Heteroepitaxial LaFeO3 (1 1 0) thin films with a thickness of 150 nm were grown on LaAlO3 (0 0 1) by reactive sputtering in an inverted cylindrical magnetron geometry. Equilibrium conductivity was measured as a function of partial pressure of oxygen at T ¼ 1000 C, and log r plotted vs. log P ðO2 Þ showed a minimum in conductivity for P (O2 ) ¼ 1011 atm and a linear response between 1010 and 1 atm. This linear response makes thin films of LaFeO3 a promising material for oxygen sensor applications. We have also measured the time response of the film conductivity upon an abrupt change in the partial pressure of ambient oxygen from 102 to 103 atm, which was determined at 60 s for T ¼ 700 C and <3.5 s at T ¼ 1000 C. 2003 Elsevier Ltd. All rights reserved. Keywords: LaFeO3 ; Thin films; High temperature conductivity; Gas sensor
1. Introduction Present research on mixed conducting 1 materials, such as alkaline earth doped LaFeO3 , is strongly motivated by their potential in gas sensors, gas separation membranes, and as electrodes in high temperature environments [1–4]. Gas sensors have a wide range of important applications, such as monitoring automobile exhaust gases, combustion in incinerator plants, and air for olfactory and explosive gases [5]. Oxygen can be gauged e.g. by a Nernst cell, measuring the difference in electromotive force between the ambient oxygen and a standard reference gas with known oxygen partial pressure. In this kind of sensor, zirconia is typically used [6]. Metal–oxide–semiconductor (MOS) gas sensors take advantage of a change in the work function difference between metal and semiconductor, e.g. upon absorption
*
Corresponding author. Fax: +47-735-91-441. E-mail address: [email protected] (I. Hole). 1 By mixed conduction in a material we understand the combination of ionic conductivity and electronic conductivity.
of hydrogen in a palladium gate MOS sensor [7]. A third class of gas sensors is based on semiconducting metal oxides, that exploit a change in conductivity as the material is exposed to the target gas. The most common commercial sensor of this type is based on polycrystalline SnO2 [8]. Thin film sensors that do not require a reference cell are simpler and thus less expensive to manufacture. Moreover, the time response can be reduced, providing for increased speed. In this regard, alkaline earth doped LaFeO3 is an interesting material system. This perovskite, an antiferromagnetic insulator at room temperature, shows significant conductivity at elevated temperatures. Extensive studies of the bulk electronic and ionic transport properties of LaFeO3 , including Seebeck measurements, show a crossover from n- to p-type conductivity as the measured conductivity passes through a minimum for a certain P (O2 ), dependent on temperature [1,2]. Conductivity measurements in air, N2 , and O2 were also reported for LaFeO3 thin films grown on MgO(1 0 0) for temperatures below 600 C, showing a near linear log r vs. 1=T behavior for T > 300 C, and with virtually no difference in conductivity between those three ambients [9]. In this paper, we
0038-1101/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00214-4
2280
I. Hole et al. / Solid-State Electronics 47 (2003) 2279–2282
explore the case for exploiting the high temperature conduction properties of a LaFeO3 thin film for oxygen sensing purposes. We report conductivity vs. oxygen partial pressure measurements at elevated temperatures, and the time response in r as the film is exposed to an abrupt change in P (O2 ).
2. Experimental Heteroepitaxial thin films of LaFeO3 were grown from a polycrystalline composite target by reactive offaxis radio frequency sputtering in an inverted cylindrical magnetron geometry. Due to its good insulating properties and high chemical stability under variable oxygen partial pressures, single-crystalline LaAlO3 (0 0 1) was chosen as the substrate material in this study. The good insulating property of this substrate was verified by the absence of a measurable conductivity over a wide range in temperature and P (O2 ). Prior to deposition, the substrates were cleaned in acetone and isopropyl alcohol, and heated to the adopted growth temperature of Tsub ¼ 775 C. The deposition chamber was filled with a 2:1 gas mixture of Ar and O2 to a total pressure of 100 /min. mTorr, and the films were grown at a rate of 8 A After deposition, the oxygen pressure in the chamber was increased to 4 Torr and the samples annealed at 775 C in order to ensure full oxygenation. This oxygenation is only important for the post growth structural characterization at room temperature, as subsequent conductivity measurements were all performed at an elevated temperature where the equilibrium oxygen concentration in the film was controlled by the ambient.
X-ray diffraction analysis revealed monocrystalline growth of (1 1 0)-oriented LaFeO3 , as shown in Fig. 1. The d1 1 0 lattice parameter of the as-grown films was . Rocking curves recorded around determined at 3.97 A the (1 1 0) peak showed an FWHM < 0.2. Atomic force microscopy measurements of the film topography unveiled flat, uniform surfaces over extended area, with a for a measured root-mean-square roughness of 6 A 2.5 · 2.5 lm2 area. The data reported in this paper were taken using 10 · 10 mm2 films with a thickness of 150 nm. The conductivity was measured with a standard dc 4-point setup. To ensure good contacts for the two current feeds, 2 mm wide stripes of Au were sputter-deposited onto each end of the film surface. Two Pt wires (u ¼ 0:5 mm) 4 mm apart, were used to measure the voltage drop. The partial pressure of ambient oxygen was controlled mechanically with mass flowmeters, providing a mixture of O2 and N2 for P (O2 ) in the range 1–105 atm; and chemically with a mixture of CO and CO2 for P (O2 ) in the range 107 –1017 atm. The minimum P (O2 ) used in this study was 1017 atm, in order to avoid decomposition of LaFeO3 to La2 O3 and Fe at lower oxygen partial pressures [1]. A ZrO2 cell was used to measure the partial pressure of oxygen, and equilibrium conductivity measurements were carried out at T ¼ 1000 C, controlled within ±1 C. Two separate gas mixtures of different P (O2 ) were prepared to measure the time response of the LaFeO3 thin film conductivity upon abrupt changes in the ambient oxygen content. Once a stable film conductivity was attained upon exposure to the higher P (O2 ) gas mix, the ambient was switched to the lower P (O2 ) mix, and the response in conductivity was
Fig. 1. H–2H X-ray diffractogram of a 150 nm LaFeO3 (1 1 0) thin film on LaAlO3 (0 0 1) substrate. The spacing of the LaFeO3 low. index planes parallel to the substrate is estimated at d1 1 0 ¼ 3:97 A
I. Hole et al. / Solid-State Electronics 47 (2003) 2279–2282
logged as a function of time. The time constant was defined by the 90–10% drop in differential conductivity between high and low P (O2 ).
The operating principle of an oxygen sensor made from a mixed conductor material such as LaFeO3d relies on variation in the concentration of oxygen vacancies with changes in the ambient oxygen pressure. When lowering the partial pressure of oxygen, the material is reduced and electrons are added to the system by the desorption reaction of oxygen, OxO þ 2FexFe ¼ 2Fe0Fe þ VO þ 1=2O2 ðgÞ using Kr€ oger–Vink notation [10]. However, the total number of free carriers is determined by the disproportion reaction of iron
1
0.5
log σ [a.u.]
3. Results and discussion
2281
0
-0.5 -1 -1.5 -17
-12
-7
-2
log P(O 2 ) [atm] Fig. 2. Equilibrium conductivities of a LaFeO3 thin film vs. oxygen partial pressure at T ¼ 1000 C. The data shows a near linear behavior for P (O2 ) ¼ 1010 –1 atm. The gap in data points between P (O2 ) ¼ 107 atm and P (O2 ) ¼ 104 atm is due to problems of controlling the oxygen partial pressure in this range.
2Fex ¼ Fe0 þ Fe 1.2
1000°C 700°C
1.0 0.8
[a.u.]
where the density of two-(Fe0 ) and four-valent iron (Fe ) corresponds to electron and hole concentrations, respectively. Depending on whether electrons or holes predominate at a given P (O2 ), lowering the oxygen content in the film ambient can either increase or decrease the conductivity, as expressed by
0.6
r / lh p þ le n
0.4
We assume that ionic conductivity can be neglected in the present experiment. In order to study the case for using LaFeO3 as an oxygen gas sensor, the equilibrium conductivities are plotted as a function of oxygen partial pressure at T ¼ 1000 C in Fig. 2. The conductivity passes through a minimum for P (O2 ) 1011 atm and displays a near linear increase in log r vs. log P ðO2 Þ in the entire range from P (O2 ) ¼ 1010 to P (O2 ) ¼ 1 atm. This linear response over 10 decades in the ambient oxygen partial pressure, with a rate of change in electronic conductivity of Dr=r 0:45/decade P (O2 ), renders LaFeO3 a most interesting material system for high temperature oxygen sensor applications. Another important aspect of a gas sensor is the time response of the system. In Fig. 3, the measured conductivity has been plotted as a function of time for a sudden change in the ambient gas mixture from P (O2 ) ¼ 103 to P (O2 ) ¼ 102 atm, at 700 and 1000 C, respectively. The time constant for the change in equilibrium conductivity was found to be 60 s for T ¼ 700 C and <3.5 s for T ¼ 1000 C. This corresponds to a chemical diffusion constant in the LaFeO3 thin film of Dch ¼ 1011:7 cm2 s1 at T ¼ 700 C and Dch > 1010:3 cm2 s1 at T ¼ 1000 C [11]. Reported time constants from similar measurements in thin film SrTiO3 are as
0.2 0.0 0
10
20
30
40
50
60
70
80
90
100
time [s]
Fig. 3. Measured conductivity vs. time in response to a sudden drop in the oxygen partial pressure from 102 to 103 atm at T ¼ 700 C (gray curve) and T ¼ 1000 C (black curve), respectively. The conductivity is normalized to 1 at the initial P (O2 ), and the change in oxygen partial pressure took place at t ¼ 0 s for both temperatures. The time constants were estimated at 60 s for T ¼ 700 C and <3.5 s at T ¼ 1000 C.
low as 5 ms at T ¼ 1000 C [5]. However, intrinsic instabilities in the electronic conductivity of SrTiO3 during thermal reduction [12], may severely limit the range in P (O2 ) for which this material can be used as an oxygen gas sensor. A 72-h measurement in pure nitrogen did not show any instability in the electronic conductivity of the LaFeO3 films. The reported time response in the present experiment is fast in comparison with LaFeO3 used as NO2 sensor at lower temperatures [13,14], where the time constants for polycrystalline thick films was found to be on the order of 10 min or more. We note that the measured time constant of 3.5 s at T ¼ 1000 C represents an upper limit imposed by gas flow impediments of
2282
I. Hole et al. / Solid-State Electronics 47 (2003) 2279–2282
our present measurement setup. By modifying the setup, a faster response to changes in the ambient oxygen content is expected. Thus, the speed of sensing a change in ambient oxygen should not be limited to the present 3.5 s, making LaFeO3 a promising material system for oxygen sensors.
[3]
[4]
4. Conclusions In this paper we have assessed the case for using LaFeO3 thin films in oxygen sensor applications. High quality epitaxial films were grown on LaAlO3 (0 0 1) substrate, and conductivity measurements were carried out in a gaseous ambient with controlled oxygen partial pressure at an elevated temperature of T ¼ 1000 C. The measurements showed a large linear response in conductivity with increasing P (O2 ) over 10 orders of magnitude. A fast time response of <3.5 s, presently limited by the geometry of our measurement setup, make this material system a promising option for high temperature oxygen sensing applications.
[5]
[6]
[7]
[8] [9]
[10]
Acknowledgement This work was funded by The Norwegian research Council under contract no. 121443/420. References [1] Mizusaki J, Sasamoto T, Cannon WR, Bowen HK. Electronic conductivity, seebeck coefficient, and defect structure of LaFeO3 . J Am Ceram Soc 1982;65(8):363–8. [2] Mizusaki J, Sasamoto T, Cannon WR, Bowen HK. Electronic conductivity, Seebeck coefficient, and defect
[11] [12]
[13]
[14]
structure of La1x Srx FeO3 (x ¼ 0:1, 0.25). J Am Ceram Soc 1983;66(4):247–52. Elshof JEten, Bouwmeester HJM, Verweij H. Oxygen transport through La1x Srx FeO3d membranes I. Permeation in air/He gradients. Solid State Ionics 1995;81:97– 109. Elshof JEten, Bouwmeester HJM, Verweij H. Oxygen transport through La1x Srx FeO3d membranes II. Permeation in air/CO, CO2 gradients. Solid State Ionics 1996;89: 81–92. Fleischer M, Meixner H. Thin-film gas sensors based on high-temperature-operated metal oxides. J Vac Sci Technol A 1999;17(4):1866–72. Moseley PT, Norris JOW, Williams DE. Techniques and mechanisms in gas sensing. Bristol: IOP Publishing Ltd.; 1991. Lundstr€ om KI, Shivaraman MS, Svensson CM. A hydrogen-sensitive Pd-gate MOS transistor. J Appl Phys 1975; 46(9):3876–81. Madou M, Morrison SR. Chemical sensing with solid state devices. New York: Academic; 1989. Yamada S, Tai H, Matsumoto Y, Koinuma M, Kamada K, Sasaki T. Preparation of LaFeO3 perovskite thin films by radio frequency magnetron sputtering and their electrical conductivities. Electrochemistry 2001;69(3):171–6. Kr€ oger FA. The chemistry of imperfect crystals, vol. 2. Amsterdam: North-Holland Publishing Company; 1974. Hole I et al., in preparation. Szot K, Speier W, Carius R, Zastrow U, Beyer W. Localized metallic conductivity and self-healing during thermal reduction of SrTiO3 . Phys Rev Lett 2002;88(7): 075508. Traversa E, Matsushima S, Okada G, Sadaoka Y, Sakai Y, Watanabe K. NO2 sensitive LaFeO3 thin films prepared by r.f. sputtering. Sens Actuat B 1995;24–25:661–4. Grilli ML, Bartolomeo ED, Traversa E. Electrochemical NOx sensors based on interfacing nanosized LaFeO3 perovskite-type oxide and ionic conductors. J Electrochem Soc 2001;148(9):H98–H102.
High temperature transport kinetics in heteroepitaxial LaFeO3 thin films I. Hole
a,*
, T. Tybell a, J.K. Grepstad a, I. Wærnhus b, T. Grande b, K. Wiik
b
a b
Department of Physical Electronics, Norwegian University of Science and Technology, 7491 Trondheim, Norway Department of Materials Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway Received 28 September 2002; received in revised form 17 November 2002; accepted 14 December 2002
Abstract Heteroepitaxial LaFeO3 (1 1 0) thin films with a thickness of 150 nm were grown on LaAlO3 (0 0 1) by reactive sputtering in an inverted cylindrical magnetron geometry. Equilibrium conductivity was measured as a function of partial pressure of oxygen at T ¼ 1000 C, and log r plotted vs. log P ðO2 Þ showed a minimum in conductivity for P (O2 ) ¼ 1011 atm and a linear response between 1010 and 1 atm. This linear response makes thin films of LaFeO3 a promising material for oxygen sensor applications. We have also measured the time response of the film conductivity upon an abrupt change in the partial pressure of ambient oxygen from 102 to 103 atm, which was determined at 60 s for T ¼ 700 C and <3.5 s at T ¼ 1000 C. 2003 Elsevier Ltd. All rights reserved. Keywords: LaFeO3 ; Thin films; High temperature conductivity; Gas sensor
1. Introduction Present research on mixed conducting 1 materials, such as alkaline earth doped LaFeO3 , is strongly motivated by their potential in gas sensors, gas separation membranes, and as electrodes in high temperature environments [1–4]. Gas sensors have a wide range of important applications, such as monitoring automobile exhaust gases, combustion in incinerator plants, and air for olfactory and explosive gases [5]. Oxygen can be gauged e.g. by a Nernst cell, measuring the difference in electromotive force between the ambient oxygen and a standard reference gas with known oxygen partial pressure. In this kind of sensor, zirconia is typically used [6]. Metal–oxide–semiconductor (MOS) gas sensors take advantage of a change in the work function difference between metal and semiconductor, e.g. upon absorption
*
Corresponding author. Fax: +47-735-91-441. E-mail address: [email protected] (I. Hole). 1 By mixed conduction in a material we understand the combination of ionic conductivity and electronic conductivity.
of hydrogen in a palladium gate MOS sensor [7]. A third class of gas sensors is based on semiconducting metal oxides, that exploit a change in conductivity as the material is exposed to the target gas. The most common commercial sensor of this type is based on polycrystalline SnO2 [8]. Thin film sensors that do not require a reference cell are simpler and thus less expensive to manufacture. Moreover, the time response can be reduced, providing for increased speed. In this regard, alkaline earth doped LaFeO3 is an interesting material system. This perovskite, an antiferromagnetic insulator at room temperature, shows significant conductivity at elevated temperatures. Extensive studies of the bulk electronic and ionic transport properties of LaFeO3 , including Seebeck measurements, show a crossover from n- to p-type conductivity as the measured conductivity passes through a minimum for a certain P (O2 ), dependent on temperature [1,2]. Conductivity measurements in air, N2 , and O2 were also reported for LaFeO3 thin films grown on MgO(1 0 0) for temperatures below 600 C, showing a near linear log r vs. 1=T behavior for T > 300 C, and with virtually no difference in conductivity between those three ambients [9]. In this paper, we
0038-1101/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00214-4
2280
I. Hole et al. / Solid-State Electronics 47 (2003) 2279–2282
explore the case for exploiting the high temperature conduction properties of a LaFeO3 thin film for oxygen sensing purposes. We report conductivity vs. oxygen partial pressure measurements at elevated temperatures, and the time response in r as the film is exposed to an abrupt change in P (O2 ).
2. Experimental Heteroepitaxial thin films of LaFeO3 were grown from a polycrystalline composite target by reactive offaxis radio frequency sputtering in an inverted cylindrical magnetron geometry. Due to its good insulating properties and high chemical stability under variable oxygen partial pressures, single-crystalline LaAlO3 (0 0 1) was chosen as the substrate material in this study. The good insulating property of this substrate was verified by the absence of a measurable conductivity over a wide range in temperature and P (O2 ). Prior to deposition, the substrates were cleaned in acetone and isopropyl alcohol, and heated to the adopted growth temperature of Tsub ¼ 775 C. The deposition chamber was filled with a 2:1 gas mixture of Ar and O2 to a total pressure of 100 /min. mTorr, and the films were grown at a rate of 8 A After deposition, the oxygen pressure in the chamber was increased to 4 Torr and the samples annealed at 775 C in order to ensure full oxygenation. This oxygenation is only important for the post growth structural characterization at room temperature, as subsequent conductivity measurements were all performed at an elevated temperature where the equilibrium oxygen concentration in the film was controlled by the ambient.
X-ray diffraction analysis revealed monocrystalline growth of (1 1 0)-oriented LaFeO3 , as shown in Fig. 1. The d1 1 0 lattice parameter of the as-grown films was . Rocking curves recorded around determined at 3.97 A the (1 1 0) peak showed an FWHM < 0.2. Atomic force microscopy measurements of the film topography unveiled flat, uniform surfaces over extended area, with a for a measured root-mean-square roughness of 6 A 2.5 · 2.5 lm2 area. The data reported in this paper were taken using 10 · 10 mm2 films with a thickness of 150 nm. The conductivity was measured with a standard dc 4-point setup. To ensure good contacts for the two current feeds, 2 mm wide stripes of Au were sputter-deposited onto each end of the film surface. Two Pt wires (u ¼ 0:5 mm) 4 mm apart, were used to measure the voltage drop. The partial pressure of ambient oxygen was controlled mechanically with mass flowmeters, providing a mixture of O2 and N2 for P (O2 ) in the range 1–105 atm; and chemically with a mixture of CO and CO2 for P (O2 ) in the range 107 –1017 atm. The minimum P (O2 ) used in this study was 1017 atm, in order to avoid decomposition of LaFeO3 to La2 O3 and Fe at lower oxygen partial pressures [1]. A ZrO2 cell was used to measure the partial pressure of oxygen, and equilibrium conductivity measurements were carried out at T ¼ 1000 C, controlled within ±1 C. Two separate gas mixtures of different P (O2 ) were prepared to measure the time response of the LaFeO3 thin film conductivity upon abrupt changes in the ambient oxygen content. Once a stable film conductivity was attained upon exposure to the higher P (O2 ) gas mix, the ambient was switched to the lower P (O2 ) mix, and the response in conductivity was
Fig. 1. H–2H X-ray diffractogram of a 150 nm LaFeO3 (1 1 0) thin film on LaAlO3 (0 0 1) substrate. The spacing of the LaFeO3 low. index planes parallel to the substrate is estimated at d1 1 0 ¼ 3:97 A
I. Hole et al. / Solid-State Electronics 47 (2003) 2279–2282
logged as a function of time. The time constant was defined by the 90–10% drop in differential conductivity between high and low P (O2 ).
The operating principle of an oxygen sensor made from a mixed conductor material such as LaFeO3d relies on variation in the concentration of oxygen vacancies with changes in the ambient oxygen pressure. When lowering the partial pressure of oxygen, the material is reduced and electrons are added to the system by the desorption reaction of oxygen, OxO þ 2FexFe ¼ 2Fe0Fe þ VO þ 1=2O2 ðgÞ using Kr€ oger–Vink notation [10]. However, the total number of free carriers is determined by the disproportion reaction of iron
1
0.5
log σ [a.u.]
3. Results and discussion
2281
0
-0.5 -1 -1.5 -17
-12
-7
-2
log P(O 2 ) [atm] Fig. 2. Equilibrium conductivities of a LaFeO3 thin film vs. oxygen partial pressure at T ¼ 1000 C. The data shows a near linear behavior for P (O2 ) ¼ 1010 –1 atm. The gap in data points between P (O2 ) ¼ 107 atm and P (O2 ) ¼ 104 atm is due to problems of controlling the oxygen partial pressure in this range.
2Fex ¼ Fe0 þ Fe 1.2
1000°C 700°C
1.0 0.8
[a.u.]
where the density of two-(Fe0 ) and four-valent iron (Fe ) corresponds to electron and hole concentrations, respectively. Depending on whether electrons or holes predominate at a given P (O2 ), lowering the oxygen content in the film ambient can either increase or decrease the conductivity, as expressed by
0.6
r / lh p þ le n
0.4
We assume that ionic conductivity can be neglected in the present experiment. In order to study the case for using LaFeO3 as an oxygen gas sensor, the equilibrium conductivities are plotted as a function of oxygen partial pressure at T ¼ 1000 C in Fig. 2. The conductivity passes through a minimum for P (O2 ) 1011 atm and displays a near linear increase in log r vs. log P ðO2 Þ in the entire range from P (O2 ) ¼ 1010 to P (O2 ) ¼ 1 atm. This linear response over 10 decades in the ambient oxygen partial pressure, with a rate of change in electronic conductivity of Dr=r 0:45/decade P (O2 ), renders LaFeO3 a most interesting material system for high temperature oxygen sensor applications. Another important aspect of a gas sensor is the time response of the system. In Fig. 3, the measured conductivity has been plotted as a function of time for a sudden change in the ambient gas mixture from P (O2 ) ¼ 103 to P (O2 ) ¼ 102 atm, at 700 and 1000 C, respectively. The time constant for the change in equilibrium conductivity was found to be 60 s for T ¼ 700 C and <3.5 s for T ¼ 1000 C. This corresponds to a chemical diffusion constant in the LaFeO3 thin film of Dch ¼ 1011:7 cm2 s1 at T ¼ 700 C and Dch > 1010:3 cm2 s1 at T ¼ 1000 C [11]. Reported time constants from similar measurements in thin film SrTiO3 are as
0.2 0.0 0
10
20
30
40
50
60
70
80
90
100
time [s]
Fig. 3. Measured conductivity vs. time in response to a sudden drop in the oxygen partial pressure from 102 to 103 atm at T ¼ 700 C (gray curve) and T ¼ 1000 C (black curve), respectively. The conductivity is normalized to 1 at the initial P (O2 ), and the change in oxygen partial pressure took place at t ¼ 0 s for both temperatures. The time constants were estimated at 60 s for T ¼ 700 C and <3.5 s at T ¼ 1000 C.
low as 5 ms at T ¼ 1000 C [5]. However, intrinsic instabilities in the electronic conductivity of SrTiO3 during thermal reduction [12], may severely limit the range in P (O2 ) for which this material can be used as an oxygen gas sensor. A 72-h measurement in pure nitrogen did not show any instability in the electronic conductivity of the LaFeO3 films. The reported time response in the present experiment is fast in comparison with LaFeO3 used as NO2 sensor at lower temperatures [13,14], where the time constants for polycrystalline thick films was found to be on the order of 10 min or more. We note that the measured time constant of 3.5 s at T ¼ 1000 C represents an upper limit imposed by gas flow impediments of
2282
I. Hole et al. / Solid-State Electronics 47 (2003) 2279–2282
our present measurement setup. By modifying the setup, a faster response to changes in the ambient oxygen content is expected. Thus, the speed of sensing a change in ambient oxygen should not be limited to the present 3.5 s, making LaFeO3 a promising material system for oxygen sensors.
[3]
[4]
4. Conclusions In this paper we have assessed the case for using LaFeO3 thin films in oxygen sensor applications. High quality epitaxial films were grown on LaAlO3 (0 0 1) substrate, and conductivity measurements were carried out in a gaseous ambient with controlled oxygen partial pressure at an elevated temperature of T ¼ 1000 C. The measurements showed a large linear response in conductivity with increasing P (O2 ) over 10 orders of magnitude. A fast time response of <3.5 s, presently limited by the geometry of our measurement setup, make this material system a promising option for high temperature oxygen sensing applications.
[5]
[6]
[7]
[8] [9]
[10]
Acknowledgement This work was funded by The Norwegian research Council under contract no. 121443/420. References [1] Mizusaki J, Sasamoto T, Cannon WR, Bowen HK. Electronic conductivity, seebeck coefficient, and defect structure of LaFeO3 . J Am Ceram Soc 1982;65(8):363–8. [2] Mizusaki J, Sasamoto T, Cannon WR, Bowen HK. Electronic conductivity, Seebeck coefficient, and defect
[11] [12]
[13]
[14]
structure of La1x Srx FeO3 (x ¼ 0:1, 0.25). J Am Ceram Soc 1983;66(4):247–52. Elshof JEten, Bouwmeester HJM, Verweij H. Oxygen transport through La1x Srx FeO3d membranes I. Permeation in air/He gradients. Solid State Ionics 1995;81:97– 109. Elshof JEten, Bouwmeester HJM, Verweij H. Oxygen transport through La1x Srx FeO3d membranes II. Permeation in air/CO, CO2 gradients. Solid State Ionics 1996;89: 81–92. Fleischer M, Meixner H. Thin-film gas sensors based on high-temperature-operated metal oxides. J Vac Sci Technol A 1999;17(4):1866–72. Moseley PT, Norris JOW, Williams DE. Techniques and mechanisms in gas sensing. Bristol: IOP Publishing Ltd.; 1991. Lundstr€ om KI, Shivaraman MS, Svensson CM. A hydrogen-sensitive Pd-gate MOS transistor. J Appl Phys 1975; 46(9):3876–81. Madou M, Morrison SR. Chemical sensing with solid state devices. New York: Academic; 1989. Yamada S, Tai H, Matsumoto Y, Koinuma M, Kamada K, Sasaki T. Preparation of LaFeO3 perovskite thin films by radio frequency magnetron sputtering and their electrical conductivities. Electrochemistry 2001;69(3):171–6. Kr€ oger FA. The chemistry of imperfect crystals, vol. 2. Amsterdam: North-Holland Publishing Company; 1974. Hole I et al., in preparation. Szot K, Speier W, Carius R, Zastrow U, Beyer W. Localized metallic conductivity and self-healing during thermal reduction of SrTiO3 . Phys Rev Lett 2002;88(7): 075508. Traversa E, Matsushima S, Okada G, Sadaoka Y, Sakai Y, Watanabe K. NO2 sensitive LaFeO3 thin films prepared by r.f. sputtering. Sens Actuat B 1995;24–25:661–4. Grilli ML, Bartolomeo ED, Traversa E. Electrochemical NOx sensors based on interfacing nanosized LaFeO3 perovskite-type oxide and ionic conductors. J Electrochem Soc 2001;148(9):H98–H102.