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Surface and microstructural properties of SnO2 thin films grown on p-InP (100) substrates at low temperature
PERGAMON
Solid State Communications 115 (2000) 503–507 www.elsevier.com/locate/ssc
Surface and microstructural properties of SnO2 thin films grown on p-InP (100) substrates at low temperature T.W. Kim a,*, D.U. Lee a, J.H. Lee a, Y.S. Yoon b b
a Department of Physics, Kwangwoon University, 447-1 Wolgye-dong, Nowon-ku, Seoul 139-701, South Korea Applied Physics Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, South Korea
Received 10 January 2000; accepted 4 March 2000 by C.N.R. Rao; received in final form by the Publisher 31 May 2000
Abstract SnO2 thin films were grown on p-InP (100) substrates by using radio-frequency magnetron sputtering at low temperature. Atomic force microscopy images showed that the root mean square of the average surface roughness of the SnO2 film was ˚ , and X-ray diffraction and transmission electron microscopy (TEM) measurements showed that the SnO2 thin films 22.6 A grown on p-InP substrates were polycrystalline. Auger electron spectroscopy and bright-field TEM measurements showed that the SnO2 thin layers grown on p-InP substrates at 200⬚C had no significant interdiffusion problems. However, a thin interfacial layer of unknown origin was detected between the SnO2 film and the substrate. These results indicate that the SnO2 epitaxial films grown on p-InP (100) substrates at low temperature hold promise for potential devices based on InP substrates, such as superior stability varistors and high-efficiency solar cells. Even the structure with the unintentionally grown interfacial layer might be used for high-efficiency solar cells. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Heterojunctions; B. Crystal growth; C. Scanning and transmission electron microscopy
Recently, SnO2 thin films have become particularly attractive due to their potential applications in electronic and optoelectronic devices, such as varistors, photodetectors, and solar cells [1–5]. A unique physical property of SnO2 thin films for device applications, in comparison to other wide band gap semiconductors, is their superior chemical stability [6], and other properties of the SnO2 thin films may prevent the inherent problem of mixed metal oxidation states in the phase present during device fabrication processes [7]. In more recent years, when SnO2 thin films were grown on Si substrates [8], the formation of an interfacial layer between the SnO2 thin films and the Si substrates prior to the creation of the SnO2 film caused deleterious reduction effects [9]. Even though some groups have investigated the growth of SnO2 thin films on various substrates [10–14], to the best of our knowledge, SnO2 thin films have not yet been fabricated on InP substrates due to the delicate problem of the interface. The fabrication of SnO2 thin films on InP substrates is particularly interesting * Corresponding author. Tel.: ⫹82-2-940-5234; fax: ⫹82-2-9420108.
due to their promising applications in new kind of highspeed varistors and digital circuits. Therefore, studies of the structural properties of SnO2/InP heterostructures with high-quality interfaces are very important for new kinds of optoelectronic devices based on InP substrates because the structural properties significantly affect the electrical and the optical properties of the thin films which are necessary for the fabrication of high-efficiency devices. This communication reports the structural properties of SnO2 thin films grown on p-InP (100) substrates by using the radio-frequency magnetron-sputtering deposition method. Atomic force microscopy (AFM) measurements were performed in order to characterize the surface smoothness of the SnO2 layer, and X-ray diffraction (XRD) measurements were carried out to investigate the crystallization of the SnO2 layer. Auger electron spectroscopy (AES) measurements were carried out to investigate the stoichiometry and the interface quality of the grown film, and transmission electron microscopy (TEM) measurements were performed to investigate the microstructure of the SnO2/p-InP (100) heterostructure. Polycrystalline stoichiometric Sn with a purity of
0038-1098/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00231-3
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T.W. Kim et al. / Solid State Communications 115 (2000) 503–507
Fig. 1. An AFM image of the SnO2/p-InP heterostructure.
99.999% was used as a source target material and was pre-cleaned by repeated sublimation. Oxygen gas with a purity of 99.999% was used as an ion source. The carrier concentration of the Zn-doped p-InP substrates with (100) orientations used in this experiment was 1 × 10 16 cm ⫺3. The InP substrates obtained from Sumitomi were mechanically polished, alternately degreased in warm acetone and trichloroethylene (TCE) three times, etched in a Br–methanol solution mechano-chemically, rinsed in deionized water thoroughly, etched in a mixture of H2SO4, H2O2, and H2O (4:1:1) at 40⬚C for 10 min, and rinsed in TCE again. After the InP wafers were cleaned chemically, they were mounted onto a susceptor in a growth chamber. After the chamber was evacuated to 1 × 10 ⫺6 Torr, the deposition was done in a substrate temperature range between 100 and 500⬚C. Ar ⫹ gas with a purity of 99.999% was used as the sputtering gas. Prior to SnO2 growth, the surface of the SnO2 was polished by Ar ⫹ sputtering. The SnO2 deposition was done at a system pressure of 2 × 10 ⫺4 Torr and a radio-frequency
power (radio frequency 13.56 MHz) of 800 W. The ˚ /s. growth rate was 0.5 A The as-grown SnO2 films had mirror-like surfaces without any indications of pinholes, which was confirmed by using Normarski optical microscopy and scanning electron microscopy (SEM) measurements. Even though the SnO2 films had been grown in the temperature range between 100 and 500⬚C, only the physical properties of the film grown at 200⬚C are reported because the films grown below 200⬚C were amorphous and had stoichiometry problems, and those grown above 400⬚C had significant interdiffusion problems. The root mean square of the average surface roughness of the SnO2 thin films grown at 200⬚C, as determined from the ˚ , as shown in Fig. 1. The AFM measurements, was 22.6 A image in Fig. 1 shows that the surface of the SnO2 thin film grown on the p-InP (100) substrate was smooth. Fig. 2 shows the XRD pattern for the SnO2 film grown on a p-InP (100) substrate at 200⬚C. The (110), (101), (211), (002), and (310) Ka diffraction peaks corresponding to the
Fig. 2. An X-ray diffraction curve of the SnO2/p-InP heterostructure.
T.W. Kim et al. / Solid State Communications 115 (2000) 503–507
505
Fig. 3. Auger electron spectra obtained from a SnO2/p-InP heterostructure. The lower curve was obtained at the surface of the as-grown SnO2/p˚. InP heterostructure, and the upper one was obtained at a depth of 500 A
SnO2 film, together with the (200) and (400) Ka1 diffraction peaks corresponding to the InP (100) substrate, are clearly observed. These XRD results indicate that SnO2 films grown on InP (100) substrates at 200⬚C are polycrystalline. The compositions and the interfacial qualities of the SnO2 thin films grown on InP substrates were investigated by AES measurements, and the results are presented in Figs. 3 and 4. Fig. 3 shows that the as-grown films consisted of tin, oxygen, and carbon at the surface and of zinc and oxygen ˚ . The carbon impurities at the SnO2 at a depth of 500 A
surface might originate from contamination due to the metal source materials or to the growth chamber; the existence of carbon was also confirmed by X-ray photoelectron spectroscopy measurements. Fig. 4 shows that the interfaces between the SnO2 and the InP were relatively abrupt and that the stoichiometry of the film was SnO2. The thickness of ˚ , and this value was the SnO2 was approximately 5000 A in reasonable agreement with that obtained from the ellipsometer and Rutherford backscattering measurements. Fig. 5 presents a bright-field TEM image showing the top
Fig. 4. An Auger depth profile of the SnO2/p-InP heterostructure.
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T.W. Kim et al. / Solid State Communications 115 (2000) 503–507
Fig. 5. A cross-sectional bright-field TEM image of the SnO2/p-InP heterostructure.
SnO2 layer and the bottom InP substrate. Even though there are no significant interface problems at the SnO2/p-InP interface grown at low temperature, the bright-field TEM image shows that a thin interfacial layer was formed at the SnO2/pInP interface. Furthermore, the interface between the SnO2/ p-InP heterostructure is more abrupt than that between the SnO2/n-Si heterostructure reported in the literature [8]. The improvement of the SnO2/p-InP heterointerface quality might be caused by the lower growth temperature of the SnO2 film. The bright-field TEM image of the SnO2 thin film shows that the grain size of the SnO2 is very small and that the average grain size is approximately 2.8 nm. Since the grain size is smaller than the Debye length of SnO2, the complete grain is depleted of electrons, and the sensitivity of sensor using SnO2/p-InP (100) will be greatly increased [15]. This small grain size might enhance the photocurrent intensity under illumination. A
selected-area electron diffraction pattern from the TEM measurements at the SnO2/p-InP heterointerface is shown in Fig. 6. The regular spots originate from the InP substrate, and the irregular spots and the diffused ring are related to the SnO2 polycrystalline film and the amorphous local interfacial layer, respectively. A SnO2/p-InP heterojunction with an interfacial layer might show the characteristics of a heterojunction photovoltaic cell, and both photocurrent suppression and photocapacitance reduction can be observed in such cells. Since the photocurrent results from photoexcited minority carrier recombination at an interfacial layer, the interfacial layer as thin as possible is required to allow maximum photocurrent. The dark current is predominantly due to electrons injected over the InP barrier and through the interfacial layer. The holes are trapped in a potential notch in the valence band [10]. Even though our main purpose was the growth of SnO2 epitaxial layers on InP substrates, the polycrystalline SnO2 thin layer was grown because of the existence of an interfacial layer prior to the growth of the SnO2 layer. Even though it is impossible to explain unambiguously from the measurements why the interfacial layer is formed, the SnO2/ thin interface layer/p-InP structures can be used as new kinds of high-efficiency solar cells. In summary, the results of AFM, XRD, AES, and TEM measurements showed that the SnO2 layers grown by the magnetron-sputtering method at low temperature were polycrystalline films. AES and TEM measurements confirmed that the SnO2/p-InP (100) heterointerface was relatively abrupt and that the interdiffusion problem was not significant. Although some details of the electrical and the optical properties remain to be clarified, these results indicate that SnO2/p-InP heterostructures grown by the magnetron-sputtering method at low temperature provide good motivation for the fabrication of InP-based optoelectronic devices, such
Fig. 6. An electron diffraction pattern from TEM of the SnO2/p-InP heterostructure. (hkl)I corresponds to the InP substrate.
T.W. Kim et al. / Solid State Communications 115 (2000) 503–507
as new kinds of superior-stability varistors and highefficiency solar cells. Acknowledgements This work was supported by the Korea Ministry of Education, Project No. KRF-99-041-D00171, in 1999. References [1] Z.M. Jarzebski, J.P. Marton, J. Electrochem. Soc. 123 (1976) 1990. [2] J. Maier, W. Gopel, J. Solid State Chem. 72 (1988) 293. [3] K.P. Kumar, A.D. Domodaran, J. Mater. Sci. 24 (1989) 220. [4] S.A. Pianaro, P.R. Bueno, E. Longo, J.A. Varela, J. Mater. Sci. Lett. 14 (1995) 692.
507
[5] P.I. Rovira, R.W. Collins, J. Appl. Phys. 85 (1999) 2015. [6] D. Narducci, G. Girardi, C.M. Mari, S. Pizzini, Sensors and Actuators B 24/25 (1995) 266. [7] J. Maier, W. Gopel, J. Solid State Chem. 72 (1988) 293. [8] C. Alfonso, A. Charai, A. Armigliato, D. Narducci, Appl. Phys. Lett. 68 (1996) 1207. [9] D. Narducci, S. Pizzini, F. Morazzoni, Proceedings of the Symposium on Chemical Sensors II, Vols. 93–97, The Electrochemical Society, Pittsburgh, PA, 1993, p. 325. [10] T. Nishino, Y. Hamakawa, Jpn. J. Appl. Phys. 9 (1970) 1085. [11] R. Kondo, H. Okimura, Y. Sakai, Jpn. J. Appl. Phys. 10 (1971) 1547. [12] R.S. Katiyar, P. Dawson, M. Hargreave, G.R. Wilkinson, J. Phys. C 4 (1971) 2421. [13] R.L. Anderson, Appl. Phys. Lett. 27 (1975) 691. [14] A. Tanaka, S. Onari, T. Arai, Phys. Rev. B 47 (1993) 1237. [15] N. Yamazoe, Sensors and Actuators B 5 (1991) 7.
Solid State Communications 115 (2000) 503–507 www.elsevier.com/locate/ssc
Surface and microstructural properties of SnO2 thin films grown on p-InP (100) substrates at low temperature T.W. Kim a,*, D.U. Lee a, J.H. Lee a, Y.S. Yoon b b
a Department of Physics, Kwangwoon University, 447-1 Wolgye-dong, Nowon-ku, Seoul 139-701, South Korea Applied Physics Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, South Korea
Received 10 January 2000; accepted 4 March 2000 by C.N.R. Rao; received in final form by the Publisher 31 May 2000
Abstract SnO2 thin films were grown on p-InP (100) substrates by using radio-frequency magnetron sputtering at low temperature. Atomic force microscopy images showed that the root mean square of the average surface roughness of the SnO2 film was ˚ , and X-ray diffraction and transmission electron microscopy (TEM) measurements showed that the SnO2 thin films 22.6 A grown on p-InP substrates were polycrystalline. Auger electron spectroscopy and bright-field TEM measurements showed that the SnO2 thin layers grown on p-InP substrates at 200⬚C had no significant interdiffusion problems. However, a thin interfacial layer of unknown origin was detected between the SnO2 film and the substrate. These results indicate that the SnO2 epitaxial films grown on p-InP (100) substrates at low temperature hold promise for potential devices based on InP substrates, such as superior stability varistors and high-efficiency solar cells. Even the structure with the unintentionally grown interfacial layer might be used for high-efficiency solar cells. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Heterojunctions; B. Crystal growth; C. Scanning and transmission electron microscopy
Recently, SnO2 thin films have become particularly attractive due to their potential applications in electronic and optoelectronic devices, such as varistors, photodetectors, and solar cells [1–5]. A unique physical property of SnO2 thin films for device applications, in comparison to other wide band gap semiconductors, is their superior chemical stability [6], and other properties of the SnO2 thin films may prevent the inherent problem of mixed metal oxidation states in the phase present during device fabrication processes [7]. In more recent years, when SnO2 thin films were grown on Si substrates [8], the formation of an interfacial layer between the SnO2 thin films and the Si substrates prior to the creation of the SnO2 film caused deleterious reduction effects [9]. Even though some groups have investigated the growth of SnO2 thin films on various substrates [10–14], to the best of our knowledge, SnO2 thin films have not yet been fabricated on InP substrates due to the delicate problem of the interface. The fabrication of SnO2 thin films on InP substrates is particularly interesting * Corresponding author. Tel.: ⫹82-2-940-5234; fax: ⫹82-2-9420108.
due to their promising applications in new kind of highspeed varistors and digital circuits. Therefore, studies of the structural properties of SnO2/InP heterostructures with high-quality interfaces are very important for new kinds of optoelectronic devices based on InP substrates because the structural properties significantly affect the electrical and the optical properties of the thin films which are necessary for the fabrication of high-efficiency devices. This communication reports the structural properties of SnO2 thin films grown on p-InP (100) substrates by using the radio-frequency magnetron-sputtering deposition method. Atomic force microscopy (AFM) measurements were performed in order to characterize the surface smoothness of the SnO2 layer, and X-ray diffraction (XRD) measurements were carried out to investigate the crystallization of the SnO2 layer. Auger electron spectroscopy (AES) measurements were carried out to investigate the stoichiometry and the interface quality of the grown film, and transmission electron microscopy (TEM) measurements were performed to investigate the microstructure of the SnO2/p-InP (100) heterostructure. Polycrystalline stoichiometric Sn with a purity of
0038-1098/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00231-3
504
T.W. Kim et al. / Solid State Communications 115 (2000) 503–507
Fig. 1. An AFM image of the SnO2/p-InP heterostructure.
99.999% was used as a source target material and was pre-cleaned by repeated sublimation. Oxygen gas with a purity of 99.999% was used as an ion source. The carrier concentration of the Zn-doped p-InP substrates with (100) orientations used in this experiment was 1 × 10 16 cm ⫺3. The InP substrates obtained from Sumitomi were mechanically polished, alternately degreased in warm acetone and trichloroethylene (TCE) three times, etched in a Br–methanol solution mechano-chemically, rinsed in deionized water thoroughly, etched in a mixture of H2SO4, H2O2, and H2O (4:1:1) at 40⬚C for 10 min, and rinsed in TCE again. After the InP wafers were cleaned chemically, they were mounted onto a susceptor in a growth chamber. After the chamber was evacuated to 1 × 10 ⫺6 Torr, the deposition was done in a substrate temperature range between 100 and 500⬚C. Ar ⫹ gas with a purity of 99.999% was used as the sputtering gas. Prior to SnO2 growth, the surface of the SnO2 was polished by Ar ⫹ sputtering. The SnO2 deposition was done at a system pressure of 2 × 10 ⫺4 Torr and a radio-frequency
power (radio frequency 13.56 MHz) of 800 W. The ˚ /s. growth rate was 0.5 A The as-grown SnO2 films had mirror-like surfaces without any indications of pinholes, which was confirmed by using Normarski optical microscopy and scanning electron microscopy (SEM) measurements. Even though the SnO2 films had been grown in the temperature range between 100 and 500⬚C, only the physical properties of the film grown at 200⬚C are reported because the films grown below 200⬚C were amorphous and had stoichiometry problems, and those grown above 400⬚C had significant interdiffusion problems. The root mean square of the average surface roughness of the SnO2 thin films grown at 200⬚C, as determined from the ˚ , as shown in Fig. 1. The AFM measurements, was 22.6 A image in Fig. 1 shows that the surface of the SnO2 thin film grown on the p-InP (100) substrate was smooth. Fig. 2 shows the XRD pattern for the SnO2 film grown on a p-InP (100) substrate at 200⬚C. The (110), (101), (211), (002), and (310) Ka diffraction peaks corresponding to the
Fig. 2. An X-ray diffraction curve of the SnO2/p-InP heterostructure.
T.W. Kim et al. / Solid State Communications 115 (2000) 503–507
505
Fig. 3. Auger electron spectra obtained from a SnO2/p-InP heterostructure. The lower curve was obtained at the surface of the as-grown SnO2/p˚. InP heterostructure, and the upper one was obtained at a depth of 500 A
SnO2 film, together with the (200) and (400) Ka1 diffraction peaks corresponding to the InP (100) substrate, are clearly observed. These XRD results indicate that SnO2 films grown on InP (100) substrates at 200⬚C are polycrystalline. The compositions and the interfacial qualities of the SnO2 thin films grown on InP substrates were investigated by AES measurements, and the results are presented in Figs. 3 and 4. Fig. 3 shows that the as-grown films consisted of tin, oxygen, and carbon at the surface and of zinc and oxygen ˚ . The carbon impurities at the SnO2 at a depth of 500 A
surface might originate from contamination due to the metal source materials or to the growth chamber; the existence of carbon was also confirmed by X-ray photoelectron spectroscopy measurements. Fig. 4 shows that the interfaces between the SnO2 and the InP were relatively abrupt and that the stoichiometry of the film was SnO2. The thickness of ˚ , and this value was the SnO2 was approximately 5000 A in reasonable agreement with that obtained from the ellipsometer and Rutherford backscattering measurements. Fig. 5 presents a bright-field TEM image showing the top
Fig. 4. An Auger depth profile of the SnO2/p-InP heterostructure.
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T.W. Kim et al. / Solid State Communications 115 (2000) 503–507
Fig. 5. A cross-sectional bright-field TEM image of the SnO2/p-InP heterostructure.
SnO2 layer and the bottom InP substrate. Even though there are no significant interface problems at the SnO2/p-InP interface grown at low temperature, the bright-field TEM image shows that a thin interfacial layer was formed at the SnO2/pInP interface. Furthermore, the interface between the SnO2/ p-InP heterostructure is more abrupt than that between the SnO2/n-Si heterostructure reported in the literature [8]. The improvement of the SnO2/p-InP heterointerface quality might be caused by the lower growth temperature of the SnO2 film. The bright-field TEM image of the SnO2 thin film shows that the grain size of the SnO2 is very small and that the average grain size is approximately 2.8 nm. Since the grain size is smaller than the Debye length of SnO2, the complete grain is depleted of electrons, and the sensitivity of sensor using SnO2/p-InP (100) will be greatly increased [15]. This small grain size might enhance the photocurrent intensity under illumination. A
selected-area electron diffraction pattern from the TEM measurements at the SnO2/p-InP heterointerface is shown in Fig. 6. The regular spots originate from the InP substrate, and the irregular spots and the diffused ring are related to the SnO2 polycrystalline film and the amorphous local interfacial layer, respectively. A SnO2/p-InP heterojunction with an interfacial layer might show the characteristics of a heterojunction photovoltaic cell, and both photocurrent suppression and photocapacitance reduction can be observed in such cells. Since the photocurrent results from photoexcited minority carrier recombination at an interfacial layer, the interfacial layer as thin as possible is required to allow maximum photocurrent. The dark current is predominantly due to electrons injected over the InP barrier and through the interfacial layer. The holes are trapped in a potential notch in the valence band [10]. Even though our main purpose was the growth of SnO2 epitaxial layers on InP substrates, the polycrystalline SnO2 thin layer was grown because of the existence of an interfacial layer prior to the growth of the SnO2 layer. Even though it is impossible to explain unambiguously from the measurements why the interfacial layer is formed, the SnO2/ thin interface layer/p-InP structures can be used as new kinds of high-efficiency solar cells. In summary, the results of AFM, XRD, AES, and TEM measurements showed that the SnO2 layers grown by the magnetron-sputtering method at low temperature were polycrystalline films. AES and TEM measurements confirmed that the SnO2/p-InP (100) heterointerface was relatively abrupt and that the interdiffusion problem was not significant. Although some details of the electrical and the optical properties remain to be clarified, these results indicate that SnO2/p-InP heterostructures grown by the magnetron-sputtering method at low temperature provide good motivation for the fabrication of InP-based optoelectronic devices, such
Fig. 6. An electron diffraction pattern from TEM of the SnO2/p-InP heterostructure. (hkl)I corresponds to the InP substrate.
T.W. Kim et al. / Solid State Communications 115 (2000) 503–507
as new kinds of superior-stability varistors and highefficiency solar cells. Acknowledgements This work was supported by the Korea Ministry of Education, Project No. KRF-99-041-D00171, in 1999. References [1] Z.M. Jarzebski, J.P. Marton, J. Electrochem. Soc. 123 (1976) 1990. [2] J. Maier, W. Gopel, J. Solid State Chem. 72 (1988) 293. [3] K.P. Kumar, A.D. Domodaran, J. Mater. Sci. 24 (1989) 220. [4] S.A. Pianaro, P.R. Bueno, E. Longo, J.A. Varela, J. Mater. Sci. Lett. 14 (1995) 692.
507
[5] P.I. Rovira, R.W. Collins, J. Appl. Phys. 85 (1999) 2015. [6] D. Narducci, G. Girardi, C.M. Mari, S. Pizzini, Sensors and Actuators B 24/25 (1995) 266. [7] J. Maier, W. Gopel, J. Solid State Chem. 72 (1988) 293. [8] C. Alfonso, A. Charai, A. Armigliato, D. Narducci, Appl. Phys. Lett. 68 (1996) 1207. [9] D. Narducci, S. Pizzini, F. Morazzoni, Proceedings of the Symposium on Chemical Sensors II, Vols. 93–97, The Electrochemical Society, Pittsburgh, PA, 1993, p. 325. [10] T. Nishino, Y. Hamakawa, Jpn. J. Appl. Phys. 9 (1970) 1085. [11] R. Kondo, H. Okimura, Y. Sakai, Jpn. J. Appl. Phys. 10 (1971) 1547. [12] R.S. Katiyar, P. Dawson, M. Hargreave, G.R. Wilkinson, J. Phys. C 4 (1971) 2421. [13] R.L. Anderson, Appl. Phys. Lett. 27 (1975) 691. [14] A. Tanaka, S. Onari, T. Arai, Phys. Rev. B 47 (1993) 1237. [15] N. Yamazoe, Sensors and Actuators B 5 (1991) 7.