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Laser-SQUID microscope for noncontact evaluation of solar cell
Physica C 471 (2011) 1249–1252
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Laser-SQUID microscope for noncontact evaluation of solar cell Y. Nakatani a,⇑, T. Hayashi b, H. Itozaki a a b
Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan Sendai National College of Technology, 4-16-1, Ayashityuou, Aoba-ku, Sendai, Miyagi 989-3128, Japan
a r t i c l e
i n f o
Article history: Available online 14 May 2011 Keywords: SQUID Microscope Laser Solar cell
a b s t r a c t A polycrystalline solar cell with several grains was investigated by the laser-SQUID (Superconducting QUantum Interference Device) microscope. The laser-SQUID microscopy detects the magnetic field generated by a photo-induced current. This technique enables nondestructive and noncontact evaluation of semiconductor samples. A needle made of high permeability material was used to transmit the magnetic field near the sample to the SQUID. The needle and the SQUID were shifted in the x- and y-directions from the center of a laser spot. The laser-SQUID microscope images varied with needle position. This indicated the possibility of current estimation using a laser-SQUID microscope. In this study the sample was also evaluated using Laser Beam Induced Current (LBIC) which is widely used for evaluation of the conversion efficiency distribution of solar cells. The laser-SQUID microscope image was compared with the LBIC image and was found to be similar. This result showed that laser-SQUID microscopy can be used for the electrical evaluation of solar cells without contact, and furthermore has the possibility of estimation of the photocurrent direction. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The conversion efficiency of the whole solar cell is the sum of the efficiency at each point of the solar cell. It is important to evaluate the distribution of the conversion efficiency over the solar cell surface. Using the localized illumination method is effective for this purpose. The Laser Beam Induced Current (LBIC) technique is widely used for evaluation of solar cells [1–5]. In this method, a laser is focused on the solar cell and the photocurrent is measured at every point on the surface. It is essential to build electrodes for collecting the photocurrent. Non-contact Laser-Beam Induced Current/Conductivity (LBIC) technique had been investigated [6,7]. Using this technique, the distribution of minority carrier lifetime on silicon wafer can be deduced. Instead of measuring the current, measuring the magnetic field from the photocurrent enables contactless measurement. A Superconducting QUantum Interference Device (SQUID) enables high sensitivity measurement of magnetic field. Laser-SQUID microscopy is a method of irradiating the sample with a laser, and measuring the magnetic field from the photocurrent using a SQUID. Doping level fluctuations in a semiconductor wafer were evaluated by a laser-SQUID microscope [8]. The spatial resolution of a laser-SQUID microscope depends
⇑ Corresponding author. Address: Division of Advanced Electronics and Optical Science, Department of Systems Innovation, Graduate School of Engineering Science, Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan. Tel.: +81 6 6850 6313; fax: +81 6 6850 6312. E-mail address: [email protected] (Y. Nakatani). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.171
mainly on the spot size of the laser. The laser-SQUID microscope has been applied to a study of solar cells [9–12]. The distribution of the energy transformation efficiency of the solar cell was evaluated by three different wavelengths [11]. The current vector distribution on the solar cell was estimated using off-axis measurement [12]. However these studies showed only laser-SQUID microscope images, and were not compared with other methodology. We compared the laser-SQUID microscope image with the LBIC image, and investigated the possibility of evaluating the localized properties of the solar cell without contact. 2. Methods 2.1. Laser-SQUID microscope measurement The laser-SQUID microscope [11] is shown schematically in Fig. 1. A semiconductor laser (AlGaAs) (Sigma Koki) was modulated at 1 kHz by a light chopper (5584A, NF Corp., Yokohama, Japan). Its wavelength was 780 nm and power was 2 mW at the sample surface. The laser was focused on the sample surface with diameter of 0.06 mm. A light was used to illuminate the sample surface, and to observe the sample surface by CCD camera (BS-41L, BITLAN). The adjustment of the size of laser spot within 0.06 mm and location on the sample surface were performed before the measurement. A needle made of permalloy 78 (The Nilaco Corp., Tokyo, Japan) was used to transmit the magnetic field at the tip of needle to the SQUID. Though the size of the magnetic sensor does not affect the spatial resolution of the laser-SQUID microscopy, the size of it
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The sample and the SQUID were set in a magnetic shield that was composed of a 3-layer high permeability cylinder. The magnetic field noise level of the SQUID with the needle was less than 1 pT Hz 1/2 at 1 kHz. The laser was modulated with 1 kHz. The signal with the frequency of 1 kHz was extracted by a digital lock-in amplifier (LI5640, NF Corp., Yokohama, Japan). It eliminates the noise from the signal which was unrelated to photocurrent generated by the laser. The time constant of the lock-in amplifier was 1 ms. 2.3. LBIC measurement The laser induced current was measured by using laser and XY scanning of SQUID microscope. The LBIC system used contacts on the solar cell and the current which was extracted outside of the solar cell through the electrodes was measured. In the LBIC measurement, because the short-circuit current should be measured, I–V conversion circuit was used and the signal was input to the lock-in amplifier. 2.4. Sample
Fig. 1. Schematic diagram of laser-SQUID microscope.
needed to be small in order to detect magnetic field at small area. The photocurrent generated inhomogeneous magnetic field and the position of the sensor was important to estimate the current vector. Fig. 2 shows the geometric configuration around the needle (/2.0 mm 7.0 mm). The needle tip of about 0.1 mm was polished mechanically. The needle was set vertically against the sample surface. The position of the needle was 2.1 mm away from a laser spot and 0.42 mm up from the surface because the magnetic field above the laser spot which is generated by the current induced by the laser does not have a vertical component of the field.
The solar cell studied in this work was a polycrystalline silicon solar cell. It was commercially available and already had several electrodes on the surface and its backside was fully covered with metal. The thickness of the metal layer was 0.02–0.13 mm. The laser modulation frequency was 1 kHz. The magnetic signal reduction due to the skin effect was negligible because the frequency and metal layer thickness were small. The size of the sample submitted for the measurement was 2.3 mm 3 mm. The thickness of silicon was 0.27 mm and the total thickness of the solar cell including electrodes was 0.4 mm. There were several grains with a typical size of a few millimeters. The sample holder was a nonmagnetic holder. The holder was mounted on an X–Y stage which was located outside of the magnetic shield. The stage was raster scanned at 0.5 mm/s in the y-direction. The sampling intervals in the x- and y-direction were 10 lm and 0.4 lm, respectively.
2.2. SQUID A SQUID was set above the needle and was mounted at the tip of the sappire rod with liquid nitrogen temperature. They were in a vacuum dewar. The SQUID detected the perpendicular component of the magnetic field generated from the sample. Type of the SQUID was a directly-coupled dc-SQUID and it was made of high transition temperature YBa2Cu3O7–d thin film on a 10 mm 10 mm SrTiO3 substrate. Its pickup loop size was 6.5 mm square. The SQUID was operated by a FFL circuit (PFL-100 operated by PCI1000, STAR Cryoelectronics, USA).
Fig. 2. Geometric configuration around needle.
3. Results and discussion 3.1. Comparison between a laser-SQUID microscope and optical image Fig. 3 shows the optical microscope image of the polycrystalline silicon solar cell. Grain boundary was clearly seen and average size of the grains was a few milimeters. We focused on the white grain which was shown at the center of Fig. 3. The white grain means
Fig. 3. Optical microscope image of the solar cell.
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that the reflection from the grain was higher than that from the other grains. This color was changed from white to black with the different angles of the incident light. This grain seemed to be uniform as far as it was observed by the optical microscope. However, the horizontal-striped pattern in the focused grain was observed which was not grain boundary by the visual inspection but images by saw cutting process. Fig. 4 is the laser-SQUID microscope image of the polycrystalline solar cell at the same area shown in the optical image in Fig. 3. The needle was shifted from the laser spot to y-direction by 2.1 mm in order to detect the magnetic field generated by the current across the laser spot. The z component of the magnetic field generated from the current across the laser spot was zero above the laser spot. Comparing the laserSQUID microscope image carefully with the optical microscope image, many grain boundaries were observed also by the laser-SQUID microscope. However, some grain boundaries were not observed in the laser-SQUID microscope image. In the focused grain, the intensity of the SQUID signal was lowered at several lines in the grains. The intensity of these lines was lower than those from the normal grain area. The wavelength of the laser was 780 nm, and the skin depth of the laser was about 10 lm for silicon. The laser was absorbed in the vicinity of the surface. Therefore, the low intensity of the SQUID signal was due to some kind of electrical defect within 10 lm from the surface. Though there were saw cutting images on the surface of the solar cell, there is no relationship with the signal. These saw cutting images were not electrically active. This signal was due to some kind of electrical defect such as dislocations or sub-grainboundaries.
Fig. 5. Laser-SQUID microscope image of the solar cell. The Needle was shifted to ydirection by 2.1 mm.
3.2. Estimation of photocurrent direction Fig. 5 shows the laser-SQUID microscope image of the solar cell when the needle was shifted from the laser spot in the y-direction by 2.1 mm. The details of the image were similar to the image in Fig. 4, although the sign of the SQUID signal became opposite. The images when the needle was shifted to ±x-direction and not shifted were also obtained (data was not shown). These images showed lower signal intensities than the image in Fig. 4. Therefore, the photocurrent was roughly estimated to flow to x-direction. 3.3. Comparison between a laser SQUID image and LBIC image Fig. 6 is a LBIC image of the same area observed in Fig. 3 and 4. The LBIC image shows the conversion efficiency map of the solar cell. The laser-SQUID microscope images corresponded to the LBIC image, and the laser-SQUID microscope images also indicated the
Fig. 6. LBIC image of the solar cell.
conversion efficiency of the solar cell. The grain boundaries seen in the optical image were observed in the LBIC image as well as the SQUID images. Low intensity lines were observed inside the grain. The LBIC image indicates the conversion efficiency and that there must be defects that could not be seen by the optical microscope inside this grain. The laser-SQUID microscope also detected these lines. This showed that laser-SQUID microscopy can be used for the detection of electrically active defects as well as the LBIC technique. Furthermore, the laser-SQUID microscope achieved contactless evaluation of the solar cell.
4. Conclusions
Fig. 4. Laser-SQUID microscope image of the solar cell. The Needle was shifted in the y-direction by +2.1 mm.
The polycrystalline silicon solar cell was evaluated by both the laser-SQUID microscopy and the LBIC technique. The laser-SQUID microscope image corresponded to the LBIC image which indicated the conversion efficiency distribution. A defect which could not be seen by optical microscope could be detected by the laser-SQUID microscope as well as the LBIC technique. Using the laser-SQUID microscope, the direction of the photocurrent was estimated by shifting the needle. The result showed that the laser-SQUID microscopy and LBIC technique evaluated a solar cell equally. Moreover laser-SQUID microscopy can evaluate a solar cell without contact unlike LBIC. This showed the possibility that the laserSQUID microscope can be applied to evaluation of a solar cells.
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Acknowledgments The authors thank Dr. Xiangyan Kong for designing and making the SQUID. She currently belongs to the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. This research was partially supported by a Grant for Osaka University Global COE Program, ‘‘Center for Electronic Devices Innovation’’, from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] N.M. Thantsha, E.Q.B. Macabebe, F.J. Vorster, F.E. Van Dyk, Physica B 404 (2009) 4445.
[2] K. Nishioka, T. Yagi, Y. Uraoka, T. Fuyuki, Sol. Energy Mater. Sol. Cells 91 (2007) 1. [3] B. Moralejo, M.A. Gonzalez, J. Jimenez, V. Parra, O. Martinez, J. Gutierrez, O. Charro, J. Electron. Mater. 39 (2010) 663. [4] J. Martin, C. Fernandez-Lorenzo, J.A. Poce-Fatou, R. Alcantara, Prog. Photovolt.: Res. Appl. 12 (2004) 283. [5] F.J. Navas, R. Alcantara, C. Fernandez-Lorenzo, J. Martin-Calleja, Rev. Sci. Instrum. 81 (2010) 035108. [6] Y. Mada, Jpn. J. Appl. Phys. 18 (1979) 2171. [7] Y. Yamaguchi, Y. Fujii, A. Usami, T. Wada, K. Matsuki, T. Takeuchi, IEICE Tech. Report SDM93-167 (1993) 95 (In Japanese). [8] J. Beyer, H. Matz, D. Drung, T. Schurig, Appl. Phys. Lett. 74 (1999) 2863. [9] J. Beyer, D. Drung, T. Schurig, IEEE. Trans. Appl. Supercond. 11 (2001) 1162. [10] T. Schurig, J. Beyer, D. Drung, F. Ludwig, A. Ludge, H. Riemann, IEICE. Trans. Electron. E85-C (2002) 665. [11] K. Kojima, S. Suda, X. Kong, H. Itozaki, Physica C 445–448 (2006) 979. [12] X.Y. Kong, K. Kojima, K. Sakuta, H. Itozaki, Physica C 463–465 (2007) 1048.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Laser-SQUID microscope for noncontact evaluation of solar cell Y. Nakatani a,⇑, T. Hayashi b, H. Itozaki a a b
Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan Sendai National College of Technology, 4-16-1, Ayashityuou, Aoba-ku, Sendai, Miyagi 989-3128, Japan
a r t i c l e
i n f o
Article history: Available online 14 May 2011 Keywords: SQUID Microscope Laser Solar cell
a b s t r a c t A polycrystalline solar cell with several grains was investigated by the laser-SQUID (Superconducting QUantum Interference Device) microscope. The laser-SQUID microscopy detects the magnetic field generated by a photo-induced current. This technique enables nondestructive and noncontact evaluation of semiconductor samples. A needle made of high permeability material was used to transmit the magnetic field near the sample to the SQUID. The needle and the SQUID were shifted in the x- and y-directions from the center of a laser spot. The laser-SQUID microscope images varied with needle position. This indicated the possibility of current estimation using a laser-SQUID microscope. In this study the sample was also evaluated using Laser Beam Induced Current (LBIC) which is widely used for evaluation of the conversion efficiency distribution of solar cells. The laser-SQUID microscope image was compared with the LBIC image and was found to be similar. This result showed that laser-SQUID microscopy can be used for the electrical evaluation of solar cells without contact, and furthermore has the possibility of estimation of the photocurrent direction. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The conversion efficiency of the whole solar cell is the sum of the efficiency at each point of the solar cell. It is important to evaluate the distribution of the conversion efficiency over the solar cell surface. Using the localized illumination method is effective for this purpose. The Laser Beam Induced Current (LBIC) technique is widely used for evaluation of solar cells [1–5]. In this method, a laser is focused on the solar cell and the photocurrent is measured at every point on the surface. It is essential to build electrodes for collecting the photocurrent. Non-contact Laser-Beam Induced Current/Conductivity (LBIC) technique had been investigated [6,7]. Using this technique, the distribution of minority carrier lifetime on silicon wafer can be deduced. Instead of measuring the current, measuring the magnetic field from the photocurrent enables contactless measurement. A Superconducting QUantum Interference Device (SQUID) enables high sensitivity measurement of magnetic field. Laser-SQUID microscopy is a method of irradiating the sample with a laser, and measuring the magnetic field from the photocurrent using a SQUID. Doping level fluctuations in a semiconductor wafer were evaluated by a laser-SQUID microscope [8]. The spatial resolution of a laser-SQUID microscope depends
⇑ Corresponding author. Address: Division of Advanced Electronics and Optical Science, Department of Systems Innovation, Graduate School of Engineering Science, Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan. Tel.: +81 6 6850 6313; fax: +81 6 6850 6312. E-mail address: [email protected] (Y. Nakatani). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.171
mainly on the spot size of the laser. The laser-SQUID microscope has been applied to a study of solar cells [9–12]. The distribution of the energy transformation efficiency of the solar cell was evaluated by three different wavelengths [11]. The current vector distribution on the solar cell was estimated using off-axis measurement [12]. However these studies showed only laser-SQUID microscope images, and were not compared with other methodology. We compared the laser-SQUID microscope image with the LBIC image, and investigated the possibility of evaluating the localized properties of the solar cell without contact. 2. Methods 2.1. Laser-SQUID microscope measurement The laser-SQUID microscope [11] is shown schematically in Fig. 1. A semiconductor laser (AlGaAs) (Sigma Koki) was modulated at 1 kHz by a light chopper (5584A, NF Corp., Yokohama, Japan). Its wavelength was 780 nm and power was 2 mW at the sample surface. The laser was focused on the sample surface with diameter of 0.06 mm. A light was used to illuminate the sample surface, and to observe the sample surface by CCD camera (BS-41L, BITLAN). The adjustment of the size of laser spot within 0.06 mm and location on the sample surface were performed before the measurement. A needle made of permalloy 78 (The Nilaco Corp., Tokyo, Japan) was used to transmit the magnetic field at the tip of needle to the SQUID. Though the size of the magnetic sensor does not affect the spatial resolution of the laser-SQUID microscopy, the size of it
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The sample and the SQUID were set in a magnetic shield that was composed of a 3-layer high permeability cylinder. The magnetic field noise level of the SQUID with the needle was less than 1 pT Hz 1/2 at 1 kHz. The laser was modulated with 1 kHz. The signal with the frequency of 1 kHz was extracted by a digital lock-in amplifier (LI5640, NF Corp., Yokohama, Japan). It eliminates the noise from the signal which was unrelated to photocurrent generated by the laser. The time constant of the lock-in amplifier was 1 ms. 2.3. LBIC measurement The laser induced current was measured by using laser and XY scanning of SQUID microscope. The LBIC system used contacts on the solar cell and the current which was extracted outside of the solar cell through the electrodes was measured. In the LBIC measurement, because the short-circuit current should be measured, I–V conversion circuit was used and the signal was input to the lock-in amplifier. 2.4. Sample
Fig. 1. Schematic diagram of laser-SQUID microscope.
needed to be small in order to detect magnetic field at small area. The photocurrent generated inhomogeneous magnetic field and the position of the sensor was important to estimate the current vector. Fig. 2 shows the geometric configuration around the needle (/2.0 mm 7.0 mm). The needle tip of about 0.1 mm was polished mechanically. The needle was set vertically against the sample surface. The position of the needle was 2.1 mm away from a laser spot and 0.42 mm up from the surface because the magnetic field above the laser spot which is generated by the current induced by the laser does not have a vertical component of the field.
The solar cell studied in this work was a polycrystalline silicon solar cell. It was commercially available and already had several electrodes on the surface and its backside was fully covered with metal. The thickness of the metal layer was 0.02–0.13 mm. The laser modulation frequency was 1 kHz. The magnetic signal reduction due to the skin effect was negligible because the frequency and metal layer thickness were small. The size of the sample submitted for the measurement was 2.3 mm 3 mm. The thickness of silicon was 0.27 mm and the total thickness of the solar cell including electrodes was 0.4 mm. There were several grains with a typical size of a few millimeters. The sample holder was a nonmagnetic holder. The holder was mounted on an X–Y stage which was located outside of the magnetic shield. The stage was raster scanned at 0.5 mm/s in the y-direction. The sampling intervals in the x- and y-direction were 10 lm and 0.4 lm, respectively.
2.2. SQUID A SQUID was set above the needle and was mounted at the tip of the sappire rod with liquid nitrogen temperature. They were in a vacuum dewar. The SQUID detected the perpendicular component of the magnetic field generated from the sample. Type of the SQUID was a directly-coupled dc-SQUID and it was made of high transition temperature YBa2Cu3O7–d thin film on a 10 mm 10 mm SrTiO3 substrate. Its pickup loop size was 6.5 mm square. The SQUID was operated by a FFL circuit (PFL-100 operated by PCI1000, STAR Cryoelectronics, USA).
Fig. 2. Geometric configuration around needle.
3. Results and discussion 3.1. Comparison between a laser-SQUID microscope and optical image Fig. 3 shows the optical microscope image of the polycrystalline silicon solar cell. Grain boundary was clearly seen and average size of the grains was a few milimeters. We focused on the white grain which was shown at the center of Fig. 3. The white grain means
Fig. 3. Optical microscope image of the solar cell.
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that the reflection from the grain was higher than that from the other grains. This color was changed from white to black with the different angles of the incident light. This grain seemed to be uniform as far as it was observed by the optical microscope. However, the horizontal-striped pattern in the focused grain was observed which was not grain boundary by the visual inspection but images by saw cutting process. Fig. 4 is the laser-SQUID microscope image of the polycrystalline solar cell at the same area shown in the optical image in Fig. 3. The needle was shifted from the laser spot to y-direction by 2.1 mm in order to detect the magnetic field generated by the current across the laser spot. The z component of the magnetic field generated from the current across the laser spot was zero above the laser spot. Comparing the laserSQUID microscope image carefully with the optical microscope image, many grain boundaries were observed also by the laser-SQUID microscope. However, some grain boundaries were not observed in the laser-SQUID microscope image. In the focused grain, the intensity of the SQUID signal was lowered at several lines in the grains. The intensity of these lines was lower than those from the normal grain area. The wavelength of the laser was 780 nm, and the skin depth of the laser was about 10 lm for silicon. The laser was absorbed in the vicinity of the surface. Therefore, the low intensity of the SQUID signal was due to some kind of electrical defect within 10 lm from the surface. Though there were saw cutting images on the surface of the solar cell, there is no relationship with the signal. These saw cutting images were not electrically active. This signal was due to some kind of electrical defect such as dislocations or sub-grainboundaries.
Fig. 5. Laser-SQUID microscope image of the solar cell. The Needle was shifted to ydirection by 2.1 mm.
3.2. Estimation of photocurrent direction Fig. 5 shows the laser-SQUID microscope image of the solar cell when the needle was shifted from the laser spot in the y-direction by 2.1 mm. The details of the image were similar to the image in Fig. 4, although the sign of the SQUID signal became opposite. The images when the needle was shifted to ±x-direction and not shifted were also obtained (data was not shown). These images showed lower signal intensities than the image in Fig. 4. Therefore, the photocurrent was roughly estimated to flow to x-direction. 3.3. Comparison between a laser SQUID image and LBIC image Fig. 6 is a LBIC image of the same area observed in Fig. 3 and 4. The LBIC image shows the conversion efficiency map of the solar cell. The laser-SQUID microscope images corresponded to the LBIC image, and the laser-SQUID microscope images also indicated the
Fig. 6. LBIC image of the solar cell.
conversion efficiency of the solar cell. The grain boundaries seen in the optical image were observed in the LBIC image as well as the SQUID images. Low intensity lines were observed inside the grain. The LBIC image indicates the conversion efficiency and that there must be defects that could not be seen by the optical microscope inside this grain. The laser-SQUID microscope also detected these lines. This showed that laser-SQUID microscopy can be used for the detection of electrically active defects as well as the LBIC technique. Furthermore, the laser-SQUID microscope achieved contactless evaluation of the solar cell.
4. Conclusions
Fig. 4. Laser-SQUID microscope image of the solar cell. The Needle was shifted in the y-direction by +2.1 mm.
The polycrystalline silicon solar cell was evaluated by both the laser-SQUID microscopy and the LBIC technique. The laser-SQUID microscope image corresponded to the LBIC image which indicated the conversion efficiency distribution. A defect which could not be seen by optical microscope could be detected by the laser-SQUID microscope as well as the LBIC technique. Using the laser-SQUID microscope, the direction of the photocurrent was estimated by shifting the needle. The result showed that the laser-SQUID microscopy and LBIC technique evaluated a solar cell equally. Moreover laser-SQUID microscopy can evaluate a solar cell without contact unlike LBIC. This showed the possibility that the laserSQUID microscope can be applied to evaluation of a solar cells.
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Y. Nakatani et al. / Physica C 471 (2011) 1249–1252
Acknowledgments The authors thank Dr. Xiangyan Kong for designing and making the SQUID. She currently belongs to the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. This research was partially supported by a Grant for Osaka University Global COE Program, ‘‘Center for Electronic Devices Innovation’’, from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] N.M. Thantsha, E.Q.B. Macabebe, F.J. Vorster, F.E. Van Dyk, Physica B 404 (2009) 4445.
[2] K. Nishioka, T. Yagi, Y. Uraoka, T. Fuyuki, Sol. Energy Mater. Sol. Cells 91 (2007) 1. [3] B. Moralejo, M.A. Gonzalez, J. Jimenez, V. Parra, O. Martinez, J. Gutierrez, O. Charro, J. Electron. Mater. 39 (2010) 663. [4] J. Martin, C. Fernandez-Lorenzo, J.A. Poce-Fatou, R. Alcantara, Prog. Photovolt.: Res. Appl. 12 (2004) 283. [5] F.J. Navas, R. Alcantara, C. Fernandez-Lorenzo, J. Martin-Calleja, Rev. Sci. Instrum. 81 (2010) 035108. [6] Y. Mada, Jpn. J. Appl. Phys. 18 (1979) 2171. [7] Y. Yamaguchi, Y. Fujii, A. Usami, T. Wada, K. Matsuki, T. Takeuchi, IEICE Tech. Report SDM93-167 (1993) 95 (In Japanese). [8] J. Beyer, H. Matz, D. Drung, T. Schurig, Appl. Phys. Lett. 74 (1999) 2863. [9] J. Beyer, D. Drung, T. Schurig, IEEE. Trans. Appl. Supercond. 11 (2001) 1162. [10] T. Schurig, J. Beyer, D. Drung, F. Ludwig, A. Ludge, H. Riemann, IEICE. Trans. Electron. E85-C (2002) 665. [11] K. Kojima, S. Suda, X. Kong, H. Itozaki, Physica C 445–448 (2006) 979. [12] X.Y. Kong, K. Kojima, K. Sakuta, H. Itozaki, Physica C 463–465 (2007) 1048.