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β-FeSi2-base MIS diodes fabricated by sputtering method
Applied Surface Science 175±176 (2001) 96±100
b-FeSi2-base MIS diodes fabricated by sputtering method Takashi Ehara*, Yuko Sasaki, Kaname Saito, Shinji Nakagomi, Yoshihiro Kokubun Department of Electronic Materials, School of Science and Engineering, Ishinomaki Senshu University, Shinmito 1, Minamisakai, Ishinomaki, Miyagi 986-8580, Japan Accepted 15 November 2000
Abstract In this paper, fabrication of beta-ironsilicide (b-FeSi2)-base metal±insulator±semiconductor diode devices is described. bFeSi2 ®lms have been prepared by co-sputtering of Fe and Si followed by thermal annealing. The [2 0 2] X-ray diffraction peak of b-FeSi2 was observed at 2y 29 when both the Fe±Si chemical composition and annealing temperature were optimized. The prepared b-FeSi2 ®lms have been thermally oxidized in an O2 gas atmosphere. Using the techniques above, we fabricated Al/oxidized-FeSi2/b-FeSi2/p-Si structured devices, which displayed diode properties. # 2001 Elsevier Science B.V. All rights reserved. PACS: 73.40.Qv; 75.50.Bb; 81.15.Cd Keywords: b-FeSi2; MIS diode; Sputtering; Oxidization; Thin ®lms
1. Introduction b-FeSi2 has gathered much interest because of its electronic properties as a semiconductor with a direct band-gap of about 0.89 eV. This value is suitable for light detectors, optoelectronic applications and optical ®ber communication systems [1,2]. b-FeSi2 thin ®lms have been prepared by various methods, although most of the b-FeSi2 reported are polycrystalline. Most of the preparation methods utilize solid-phase reactions. For example, formation of b-FeSi2 layers on Si substrates by reactive deposition epitaxy was reported [3,4]. b-FeSi2 layers formed by the thermal reaction of a Si substrate, a Si layer with a Fe layer have also been
*
Corresponding author. Tel.: 81-225-22-7716, ext.: 3177; fax: 81-225-22-7746. E-mail address: [email protected] (T. Ehara).
reported [5]. In their reports, the formation of a bFeSi2 layer has been con®rmed by X-ray diffraction and the properties of the ®lms have shown a dependency on the reaction temperature. As an another preparation method, co-sputtering from a Si target and Fe chips placed on the Si target has also been reported. The method is adequate to prepare ®lms with various chemical compositions by changing the amount of Fe chips [6]. In contrast, device applications of b-FeSi2 have not been well studied. Although the electronic properties of b-FeSi2/Si heterojunction have been reported, bFeSi2 homojunction or metal±insulator±semiconductor (MIS) devices have not been well studied, because doping and oxidation of b-FeSi2 ®lms have not been established. In the present work, we report on the fabrication technique of Al/oxidized-FeSi2/b-FeSi2/ p-Si MIS structures. Properties of the oxidized b-FeSi2 as an insulating layer are discussed.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 0 3 9 - 3
T. Ehara et al. / Applied Surface Science 175±176 (2001) 96±100
2. Experimental The FeSi2 ®lms were prepared by the RF co-sputtering method and thermal annealing. Fe chips, 10 mm 10 mm 1 mm in size were placed on a Si target (100 mm in diameter, purity of 99.999%) and co-sputtered. The ®lms were deposited at an RF power of 100 W (13.56 MHz) in Ar at a sputtering pressure of 1.33 Pa for 2 h using an HSR551 sputtering system (Shimadzu). The ®lms were sputtered on (1 0 0)-oriented Si substrates with two types of conductivities and on SiO2 substrates. Highly p-typedoped substrates (less than 0.018 O cm) were used for fabricating devices, while undoped (high resistivity) substrates were employed for the layer characterization by infrared (IR) absorption spectroscopy. Samples prepared on the SiO2 substrates were used for X-ray diffraction and conductivity measurements. Thermal annealing for 1 h was carried out at temperature between 700 and 10008C in a nitrogen atmosphere to form the FeSi2 structure. The crystal quality of the samples was studied by X-ray diffraction using CuKa radiation. MIS diodes were fabricated as follows. The b-FeSi2 samples were oxidized in a furnace at 6508C for various time periods ranging from 0.5 to 2 h. The properties of the oxide layers were studied using IR absorption spectra. The IR spectra were obtained using a Fourier transfer infrared (FT-IR) spectrometer (Shimadzu 8100A). After the thermal oxidation, backside oxide layer were removed using hydro¯uoric acid and backside Al electrodes were formed by vacuum evaporation and followed by annealing at 5008C for 10 min. Subsequently, Al dot electrodes 0.5 mm in diameter were formed on the oxide layer by vacuum evaporation. I±V characteristic curves of the devices were obtained using TT-506 curve tracer (Iwatsu). 3. Results and discussion 3.1. FeSi2 ®lm preparation In the present work, the b-FeSi2 ®lm preparation condition is very important. Thus, the optimization of the FeSi2 ®lm preparation was performed ®rst. The chemical composition was optimized by changing the amount of the Fe chips on the Si target. In Fig. 1, X-ray
97
diffraction patterns of samples prepared using various amounts of Fe chips and annealed at 8008C are shown. For samples prepared use of two Fe chips, very wide diffraction peak was observed near 2y 29. The peak was identi®ed as the overlap of the b-FeSi2 (2 0 2) [5] and Si (1 1 1) located at 298 and 278, respectively [7]. This result indicates that using two Fe chips is insuf®cient for the co-sputtering of FeSi2. In contrast, the X-ray diffraction patterns of samples prepared using six or eight Fe chips display peaks of FeSi at 2y 34 (1 1 1) and 468 (2 1 0). The results suggest the existence of excess Fe in the ®lms. As a result, we conclude that four Fe chips are adequate for the preparation of FeSi2 ®lms. In Fig. 2, X-ray diffraction patterns of co-sputtered ®lms using four Fe chips on the Si target annealed at various temperatures are shown. For the sample annealed at 700, 800, and 9008C, the (2 0 2) and (4 0 2) diffraction peak of b-FeSi2 is observed at 2y 29 and 498, respectively. Crystallite sizes extracted from the width of the b-FeSi2 (2 0 2) peak were 28, 31, and 33 nm at 700, 800, and 9008C, respectively. Only a very weak b-FeSi2 (2 0 2) peak was observed for samples annealed at 6008C. The result indicates that the annealing temperature of 6008C was not high enough to form crystalline ®lms. The X-ray diffraction pattern changed signi®cantly with increased annealing temperatures. The sample annealed at 10008C showed the diffraction peaks of aFeSi2 at 388 (1 0 1) and 488 (1 1 0), indicating a change in ®lm structure from b-FeSi2 to a-FeSi2. We conclude that annealing temperature between 700 and 9008C are required to prepare crystalline b-FeSi2 ®lms. In Fig. 3, the surface of the samples annealed at 700 and 8008C are shown in Nomarski micrograph. As shown in the ®gure, cracks appeared at the surface of the sample annealed at 8008C. It is believed that the cracks are caused during the thermal annealing by the difference of the thermal expansion coef®cient between the Si substrate and b-FeSi2 ®lms. The ®lm that has a lot of cracks on its surface is not adequate for the device fabrications. Films annealed at 8008C have peeled off after the oxidation. In contrast, the sample annealed at 7008C showed no cracks as far as observed by a Nomarski microscope. According to the results shown above, the annealing temperature adequate for the preparation of the b-FeSi2 ®lms was 7008C. The
98
T. Ehara et al. / Applied Surface Science 175±176 (2001) 96±100
Fig. 1. X-ray diffraction patterns of ®lms co-sputtered from a Si target with various amounts of Fe chips. The samples have been annealed at 8008C for 1 h.
Fig. 2. X-ray diffraction patterns of ®lms co-sputtered from a Si target and four Fe chips annealed at 700, 800, 900, and 10008C.
T. Ehara et al. / Applied Surface Science 175±176 (2001) 96±100
99
Fig. 3. Surface images of the samples annealed at (a) 7008C and (b) 8008C taken with a Nomarski microscope.
®lms prepared under optimized preparation conditions experienced an optical band-gap and resistivity of bFeSi2, 0.85±0.9 eV and 0.025 O cm, respectively. The resistivity measured in the present work is consistent with the samples reported previously [6].
the peak at 1080 cm 1 increased with oxidation time. After 1 h of oxidation, the peak intensity was 1.3 times higher, as compared to the peak intensity after 30 min. We assigned this peak to an oxide layer on the ®lm surface. Due to this behavior, we conclude that the
3.2. Fabrication of MIS diodes Fig. 4 depicts the IR transmission spectra of the samples before and after the thermal oxidation of the b-FeSi2 ®lms. Although we measured as-sputtered sample, no peaks were observed. In the ®gure, the scans of the samples oxidized for 0.5 and 1 h at 6508C are shown. The ®lms used for oxidation were prepared by co-sputtering of four Fe chips on a Si target, followed by annealing at 7008C in nitrogen. For the annealed sample, a weak peak was observed at 1050 cm 1. This wavenumber corresponds to the stretching mode of Si±O±Si, possibly due to an oxide formed by residual oxygen during annealing. The IR transmission spectrum of the annealed sample showed no further peaks, while it changed signi®cantly after thermal oxidation, where a strong peak appeared at 1080 cm 1. The observed peak position is consistent with the IR spectra of the SiOx x < 2 ®lms prepared by sputtering [8]. The presence of SiOx and absence of Fe-related peaks suggest that the Fe atoms in the ®lms do not affect the IR spectra. We also observed a dependency on the oxidation time. The intensity of
Fig. 4. IR transmission spectra of annealed and oxidized samples.
100
T. Ehara et al. / Applied Surface Science 175±176 (2001) 96±100
was due to a low current under both forward and reverse biases. Most likely, the surface oxide layer was too thick to allow for tunneling currents. These results indicate that oxidized FeSi2 layers behave much like insulators. However, the diode devices fabricated in the present work showed a poor stability against voltage, and the yield was rather low. The authors believe that the existence of Fe atoms in the ®lms and the structural disorder of the oxide layer may be the cause of the poor stability of the devices. Further optimization of the device fabrication is required to enable a detailed study of the electronic properties of the MIS diode devices. 4. Conclusions
Fig. 5. I±V characteristics of an MIS diode sample prepared by oxidation for 1 h.
oxidation of the FeSi2 ®lms can be achieved by this method. After the optimization of the annealing and oxidization step, we prepared MIS diode devices and measured its I±V characteristics. In Fig. 5, typical dark I±V characteristic curve of the devices is depicted. The devices have been prepared under optimized b-FeSi2 ®lm preparation conditions, co-sputtering of four Fe chips on a Si target for 2 h, followed by annealing at 7008C for 1 h. Thickness of the annealed ®lm was 1.32 mm measured by a DEKTAK stylus pro®le. The ®lms were thermally oxidized for 1 h at 6508C. As shown in the ®gure, typical rectifying diode characteristics were observed, showing that the formation of a thin enough oxide layer for MIS diode devices was possible by thermally oxidizing the surface for 1 h. In contrast, no rectifying behavior was observed using a curve tracer for samples thermally oxidized for 2 h. The absence of diode characteristics
We optimized the deposition and annealing conditions to yield crystalline b-FeSi2 on p-type (1 0 0) Si substrates using X-ray diffraction and FT-IR spectroscopy. Films prepared by co-sputtering of four Fe chips on a Si target in 1.33 Pa of Ar, annealed at 7008C for 30 min in N2, and oxidized at 6508C revealed the lowest amount of visible defects and were utilized in MIS diodes. The diodes showed good rectifying behavior during current±voltage measurement. References [1] M.C. Bost, J.E. Mahan, J. Appl. Phys. 58 (1985) 2696. [2] M.C. Bost, J.E. Mahan, J. Vac. Sci. Technol. B 4 (1986) 1336. [3] K.M. Geib, J.E. Mahan, R.G. Long, M. Nathan, G. Bai, J. Appl. Phys. 70 (1991) 1730. [4] A.H. Reader, J.P.W.B. Duchateaun, J. Timmers, F.J.G. Hakkens, Appl. Surf. Phys. 73 (1993) 131. [5] M. Tanaka, Y. Kumagai, T. Suemasu, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) 3620. [6] M. Komabayashi, K. Hijikata, S. Ido, Jpn. J. Appl. Phys. 29 (1990) 1118. [7] T. Ehara, T. Nagasawa, Mater. Lett. 44 (2000) 223. [8] T. Ehara, S. Machida, Thin Solid Films 346 (1999) 275.
b-FeSi2-base MIS diodes fabricated by sputtering method Takashi Ehara*, Yuko Sasaki, Kaname Saito, Shinji Nakagomi, Yoshihiro Kokubun Department of Electronic Materials, School of Science and Engineering, Ishinomaki Senshu University, Shinmito 1, Minamisakai, Ishinomaki, Miyagi 986-8580, Japan Accepted 15 November 2000
Abstract In this paper, fabrication of beta-ironsilicide (b-FeSi2)-base metal±insulator±semiconductor diode devices is described. bFeSi2 ®lms have been prepared by co-sputtering of Fe and Si followed by thermal annealing. The [2 0 2] X-ray diffraction peak of b-FeSi2 was observed at 2y 29 when both the Fe±Si chemical composition and annealing temperature were optimized. The prepared b-FeSi2 ®lms have been thermally oxidized in an O2 gas atmosphere. Using the techniques above, we fabricated Al/oxidized-FeSi2/b-FeSi2/p-Si structured devices, which displayed diode properties. # 2001 Elsevier Science B.V. All rights reserved. PACS: 73.40.Qv; 75.50.Bb; 81.15.Cd Keywords: b-FeSi2; MIS diode; Sputtering; Oxidization; Thin ®lms
1. Introduction b-FeSi2 has gathered much interest because of its electronic properties as a semiconductor with a direct band-gap of about 0.89 eV. This value is suitable for light detectors, optoelectronic applications and optical ®ber communication systems [1,2]. b-FeSi2 thin ®lms have been prepared by various methods, although most of the b-FeSi2 reported are polycrystalline. Most of the preparation methods utilize solid-phase reactions. For example, formation of b-FeSi2 layers on Si substrates by reactive deposition epitaxy was reported [3,4]. b-FeSi2 layers formed by the thermal reaction of a Si substrate, a Si layer with a Fe layer have also been
*
Corresponding author. Tel.: 81-225-22-7716, ext.: 3177; fax: 81-225-22-7746. E-mail address: [email protected] (T. Ehara).
reported [5]. In their reports, the formation of a bFeSi2 layer has been con®rmed by X-ray diffraction and the properties of the ®lms have shown a dependency on the reaction temperature. As an another preparation method, co-sputtering from a Si target and Fe chips placed on the Si target has also been reported. The method is adequate to prepare ®lms with various chemical compositions by changing the amount of Fe chips [6]. In contrast, device applications of b-FeSi2 have not been well studied. Although the electronic properties of b-FeSi2/Si heterojunction have been reported, bFeSi2 homojunction or metal±insulator±semiconductor (MIS) devices have not been well studied, because doping and oxidation of b-FeSi2 ®lms have not been established. In the present work, we report on the fabrication technique of Al/oxidized-FeSi2/b-FeSi2/ p-Si MIS structures. Properties of the oxidized b-FeSi2 as an insulating layer are discussed.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 0 3 9 - 3
T. Ehara et al. / Applied Surface Science 175±176 (2001) 96±100
2. Experimental The FeSi2 ®lms were prepared by the RF co-sputtering method and thermal annealing. Fe chips, 10 mm 10 mm 1 mm in size were placed on a Si target (100 mm in diameter, purity of 99.999%) and co-sputtered. The ®lms were deposited at an RF power of 100 W (13.56 MHz) in Ar at a sputtering pressure of 1.33 Pa for 2 h using an HSR551 sputtering system (Shimadzu). The ®lms were sputtered on (1 0 0)-oriented Si substrates with two types of conductivities and on SiO2 substrates. Highly p-typedoped substrates (less than 0.018 O cm) were used for fabricating devices, while undoped (high resistivity) substrates were employed for the layer characterization by infrared (IR) absorption spectroscopy. Samples prepared on the SiO2 substrates were used for X-ray diffraction and conductivity measurements. Thermal annealing for 1 h was carried out at temperature between 700 and 10008C in a nitrogen atmosphere to form the FeSi2 structure. The crystal quality of the samples was studied by X-ray diffraction using CuKa radiation. MIS diodes were fabricated as follows. The b-FeSi2 samples were oxidized in a furnace at 6508C for various time periods ranging from 0.5 to 2 h. The properties of the oxide layers were studied using IR absorption spectra. The IR spectra were obtained using a Fourier transfer infrared (FT-IR) spectrometer (Shimadzu 8100A). After the thermal oxidation, backside oxide layer were removed using hydro¯uoric acid and backside Al electrodes were formed by vacuum evaporation and followed by annealing at 5008C for 10 min. Subsequently, Al dot electrodes 0.5 mm in diameter were formed on the oxide layer by vacuum evaporation. I±V characteristic curves of the devices were obtained using TT-506 curve tracer (Iwatsu). 3. Results and discussion 3.1. FeSi2 ®lm preparation In the present work, the b-FeSi2 ®lm preparation condition is very important. Thus, the optimization of the FeSi2 ®lm preparation was performed ®rst. The chemical composition was optimized by changing the amount of the Fe chips on the Si target. In Fig. 1, X-ray
97
diffraction patterns of samples prepared using various amounts of Fe chips and annealed at 8008C are shown. For samples prepared use of two Fe chips, very wide diffraction peak was observed near 2y 29. The peak was identi®ed as the overlap of the b-FeSi2 (2 0 2) [5] and Si (1 1 1) located at 298 and 278, respectively [7]. This result indicates that using two Fe chips is insuf®cient for the co-sputtering of FeSi2. In contrast, the X-ray diffraction patterns of samples prepared using six or eight Fe chips display peaks of FeSi at 2y 34 (1 1 1) and 468 (2 1 0). The results suggest the existence of excess Fe in the ®lms. As a result, we conclude that four Fe chips are adequate for the preparation of FeSi2 ®lms. In Fig. 2, X-ray diffraction patterns of co-sputtered ®lms using four Fe chips on the Si target annealed at various temperatures are shown. For the sample annealed at 700, 800, and 9008C, the (2 0 2) and (4 0 2) diffraction peak of b-FeSi2 is observed at 2y 29 and 498, respectively. Crystallite sizes extracted from the width of the b-FeSi2 (2 0 2) peak were 28, 31, and 33 nm at 700, 800, and 9008C, respectively. Only a very weak b-FeSi2 (2 0 2) peak was observed for samples annealed at 6008C. The result indicates that the annealing temperature of 6008C was not high enough to form crystalline ®lms. The X-ray diffraction pattern changed signi®cantly with increased annealing temperatures. The sample annealed at 10008C showed the diffraction peaks of aFeSi2 at 388 (1 0 1) and 488 (1 1 0), indicating a change in ®lm structure from b-FeSi2 to a-FeSi2. We conclude that annealing temperature between 700 and 9008C are required to prepare crystalline b-FeSi2 ®lms. In Fig. 3, the surface of the samples annealed at 700 and 8008C are shown in Nomarski micrograph. As shown in the ®gure, cracks appeared at the surface of the sample annealed at 8008C. It is believed that the cracks are caused during the thermal annealing by the difference of the thermal expansion coef®cient between the Si substrate and b-FeSi2 ®lms. The ®lm that has a lot of cracks on its surface is not adequate for the device fabrications. Films annealed at 8008C have peeled off after the oxidation. In contrast, the sample annealed at 7008C showed no cracks as far as observed by a Nomarski microscope. According to the results shown above, the annealing temperature adequate for the preparation of the b-FeSi2 ®lms was 7008C. The
98
T. Ehara et al. / Applied Surface Science 175±176 (2001) 96±100
Fig. 1. X-ray diffraction patterns of ®lms co-sputtered from a Si target with various amounts of Fe chips. The samples have been annealed at 8008C for 1 h.
Fig. 2. X-ray diffraction patterns of ®lms co-sputtered from a Si target and four Fe chips annealed at 700, 800, 900, and 10008C.
T. Ehara et al. / Applied Surface Science 175±176 (2001) 96±100
99
Fig. 3. Surface images of the samples annealed at (a) 7008C and (b) 8008C taken with a Nomarski microscope.
®lms prepared under optimized preparation conditions experienced an optical band-gap and resistivity of bFeSi2, 0.85±0.9 eV and 0.025 O cm, respectively. The resistivity measured in the present work is consistent with the samples reported previously [6].
the peak at 1080 cm 1 increased with oxidation time. After 1 h of oxidation, the peak intensity was 1.3 times higher, as compared to the peak intensity after 30 min. We assigned this peak to an oxide layer on the ®lm surface. Due to this behavior, we conclude that the
3.2. Fabrication of MIS diodes Fig. 4 depicts the IR transmission spectra of the samples before and after the thermal oxidation of the b-FeSi2 ®lms. Although we measured as-sputtered sample, no peaks were observed. In the ®gure, the scans of the samples oxidized for 0.5 and 1 h at 6508C are shown. The ®lms used for oxidation were prepared by co-sputtering of four Fe chips on a Si target, followed by annealing at 7008C in nitrogen. For the annealed sample, a weak peak was observed at 1050 cm 1. This wavenumber corresponds to the stretching mode of Si±O±Si, possibly due to an oxide formed by residual oxygen during annealing. The IR transmission spectrum of the annealed sample showed no further peaks, while it changed signi®cantly after thermal oxidation, where a strong peak appeared at 1080 cm 1. The observed peak position is consistent with the IR spectra of the SiOx x < 2 ®lms prepared by sputtering [8]. The presence of SiOx and absence of Fe-related peaks suggest that the Fe atoms in the ®lms do not affect the IR spectra. We also observed a dependency on the oxidation time. The intensity of
Fig. 4. IR transmission spectra of annealed and oxidized samples.
100
T. Ehara et al. / Applied Surface Science 175±176 (2001) 96±100
was due to a low current under both forward and reverse biases. Most likely, the surface oxide layer was too thick to allow for tunneling currents. These results indicate that oxidized FeSi2 layers behave much like insulators. However, the diode devices fabricated in the present work showed a poor stability against voltage, and the yield was rather low. The authors believe that the existence of Fe atoms in the ®lms and the structural disorder of the oxide layer may be the cause of the poor stability of the devices. Further optimization of the device fabrication is required to enable a detailed study of the electronic properties of the MIS diode devices. 4. Conclusions
Fig. 5. I±V characteristics of an MIS diode sample prepared by oxidation for 1 h.
oxidation of the FeSi2 ®lms can be achieved by this method. After the optimization of the annealing and oxidization step, we prepared MIS diode devices and measured its I±V characteristics. In Fig. 5, typical dark I±V characteristic curve of the devices is depicted. The devices have been prepared under optimized b-FeSi2 ®lm preparation conditions, co-sputtering of four Fe chips on a Si target for 2 h, followed by annealing at 7008C for 1 h. Thickness of the annealed ®lm was 1.32 mm measured by a DEKTAK stylus pro®le. The ®lms were thermally oxidized for 1 h at 6508C. As shown in the ®gure, typical rectifying diode characteristics were observed, showing that the formation of a thin enough oxide layer for MIS diode devices was possible by thermally oxidizing the surface for 1 h. In contrast, no rectifying behavior was observed using a curve tracer for samples thermally oxidized for 2 h. The absence of diode characteristics
We optimized the deposition and annealing conditions to yield crystalline b-FeSi2 on p-type (1 0 0) Si substrates using X-ray diffraction and FT-IR spectroscopy. Films prepared by co-sputtering of four Fe chips on a Si target in 1.33 Pa of Ar, annealed at 7008C for 30 min in N2, and oxidized at 6508C revealed the lowest amount of visible defects and were utilized in MIS diodes. The diodes showed good rectifying behavior during current±voltage measurement. References [1] M.C. Bost, J.E. Mahan, J. Appl. Phys. 58 (1985) 2696. [2] M.C. Bost, J.E. Mahan, J. Vac. Sci. Technol. B 4 (1986) 1336. [3] K.M. Geib, J.E. Mahan, R.G. Long, M. Nathan, G. Bai, J. Appl. Phys. 70 (1991) 1730. [4] A.H. Reader, J.P.W.B. Duchateaun, J. Timmers, F.J.G. Hakkens, Appl. Surf. Phys. 73 (1993) 131. [5] M. Tanaka, Y. Kumagai, T. Suemasu, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) 3620. [6] M. Komabayashi, K. Hijikata, S. Ido, Jpn. J. Appl. Phys. 29 (1990) 1118. [7] T. Ehara, T. Nagasawa, Mater. Lett. 44 (2000) 223. [8] T. Ehara, S. Machida, Thin Solid Films 346 (1999) 275.