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Growth condition dependence of direct bandgap in β-FeSi2 epitaxial films grown by molecular beam epitaxy
Available online at www.sciencedirect.com
Physics Procedia 0 (2011) 000–000 Physics Procedia 23 (2012) 5 – 8
Physics Procedia www.elsevier.com/locate/procedia
Asian School-Conference on Physics and Technology of Nanostructured Materials (ASCO-NANOMAT 2011)
Growth condition dependence of direct bandgap in β-FeSi2 epitaxial films grown by molecular beam epitaxy K. Nodaa, Y. Teraia,*, N. Miuraa, H. Udonob, Y. Fujiwaraa a
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan b Graduate School of Science and Engineering, Ibaraki University, 4-12-1 Nakanarusawa Hitachi, Ibaraki 316-8511, Japan
Abstract Direct bandgap energy (Eg) and lattice deformations were investigated in β-FeSi2 epitaxial films grown by molecular beam epitaxy (MBE) with different growth condition. As Si/Fe flux ratio during the MBE growth became smaller than Si/Fe = 2.0, the lattice constants deviated from those of β-FeSi2 single crystal, which indicated an enhanced lattice deformation at the lower Si/Fe ratio. In photoreflectance (PR) measurements, the PR spectra shifted to lower photon energy with the enhanced lattice deformation. These results revealed that the Eg of β-FeSi2 epitaxial film was modified by the lattice deformation depending on the growth condition.
© B.V. Selection and/or peer-review under responsibility of Publication ©2011 2011Published PublishedbybyElsevier Elsevier Ltd. Selection and/or peer-review under responsibility of Guest Committee of ASCO-NANOMAT 2011 andCommittee Far EasternofFederal University (FEFU) Editors of Physics Procedia, Publication ASCO-NANOMAT 2011 Keywords: β-FeSi2; iron silicides; photoreflectance; bandgap modification; band engineering
1. Introduction Semiconducting β-FeSi2 thin films on Si substrates have attracted much interest as silicon-based optoelectronics materials as it shows photoluminescence (PL) at 1.54 μm [1]. For the application to Si-based light emitter using β-FeSi2, a technique to enhance the 1.54 μm-PL intensity should be developed. In PL and photoreflectance (PR) studies in the β-FeSi2 thin films, the 1.54 μm-PL was found to be due to the indirect transition [1-5]. On the other hand, the theoretical calculations have predicted a band-structure change from indirect to direct transition by the stress at the heteroepitaxial interface of β-FeSi2/Si [6]. Therefore, it is necessary to investigate the dependence of band structure on the stress in the β-FeSi2 epitaxial films on Si. Recently, we have investigated * Corresponding author. Tel.: +8-166-879-7548; fax:+8-166-879-7536. E-mail address: [email protected].
1875-3892 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Publication Committee of ASCO-NANOMAT 2011 and Far Eastern Federal University (FEFU) doi:10.1016/j.phpro.2012.01.002
6 2
K. et Noda al. / Physics Procedia 23 (2012) 5–8 K. Noda al./ et Physics Procedia 00 (2011) 000–000
lattice deformation and direct bandgap (Eg) in β-FeSi2 epitaxial films on Si(111) and Si(001) substrates [2,3]. As a result, the change of Eg was successfully observed when the lattice deformation was induced by thermal annealing. This is the first experimental results which show the band-structure change by the strain at the β-FeSi2/Si interface. In this study, we fabricated the β-FeSi2 epitaxial films on Si(001) substrates with different Si/Fe flux ratio. In the films, the lattice deformation and Eg were investigated to confirm the bandstructure change depending on the Si/Fe ratio.
2. Experiments β-FeSi2 epitaxial films were grown on n-type floating-zone Si(001) substrate (ρ = 2.0-3.0 kΩ cm) by MBE using template technique. After the cleaning by the RCA method, the substrates were heated to 800°C for 30 min to remove the surface oxide layer. A 24 nm Si buffer layer was grown at 550°C, and then, a 20-nm-thick (100)-oriented β-FeSi2 epitaxial template was formed by reactive deposition epitaxy (RDE) at 550 °C. After that, a 130-nm-thick β-FeSi2 epitaxial film was grown on the template by co-deposition of Si and Fe at 550 °C. During the MBE growth, the Si/Fe flux ratio was changed from 0.4 to 2.0 by controlling deposition rate of Si. The crystal structure of the films was analyzed by high-resolution X-ray diffraction (HRXRD : X’pert Pro, PANalytical) at room temperature. The crystal orientation was confirmed by the conventional 2θ-θ scan. The quality of the films was evaluated by the full width at half maximum (FWHM) of the rocking curve of the β-FeSi2(800) peak. The lattice constant of a-axis was obtained by the β-FeSi2(800) diffraction peak position, and the average lattice constant of b- and c-axis was obtained by the βFeSi2 (220)(202) peaks . Detailed direct bandgap energies were obtained by PR spectra measured at 5.5 K. In PR measurements, a halogen lamp in conjunction with a single grating monochromator was used as a probe source. The pump source was 532 nm laser mechanically chopped at a frequency of 140 Hz. The PR spectra (ΔR/R) were expressed as the ratio of a small reflectance change (ΔR) caused by the modulation laser to the reflectance (R). The ΔR/R was detected by an InGaAs photodiode. The PR lineshape at low-electric-field could be expressed by a generalized Lorentzian function called the Aspnes third derivative functional form [7], n i R / R Re C j e j ( E E j iΓ j ) m (1) j 1 where Ej is direct bandgap energy, Γj is broadening parameter at interband transition. Cj and θj are amplitude and phase factor, respectively. The exponent m depends on the type of the transition. In this study, m is fixed at 2.5 for a three-dimensional band-to-band transition.
3. Results and discussion In 2θ-θ XRD measurements, all the samples grown at the different Si/Fe ratio showed the epitaxial growth of β-FeSi2(100)//Si(001). In rocking curves corresponding to β-FeSi2(800) diffraction peak, FWHM was 1.76° at Si/Fe = 2.0. The FWHM became small at lower Si/Fe ratio. These results indicate that the highest crystalline orientation is achieved at the lowest Si/Fe ratio of 0.5. Figure 1 shows the dependence of (a) the lattice constant in a-axis, (b) the average lattice constant in b-,caxis on the Si/Fe ratio. As the Si/Fe ratio decreased, the lattice constant in a-axis expanded and that in b, c-axis shrunk. The dependence of lattice constants on Si/Fe ratio clearly shows that the lattice deformation depends on the Si/Fe ratio during the MBE growth. In comparison with the lattice constants of β-FeSi2 single crystal (abulk = 9.881 Å, b, cbulk = 7.824 Å), the difference between the epitaxial films and the single crystal was largest at Si/Fe = 0.5. These results show the enhanced lattice deformation at the lower Si/Fe ratio.
Nodaetetal. al./ PhysicsProcedia Procedia23 00(2012) (2011)5000–000 K.K.Noda / Physics –8
73
In order to investigate the effect of lattice deformation on band-structure change, we conducted the PR measurements. In Fig.2, PR spectra of β-FeSi2 epitaxial films and the single crystal are shown. In the single crystal, the direct bandgap of 0.938 eV was obtained by a least-square fitting using Aspnes equation (1). In the epitaxial films, two direct bandgap energies defined as E1, E2 were obtained [3]. In the sample grown at Si/Fe = 2.0, the E1 and E2 were 0.934 eV and 0.917 eV respectively. These direct bandgap energies were smaller than that in the single crystal. As the Si/Fe ratio decreased, both E1 and E2 shifted to lower energy. These results revealed that the direct bandgap was shifted by the enhanced lattice deformation depending on the Si/Fe ratio. In Fig. 3, the obtained E1 and E2 are plotted as a function of variation from the lattice constant of the single crystal in a-axis (Δa/abulk). The circle points (●:E1, ○:E2) represent the direct bandgap energies obtained in Fig. 2. The square points (■:E1, □:E2) are the results of the annealing-induced lattice deformation in previous reports [3]. As seen in the figure, the direct bandgap energies of E1 and E2 in both cases shifted to lower energy as Δa/abulk increased. Therefore, it was found that there is an universal relationship between the direct bandgap energy and the lattice deformation although the origin of the lattice deformation is different. Migas et al. have calculated the band structure of strain-free and highly-strained β-FeSi2 [6]. In the strain-free β-FeSi2 (single crystal), the band structure is indirect transition type. As a large lattice deformation of Δa/abulk = +8.0% Δb/bbulk = +2.3%, and Δc/cbulk = –1.9 % (lattice volume is constant) is introduced, the direct transition energy at Y-point in the Brillouin zone decreases from 0.7 eV to 0.25 eV. As a result of the bandgap modification, the band structure changes to direct transition type. The calculated result could not be quantitatively compared with experimental results of Fig. 3 because the lattice deformations in this study was much small and the lattice volume changed with Si/Fe ratio. However, the shift of Eg in Fig. 3 is qualitatively consistent with a common feature of the theoretical results that the conduction band shifts to lower energy by the lattice deformations [6,8]. Therefore, these results convince us that the band structure of β-FeSi2 epitaxial film is modified by the lattice deformation. As described before, the highest crystalline orientation of β-FeSi2(100)//Si(001) is achieved at Si/Fe = 0.5. The result indicates that the stoicheiometry of Si/Fe = 2 is accomplished at the Si/Fe flux ratio of 0.5, because Si atoms are also supplied by thermal diffusion from the heated Si substrate in addition to the Si supply at the growth surface. In the case of Si/Fe ratio > 0.5, it is considered that the excess Si atoms exist as interstitial atoms or Si aggregation. The origin of the lattice deformation depending on the Si/Fe ratio is unclear at present, but there is a possibility that the excess Si atoms affect the lattice deformation. 㻌
abulk = 9.881Å
9.87 9.86
b,c㻌(Å)
7.85 7.84 7.83 7.82
as-grown E2 E1
(b) 0.5
Si/Fe = 0.5
1.4
2.0
b,cbulk = 7.824Å
1.0
1.5
2.0
Si/Fe flux ratio Fig.1. Si/Fe flux dependence of lattice constants in(a) aaxis, (b) b or c-axis
0.85
㻌
(a)
5.5 K
R/R (arb. units) 㻌
a㻌㻌(Å)
9.88
0.90
0.95
Photon Energy (eV)
bulk 1.00
Fig.2. PR spectra of β-FeSi2 single crystal and epitaxial films grown at different Si/Fe ratio
K. Noda et al./ Physics Procedia 00 (2011) 000–000 K. Noda et al. / Physics Procedia 23 (2012) 5 – 8
4
Direct Transition Energy (eV)
8
0,94
bulk 0,93
E1
0,92
E2
0,91 -0,3
-0,2
-0,1
0,0
a/abulk (%)
Fig.3. Relationship between lattice constants and direct bandgap energy
4. Conclusion We fabricated the β-FeSi2 epitaxial films with different Si/Fe ratio. Lattice constants in both aaxis and b-, c-axis deviated from those of the single crystal as Si/Fe flux ratio decreased. Corresponding to the lattice deformation, the redshift of direct transition energies was observed in PR measurements. These results are new verification of band-structure modifications caused by the lattice deformation. Compared with the results of annealing-induced lattice deformation reported before, the universal relationship between the lattice deformation and the direct transition energy was confirmed in the β-FeSi2 epitaxial films. Acknowledgements This work was supported in part by Grant-in-Aid for JSPS Fellows No. 10J00775 and by Grantin-Aid for Young Scientists (A) No. 22686032 from the Ministry of Education, Culture, Sports, Science and Technology. This work was also supported by Priority Assistance for the Formation of Worldwide Renowned Centers of Research - The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] Christensen NE. Electronic structure of β-FeSi2. Phys. Rev. B 1990;42:7148-53. [2] Noda K, Terai Y, Hashimoto S, Yoneda K, Fujiwara Y. Modifications of direct transition energies in β-FeSi2 epitaxial films grown by molecular beam epitaxy. Appl. Phys. Lett. 2009;94:241907-1-3. [3] Terai Y, Noda K, Yoneda K, Udono H, Maeda Y, Fujiwara Y. Bandgap modifications by lattice deformations in βFeSi2 epitaxial films. Thin Solid Films 2011;519:8468-72. [4] Noda K, Terai Y, Yoneda K, Fujiwara Y. Temperature dependence of direct transition energies in β-FeSi2 epitaxial films on Si(111) substrate. Physics Procedia 2011;11:181-4. [5] Yoneda K, Terai Y, Noda K, Miura N, Fujiwara Y. Photoluminescence and photoreflectance studies in Si/βFeSi2/Si(001) double heterostructure. Physics Procedia 2011;11:185-8. [6] Migas DB, Miglio L. Band-gap modifications of β-FeSi2 with lattice distortions corresponding to the epitaxial relationships on Si(111). Phys. Rev. B 2000;62:11063-70. [7] Aspnes D E. In Handbook on Semiconductors, edited by T. S. Moss North-Holland, Amsterdam, 1980;2,109. [8] Yamaguchi K, Mizushima K. Luminescent FeSi2 Crystal Structures Induced by Heteroepitaxial Stress on Si(111). Phys. Rev. Lett. 2001;86:6006-9.
Physics Procedia 0 (2011) 000–000 Physics Procedia 23 (2012) 5 – 8
Physics Procedia www.elsevier.com/locate/procedia
Asian School-Conference on Physics and Technology of Nanostructured Materials (ASCO-NANOMAT 2011)
Growth condition dependence of direct bandgap in β-FeSi2 epitaxial films grown by molecular beam epitaxy K. Nodaa, Y. Teraia,*, N. Miuraa, H. Udonob, Y. Fujiwaraa a
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan b Graduate School of Science and Engineering, Ibaraki University, 4-12-1 Nakanarusawa Hitachi, Ibaraki 316-8511, Japan
Abstract Direct bandgap energy (Eg) and lattice deformations were investigated in β-FeSi2 epitaxial films grown by molecular beam epitaxy (MBE) with different growth condition. As Si/Fe flux ratio during the MBE growth became smaller than Si/Fe = 2.0, the lattice constants deviated from those of β-FeSi2 single crystal, which indicated an enhanced lattice deformation at the lower Si/Fe ratio. In photoreflectance (PR) measurements, the PR spectra shifted to lower photon energy with the enhanced lattice deformation. These results revealed that the Eg of β-FeSi2 epitaxial film was modified by the lattice deformation depending on the growth condition.
© B.V. Selection and/or peer-review under responsibility of Publication ©2011 2011Published PublishedbybyElsevier Elsevier Ltd. Selection and/or peer-review under responsibility of Guest Committee of ASCO-NANOMAT 2011 andCommittee Far EasternofFederal University (FEFU) Editors of Physics Procedia, Publication ASCO-NANOMAT 2011 Keywords: β-FeSi2; iron silicides; photoreflectance; bandgap modification; band engineering
1. Introduction Semiconducting β-FeSi2 thin films on Si substrates have attracted much interest as silicon-based optoelectronics materials as it shows photoluminescence (PL) at 1.54 μm [1]. For the application to Si-based light emitter using β-FeSi2, a technique to enhance the 1.54 μm-PL intensity should be developed. In PL and photoreflectance (PR) studies in the β-FeSi2 thin films, the 1.54 μm-PL was found to be due to the indirect transition [1-5]. On the other hand, the theoretical calculations have predicted a band-structure change from indirect to direct transition by the stress at the heteroepitaxial interface of β-FeSi2/Si [6]. Therefore, it is necessary to investigate the dependence of band structure on the stress in the β-FeSi2 epitaxial films on Si. Recently, we have investigated * Corresponding author. Tel.: +8-166-879-7548; fax:+8-166-879-7536. E-mail address: [email protected].
1875-3892 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Publication Committee of ASCO-NANOMAT 2011 and Far Eastern Federal University (FEFU) doi:10.1016/j.phpro.2012.01.002
6 2
K. et Noda al. / Physics Procedia 23 (2012) 5–8 K. Noda al./ et Physics Procedia 00 (2011) 000–000
lattice deformation and direct bandgap (Eg) in β-FeSi2 epitaxial films on Si(111) and Si(001) substrates [2,3]. As a result, the change of Eg was successfully observed when the lattice deformation was induced by thermal annealing. This is the first experimental results which show the band-structure change by the strain at the β-FeSi2/Si interface. In this study, we fabricated the β-FeSi2 epitaxial films on Si(001) substrates with different Si/Fe flux ratio. In the films, the lattice deformation and Eg were investigated to confirm the bandstructure change depending on the Si/Fe ratio.
2. Experiments β-FeSi2 epitaxial films were grown on n-type floating-zone Si(001) substrate (ρ = 2.0-3.0 kΩ cm) by MBE using template technique. After the cleaning by the RCA method, the substrates were heated to 800°C for 30 min to remove the surface oxide layer. A 24 nm Si buffer layer was grown at 550°C, and then, a 20-nm-thick (100)-oriented β-FeSi2 epitaxial template was formed by reactive deposition epitaxy (RDE) at 550 °C. After that, a 130-nm-thick β-FeSi2 epitaxial film was grown on the template by co-deposition of Si and Fe at 550 °C. During the MBE growth, the Si/Fe flux ratio was changed from 0.4 to 2.0 by controlling deposition rate of Si. The crystal structure of the films was analyzed by high-resolution X-ray diffraction (HRXRD : X’pert Pro, PANalytical) at room temperature. The crystal orientation was confirmed by the conventional 2θ-θ scan. The quality of the films was evaluated by the full width at half maximum (FWHM) of the rocking curve of the β-FeSi2(800) peak. The lattice constant of a-axis was obtained by the β-FeSi2(800) diffraction peak position, and the average lattice constant of b- and c-axis was obtained by the βFeSi2 (220)(202) peaks . Detailed direct bandgap energies were obtained by PR spectra measured at 5.5 K. In PR measurements, a halogen lamp in conjunction with a single grating monochromator was used as a probe source. The pump source was 532 nm laser mechanically chopped at a frequency of 140 Hz. The PR spectra (ΔR/R) were expressed as the ratio of a small reflectance change (ΔR) caused by the modulation laser to the reflectance (R). The ΔR/R was detected by an InGaAs photodiode. The PR lineshape at low-electric-field could be expressed by a generalized Lorentzian function called the Aspnes third derivative functional form [7], n i R / R Re C j e j ( E E j iΓ j ) m (1) j 1 where Ej is direct bandgap energy, Γj is broadening parameter at interband transition. Cj and θj are amplitude and phase factor, respectively. The exponent m depends on the type of the transition. In this study, m is fixed at 2.5 for a three-dimensional band-to-band transition.
3. Results and discussion In 2θ-θ XRD measurements, all the samples grown at the different Si/Fe ratio showed the epitaxial growth of β-FeSi2(100)//Si(001). In rocking curves corresponding to β-FeSi2(800) diffraction peak, FWHM was 1.76° at Si/Fe = 2.0. The FWHM became small at lower Si/Fe ratio. These results indicate that the highest crystalline orientation is achieved at the lowest Si/Fe ratio of 0.5. Figure 1 shows the dependence of (a) the lattice constant in a-axis, (b) the average lattice constant in b-,caxis on the Si/Fe ratio. As the Si/Fe ratio decreased, the lattice constant in a-axis expanded and that in b, c-axis shrunk. The dependence of lattice constants on Si/Fe ratio clearly shows that the lattice deformation depends on the Si/Fe ratio during the MBE growth. In comparison with the lattice constants of β-FeSi2 single crystal (abulk = 9.881 Å, b, cbulk = 7.824 Å), the difference between the epitaxial films and the single crystal was largest at Si/Fe = 0.5. These results show the enhanced lattice deformation at the lower Si/Fe ratio.
Nodaetetal. al./ PhysicsProcedia Procedia23 00(2012) (2011)5000–000 K.K.Noda / Physics –8
73
In order to investigate the effect of lattice deformation on band-structure change, we conducted the PR measurements. In Fig.2, PR spectra of β-FeSi2 epitaxial films and the single crystal are shown. In the single crystal, the direct bandgap of 0.938 eV was obtained by a least-square fitting using Aspnes equation (1). In the epitaxial films, two direct bandgap energies defined as E1, E2 were obtained [3]. In the sample grown at Si/Fe = 2.0, the E1 and E2 were 0.934 eV and 0.917 eV respectively. These direct bandgap energies were smaller than that in the single crystal. As the Si/Fe ratio decreased, both E1 and E2 shifted to lower energy. These results revealed that the direct bandgap was shifted by the enhanced lattice deformation depending on the Si/Fe ratio. In Fig. 3, the obtained E1 and E2 are plotted as a function of variation from the lattice constant of the single crystal in a-axis (Δa/abulk). The circle points (●:E1, ○:E2) represent the direct bandgap energies obtained in Fig. 2. The square points (■:E1, □:E2) are the results of the annealing-induced lattice deformation in previous reports [3]. As seen in the figure, the direct bandgap energies of E1 and E2 in both cases shifted to lower energy as Δa/abulk increased. Therefore, it was found that there is an universal relationship between the direct bandgap energy and the lattice deformation although the origin of the lattice deformation is different. Migas et al. have calculated the band structure of strain-free and highly-strained β-FeSi2 [6]. In the strain-free β-FeSi2 (single crystal), the band structure is indirect transition type. As a large lattice deformation of Δa/abulk = +8.0% Δb/bbulk = +2.3%, and Δc/cbulk = –1.9 % (lattice volume is constant) is introduced, the direct transition energy at Y-point in the Brillouin zone decreases from 0.7 eV to 0.25 eV. As a result of the bandgap modification, the band structure changes to direct transition type. The calculated result could not be quantitatively compared with experimental results of Fig. 3 because the lattice deformations in this study was much small and the lattice volume changed with Si/Fe ratio. However, the shift of Eg in Fig. 3 is qualitatively consistent with a common feature of the theoretical results that the conduction band shifts to lower energy by the lattice deformations [6,8]. Therefore, these results convince us that the band structure of β-FeSi2 epitaxial film is modified by the lattice deformation. As described before, the highest crystalline orientation of β-FeSi2(100)//Si(001) is achieved at Si/Fe = 0.5. The result indicates that the stoicheiometry of Si/Fe = 2 is accomplished at the Si/Fe flux ratio of 0.5, because Si atoms are also supplied by thermal diffusion from the heated Si substrate in addition to the Si supply at the growth surface. In the case of Si/Fe ratio > 0.5, it is considered that the excess Si atoms exist as interstitial atoms or Si aggregation. The origin of the lattice deformation depending on the Si/Fe ratio is unclear at present, but there is a possibility that the excess Si atoms affect the lattice deformation. 㻌
abulk = 9.881Å
9.87 9.86
b,c㻌(Å)
7.85 7.84 7.83 7.82
as-grown E2 E1
(b) 0.5
Si/Fe = 0.5
1.4
2.0
b,cbulk = 7.824Å
1.0
1.5
2.0
Si/Fe flux ratio Fig.1. Si/Fe flux dependence of lattice constants in(a) aaxis, (b) b or c-axis
0.85
㻌
(a)
5.5 K
R/R (arb. units) 㻌
a㻌㻌(Å)
9.88
0.90
0.95
Photon Energy (eV)
bulk 1.00
Fig.2. PR spectra of β-FeSi2 single crystal and epitaxial films grown at different Si/Fe ratio
K. Noda et al./ Physics Procedia 00 (2011) 000–000 K. Noda et al. / Physics Procedia 23 (2012) 5 – 8
4
Direct Transition Energy (eV)
8
0,94
bulk 0,93
E1
0,92
E2
0,91 -0,3
-0,2
-0,1
0,0
a/abulk (%)
Fig.3. Relationship between lattice constants and direct bandgap energy
4. Conclusion We fabricated the β-FeSi2 epitaxial films with different Si/Fe ratio. Lattice constants in both aaxis and b-, c-axis deviated from those of the single crystal as Si/Fe flux ratio decreased. Corresponding to the lattice deformation, the redshift of direct transition energies was observed in PR measurements. These results are new verification of band-structure modifications caused by the lattice deformation. Compared with the results of annealing-induced lattice deformation reported before, the universal relationship between the lattice deformation and the direct transition energy was confirmed in the β-FeSi2 epitaxial films. Acknowledgements This work was supported in part by Grant-in-Aid for JSPS Fellows No. 10J00775 and by Grantin-Aid for Young Scientists (A) No. 22686032 from the Ministry of Education, Culture, Sports, Science and Technology. This work was also supported by Priority Assistance for the Formation of Worldwide Renowned Centers of Research - The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] Christensen NE. Electronic structure of β-FeSi2. Phys. Rev. B 1990;42:7148-53. [2] Noda K, Terai Y, Hashimoto S, Yoneda K, Fujiwara Y. Modifications of direct transition energies in β-FeSi2 epitaxial films grown by molecular beam epitaxy. Appl. Phys. Lett. 2009;94:241907-1-3. [3] Terai Y, Noda K, Yoneda K, Udono H, Maeda Y, Fujiwara Y. Bandgap modifications by lattice deformations in βFeSi2 epitaxial films. Thin Solid Films 2011;519:8468-72. [4] Noda K, Terai Y, Yoneda K, Fujiwara Y. Temperature dependence of direct transition energies in β-FeSi2 epitaxial films on Si(111) substrate. Physics Procedia 2011;11:181-4. [5] Yoneda K, Terai Y, Noda K, Miura N, Fujiwara Y. Photoluminescence and photoreflectance studies in Si/βFeSi2/Si(001) double heterostructure. Physics Procedia 2011;11:185-8. [6] Migas DB, Miglio L. Band-gap modifications of β-FeSi2 with lattice distortions corresponding to the epitaxial relationships on Si(111). Phys. Rev. B 2000;62:11063-70. [7] Aspnes D E. In Handbook on Semiconductors, edited by T. S. Moss North-Holland, Amsterdam, 1980;2,109. [8] Yamaguchi K, Mizushima K. Luminescent FeSi2 Crystal Structures Induced by Heteroepitaxial Stress on Si(111). Phys. Rev. Lett. 2001;86:6006-9.