An approach to device grade amorphous and microcrystalline silicon thin films fabricated at higher deposition rates

Current Opinion in Solid State and Materials Science 6 (2002) 445–453

An approach to device grade amorphous and microcrystalline silicon thin films fabricated at higher deposition rates Michio Kondo*, Akihisa Matsuda National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305 -8568, Japan Received 6 August 2002; accepted 27 August 2002

Abstract We demonstrate the recent developments in the high-rate deposition technique of thin film silicon using a novel approach of plasma enhanced chemical vapor deposition. Hydrogenated amorphous silicon has been developed for thin film solar cells in the past three decades, however the severe photodegradation of the film as well as of the devices prepared at a high deposition rate has hindered the industrialization of this material. The SiH 2 structure in the film has been identified as a cause of the photodegradation, and its formation mechanism has been clarified in terms of the electron temperature of the plasma, gaseous phase reactions and surface reactions. We found that the combination of a VHF plasma, proper hydrogen dilution and higher deposition temperature improves the efficiency of solar cells after photodegradation up to 8.2%, which is the highest efficiency under high deposition rate conditions. The high deposition rate technique for microcrystalline silicon has also been developed on the basis of the knowledge that a sufficiently high amount of atomic hydrogen and the reduction in the ion bombardment are the key factors for high quality microcrystalline silicon. A novel deposition technique under high-pressure depletion conditions has been developed in combination with a VHF plasma to obtain a deposition rate as high as 5 nm / s, and further development has been achieved by means of a hollow mesh so that a lot of migrating bright spot appear on the mesh to obtain a deposition rate of 5.8 nm / s with good crystallinity and a defect density as low as 2.6310 16 cm 23 , which implies a device grade material. A solar cell with an efficiency of 8.1% has been prepared at a deposition rate of 1.2 nm / s.  2002 Published by Elsevier Science Ltd. Keywords: Amorphous silicon; Microcrystalline silicon; Plasma CVD; Solar cells

1. Introduction In the present semiconductor device industry, thin film devices such as flat panel display and solar cells have obtained increased significance. In particular, photovoltaic energy is of high importance not only for energy security but also from an environmental point of view. Among a variety of materials for solar cells, silicon is the most abundant and safe element. The dominant solar cells currently in the market are made of monocrystalline or polycrystalline bulk materials, which lead to high power generation cost as well as limited feedstock. For a vast increase of the solar cell market it is crucial that the cost demand of solar electric power becomes competitive with conventional electricity. Thin film silicon, therefore, is considered to be a promising material for photovoltaic energy in the next-generation because of the cost reduction *Corresponding author.

as well as the large-scale production since Spear and LeComber succeeded in controlling the p–n doping of hydrogenated amorphous silicon [1], while the drawback has been its lower efficiency than that for bulk materials. An attempt to improve efficiency has been made using a tandem cell structure consisting of hydrogenated amorphous silicon (a-Si:H) for the top cell and its alloys (a-SiGe) or microcrystalline silicon (mc-Si:H) for the bottom cell. An initial efficiency of 15% for a-Si:H / aSiGe:H / a-SiGe:H triple tandem cell and 14.5% for a-Si:H / mc-Si:H double tandem cell have been reported to date [2,**3]. The advantages of the microcrystalline silicon are the low temperature process compatible with a-Si:H for low cost processes and the stability against light exposure, while the disadvantage is the much larger thickness than a-Si:H to absorb sunlight due to the indirect band gap. For a high throughput process, deposition rate is a crucial factor, as the material properties and device performance are monotonically decreasing functions with the

1359-0286 / 02 / $ – see front matter  2002 Published by Elsevier Science Ltd. PII: S1359-0286( 02 )00113-4

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deposition rate both for a-Si:H and mc-Si:H. In this review, our recent development of the high-rate deposition of device-grade amorphous and microcrystalline silicon using plasma enhanced chemical vapor deposition (PECVD) is demonstrated.

2. Deposition processes Plasma enhanced chemical vapor deposition (PECVD) has been widely and successfully employed for the fabrication of amorphous and microcrystalline silicon since Spear and LeComber have succeeded in controlling the p- and n-type doping of a-Si:H using monosilane as a source gas. A typical RF-PECVD system is shown in Fig. 1. The plasma is generated between two parallel electrodes and the substrate is placed at the grounded electrode (anode). The RF power is fed to the other electrode (cathode) through the matching network and the blocking capacitor. For a-Si:H deposition, monosilane, SiH 4 has been most commonly employed together with proper hydrogen dilution. With increasing hydrogen dilution under otherwise identical deposition conditions, the film structure changes from amorphous to microcrystalline above a certain threshold dilution ratio, R5H 2 / SiH 4 . This will be discussed later. As shown in Fig. 2, a silane molecule is decomposed

Fig. 2. Schematics for the dissociation pathways of SiH 4 molecules excited by the electron impact.

into various radicals by electron impact [4]. Under the usual deposition conditions, the plasma is in a non-equilibrium state, that is, the electron temperature, T e , is of the order of eV and is much higher than the gas temperature, T g , (T g |room temperature). The non-equilibrium plasma is

Fig. 1. Schematic diagram of the PECVD reactor and a picture of a typical example.

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suitable for synthesizing high temperature materials on a low temperature substrate without thermal damage. Thermal dissociation of monosilane occurs above |600 8C, while in the plasma CVD process the deposition occurs even at room temperature. Device grade a-Si:H and mcSi:H is typically deposited at around 200 8C and this capability of a low temperature process enables us to employ a variety of materials as a substrate such as glass and polymer. In particular, crystalline silicon requires much higher temperature than amorphous silicon, for which the non-equilibrium process is a necessary condition. The standard excitation frequency of the plasma has been 13.56 MHz, and recently higher frequencies have become popular because of the better film properties and higher deposition rate [5–7]. In the VHF plasma, the electron temperature and plasma potential decrease while the electron density increases because of a more efficient energy gain mechanism of electrons by a wave-riding effect [8]. The primary decomposition of monosilane is as follows; e1SiH 4 →n H1SiH 32n (n51, 2, 3) depending on the electron energy as shown in Fig. 2. The generation of SiH 3 has the lowest threshold energy, 8.75 eV, and those for SiH 2 and SiH are 9.47 and 10.33 eV, respectively [4]. The reaction rate constant with the parent molecule SiH 4 is much lower for SiH 3 than for SiH 2 and SiH, because of the recombination with SiH 4 through the insertion reaction. The successive insertion reaction, SiH 2 1Si n H m → Si n11 H m12 , results in the formation of higher silane species and finally in powder formation [9]. Therefore, the SiH 3 radical is the most predominant film precursor in the PECVD process [10], and its density has been measured to be of the order of 10 12 cm 23 by infrared laser absorption spectroscopy (IR-LAS), which reasonably accounts for the actual deposition rate [11]. Although the contribution of other radicals is much smaller, they contribute to the film growth indirectly and remarkably influence the electronic properties, as discussed later. Under high deposition rate conditions, a-Si:H and mcSi:H present different issues: In the case of a-Si:H, the degree of photodegradation becomes more significant, while in mc-Si:H, the volume fraction of crystalline phase and defect density deteriorate with increasing deposition rate. In the following sections, we discuss these issues individually.

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however, that the photodegradation becomes more significant with increasing deposition rate [14]. The decrease in the fill factor and the density of the photoinduced defects has a good correlation with the SiH 2 density in the film as shown in Fig. 3 [15–17]. These results imply that the SiH 2 density increases with increasing deposition rate under the conventional method and that the photoinduced defect density increases as well, resulting in a decrease in the cell efficiency. A plausible explanation for the defect formation related to the SiH 2 invokes the structural flexibility of the Si network containing SiH 2 clusters, because the presence of SiH 2 decreases the coordination number of Si and increases the flexibility similar to chalcogenide glasses, which are well known to show photo-induced structural changes. The relation between the deposition rate and SiH 2 density in the film can be explained by two different models, which may be correlated with each other. One is the generation of different film precursors in the gaseous phase, e.g. higher silane radicals. The higher silane radicals are formed by the successive insertion reaction of SiH 2 , SiH 2 1Si n H m →Si n11 H m 12 , and such a higher order reaction depends nonlinearly on the radical density, i.e. it is more significant under high deposition rate conditions. Another model assumes that the hydrogen content in the film is limited by the hydrogen desorption (or evolution) from the sub-surface region during the network formation. The high-density flux of SiH 3 can be generated by using a remote plasma of a cascaded arc discharge of Ar [**18]. The adsorbed SiH 3 radicals on the growing surface give rise to the Si–Si network formation through hydrogen desorption, and only a small part of the hydrogen remains in the film. Since the hydrogen desorption reaction is a

3. Photodegradation of a-Si:H as a function of deposition rate Since the first report on amorphous silicon solar cells [12], photodegradation and its suppression have been the most crucial problem for the industrialization [13]. In the actual manufacturing process, a deposition rate of 1|2 nm / s is required for a-Si:H. It is empirical knowledge,

Fig. 3. The decrease of the fill factor and the increase in the defect density after the photodegradation of a-Si:H as a function of the SiH 2 density in the film.

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thermally activated process, the amount of remaining hydrogen decreases by elevating the deposition temperature. In particular, the density of SiH 2 configurations is more sensitive to the temperature than that of SiH configurations, probably because of the presence of the two adjacent hydrogens in the SiH 2 configuration. With higher silane related radicals on the surface it is difficult to give rise to sufficient surface diffusion to reach proper sites followed by Si–Si network formation [19]. Therefore, the contribution of the higher silane related radicals results in an increase in the amount of hydrogen in the film. A higher deposition temperature can recover the contribution of higher silane radicals, because the surface reactions as well as hydrogen elimination reaction can be facilitated to form a compact Si network with smaller SiH 2 density. Thus, the hydrogen content in the film is determined by the balance of the flux density, which increases the hydrogen content, and the deposition temperature, which decreases the hydrogen content. Several possible approaches to decrease the SiH 2 density in the film have been attempted on the basis of the above arguments: reduction in the electron temperature using very high frequency, hydrogen dilution, and the increment of deposition temperature. Hydrogen dilution suppresses the higher silane formation because of the recombination between H 2 and SiH 2 . It has been reported that hydrogen dilution decreases the surface roughness, probably due to the enhancement of the surface diffusion of precursors [20]. It has been reported that the VHF plasma is advantageous to reduce the higher silane and its related radicals due to the lower electron temperature than the RF plasma [21]. The effects of the deposition temperature and hydrogen dilution are evaluated by the degradation of solar cell efficiency as shown in Fig. 4 [**22]. The substrate type structure was employed for the solar cells because it allows a wider processing temperature for the i-layer. All the cells were prepared under a nearly constant deposition rate of 2 nm / s and have the same device configuration. The degradation ratio of the cell prepared at a lower temperature of 220 8C using pure silane source gas is about 38%, and hydrogen dilution as well as higher deposition temperature improves the stability. The best stabilized efficiency was obtained at a higher deposition temperature of 340 8C and moderate hydrogen dilution of H 2 / SiH 4 53. The plasma excitation frequency was 80 MHz. The obtained stabilized efficiency of 8.2% is shown in Fig. 5, and is the highest among cells prepared at a deposition rate higher than 2 nm / s. The effect of the higher deposition temperature may support the hydrogen elimination reaction in the surface region. However, hydrogen dilution in combination with the VHF plasma cannot suppress the higher silane formation completely, and therefore the above result cannot discriminate between two mechanisms to determine the SiH 2 density as mentioned above.

Fig. 4. The time evolution of the efficiency during light exposure for solar cells prepared under three different deposition conditions.

4. Structural and electrical properties of mc-Si:H Microcrystalline silicon can be fabricated at temperatures much lower than the melting temperature using high hydrogen dilution, typically H 2 / SiH 4 gas flow ratios of 10 or higher [23]. Since the microcrystalline silicon solar cell

Fig. 5. The I–V characteristics of a-Si:H solar cell prepared at a deposition rate of 2 nm / s after light exposure (AM1.5, 100 mW/ cm 2 , 500 h at 60 8C). The i-layer thickness is 270 nm.

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needs a thick layer over several microns, there is an important industrial demand for high deposition rate. In addition, high hydrogen dilution readily leads to a lower deposition rate. Therefore, to obtain a high deposition rate of mc-Si:H, an understanding of the formation mechanism is vital. From optical emission spectroscopy (OES) measurements, it has been clarified that atomic hydrogen plays a crucial role in crystal formation and that there exists a clear threshold for the atomic hydrogen flux density to the growing surface, although its amount depends on other deposition conditions such as gaseous pressure and deposition temperature [23]. It is noteworthy that atomic hydrogen can be generated from either SiH 4 or H 2 . However, atomic hydrogen is annihilated by the recombination reaction with SiH 4 , H1SiH 4 →H 2 1SiH 3 . The high hydrogen dilution, therefore, compensates for the annihilation reaction and allows a sufficient amount of atomic hydrogen flux to the growing surface. Alternatively, another possible way to increase the atomic hydrogen density is the silane depletion (or starvation) condition, where the decomposition of silane is faster than the gas supply, which suppresses the annihilation reaction. High-density plasmas, where the silane depletion can be easily realized, have been applied for fabricating microcrystalline silicon [24–28], even with the pure silane [24,28]. The microscopic mechanism of the formation of microcrystalline silicon has been extensively argued in terms of the etching model [29,30], chemical heating model [31], surface diffusion model [32] and subsurface reaction model [33]. Although the role of hydrogen is still controversial among these models, the importance of atomic hydrogen is in agreement. Another important aspect of the plasma process is the role of the ion bombardment onto the surface. The ion sometimes plays a beneficial role [34] and sometimes a disrupting role for good quality materials [35]. In mc-Si:H deposition, an important result is that the ion energy is more effective than ion density when the ion energy exceeds a certain threshold value [**36]. The degree of the ion damage is evaluated by the product of the ion energy and ion flux density during the deposition of a monolayer. For obtaining a higher deposition rate, it is necessary to increase the power density fed to the plasma. The ion bombardment per unit deposition layer increases in the depletion regime because the plasma density increases without an increase of the deposition rate. On the basis of the above knowledge, we have developed a novel approach for this issue using a highpressure depletion condition [37,**38]. The basic idea is that the ion bombardment is suppressed under the highpressure condition and that the atomic hydrogen density can be increased under the depletion conditions. The deposition rate and the crystalline volume fraction as a function of the RF power are shown in Fig. 6(a) and (b) for

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Fig. 6. Deposition rate and the crystalline volume fraction as a function of the RF power for RF plasma (a) and for VHF plasma (b).

RF and VHF plasmas, respectively. In the low RF power region, the structure is amorphous, and at the onset power of the saturation of the deposition rate the crystalline phase starts to appear. This is due to the increase of the atomic hydrogen density under the depletion condition of silane. In the high power region, the deposition rate decreases due to the etching by atomic hydrogen. The etching is more pronounced for the VHF plasma, suggesting a higher atomic hydrogen density in the VHF plasma. The preferential orientation along [110] direction is more pronounced in the sample prepared using a VHF plasma, as reported [39–41]. This high-pressure depletion with the VHF technique improves the crystalline volume fraction in the very high deposition rate regime over 2 nm / s, which was the upper limit of the microcrystalline silicon at around 200 8C [**41]. The Raman spectra as a function of the deposition rate deteriorate significantly at a deposition rate of 5 nm / s, as shown in Fig. 7, even using the high-pressure depletion method. This suggests the presence of a disrupting factor in the very high deposition rate region. We have considered three possible causes: ion bombardment, higher silanes, and the intrinsic upper limit. The intrinsic limit of the growth rate has been observed in molecular beam epitaxy at low temperatures [42]. Another important issue is that of defects in the film, probably in the grain boundary region.

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Fig. 8. Defect density measured by ESR as a function of deposition rate using three different deposition methods; (A) conventional method, (B) high pressure depletion method, (C) hollow mesh method.

Fig. 7. The Raman spectra of microcrystalline silicon film as a function of the deposition rate. The film thickness is around 1 mm. The sample for 0.006 nm / s (the bottom trace) is prepared by a conventional method, and the deposition methods used are indicated in the parenthesis.

For solar cell application, the recombination of photocarriers is a key factor and this recombination is mainly dominated by the mid gap states arising from dangling bonds. The defect density in the film measured by electron spin resonance monotonically increases with the growth rate as shown in Fig. 8. From a study of anisotropy of the defects, these defects are located on the surface of the grain and in the amorphous tissue region [43,44]. The high efficiency solar cell with an efficiency of nearly 10% contains a defect density as low as 2310 16 cm 23 or less [**45]. As compared to the conventional method, the high-pressure depletion method can reduce the defect density by about one order of magnitude, probably due to the lower ion bombardment. However, the defect density still increases monotonically with the growth rate, i.e. with the RF power as shown in Fig. 8. At a growth rat of 5 nm / s, however, the defect density exceeds 10 17 cm 23 , which predicts a poor device performance. This implies that further reduction of the ion bombardment is necessary. We have attempted the triode method to reduce the ion bombardment using a mesh electrode between the cathode

and anode electrodes [**46]. The distance between the mesh and the substrate was about 2 mm. The deposition rate usually decreases by shadowing of the substrate due to the mesh. As shown in Fig. 9, the deposition rate is reduced to less than half by the mesh. With increasing RF power, however, the deposition rate abruptly increases and reaches 90% of the value for the diode case accompanied by a lot of migrating bright spots above the mesh. Interestingly, the film structure is crystalline under such conditions, while only the amorphous phase is obtained without the mesh, and the defect density shows to have a nearly one order of magnitude lower value as compared to the high-pressure depletion method, as shown in Fig. 8. The origin of the bright spots has been investigated using the optical emission spectra from the plasma and its spatial distribution. The emission intensities for Si* (288 nm), SiH* (414 nm), H a (656 nm), and H b (486 nm) lines as a function of the RF power are shown in Fig. 10. When the bright spots appear, the intensity of Si* above the mesh rapidly increases and the intensities of the H a and H b lines show a similar increase, while the SiH* intensity does not show any anomalous increase accompanied with the bright spots. The Si* intensity near the cathode shows a behavior complementary to that near the mesh electrode. As the intensity near the mesh side increases, that near the cathode side decreases. The bright spot is considered to consist of a local high-density plasma and the decomposition rate significantly increases around the bright spots. Since the spots occur closer to the substrate than the cathode, the deposition rate effectively increases. The film structure, on the other hand, might be explained in terms of the depletion of the silane due to the

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Fig. 10. Optical emission intensity for various emission lines as a function of the RF power. The emission intensity is measured near the mesh.

Fig. 9. The deposition rate and the crystalline volume fraction as a function of the RF power under high-pressure depletion conditions using VHF plasma with the hollow mesh and without mesh. The film structure is shown only for the case with the mesh because the film structure without using the mesh is always amorphous.

high-density plasma and the resultant increase in the atomic hydrogen density. In addition, the ion damage can be reduced by the mesh because of the dark space between the mesh and the substrate. The fact that the bright spots are centered at the mesh holes and that the hole size is very critical for the appearance of the spots suggest a kind of a hollow effect in the mesh hole. In the literature, such a hollow cathode effect is regarded as a kind of instability that should be avoided. In this case, however, the depletion due to the hollow effect may play a beneficial role for microcrystalline growth. The defect density is also remarkably improved using the hollow-mesh as shown in Fig. 8. The defect density is estimated to be 2.6310 16 cm 23 , which is nearly one order of magnitude lower at a deposition rate of 5|6 nm / s, as compared to the highpressure depletion method. Other advantages of the hollow-like effect other than the crystallinity are the powder formation and the defect density. The powder formation involves a higher order reaction through the SiH 2 insertion reaction. This is of

high practical importance although its mechanism is not so clear. From a material point of view, the structural and electrical properties have sufficient quality for device application by using the high-pressure depletion method. For actual device application, however, we have found a couple of problems; one is the interface damage and the other is the amorphous incubation layer formed in the beginning of the deposition. Usually, the efficiency is a decreasing function of the deposition rate similar to the defect density. The high-pressure depletion method is effective to maintain solar cell efficiency at higher deposition rate. Our best efficiency for a single junction cell thus far was 9.4% prepared at 0.15 nm / s [**47] and the efficiency slightly goes down to 8.1% at 1.2 nm / s, as shown in Fig. 11. This number is the highest among those ever reported [48,49]. This result is considered to correspond to the improvement of the film quality. For a deposition rate around 5 nm / s, the interface problem arises, i.e. a thick amorphous incubation layer is formed on the doped layer in the beginning of the deposition of the i-layer. In order to avoid the formation of the incubation layer, we have attempted to use a two-step growth where the low deposition rate is employed in the initial stage followed by the high-rate deposition. The spectral response is remarkably improved, showing a typical microcrystalline silicon solar cell. The efficiency is still around 4%, in contrast to the efficiency of nearly 10% for the high quality cells. The improvement of the interface is a problem to be addressed in the future. As discussed in the scope of high rate deposition of

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5. Summary

Fig. 11. The I–V characteristics of microcrystalline silicon solar cells prepared at 1.2 nm / s. The thickness of the i-layer is 2 mm.

a-Si:H, the growth of microcrystalline silicon involves surface reactions such as diffusion and hydrogen elimination, which are thermally activated. In addition, the substrate temperature influences the electron temperature of the plasma and the higher silane formation. Therefore the deposition temperature tends to be higher with increasing the deposition rate. In microcrystalline silicon solar cells, however, the deposition temperature causes another problem related to impurity contamination. Crystalline materials are sensitive to impurities affecting the electrical conductivity because of the high doping efficiency. It has also been reported that oxygen at the grain boundaries is electrically active [50,**51]. Therefore, oxygen-related donors are expected to precipitate at grain boundary regions. If that is the case, the accumulated donors at grain boundaries should be expected to form a shunt leakage path along the columnar structure, which is commonly observed in mc-Si:H. This shunt leakage results in lower Voc and FF at temperatures higher than 200 8C. A possible way to avoid the impurity effect is to prepare the purified material without oxygen using a ultra high vacuum system [52]. We have found an alternative way, by employing a low temperature deposition below 180 8C. At low temperatures, the excess amount of hydrogen is expected to passivate the oxygen donors as observed for shallow impurities in single crystalline silicon [53,54]. Therefore, the lower processing temperature of microcrystalline silicon solar cells at high deposition rate is a future problem, if a purified system is not employed.

In this review, we demonstrate the recent development in the deposition technique of thin film silicon at a high rate using a novel approach of plasma enhanced chemical vapor deposition. Hydrogenated amorphous silicon has been developed for thin film solar cells in the past three decades, however the severe photodegradation of the film prepared at a high deposition rate has hindered the industrialization of this material. The SiH 2 structure in the film has been identified as a cause of the photodegradation, and its formation mechanism has been clarified in terms of the electron temperature, gaseous phase reactions, and surface reactions. The combination of the VHF plasma, proper hydrogen dilution, and higher deposition temperature improve the efficiency after photodegradation to up to 8.2% for a deposition rate of 2 nm / s, which is the highest efficiency under high deposition rate conditions. The high deposition rate technique for microcrystalline silicon has been developed on the basis of the knowledge that a sufficient amount of atomic hydrogen and the reduction in the ion bombardment are the key factors for high quality microcrystalline silicon. A novel deposition technique under high-pressure depletion method in combination with the VHF plasma has been developed to obtain a deposition rate as high as 5 nm / s. Further development has been achieved by means of a hollow mesh, where a lot of migrating bright spot appear on the mesh, to obtain a deposition rate of 5.8 nm / s with good crystallinity and a defect density as low as 2.6310 16 cm 23 , which implies a device grade material. A solar cell with an efficiency of 8.1% has been prepared at a deposition rate of 1.2 nm / s.

Acknowledgements We would like to thank Drs. R. Hayashi, T. Nishimoto, T. Takagi, M. Takai, L.H. Guo, M. Fukawa, S. Suzuki, M. Tanda and Y. Nasuno for invaluable collaboration and discussion. We are also indebted to Profs. H. Sugai (Nagoya University), Y. Watanabe, M. Shiratani (Kyushu University) for helpful suggestions. This work is partly supported by NEDO.

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