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ABSTRACT: Fundamental understanding of crystal growth behaviors from Si melt is significant for the researchers, who are involved in
the development of crystal growth technologies. In the light of Si crystals for solar cells, it is imperative to improve the
crystal-quality of Si-multicrystal ingot grown by casting, because it is widely used for solar cell substrates in the present
and future. Faceted dendrite has unique structural features and has a potential to be used for controlling the crystal structure
in Si-multicrystal ingots. In addition, basically, its growth behavior is fascinating. In this chapter, the growth mechanism
of Si faceted dendrite will be described with recent experimental results.
03/2010: pages 71-82;
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ABSTRACT: Generally, Si multicrystals, which are grown by a casting method using a crucible, contain many grain boundaries (GBs) and
crystal grains with various orientations. Since the grain size has increased as a result of improving in the growth technique,
instead of GBs, subgrain boundaries (sub-GBs) have become major defects acting as recombination centers for photogenerated
carriers. In this chapter, the study of sub-GBs in Si multicrystals is comprehensively reviewed with the authors’ current
results.
03/2010: pages 83-95;
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ABSTRACT: We attempted to clarify relationship between grain boundary structures in Si multicrystals and generation of dislocations during crystal growth. Systematic variation of grain boundary structures was realized by employing dendritic nucleation at the initial stage of crystal growth. Etch-pit observation revealed that the contact angle of adjacent dendrite crystals to form a grain boundary affects generation of dislocations. Experimentally observed dislocation density was found to be well correlated with shear stress around the grain boundary calculated by finite element analysis.
Journal of Applied Physics 02/2010; · 2.17 Impact Factor
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ABSTRACT: We implement spatially resolved x-ray rocking curve for quantitative analysis of subgrain boundary (sub-GB) density in Si multicrystals with controlled microstructures by “dendritic casting method.” The oriented crystal grains made it possible to analyze a large area over different crystal grains by using the same optical configuration. Sub-GBs were found to be spatially localized in a particular region and spread in the growth direction of the ingot. By combining sub-GB density with a separate measurement of minority carrier diffusion length, carrier recombination velocity at sub-GBs was revealed to be at the same order as random GBs. Furthermore, by analyzing electroluminescence images of a solar cell with different detection wavelengths, sub-GBs were shown to behave as shunts with stronger activity than random GBs.
Journal of Applied Physics 03/2009; · 2.17 Impact Factor
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ABSTRACT: Pre-polished Ge(111) single-crystal wafers were deformed just below the melting temperature to prepare point-focusing Johansson monochromator crystals. The (111) lattice plane had curvature 2R in the focusing plane and R perpendicular to it, with a hemispherical inner surface with a radius of R = 600 mm. By using Cu Kalpha radiation, the diverging X-ray beam was focused onto a small spot.
Journal of Applied Crystallography 09/2008; 41(Pt 4):798-799. · 5.15 Impact Factor
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ABSTRACT: Si and Ge are widely used as analyzing crystals for x-rays. Drastic and accurate shaping of Si or Ge gives significant advance in the x-ray field, although covalently bonded Si or Ge crystals have long been believed to be not deformable to various shapes. Recently, we developed a deformation technique for obtaining strongly and accurately shaped Si or Ge wafers of high crystal quality, and the use of the deformed wafer made it possible to produce fine-focused x-rays. In the present study, we prepared a cylindrical Ge wafer with a radius of curvature of 50 mm, and acquired fluorescent x-rays simultaneously from four elements by combining the cylindrical Ge wafer with a position-sensitive detector. The energy resolution of the x-ray fluorescence spectrum was as good as that obtained using a flat single crystal, and its gain was over 100. The demonstration of the simultaneous acquisition of high-resolution x-ray fluorescence spectra indicated various possibilities of x-ray spectrometry, such as one-shot x-ray spectroscopy and highly efficient wave-dispersive x-ray spectrometers.
Review of Scientific Instruments 04/2008; 79(3):033110. · 1.37 Impact Factor
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ABSTRACT: We show that spatially resolved x-ray rocking curve analysis can be a useful technique to investigate local structures in bulk multicrystals and their modification during crystal growth when combined with appropriate samples. For this purpose, a model crystal growth experiment was carried out using a Si seed crystal with artificially controlled 〈110〉 tilted grain boundaries. The growth orientation was chosen as the scan axis for rocking curve measurements in different crystal grains. Thanks to the superior angle resolution, the gradual structural changes can be monitored by the changes in the rocking curve profile such as the peak shift and the peak splitting. The amount as well as the sign of the peak shift was found to be strongly dependent on the initial grain boundary structure. Furthermore, the technique was applied to investigate local structures of Si multicrystals with controlled grain orientation grown without any seeds.
Journal of Applied Physics 11/2007; 102(10):103504-103504-4. · 2.17 Impact Factor
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ABSTRACT: Bulk multicrystalline Si with {310} Σ5 grain boundaries (GBs) was grown by Bridgman growth method using seed crystals with artificially controlled configuration. The structure of the GBs was preserved in the epitaxial growth region without formation of more GBs. However, the GBs were revealed to contain small-angle deviation of ∼ 5º from the perfect Σ5 relative crystal orientation both in tilt and twist components due to the inaccuracy of the seed crystal arrangement. Such an unintentional misalignment was utilized to investigate the relationship between electrical activity and the deviation angle of {310} Σ5 GB. Electron beam-induced current measurement clarified that carrier recombination velocity at the {310} Σ5 GBs decreases with decreasing deviation angle and shows a minimum at the perfect Σ5 relationship. This tendency suggests that {310} Σ5 GB is electrically inactive, as well as Σ3 and Σ9 GBs.
Journal of Applied Physics 03/2007; 101(6):063509-063509-5. · 2.17 Impact Factor
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ABSTRACT: We utilized bi-crystals with an artificially designed grain boundary configuration as a seed for floating-zone growth of multicrystalline Si (mc-Si) in order to clarify the impact of the initial grain boundary configuration on the crystal growth. Intentionally introduced large tilt deviation from Sigma3 was found to lead to appearance of new crystal grains accompanied by spontaneous modification of the grain boundary to electrically inactive Sigma3 under appropriate amount of the supercooling. Based on this mechanism, a model crystal growth experiment was performed to realize mc-Si with electrically inactive grain boundaries using purposely designed lang110rang-oriented seed crystal with an array of random grain boundaries. After 40 mm growth, all the grain boundaries were revealed to be modified to electrically inactive. These results suggest that bulk mc-Si with electrically inactive grain boundaries could be grown even by the practical casting method if oriented crystal grains with random grain boundaries could be somehow realized at the bottom of the crucible
Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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ABSTRACT: The dendrite casting method has been developed to constantly obtain high-quality Si multicrystal ingots, on the basis of our new concept to control the grain orientation and grain size. Using this method, textured high-quality Si multicrystal ingots (15cm<sup>oslash </sup>) have been grown on the {110} or {112} upper surface of dendrite crystals. So, the Si multicrystal ingots have same grain orientation and quite large grain sizes (ges3cm), and have the longer lifetime more than 35mus without hydrogen passivation. The conversion efficiency of solar cells prepared by the Si multicrystal with the same orientation is found to be higher than that of the solar cell with different orientations. The conversion efficiency of about 17% has been obtained without texture structure and hydrogen passivation. So, this dendritic casting method is very promising in the field of the Si multicrystal solar cells
Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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ABSTRACT: Hemisphere-shaped crystal wafers can be prepared by the plastic deformation of Si crystal wafers. To obtain hemispherical
Si wafers, graphite convex and concave dies were used. A Si wafer was set between dies and pressed at high temperatures. The
Si wafer was pressed by an overweight of 200 N at various temperatures. The deformation regions in which well-shaped (100)
and (111) wafers can be obtained by plastic deformation were determined using parameters of thickness and temperature. In
order to demonstrate that the shaped wafers are of sufficiently high quality to be used in the preparation of devices, solar
cells were fabricated using the hemispherical Si wafers pressed at 1,120°C and 1,200°C. The conversion efficiency of the hemispherical
solar cells is 8.5–11.5%. It was clarified from the conversion efficiency of solar cells that the quality of the shaped crystal
wafers can be improved by a proper annealing process. Thus, the hemispherical shaped wafers are of high quality to be used
in the preparation of devices.
Journal of Electronic Materials 06/2005; 34(7):1047-1052. · 1.47 Impact Factor
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ABSTRACT: Plastic deformation is an unlikely process by which to mould pristine silicon wafers into three-dimensional shapes owing to the inevitable detrimental impact that the resulting mechanically induced defects would have on their electrical properties. However, if one were to find a way of doing so without substantial degradation of these properties, a range of new applications might be opened up. Here we report on the successful plastic deformation of silicon crystal wafers for the preparation of wafers with various shapes. A silicon wafer was set between dies and pressed at high temperatures. One application of shaped wafers is as well-shaped concave silicon crystal lenses or mirrors. The lattice plane of such a crystal lens has a curvature exactly along the surface. A concave spheroidal X-ray lens, in the form of two-dimensional Johann and Johansson's monochromators, is proposed for an X-ray optical component system. We propose and demonstrate a new solar cell system with the concave silicon crystal mirror used as both a solar cell and a focused mirror. This system can make use of the reflected photons from solar cells.
Nature Material 02/2005; 4(1):47-50. · 32.84 Impact Factor
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ABSTRACT: We investigated the microscopic strain fluctuation in strained-Si grown on SiGe-on-insulator (SGOI) and SiGe virtual substrates, and clarified the origins of the strain fluctuation in the strained-Si film. A periodic strain fluctuation, which reflects a cross-hatch pattern of the substrate, was observed in the sample on the virtual substrate. On the other hand, a featureless strain fluctuation with suppressed amplitude was observed in the sample on SGOI substrate. By analyzing the correlation of the Raman peak positions of the Si–Si modes in strained-Si and SiGe, the dominant mechanism of the strain fluctuation in the strained Si film was found to be the compositional fluctuation in underlying SiGe for the sample on SGOI, and the strain fluctuation reflecting the cross-hatch pattern for the sample on the virtual substrate, respectively.
Applied Physics Letters 08/2004; 85(8):1335-1337. · 3.84 Impact Factor
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ABSTRACT: We present device performance of solar cells based on multicrystalline SiGe (mc-SiGe) bulk crystal with microscopic compositional distribution grown by the casting method. The average Ge composition was systematically changed in the range between 0% and 10%. A small addition of Ge to multicrystalline Si (mc-Si) was found to be very effective to increase the short-circuit current density without affecting the open-circuit voltage. As a consequence, the overall efficiency of a solar cell based on mc-SiGe was improved compared with that based on mc-Si. This result demonstrates that mc-SiGe is a promising candidate to replace mc-Si since it could achieve higher conversion efficiency without drastic increase of the production cost. © 2004 American Institute of Physics.
Journal of Applied Physics 07/2004; 96(2):1238-1241. · 2.17 Impact Factor
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ABSTRACT: We report on the performance of solar cells with stacked self-assembled Ge dots in the intrinsic region of Si-based p-i-n diode. These dots were epitaxially grown on p -type Si(100) substrate via the Stranski–Krastanov growth mode by gas-source molecular beam epitaxy. Enhanced external quantum efficiency (EQE) in the infrared region up to 1.45 μm was observed for the solar cells with stacked self-assembled Ge dots compared with that without Ge dots. Furthermore, the EQE was found to increase with increasing number of stacking. These results show that electron-hole pairs generated in Ge dots can be efficiently separated by the internal electric field, and can contribute to the photocurrent without considerable recombination in Ge dots or at Ge/Si interfaces. © 2003 American Institute of Physics.
Applied Physics Letters 09/2003; · 3.84 Impact Factor
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ABSTRACT: We report on growth and characterizations of Si/multicrystalline-SiGe (mc-SiGe) heterostructure as a promising candidate to surpass mc-Si solar cells. Spatial distribution of the status of strain in Si was investigated using microscopic Raman spectroscopy. The strain was found to be strongly influenced by the composition and microstructure of underlying mc-SiGe. Spatial variation of the strain as well as strain relaxation was found to be suppressed by decreasing average Ge composition of underlying SiGe. © 2002 American Institute of Physics.
Journal of Applied Physics 12/2002; 92(12):7098-7101. · 2.17 Impact Factor
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ABSTRACT: We proposed a simple lattice model to describe a solid–liquid interface of silicon based on experimental facts and molecular dynamics simulation results, and evaluated the relationship between the interface structure and the interfacial tension by comparing the model with experimental values. As a result, the entropy was found to give a major contribution to the interfacial tension, and it was revealed that the difference of entropy due to lattice disorder of bulk liquid and interface structure is the dominant factor of the entropy contribution. Moreover, the solid–liquid bond energy, which is crucial to estimate the contribution of the enthalpy, was successfully derived. The present model can be also applied to be the semiconductor material which has a diamond structure or a zinc blende structure. © 2001 American Institute of Physics.
Journal of Applied Physics 08/2001; · 2.17 Impact Factor
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ABSTRACT: Growth of binary and ternary single crystals which have complete miscibility in the phase diagrams, have been studied by several newly developed methods such as the liquid encapsulated Czochralski, Bridgman, and multi-component zone melting methods for InGaAs bulk crystals, and the multi-component zone melting methods and Bridgman methods for Ge-rich and Si-rich SiGe bulk crystals. Crystals grown by these methods are compared with each other, to find the proper growth conditions to obtain single crystals with uniform composition. Techniques for the precise control of the temperature at the growing interface and for the continuous supply of the depleted solute elements to the growth melt were developed. InGaAs bulk crystals with uniform composition were obtained by the multicomponent zone melting method. Si-rich and Ge-rich SiGe bulk crystals with uniform composition were obtained in the Ge compositional range from 10 to 78%. The advanced technologies to obtain the InGaAs and SiGe bulk crystals with much higher-quality are discussed in this paper.
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ABSTRACT: The growth technique and physical properties of SiGe multicrystals with microscopic compositional distribution are demonstrated for new high-efficiency solar cells in which the wavelength dependence of the absorption coefficient can be freely designed by controlling the compositional distribution in the SiGe multicrystals. This growth technique is suitable for the practical casting method, and it is made up of melt growth of SiGe multicrystals with wide and microscopic distribution of the composition from Si to Ge all over the crystals. It is studied how much widely the microscopic compositional distribution in SiGe multicrystals grown from binary Si–Ge melts can be controlled by the melt composition and the cooling process. The range of the microscopic compositional distribution becomes wider as the starting Si concentration in the growth melt becomes larger. SiGe multicrystals with various microscopic compositional distribution can be freely controlled by optimizing the melt composition and the cooling process. The wavelength dependence of the absorption coefficient of such SiGe multicrystals can also be freely designed. Using the experimentally determined absorption coefficient of a SiGe crystal with microscopic compositional distribution, the short circuit photo-current of solar cells was calculated and it is demonstrated that the short circuit photo-current can be much larger for SiGe with microscopic compositional distribution than for SiGe with uniform composition. Si thin film can be easily grown on such a SiGe multicrystal and the Si/SiGe heterostructure can be obtained. These results show that SiGe multicrystals with microscopic compositional distribution are hopeful for new high-efficiency solar cell applications by using the practical casting method.
Solar Energy Materials and Solar Cells 73(3):305-320. · 4.54 Impact Factor
