3FTFBSDI

Dendritic Casting Method as a Novel Crystal Growth Technique toRealize High-quality Si Multicrystals for High-efficiency Solar Cells
In the face of the destruction of the global environment, thedegradation of world-wide natural resources and the exhaustionof energy sources in the 21st century, we must establish asustainable society. Development of high-efficiency solar cells
is one of the key technologies to solve the problems of the
global environment and energy to aim at sustaining humandevelopment and providing high living standards.
In comparison with the traditional energy sources, solar
energy is the only ultimate natural energy source. Although~30% of total solar energy is reflected from the earth surface,
the most part of ~70% is utilized by human being. The
shipped total power of solar cells in the world rapidly increases
in the recent years. The share of the Si multicrystal solar cells
is the largest one (~60%) in all types of solar cells. To largely
promote the applications of solar cells as an energy source,
the present energy cost of the solar cells should be halved.
So, the main technology to be developed is the growthmethod to largely increase the yield of high-quality Si
multicrystal ingots, and the growth method to obtain extremely
high-quality Si multicrystal ingots.
We have recently developed a reliable method for both
purification and structure control to obtain high-quality Si
multicrystals with the same grain orientation and large grain
sizes by the newly developed dendritic casting method basedon deep understanding of crystal growth mechanisms.
To explore the growth mechanism to control
microstructures, we newly developed an in-situ observation
system to directly observe the growing interface of Si attemperature higher than 1400°C. By utilizing the system, wefound that a Si dendrite crystal grew along the bottom of the
crucible at the initial stage of the growth followed by the

directional growth on the upper surface of the dendrite crystal
under particular growth conditions. Importantly, the uppersurface of dendrite crystals can be controlled either {110} or{112} depending on the amount of the supercooling.
This knowledge obtained by the fundamental researchlead to develop a novel growth method named as “dendritic
casting method” to realize Si multicrystals with the same grainorientation and large grain size. At first, dendrite crystals withthe controlled upper plane are grown along the bottom of the
crucible under appropriate growth conditions. Subsequently,
the growth conditions are controlled to cause directional growthon the upper surface of the dendrite crystals.
By the dendritic casting method, we succeeded in
obtaining extremely high-quality Si multicrystal ingots withthe same grain orientation and very large grain size as
demonstrated by structural analysis and improved conversion
efficiency of solar cells. Furthermore, it was found that theconversion efficiency of solar cells is kept high even at the
top part of the ingot. This is in contrast with the case for theconventional casting method, where the top part of the ingot
is not applicable for solar cells due to segregated impurities.
The dendritic casting method is concluded to be promising
not only for producing high-quality Si multicrystal ingot but
also for increasing yield of high-quality part.

Fig.2 Outlook of an extremely high-quality Si multicrystal
ingot with the same grain orientation and large grain sizegrown by the dendritic casting method.
References[1] K. Fujiwara, W. Pan, N. Usami, K. Sawada, M. Tokairin,
Y. Nose, A. Nomura, T. Shishido and K. Nakajima, Acta
Materialia 54, 3191 (2006).
[2] K. Fujiwara, W. Pan, K. Sawada, M. Tokairin, N. Usami,
Y. Nose, A. Nomura, T. Shishido and K. Nakajima, J. Cryst.
Growth 292, 282 (2006).
[3] K. Fujiwara, Y. Obinata, T. Ujihara, N. Usami, G. Sazaki
and K. Nakajima, J. Cryst. Growth 262, 124 (2004).

Fig.1 Concept of the dendritic casting method. Dendrite
growth along the bottom wall of the crucible is used at theinitial stage followed by directional growth.

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Contact toKazuo Nakajima (Crystal Physics Division)e-mail: nakasisc@imr.tohoku.ac.jp

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High Brightness Light Emitting Diodes by Metal buffer and Chemical
Lift-Off Techniques
Device quality GaN layers and LEDs (Light Emitting
Diodes) were successfully grown on sapphire substrate withmetal buffer layers. GaN layers and LED structure wereseparated from sapphire substrate by chemically selective
etching of metal buffer layer. This chemical lift-off (CLO)
process is easily transferred to the vertical LED process andvery powerful for mass-production. CLO processes should
also be applicable and suitable for high-power and largearea LEDs.
Recently, III-nitrides have promoted great advances inLEDs. High efficiency GaN-based LEDs attract great interest
for applications such as displays, traffic signals, back light for
Liquid crystal display (LCD) and white-light sources. However,
the total light output from these LEDs is still rather low. Many
approaches have been carried out to increase the external
efficiency, including roughening the surface of LEDs, p-side
down LEDs, patterned substrate, photonic crystal LEDs,
reflective metals and flip-chip techniques. Kelly et al. first
demonstrated the laser lift-off (LLO) technique to separate a
HVPE grown GaN layer from sapphire. The LLO technique
has been used to fabricate the vertical type GaN/InGaN
LEDs. The luminance intensity is over two times stronger
than that of the original planar GaN/sapphire LED. However,the LLO technique has some problems; 1) complicatedprocess, 2) not suitable for mass production, 3) highexpensive laser system, 4) formation of micro cracks on debonding and 5) lift-off side damage by laser.
We succeed in growing high crystallinity GaN by inserting
metal buffer layers between sapphire substrates and GaN
epitaxial layers. GaN layers and LED structure wereseparated from sapphire substrate by chemically selective
etching of metal buffer layer, i.e, CLO process. The
development of metal buffer layers and chemical lift-off
process can supply new process for high brightness verticalLEDs, which results in low cost process and strong impacts
on optical and electronic device structure. Figure 1 shows the
CLO process flow for fabrication of vertical LEDs.

Fig.1. CLO processes for fabrication of vertical LED
The CLO technique has advantages of damage-free andsimple process. Hence, the V-LEDs manufactured by CLO
processes have a high-efficient and reliable characteristic forhigh-brightness LED applications, such as backlights for flat
panel display in LCD, automotive headlights and generalroom lightings.

Future Energy / Environmental Materials

Based on GaN growth on metal buffer and CLO technique,we fabricated vertical LEDs as shown in fig. 2.

Fig.2. (a) Selective metal buffer etching. (b) n-GaN surface
separated from sapphire by CLO process. (c) vertical LEDs.
Fig. 2(a) is a picture of a laser scribed GaN wafer grown
on metal buffer, the bottom images show etching progress ofa chip. The metal buffer layer is being etched out by etchantas time goes. The impact of this achievement is that any postprocesses such as lapping, polishing are not needed any
more for device manufacturing, if the CLO process is applied.
Fig. 2 (b) shows an n-GaN back surface of a LED structure
separated from sapphire after CLO process. The surface
shows very smooth morphology without any cracks. Fig. 2 (c)shows surface emission of a V-LED with top-down
electrodes.
The crystalline quality of GaN layers grown on sapphirewith metal buffer is even better than those grown with low
temperature GaN or AlN buffer layers. Hence, both highoperation current and substantial improvement in light output
power can be realized with CLO process. Based on thepresent achievements, the CLO process is applicable andsuitable for the fabrication of high-power and large-areaLEDs. It offers simpler processes for mass production of VLEDs, which should lead to cost-down of V-LEDs, in turnshould drive solid-state lighting in the world.
References[1] S. W. Lee, D. C. Oh, H. Goto, J. S. Ha, H. J. Lee, T. Hanada,
M. W. Cho, S. K. Hong, H. Y. Lee, S. R. Cho, J. W. Choi, J. H.
Choi, J. H. Jang, J. E. Shin, J. S. Lee and T. Yao, Phys. stat.
sol. (c) 4, 37 (2007)[2] S. W. Lee, D. C. Oh, H. Goto, J. S. Ha, H. J. Lee, T. Hanada,
M. W. Cho, T. Yao, S. K. Hong, H. Y. Lee, S. R. Cho, J. W.
Choi, J. H. Choi, J. H. Jang, J. E. Shin, Appl. Phys. Lett. 89,
132117 (2006)[3] W. H. Lee, S. W. Lee, H. Goto, H. C. Ko, M. W. Cho, T. Yao,
Phys. stat. sol. (c) 3, 1388 (2006)
Contact toTakafumi Yao (Physics of Electronic Materials Division)e-mail: yao@cir.tohoku.ac.jp

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How is Crystal Growth Affected by an External Electric Field?
A nanoscopic approach was applied to bulk crystal
growth by sustaining a large electric field in the electric
double layer induced by an external electric field. Thisaffected the phase equilibria and growth dynamics andrepresents a new approach to crystal growth.
Bulk crystals are frequently grown by the conventional
Czochralski or Bridgman method, in which the temperature
gradient, pulling rate, and rotation rate, etc. are the only
parameters that can be optimized for producing “high-quality”
crystals. Researchers have been elaborating theseparameters on crystal growth in systems that are normallyconstrained by two variables; “composition” and “temperature”
at a fixed pressure. In order to unbind the restriction of thelimited freedom of “composition” and “pressure”, we haveintroduced a third parameter, namely an external electricfield, to add an additional degree of freedom to crystal
growth.
Figure 1 shows that the relationship between the energiesof the solid and the melt is critical for improving crystal growth.
One should consider phase equilibria in the case when there
is no energy difference, and growth dynamics in which theenergy difference is a driving force for nucleation and growth.
Applying an external electric field to the growth system altersboth the phase equilibria and growth dynamics, which has
the potential to lead to a new crystal growth mechanism. We
have demonstrated the conversion of the melting state of
langasite (La3Ga5SiO14) from incongruent to congruent [1] by
the imposition of an electric field as large as 104 V/cm which
is sustained in the electric double layer between a platinum
crucible wall and langasite melt [2]. This is a good news forreviving many incongruent-melting materials which are
difficult to grow and thus have been unused even though
they have superior functional properties.
Concerning the effect of an applied electric field on growthFig. 1 Relationship between the energies of the solid andmelt in association with phase equilibria and growth dynamics.
An external electric field significantly modifies this relationship,and so represents a new fresh approach to crystal growth.

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IMR KINKEN Research Highlights 2007

dynamics, we have demonstrated that the dissolution of the
211 phase into solution was greatly enhanced by applying anelectric field in the case of the peritectic reaction of the 211
phase with a solution to form YBCO (123 phase) [3]. Thischange occurred at the growth front and is attributed to a
20% reduction in the activation energy for dissolution (Fig.
2).
It should be noted that these modifications of crystal
growth by an applied electric field occurred in an ultra-thinlayer having a thickness of the order of nanometers, the socalled “electric double layer”. Because the oxide melt iselectrically conductive due to the presence of ionic species in
it, an external applied field does not penetrate into the bulk
melt, but rather it generates a very large electric field in theelectrical double layer. This phenomenon is interesting since
it demonstrates that macroscopic operation can lead tonanoscopic control. This study could be said to be the firstattempt of performing bulk crystal growth in a nanoscopic
manner.
References[1] S. Uda, X. Huang and S. Koh, J. Cryst. Growth 281, 481(2005).
[2] S. Uda, S. Koh and X. Huang, J. Cryst. Growth 292, 1(2006).
[3] X. Huang, S. Uda, X. Yao and S. Koh, J. Cryst. Growth294, 420 (2006).
Contact toSatoshi Uda (Crystal Chemistry Division)e-mail: uda@imr.tohoku.ac.jp

Fig. 2 Relationship between ln(v/Tp2112) and 1/Tp211 forinvestigating the activation energy of dissolution of 211.

Future Energy / Environmental Materials

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Materials Designing of Metal Borohydrides
as Advanced Hydrogen Storage Materials
Systematic research and development on borohydrides,
promising candidates as high-density hydrogen storage
materials, were performed from experimental and theoretical
viewpoints.
Some types of borohydrides such as LiBH4, Mg(BH4)2,and Ca(BH4)2 [1, 2] have attracted considerable attention as
high-density hydrogen storage materials, especially for fuelcell applications. Crystal structure of LiBH4 is shown in Fig. 1.

Fig. 1 Crystal structure of low-temperature phase of LiBH4.
a = 0.718 nm, b = 0.444 nm, and c = 0.680 nm (space group:
Pnma (#62). Red (large), green (middle), and blue (small)
spheres represent Li, B, and H atoms, respectively.
We investigated thermodynamical stabilities of a seriesof alkali-, alkaline-earth-, and transition-metals borohydrides(M(BH4)n with M = Li, Na, Mg, Sc, Zn, and Zr; n = 1~4) byusing both the first-principles studies and hydrogen desorption
measurements [1]. The theoretical study indicated that the
charge transfer from cation Mn+ to complex anion [BH4]– is a
key feature for the stability of M(BH4)n, and also that there
exists a linear relationship between calculated heat offormation ΔHform. of M(BH4)n and Pauling electronegativity χPof M. Experimentally, M(BH4)n was successfully synthesizedon the basis of the following solid-state metathesis (SSM)
reaction:
MCln + nLiBH4 → M(BH4)n + nLiCl(1)

Hydrogen desorption temperature Td of M(BH4)ndetermined using gas-chromatography was also closely
correlated with χP, as shown in Fig. 2. Td decreases with anincrease in the value of χP. Therefore, we conclude that thevalue of χP is an useful indicator that assists in the estimationof the thermodynamical stabilities of “single-cation”
borohydrides M(BH4)n with the corresponding value of Td invarious temperature ranges.
Thermodynamical stabilities of “multi-cation” borohydrides MM’(BH4)n were also investigated. For example,hydrogen desorption temperature Td of ZrLin–4(BH4)n (n = 4,5, and 6) increases from 440 K to 650 K as the value ofcomposition n increases, and continuously approaches
toward 740 K — Td of LiBH4. Thus, the correlation between
Td and χP observed in “single-cation” borohydrides can bereasonably extended to “multi-cation” ones; Td correlateswith the averaged value of χP calculated from n.
In addition to the materials designing of borohydrides
M(BH4)n focusing on electronegativity of M, formation ofintermediate compounds are found to be of great importanceto promote their hydrogen desorption reaction [3, 4]. For
example, LiBH4 starts to change into an intermediate
compound at the temperature range over 700 K,
accompanying the partial hydrogen desorption reaction.
Although the compound does not show long-range ordering
for the powder X-ray diffraction measurement, the local
atomistic structure can be investigated by the laser Raman
spectroscopy; the B-H bending modes have lower frequencies
as compared to those of LiBH4, while the B-H stretchingmodes appear at higher frequencies [3]. These features are
fairly consistent with the theoretical calculation on the
monoclinic Li2B12H12, consisting of Li+ and [B12H12]2- ions, asa possible intermediate compound of LiBH4 [4] The further
experimental studies on the atomistic and electronic
structures of the compound, and also on hydrogen desorption
and re-adsorption reactions of LiBH4 using appropriate
intermediate compounds are now in progress.
Neutronics assessment as advanced neutron shieldmaterials for fusion reactors [5], and combination research
between hydrogen storage and microwave technologies [6]were also successively carried out using the series ofborohydrides.
References[1] Y. Nakamori et al., Phys. Rev. B 74, 045126 (2006).
[2] K. Miwa et al., Phys. Rev. B 74, 155122 (2006).
[3] S. Orimo et al., Appl. Phys. Lett. 89, 021920 (2006).
[4] N. Ohba et al., Phys. Rev. B 74, 075110 (2006).
[5] T. Hayashi et al., Fusion Eng. Design 81, 1285-1290 (2006)[6] Y. Nakamori et al., Appl. Phys. Lett. 88, 112104 (2006).

Fig. 2 Hydrogen desorption temperature Td of M(BH4)n as afunction of Pauling electronegativity χP of M. Inset shows thecorrelation between Td and estimated enthalpy change ofhydrogen desorption reaction ΔHdes..

Future Energy / Environmental Materials

Contact toShin-ichi Orimo (High-Temperature Materials Science(Environmental Materials Science) Division)e-mail: orimo@imr.tohoku.ac.jp

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Two-dimensional Images of Emission Spectra from Laser Induced
Plasmas and the Application to Emission Spectrometric Analysis
Spatial distribution analysis of emission signals from a laserinduced plasma can provide information on the optimization of theanalytical conditions when it is employed as a sampling and excitation
source in optical emission spectrometry. A two-dimensionally imaging
spectrometer system is employed to measure the spatial variations in
the emission intensities of analytes.
Laser-induced plasma spectrometry (LIPS) is a powerful techniquefor direct analysis of various materials because of the rapid response
without any sample pre-treatment. A Q-switched Nd:YAG laser, whichis irradiated onto a sample surface during a very short period but with alarge power density, produces a pulsated laser-induced plasma (LIP) at
the repetition frequency of the laser. Sample atoms are ejected from thesurface due to laser ablation and are subsequently ionized and excitedin LIP; therefore, this plasma acts as an excitation source as well as asampling source for atomic emission spectrometry.
The LIP has both spatial and temporal variations depending on
various experimental parameters, such as the kind and the pressure of
the plasma gas, which is generally called an expansion of the plasma
plume. Previous papers reported that the emission zone varied with theprogress of the plasma plume, which was strongly dependent on thenature of the plasma gas. It is further known that LIPs produced at
reduced pressures have favorable features for emission analysis
because of the relatively low fluctuation of the emission intensities aswell as the large signal-to-background ratio. Time-resolved
measurement of the emission spectra thus gives useful information on
optimization of the experimental conditions in the emission spectrometricapplications, such as the delay time as well as the exposure time on thedata acquisition [1,2]; however, it cannot provide the overall variationsof the plasma expansion. For this purpose, the two-dimensional (2D)
image of a spectral line emitted from the LIP should be observed, whichis expected to yield information on the fundamental processes occurringin the LIP. However, few studies have been published mainly due tolack of spectrometers enabling 2D observation having good spectral
resolution.
In this study, we investigate 2D images of emission lines of copperfrom a reduced-pressure LIP when krypton, argon, or helium is employed
as the plasma gas and their variations against the gas pressures inorder to discuss the excitation mechanism for each plasma gas.
A schematic diagram of the apparatus is shown in Fig. 1. A Nd:
YAG laser (LOTIS T II U LS-2135, Japan) was employed at the
wavelength of 532 nm (SHG mode). The laser energy of 40 mJ/pulse
was set to obtain sufficient sample ablation. The pulse duration of about
10 ns and the repetition rate of 10 Hz were employed. A spherical lenswith a 200-nm focal length was used to focus the laser beam onto thetarget surface. High-purity copper plates (99.99%) were used as the
sample. High-purity argon (>99.99995%), krypton (>99.999 %), andhelium (>99.99999 %) were introduced as the plasma gas after
evacuation of the chamber below 7 Pa. On keeping 2D spatial
information, emission signals from the LIP were conducted through atelescope optics onto the entrance slit of an image spectrometer (Model
12580, BunkoKeiki Corp., Japan), and the emission image is thendispersed and detected on a charge-coupled device detector(SensiCam QE Model, PCO Imaging Corp., Germany). The spectral
resolution was 0.1-0.5 nm depending on the slit width. The plasma
images were recorded for the period of 0.01 ms (gate width).
Figure 1 shows 2D emission images of the Cu I 324.75 nm and thebackground at 315.0 nm when argon gas was introduced at variouspressures [3]. It is found from these coloured maps that the intensities

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IMR KINKEN Research Highlights 2007

are larger at higher gas pressures and that the images roughly comprise
two portions: a narrow spot near the sample surface occurring the
breakdown of LIP and a portion expanding towards the surrounding
gas which is called a plasma plume. The shape and the intensities ofthe emission zone are considered to be determined through theinteraction between energetic particles produced immediately after thelaser breakdown and the surrounding gas. In this case, the copper
emission from the plasma plume should be measured for emission
analysis due to the small background level, compared to the breakdown
position. It is expected that the 2D imaging of LIP can yield an optimum
condition regarding the plasma portion observed when it is employed
for the analytical application.

Fig 1. Experimental setup for 2D-imaging measurement.

Fig 2. Two-dimensional images of laser-induced plasmas measured at
Cu I 324.75 nm and BG 315.0 nm at various argon pressures of 10, 50,
80, 100, 300, and 500 Torr.
References[1] Y. Ushirozawa and K. Wagatsuma, Spectrosc. Lett. 28, 539 (2005).
[2] Y. Ushirozawa and K. Wagatsuma, Anal. Sci. 22, 1011 (2006).
[3] C. Kitaoka and K. Wagatsuma, Proc. of 12th International Symposium
on Advanced Technology and Applications, 72 (2006).

Contact toKazuaki Wagatsuma (Analytical Science Division)e-mail: kwagatsuma@mm.neweb.ne.jp

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Improvements of Superconducting Properties
for Practical Nb3Sn Wires by the Mechanical Treatment

non-Cu Jc(A/mm2)

Nb3Sn superconducting wires are now utilized widely formost superconducting power devices with high magnetic
fields above 10 T. Generally the superconducting wires havea composite architecture of the fine Nb3Sn filaments with Cu,
Nb/Ta and sometime high strength materials as shown in
Fig.1, from the requirement of a formation of Nb3Sn phase, astability of the superconductivity, a diffusion barrier and areinforcement. However, it is well known that some unsolved
problems such as a non-stoichometry, residual strain andmechanical properties are still remained. Hence it is expectedthat the superconducting performance of the practical Nb3Sn
wires would be improved drastically, if those problems canbe solved. We have already developed the high strength
Nb3Sn wire reinforced by the Cu-based micro-composite
materials such as CuNb [1]. Figure 1 represents the crosssectional view of the typical high strength Nb3Sn wiredeveloped by our group. Due to the reinforcement, the stressdependence of critical current density Jc is improved by about
twice of the stress due to the reinforcement in comparison
with the conventional Nb3Sn wires (blue triangle) as shown in
Fig. 2 (b). In other words, the high strength Nb3Sn wires canbe utilized in high stress states up to 250-300 MPa. However,
the residual strain induced by the thermal contraction
difference between Nb3Sn and materials composing the wire
still deteriorates the superconducting parameters, i.e., acritical current density, an upper critical field and a criticaltemperature, when the wires are cooled from the heat
treatment temperature to 4.2 K. When the axial tensile stressis given to the Nb3Sn wires, the Jc values usually increase
first and become the maximum at the certain stress/strain asshown in Fig.2 (b). This stress/strain at the maximum Jc
corresponds to the thermal residual stress/strain.
Recently, we found that the mechanical treatment of therepeated bending at room temperature enhances Jc as

shown in Fig. 2(a). For example the Jc value becomes twice
after the repeated bending. We call this phenomenon “prebending” effect. Due to the prebending treatment, the strain
dependence of Jc shifts to lower strain and the maximum Jc
value also increases at the same time as shown in Fig. 2(b).
This suggests that the reduction of not only the axial residualstrain but also the radial one due to the prebending treatment.
We successfully confirmed the change of the 3-dimensinal
strain states directly by the neutron diffraction at the first in theworld [3]. Finally, we got high performance of Jc in the wide
stress region as shown in Fig.2 (b) by both of the high-strengthand the prebending treatment. In addition, the prebending
effect would be a key technique to clarify the strain dependenceof Jc for the strain sensitive superconducting materials.

non-Cu Jc (A/mm2)

It is well known that practical Nb3Sn superconducting
wires are deteriorated by the residual strain, because of
their large strain sensitivity. We found that the mechanicaltreatment at room temperature improves the superconducting
performance drastically. This new but simple technique cancontrol and optimize the residual strain state and is effective
the improvement of the strain dependence of thesuperconducting parameters

700(a)0.8% prebent high-strength Nb3Sn wire
600
as-reacted high-strength Nb3Sn wire
as-reacted standard Nb3Sn wire
500
400
300
200
100
T = 4.2 K
0
14
16
18
20
22
24
B
(T)
700
(b)
pre-bent
600
500
400
300
high200
strengthened
100
4.2K 14.5T
00 100 200 300 400 500 600
axial tensile stress (MPa)Fig. 2 (a) Field dependence and (b) axial stress dependenceof Jc for the standard Nb3Sn without reinforcement and thehigh-strength Nb3Sn wires with and without prebending.
References[1] K. Watanabe, S. Awaji, K. Katagiri, K. Noto, K. Goto, M.
Sugimoto, T. Saito, and O. Kohno, IEEE Trans. Magn. 30,
1871 (1994).
[2] S. Awaji, K. Watanabe, H. Oguro, G. Nishijima, T. Tsubouchi,
K. Miyoshi, S. Meguro, Fusion Eng. Design 81, 2473 (2006).
[3] H. Oguro, S. Awaji, G. Nishijima, P. Badica, K. Watanabe,
F. Shikanai, T. Kamiyama, K. Katagiri, J. Appl. Phys. 101,
103913 (2007).
Contact toSatoshi Awaji (High Field Lab. for Superconducting Materials)
e-mail: awaji@imr.tohoku.ac.jp

Fig. 1 Cross-section of the typical high strength Nb3Sn wire.

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