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Abstract : Vol.41No.2(2006.6)
Special Issue：Processing
Design of Single Crystals and Textured Polycrystals
for Advanced Electronic Devices
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Review
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Processing
Design of Single Crystals and Textured Polycrystals for
Advanced Electronic Devices
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Development of high performance materials with environmental
compatibility has been strongly desired for electronic
device elements. One of the strategies is the design
of a novel processing method to produce a material with
the optimum microstructure for enhanced physical properties.
Examples are shown as recently developed processing
techniques; a single crystal growth method to dramatically
decrease the dislocation density of compound semiconductors,
and a texture engineering method to enhance anisotropic
properties of polycrystals.
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Research Reports
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| P.5 |
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Daisuke Nakamura, Itaru Gunjishima,
Satoshi Yamaguchi, Tadashi Ito,
Atsuto Okamoto, Hiroyuki Kondo,
Shoichi Onda, Kazumasa Takatori
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Silicon carbide (SiC) has a range of useful physical,
mechanical and electronic properties that make it a
promising material for next-generation electronic devices.1,
2) Careful consideration of the thermal conditions3-6)
in which SiC {0001} is grown has resulted in improvements
in crystal diameter and quality: the quantity of macroscopic
defects such as hollow core dislocations (micropipes),7-9)
inclusions, small-angle boundaries and long-range lattice
warp has been reduced.10,11) But some macroscopic
defects (about 1-10 cm-2) and a large density
of elementary dislocations (~104 cm-2),
such as edge, basal plane and screw dislocations, remain
within the crystal, and have so far prevented the realization
of high-efficiency, reliable electronic devices in SiC
(Refs. 12-16). Here we report a method, inspired by
the dislocation structure of SiC grown perpendicular
to the c-axis (a-face growth),17)
to reduce the number of dislocations in SiC single crystals
by two to three orders of magnitude, rendering them
virtually dislocation-free. These substrates will promote
the development of high-power SiC devices and reduce
energy losses of the resulting electrical systems.
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| P.11 |
Design
of Grain Oriented Microstructure by the Monte Carlo Simulation
of Sintering and Isotropic Grain Growth |
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Hiroshi Itahara, Hiroshi Nomura,
Toshihiko Tani, Hideaki Matsubara
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Fabrication of dense and highly textured ceramics is
significantly important for the improvement of their
anisotropic properties by the templated grain growth
(TGG) method. Monte Carlo simulation based on the Potts
model was carried out through computation for the design
of TGG processing, in which anisometric template particles
are mixed with fine matrix particles and organic binder
and aligned by tape casting before heat treatment. Thus,
four initial parameters affecting the microstructure
after sintering with isotropic grain growth were investigated;
1) mixing ratio of template/matrix particles, 2) size
and 3) aspect ratio of the template particles, and 4)
initial porosity (caused by the removal of binder).
The fraction of oriented grains in the simulated microstructure
was found to increase with increasing template/matrix
mixing ratio and aspect ratio of template, and with
decreasing template size and initial porosity. The residual
porosity was shown to decrease with decreasing template/matrix
mixing ratio and template size, and with decreasing
initial porosity. The study suggests that computational
design would give a guiding principle in terms of the
initial preparation conditions for the ceramics having
both a large fraction of oriented grains and low residual
porosity.
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| P.22 |
High
Performance Lead-free Piezoelectric Material |
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Yasuyoshi Saito, Hisaaki Takao,
Toshihiko Tani, Tatsuhiko Nonoyama,
Kazumasa Takatori, Takahiko Homma,
Toshiatsu Nagaya, Masaya Nakamura
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Lead has recently been expelled from
many commercial applications and materials (for example,
from solder, glass and pottery glaze) owing to concerns
regarding its toxicity. Lead zirconium titanate (PZT)
ceramics are high-performance piezoelectric materials,
which are widely used in sensors, actuators and other
electronic devices; they contain more than 60 weight
per cent lead. Although there has been a concerted effort
to develop lead-free piezoelectric ceramics, no effective
alternative to PZT has yet been found.1-14)
Here we report a lead-free piezoelectric ceramic with
an electric-field-induced strain comparable to typical
actuator-grade PZT. We achieved this through the combination
of the discovery of a morphotropic phase boundary in
an alkaline niobate-based perovskite solid solution,
and the development of a processing route leading to
highly <001> textured polycrystals. The ceramic
exhibits a piezoelectric constant
d33 (the induced charge per unit force
applied in the same direction) of above 300 picocoulombs
per newton (pC N-1), and texturing the material
leads to a peak d33 of 416 pC N-1.
The textured material also exhibits temperature-independent
field-induced strain characteristics.
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| P.29 |
Elucidation
of the Formation Mechanism of a Textured Ceramic of a
P-type Thermoelectric Layered Oxide [Ca2CoO3]0.62[CoO2]
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Hiroshi Itahara, Won-Seon Seo,
Sujeong Lee, Hiroshi Nozaki,
Toshihiko Tani, Kunihito Koumoto
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The reactive-templated grain growth
(RTGG) method is a powerful fabrication technique for
producing textured ceramics having enhanced performance
compared to those of conventionally prepared non-textured
ceramics, for various functional materials. Its wide
applicability is demonstrated by the fact that the RTGG
method using β-Co(OH)2 templates
gave textured ceramics of p-type thermoelectric layered
cobaltites having various compositions. The orientation
degree of a prepared ceramic, which influences its performance,
depends on the composition of the ceramic. Thus, in
order to determine general guidelines for the production
of a highly textured ceramic, we analyzed the formation
mechanism of the model system [Ca2CoO3]0.62[CoO2]
(CCO: Ca2CoO3 layer + CoO2
layer) ceramic on β-Co(OH)2 templates
by using high-temperature X-ray diffraction (XRD), pole
figure, scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). We demonstrated that a textured
CCO ceramic is formed through a series of in-situ
topotactic conversions via intermediate phases with
a preserved CoO2 layer of β-Co(OH)2
templates. In general, we showed, for the first time,
that 'a reaction design with partially preserved crystallographic
similarities' is essential for the fabrication scheme
of a highly textured ceramic with enhanced performance.
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| P.37 |
Thermoelectric
Properties of Highly Textured Ca-doped
(ZnO)mIn2O3 Ceramics
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Hisashi Kaga, Ryoji Asahi, Toshihiko Tani
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Highly textured Ca-doped (ZnO)mIn2O3
(m is an integer) ceramics were fabricated by
the reactive templated grain growth (RTGG) method and
their thermoelectric properties were examined. Platelike
ZnSO4 ・3Zn(OH)2 particles were
used as reactive templates and mixed with
In2O3 and CaCO3 powders
into a stack of tapes. In situ formation and
subsequent sintering resulted in textured Ca-doped (ZnO)mIn2O3
ceramics. The electrical conductivity of the textured
specimen along the ab-plane was almost two times
larger than that of the textured specimen along the
c-axis and about 30% larger than that of a nontextured
specimen. On the other hand, the Seebeck coefficients
of the textured specimen exhibited a small anisotropy.
The thermal conductivity of the RTGG specimen along
the ab-plane was higher than that of the RTGG
specimen along the c-axis. However, both specimens
showed similar values at high temperatures. As a result,
the Ca-doped specimen along the ab-plane with a composite
phase of (ZnO)3In2O3
and (ZnO)4In2O3 showed
a ZT value of 0.31 (at 1053 K), compared with
0.23 (at 1053 K) for the nontextured specimen.
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