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| RESEARCH ACTIVITIES |
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| Group IV Semiconductor Nanowires |
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119. Takahiro Kawashima, Tohru Saito, Kazunori Komori, and Minoru Fujii,
Synthesis of a Si Nanowire with Thermally-oxidized Shell - Effects of the
Shell on Transistor Characteristics
Thin Solid Films, Vol. 57, pp. 4520-4526 (2009).
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![[ABSTRACT]](logo312.gif) Silicon nanowires (SiNWs) with a single-crystalline Si core and a thermally
oxidized shell (core-shell SiNWs) were synthesized by gold-catalyzed chemical
vapor deposition followed by rapid thermal oxidation. To synthesize high-quality
core-shell SiNWs, the relationship between the growth parameters and the
crystallinity was studied. Furthermore, the formation process of the oxide
shell was analyzed in detail by transmission electron microscopy. Using
SiNWs as channels, back gate-type field effect transistors (FETs) were
fabricated on p-type silicon wafers. FETs with core-shell SiNWs channels
exhibited smaller hysteresis in the drain current (Id) vs. gate voltage
(VGS) characteristics and higher on/off drain current ratio (ION/IOFF)
than those with bare SiNWs channels. |
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114. Chiharu Nishimura, Go Imamura, Minoru Fujii, Takahiro Kawashima, Tohru Saitoh, and Shinji Hayashi,
“Raman Characterization of B and Ge Distribution in Individual B-doped Si1-xGex Alloy Nanowires,”
The Journal of Physical Chemistry C, Vol. 113, No. 14, pp. 5467-5471 (2009). |
| The distribution of Ge composition and active B concentration in individual
B-doped Si1−xGex alloy nanowires (SiGeNWs) grown by the vapor liquid solid
(VLS) process is studied by micro Raman spectroscopy by shifting the measurement
position from the catalyst side to the substrate side. The Raman data reveal
that there is strong correlation between the Ge composition and the active
B concentration within individual SiGeNWs. We show that B-doped SiGeNWs
have a core−shell structure with a low-Ge composition and low-B concentration
core grown by the VLS process covered with a high-Ge composition and high-B
concentration shell grown by the conformal deposition. |
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110. Chiharu Nishimura, Go Imamura, Minoru Fujii,
Takahiro Kawashima, Tohru Saitoh, and Shinji Hayashi
"Raman Characterization of Ge Distribution in Individual Si1-xGex Alloy Nanowires,”
Applied Physics Letters, Vol. 93, 203101, pp. 1-3 (2008). |
| The distribution of Ge composition in an individual Si1−xGex alloy nanowire
(SiGeNW) grown by the vapor liquid solid (VLS) process was studied by micro-Raman
spectroscopy by changing the measurement position from the catalyst side
to the substrate side. The Si–Si mode in Raman spectra were found to split
into two peaks and the intensity ratio as well as the wavenumbers of the
two peaks depended on the measurement position. The Raman data revealed
that SiGeNWs have a core-shell structure with a low-Ge composition core
grown by the VLS process covered with a high-Ge composition shell grown
by the conformal deposition. |
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109. Takahiro Kawashima, Tatsunori Mizutani, Hiroyuki Matsuda, Tohru Saitoh, and Minoru Fujii
"Initial Stage of Vapor-Liquid-Solid Growth of Si Nanowires,”
The Journal of Physical Chemistry C, Vol. 112, pp. 17121-17126 (2008). |
| The initial stage of gold (Au)-catalyzed vapor−liquid−solid (VLS) growth
of Si nanowires (SiNWs) was studied by transmission electron microscope
(TEM) and atomic force microscope (AFM). Analysis and classification of
the shape of SiNWs in TEM images revealed that there is strong correlation
between the shape and crystallinity of Au catalysts and those of SiNWs,
and the morphology of SiNWs is determined at a very early stage of the
growth. The transition from initial horizontal growth of SiNWs on SiO2
substrates to vertical growth was studied by AFM, and it was demonstrated
that SiNWs change growth direction abruptly by the formation of kinks. |
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107. Go. Imamura, Takahiro Kawashima, Minoru Fujii, Chiharu Nishimura, Tohru Saitoh, and Shinji Hayashi,
"Distribution of Active Impurities in Single Silicon Nanowires,”
Nano Letters, Vol. 8, No. 9, pp. 2620-2624 (2008). |
| The distribution of electrically active B concentration in single SiNWs (nanowires) grown by a vapor−liquid−solid (VLS) process was studied by analyzing Fano resonance in Raman spectra. We found a gradient of active B concentration along the growth direction; the B concentration was the largest at the substrate side and the smallest at the catalyst side. The observed concentration gradient suggests the conformal growth of a high B concentration layer during a VLS process. To confirm this effect, we grew SiNWs with controlled impurity profiles, that is, p-type/intrinsic (p-i) and intrinsic/p-type (i-p) SiNWs, by controlling the supply of B source during SiNWs growth. We found that p-i SiNWs can be grown by just stopping the supply of B source in the middle of the growth, while i-p SiNWs were not realized; that is, the whole region of nominal “i-p” SiNWs was B-doped even if we started the supply of B source in the middle of the growth. These results confirm the above doping model. We also found that the distribution of active B concentration was significantly modified by high temperature annealing. By annealing at 1100 °C for 1 min, B concentration became almost uniform along 10 μm long SiNWs irrespective of initial B profiles. This suggests very efficient diffusion of B atoms in a defective high B concentration surface layer of SiNWs. |
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99. Takahiro Kawashima, Tatsunori Mizutani, Tohru Nakagawa, Hideo Torii,
Tohru Saitoh, Kazunori Komori, and Minoru Fujii,
"Control of Surface Migration of Gold Particles on Si Nanowires,”
Nano Letters Vol. 8, No. 1, pp. 362-368 (2008). |
| On the surface of silicon nanowires (SiNWs) synthesized by gold (Au)-catalyzed
chemical vapor deposition (CVD), Au particles 5−20 nm in diameter are formed
if the growth conditions are within a specific range. We studied the mechanism
of Au particle formation by growing SiNWs under different conditions, specifically
by dynamically changing the growth parameters during the growth process.
We show that insufficient supply of Si source to the Au−Si eutectic on
top of the SiNWs enhances the migration of Au atoms on the surface of SiNWs
in the form of Au−Si eutectic, which is precipitated on the surface as
Au particles during cooling. We also show that using Au−Si eutectic on
the surface of SiNWs as a catalyst enables one-step growth of branched
SiNWs. |
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98. Takahiro Kawashima, Goh Imamura, Minoru Fujii, Shinji Hayashi, Tohru Saitoh, and Kazunori Komori,
“Raman and Electron Microscopic Studies of Si1-xGex Alloy Nanowires Grown by Chemical Vapor Deposition,”
Journal of Applied Physics, Vol. 102, 124307, pp. 1-6 (2007). |
| Si1−xGex alloy nanowires (SiGeNWs) were grown by Au-catalyzed chemical
vapor deposition and studied by Raman spectroscopy, transmission electron
microscopy (TEM), and energy-dispersive x-ray spectroscopy (EDS) in TEM
(TEM-EDS). The relationship between the growth parameters and the structure
of the SiGeNWs was clarified by systematically changing the growth conditions
over a wide range. Raman and TEM-EDS results demonstrated that the SiGeNWs
consist of a lower Ge composition core and a higher Ge composition shell
epitaxially grown on the surface of the core. The effects of oxidation
on the structure of the SiGeNWs were studied. It was found that oxidation
leads to segregation of the Ge atoms at the interface between the SiGeNWs
and SiO2, which in turn results in a large inhomogeneity in Ge composition.
Oxidation at a very low rate in a diluted oxygen gas atmosphere is required
to avoid the formation of Ge particles and minimize the inhomogeneity. |
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94. Takahiro Kawashima, Goh Imamura, Tohru Saitoh, Kazunori Komori, Minoru Fujii, and Shinji Hayashi,
"Raman Scattering Studies of Electrically Active Impurities in In-situ B-Doped Silicon Nanowires: Effects of Annealing and
Oxidation,”
Journal of Physical Chemistry C, Vol. 111, pp. 15160-15165, October (2007).
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| The concentration of electrically active impurities in in situ boron (B)-doped
silicon (Si) nanowires (SiNWs) synthesized by gold (Au)-catalyzed chemical
vapor deposition (CVD) is studied by Raman spectroscopy. B-doped SiNWs
exhibit an asymmetric Raman spectrum due to Fano resonance between discrete
phonon Raman scattering and continuous electric Raman scattering caused
by the excitation of holes in the valence band. To quantitatively evaluate
the concentration of active B atoms from the asymmetric spectral shape,
the spectra are fitted by a Fano resonance formula and asymmetry parameters
are extracted. From the comparison of the asymmetry parameter with those
obtained for reference samples, the concentration of active B atoms in
SiNWs is estimated. The effects of thermal annealing in nitrogen and oxygen
gases on the active B concentration are also studied. The annealing in
nitrogen ambience for a short period significantly increases the concentration
of active B atoms especially when the doping level is high, while longer
period annealing decreases the active B concentration. The reduction of
active B concentration is more significant when SiNWs are annealed in oxygen
gas ambience. By combining Raman results with high-resolution transmission
electron microscope observations, the growth mechanism of B-doped SiNWs
is discussed. We show that the number of B atoms doped into SiNWs via Au
catalysts is very limited and high B concentration layers are grown by
conformal growth on the sidewall of SiNWs. |
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