Akito Hara
Tohoku Gakuin University
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Featured researches published by Akito Hara.
Japanese Journal of Applied Physics | 2002
Akito Hara; Fumiyo Takeuchi; Michiko Takei; Katsuyuki Suga; Kenichi Yoshino; Mitsuru Chida; Yasuyuki Sano; Nobuo Sasaki
We have developed high-performance polycrystalline silicon (poly-Si) thin film transistors (TFTs) with a field-effect mobility of 566 cm2/Vs for n-channel TFT and 200 cm2/Vs for p-channel TFT on 300 mm×300 mm non-alkali glass substrate. The TFTs were developed using a stable diode pumped solid state (DPSS) continuous-wave laser lateral crystallization (CLC) method at a temperature below 450°C. The high performance of the TFTs was attributed to the very large predominantly (100)-oriented grain. This crystallization method will enable high-performance Si-LSI circuits to be fabricated on large non-alkali glass substrates.
Japanese Journal of Applied Physics | 2004
Akito Hara; Michiko Takei; Fumiyo Takeuchi; Katsuyuki Suga; Kenichi Yoshino; Mitsuru Chida; Tatsuya Kakehi; Yoshiki Ebiko; Yasuyuki Sano; Nobuo Sasaki
High performance low temperature polycrystalline silicon (poly-Si) thin film transistors (TFTs) with large grains were created using diode pumped solid state (DPSS) continuous wave (CW) laser lateral crystallization (CLC), employing fabrication processes at 450°C. Field-effect mobilities of 566 cm2/Vs for the n-channel and 200 cm2/Vs for the p-channel were obtained for a thick Si film (100–150 nm) on a 300×300 mm non-alkaline glass substrate. The high performance of the TFTs is attributed to the predominantly (100)-oriented very large grains. With a decreasing Si-film thickness, the grain size decreases, and the surface orientation of the grain changes from (100) to other orientations. These effects lead to reduced field-effect mobility with decreasing Si-film thickness, but it is easy to obtain a high field-effect mobility of over 300 cm2/Vs, even with a 50 nm thick Si film, without special processing techniques. A complementary metal oxide semiconductor (CMOS) ring oscillator was fabricated using a thin Si film 65 nm thick to demonstrate the high circuit performance of CLC poly-Si TFTs by applying the simplest CMOS process technology. A delay of 400 ps/stage at a gate length of 1.5 µm and a supply voltage of Vdd=5.0 (V) was produced on a large non-alkaline glass substrate utilizing a fabrication temperature of 450°C. This crystallization method will lead to the fabrication of high-performance and cheap Si-LSI circuits on large non-alkaline glass substrates.
Japanese Journal of Applied Physics | 1989
Akito Hara; Tetsuo Fukuda; Toru Miyabo; Iesada Hirai
We observed the electron spin resonance of oxygen-nitrogen complexes (ONCs) and found that they have C2V symmetry. Although they contain nitrogen, hyperfine interaction (hf) with nitrogen cannot be clearly observed. These characters of ONCs resemble thermal donors (TDs) very closely.
Japanese Journal of Applied Physics | 2003
Akito Hara; Kenichi Yoshino; Fumiyo Takeuchi; Nobuo Sasaki
We have developed a new silicon (Si) crystallization method that makes it possible to form single-crystalline Si in the channel regions of thin-film transistors (TFTs) on non-alkali glass without introducing thermal damage. The method includes using a frequency-doubled (2ω) diode-pumped solid-state (DPSS) Nd:YVO4 continuous-wave (CW) laser (λ=532 nm). The unique characteristics of this crystallization method are the introduction of a pre-defined thick capping-Si layer on a pre-patterned channel region and laser irradiation from the back surface through the glass substrate. We succeeded in forming 2-µm-wide and 20-µm-long single-crystalline Si in the channel region of a TFT. A high-performance n-channel TFT on a glass substrate was obtained using a 450°C fabrication process. The TFT had a field-effect mobility of 400 cm2/Vs, a subthreshold swing of 0.16 V/dec, and a threshold voltage of 0.24 V.
Japanese Journal of Applied Physics | 1994
Akito Hara; Masaaki Koizuka; Masaki Aoki; Tetsuo Fukuda; Hiroshi Yamada-Kaneta
Using hydrogen-enhanced thermal donor formation and hydrogen-enhanced oxygen precipitate nuclei formation, we confirmed the presence of hydrogen in as-grown Czochralski (CZ) silicon (Si) ingots. Hydrogen concentrations in as-grown ingots were very low at 2.5×1011 cm-3. We found that even such a small amount of hydrogen influences the quality of as-grown CZ Si crystals due to hydrogen-enhanced oxygen precipitate nuclei formation caused by in situ annealing during crystal growth. Reducing hydrogen contamination during crystal growth is important in obtaining high-quality CZ Si crystals.
Japanese Journal of Applied Physics | 1991
Masaki Aoki; Akito Hara; Akira Ohsawa
We present a new experimental approach to using intrinsic gettering to remove Fe impurities. We annealed samples, followed by quenching, and measured the Fe concentration near the surface using deep-level transient spectroscopy. The supersaturation of Fe impurities is necessary for the intrinsic gettering of Fe. However, for a higher supersaturation, Fe impurities precipitate faster than gettering. The optimum degree of supersaturation is one order of magnitude. Gettering is limited by the reaction of Fe with the oxygen precipitates in the defect region, rather than by Fe diffusion to the defect region.
Japanese Journal of Applied Physics | 2000
Akito Hara; Nobuo Sasaki
The nucleation site and solidification direction of polycrystalline silicon were controlled by excimer laser crystallization. The sidewall and the top of the amorphous silicon island, which includes a gradually narrowing region, were covered with a thick layer of polycrystalline silicon, and single-shot irradiation was performed from the back surface. The formation of only one nucleus was observed in a gradually narrowing region of width two times that of the lateral growth distance. Solidification from the nucleus toward a narrower width region was then effected in a region 2 µm in width and 3 µm in length. The growth mechanism is explained on the basis of the temperature gradient formed by the covering of the polycrystalline silicon and the gradually narrowing structure.
Japanese Journal of Applied Physics | 1996
Kuninori Kitahara; Katsuyuki Suga; Akito Hara; Kazuo Nakajima
Excimer laser induced crystallization of Si thin films was studied by Raman spectroscopy and transmission electron microscopy. Single pulses of laser with various energies were irradiated on two types of starting materials to analyze complicated effects of the laser crystallization. For energies higher than a threshold value, an obvious difference in crystallization was found between starting materials of amorphous Si (a-Si) and laser-crystallized Si (lc-Si). Microcrystals were generated in the former but not in the latter, which is attributed to the presence of nuclei which remained deep within the lc-Si film during the film melting. Phase variation of a-Si and degradation of lc-Si induced by small-energy irradiation were also demonstrated.
Japanese Journal of Applied Physics | 2013
Yusuke Shika; Takuro Bessho; Yasunori Okabe; Hiroyuki Ogata; Shinya Kamo; Kuninori Kitahara; Akito Hara
We investigated hydrogenation of low-temperature (LT) polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) from the point of view of the gettering phenomenon, specifically, using self-aligned metal double-gate p-channel LT poly-Si TFTs that had a small subthreshold swing value and a high field-effect mobility. Hydrogenation of TFTs was carried out by forming gas annealing. Our results indicate that the conventionally used hydrogenation temperature of 400 °C is considerably high because annealing at this temperature results in the re-emission of gettered hydrogen. Moreover, when annealing in forming gas, hydrogenation actually occurs during cooling from 400 °C, but not at 400 °C. The most important parameter for effective hydrogenation is the rate of cooling from 400 °C, but not the hydrogenation temperature of 400 °C.
Japanese Journal of Applied Physics | 2007
Akito Hara
It has been reported that shallow thermal donors (STDs) are formed in oxygen-rich silicon (Si) crystals preannealed in nitrogen gas (N-gas-doped) and also in hydrogen-doped (H-doped) oxygen-rich Si crystals. The STDs formed in these crystals exhibit very similar electronic structures. Experiments using far-infrared optical absorption showed that several hydrogen-like STDs were formed at the same time and their energy levels in both the above-mentioned crystals were very similar. It has also been reported that the g-values of the STDs formed in both the crystals were identical. On the basis of electron–nucleus double resonance results, it has been strongly suggested that a hydrogen impurity is incorporated as a structural element of the STDs formed in the H-doped Si crystals. However, the origin of the STDs that are formed in N-gas-doped Si crystals is still unclear. To clarify this point, hydrogen detection in N-gas-doped Si was conducted and the annealing behaviors of STDs in N-gas-doped Si and H-doped Si were compared by electron spin resonance and far-infrared optical absorption measurement. It was concluded that the origin of the STDs formed in N-gas-doped Si crystals is not related to the hydrogen impurity.