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Featured researches published by K Nakahata.


Journal of Applied Physics | 2001

Growth, structure, and transport properties of thin (>10 nm) n-type microcrystalline silicon prepared on silicon oxide and its application to single-electron transistor

Toshio Kamiya; K Nakahata; Y. T. Tan; Z. A. K. Durrani; Isamu Shimizu

Microcrystalline silicon (μc-Si:H) thin films were prepared at 300 °C on glass. Their structure and transport properties were studied in a wide range of film thickness ranging from 10 nm to 1 μm. The crystal fraction increases monotonously from ∼64% to ∼100% as film thickness increases. Electron mobility first increases with increasing film thickness at thicknesses smaller than 50 nm but saturates at larger thickness. This mobility behavior is explained by percolation transport through crystalline grains. These results are different from those obtained with preferentially oriented polycrystalline silicon films. It is related to the difference in the microstructure evolution in which subsequent film growth is influenced by the growth surface structure. A single-electron transistor fabricated in 30-nm-thick μc-Si:H exhibits Coulomb blockade effects at 4.2 K. This result indicates that amorphous phases which exist between crystalline grains behave as tunnel barrier for electrons.


Thin Solid Films | 1999

Control of orientation from random to (220) or (400) in polycrystalline silicon films

Toshio Kamiya; K Nakahata; A Miida; Charles M. Fortmann; Isamu Shimizu

The control of grain orientation in polycrystalline silicon thin films on glass substrates by low-temperature techniques was investigated. Either (220) or (400) preferential grain orientation could be attained by control of source gas ratio over substrate temperatures between 250°C and 360°C. A remote type plasma chemical vapor deposition system was used with source gas mixtures of SiF 4 , H 2 and Ar. The (220) preferential films were obtained with Ar/H 2 /SiF 4 gas flow rates of 60/15/30 seem (respectively), while the (400) preferential oriented films were obtained at higher SiF 4 /H 2 ratios (SiF 4 /H 2 = 90/10 sccm). At the higher SiF 4 /H 2 ratio during the crystal nucleation stage, either randomly oriented or (400) grains formed followed by the highly preferred deposition of (400) oriented crystallites. Raman scattering and ellipsometry spectra indicated that the (400) oriented films had a very smooth surface.


Thin Solid Films | 1999

Carrier transport, structure and orientation in polycrystalline silicon on glass

K Nakahata; A Miida; Toshio Kamiya; Charles M. Fortmann; Isamu Shimizu

Abstract Polycrystalline silicon films exhibiting (220) and (400) preferential orientation in X-ray diffraction (XRD) were grown on glass substrate from gaseous mixture of SiF 4 and H 2 , respectively, using a remote type plasma enhanced chemical vapor deposition (PECVD). In particular, the grains of (400) oriented texture showed smooth surface resulting from its highly selective sticking of deposition precursor on a certain site of (100) surface. Hall mobility at room temperature of (220) and (400) oriented films rose by 12 and 7 cm 2 /Vs, respectively, with increasing the grain size and decreasing the structure fluctuation. In addition, the Hall mobility observed in these films is characterized by a thermally activated process given by the equation, μ = μ 0 exp (− Eμ /kT) where μ 0 , E μ, k and T are the extended mobility, activation energy, Boltzmann constant and temperature, respectively. The μ 0 increased with increasing the grain size up to 40 cm 2 /Vs at the grain size of 250 nm diameter, whereas E μ was kept constant at around 35 meV being independent of grain size, where a part of the free electrons is considered to be localized in the shallow traps being in ‘thermal contact’ with the conduction band.


Journal of Applied Physics | 2000

Optical absorption and Hall effect in (220) and (400) oriented polycrystalline silicon films

Toshio Kamiya; K Nakahata; Toshiyuki Sameshima; Takato Watanabe; T. Mouri; Isamu Shimizu

Carrier transport properties were investigated for polycrystalline silicon (poly-Si:H:F) films fabricated at 300 °C by 100 MHz plasma enhanced chemical vapor deposition from gaseous mixture of SiF4 and H2. Analysis of free carrier optical absorption (FCA) revealed that 1 μm thick (400) oriented phosphorus-doped poly-Si:H:F films with a carrier concentration of 5×1019 cm−3 had the average electron mobility in crystalline grains at 40 cm2/V s, while the electron mobility of the (220) oriented phosphorus-doped poly-Si:H:F films was only 12 cm2/V s. These results indicated that (400) oriented poly-Si:H:F films had excellent quality crystalline grains. Analyses of the FCA combined with Hall effect current measurements revealed that the electrical conductivity at grain boundaries of top doped films increased as the underlying film thickness increased from 0 to 280 nm for (400) oriented phosphorus-doped/undoped double layered samples, but grain boundaries still acted as large resistive regions limiting the effec...


Journal of Non-crystalline Solids | 2000

Microstructure and photovoltaic properties of low temperature polycrystalline silicon solar cells fabricated by VHF-GD CVD using fluorinated gas

Kazuyoshi Ro; K Nakahata; Toshio Kamiya; Charles M. Fortmann; Isamu Shimizu

Abstract Polycrystalline silicon (poly-Si) photovoltaic devices were fabricated from SiF 4 , H 2 and SiH 4 gas mixtures using very high frequency (100 MHz) chemical vapor deposition (VHF CVD). The gas flow rate and the SiF 4 /H 2 /SiH 4 ratio were optimized for low temperature, device quality poly-Si growth. The n-layers used for the n/i/Pt Schottky diodes and n/i/p solar cells were deposited at 300°C and i-layers at 100–300°C to provide a seed layer for subsequent i-layer growth. The open circuit voltage increased with i-layer growth temperature in the n/i/Pt Schottky diodes. However, the optimal fill factor (FF) occurred at a ∼200°C i-layer growth temperature. The FF decreased at temperatures >250°C. The best solar cells had short circuit current of 24 mA/cm 2 and energy conversion efficiency of 6.22%. Also, it was possible to prepare high efficiency solar cells at relatively high growth rates, 0.5 nm/s, by optimizing the SiH 4 flow rate.


MRS Proceedings | 1999

High Rates and Very Low Temperature Fabrication of Polycrystalline Silicon From Fluorinated Source GAS and Their Transport Properties

Toshio Kamiya; K Nakahata; Kazuyoshi Ro; Charles M. Fortmann; Isamu Shimizu

Low temperature (50--300 C) growth of polycrystalline silicon (poly-Si) by very high frequency (100 MHz) glow-discharge plasma enhanced CVD using SiF{sub 4} and H{sub 2} mixtures was studied. The poly-Si microstructure was strongly affected by the SiF{sub 4}/H{sub 2} gas flow ratio. For example, either (220) or (400) preferentially oriented films were prepared by appropriate SiF{sub 4}/H{sub 2} ratio selection. The addition of small SiH{sub 4} flows to the SiF{sub 4}/H{sub 2} mixtures could be used to increase the growth rate while the SiF{sub 4}/H{sub 2} continued to control the film structures such as preferential orientation. Highly crystalline films were grown at a growth rate of 0.52 nm/s using SiF{sub 4}/H{sub 2}/SiH{sub 4} flow rates of 30/90/2.0sccm (respectively). However, at higher SiH{sub 4} flows amorphous films were deposited. Under the small SiF{sub 4}/H{sub 2} ratio condition, highly crystallized poly-Si was grown at temperatures as low as 50 C. N/i/Pt Schottky diode solar cells were prepared using these poly-Si for both the n- and the i-layers. These solar cells exhibited good performance; for example, open circuit voltages over 0.32 V. N-i-p solar cell results are very promising with 6.2% of conversion efficiency being achieved in the initial trials.


Thin Solid Films | 2001

Free carrier optical absorption used to analyze the electrical properties of polycrystalline silicon films formed by plasma enhanced chemical vapor deposition

Toshiyuki Watanabe; Toshiyuki Sameshima; K Nakahata; Toshio Kamiya; Isamu Shimizu

Ž. The electrical properties of 400 oriented polycrystalline silicon films fabricated at 300C by 100-MHz plasma enhanced chemical vapor deposition from gaseous mixture of SiF H SiH are reported. A double layered structure of phosphorus-doped 42 4 Ž. poly-SiHF film 200 nm undoped poly-SiHF film was adopted to research the changes in electrical properties in the doped layer induced by the undoped layer thickness. The carrier mobility in the crystalline grain of the doped layer, analyzed by free carrier optical absorption, increased from 10 to 35 cm 2 Vs as the undoped film thickness increased from 0 to 1000 nm. The carrier density in the crystalline grain was 2.5 10 20 cm 3 for each sample. The grain properties in the doped layer improved as the undoped film thickness increased. 2001 Elsevier Science B.V. All rights reserved. Ž.


Thin Solid Films | 2001

Structure and transport properties of p-type polycrystalline silicon fabricated from fluorinated source gas

Toshio Kamiya; K Nakahata; Kazuyoshi Ro; Isamu Shimizu

P-type polycrystalline silicon (poly-Si) thin films are fabricated from SiF4, H2 and SiH4 gas mixtures by plasma-enhanced chemical vapor deposition using in situ doping of B2H6 or BF3. Relationships between deposition condition, film structure and transport properties are focused on. In addition, ionization efficiency is discussed in connection with deposition condition and with B and F concentrations. When poly-Si is doped with B, crystal fractions are lower for 300°C-grown films than for 200°C-grown films. High B doping results in (220) preferential orientation even when no fluorinated gas is used in the gas source. These trends are very similar among films prepared using different doping sources, i.e. B2H6 and BF3. When SiF4 is used, the B ionization efficiency is very low, ∼10%. It is improved to ∼50% by removing SiF4 from the gas source. This low ionization efficiency shows good correlation with atomic concentration ratio of B/F in the poly-Si films, suggesting that some high incorporation of F compared to B inhibits the ionization of B. Using SiH4, H2 and BF3 without SiF4, highly (220) oriented, large grain poly-Si is obtained and it exhibits large mobility of 3.7 cm2/Vs.


Journal of Non-crystalline Solids | 2000

Anisotropic carrier transport in preferentially oriented polycrystalline silicon films fabricated by very-high-frequency plasma enhanced chemical vapor deposition using fluorinated source gas

K Nakahata; Toshio Kamiya; Charles M. Fortmann; Isamu Shimizu; H Stuchlı́ková; A Fejfar; J Kočka


Journal of Non-crystalline Solids | 2000

Structural properties of polycrystalline silicon films having varied textures fabricated with intentional control of surface reactions using SiF4/H2/SiH4 mixing gas

Toshio Kamiya; K Nakahata; Charles M. Fortmann; Isamu Shimizu

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Isamu Shimizu

Tokyo Institute of Technology

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Toshio Kamiya

Tokyo Institute of Technology

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Charles M. Fortmann

Tokyo Institute of Technology

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Kazuyoshi Ro

Tokyo Institute of Technology

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A Miida

Tokyo Institute of Technology

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Atsushi Suemasu

Tokyo Institute of Technology

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Toshiyuki Sameshima

Tokyo University of Agriculture and Technology

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T. Mouri

Tokyo University of Agriculture and Technology

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Takashi Komaru

Tokyo Institute of Technology

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Takato Watanabe

Tokyo University of Agriculture and Technology

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