九州大学 研究者情報
論文一覧
西澤 伸一(にしざわ しんいち) データ更新日:2019.04.20

教授 /  応用力学研究所 附属自然エネルギー統合利用センター 新エネルギーシステム工学分野


原著論文
1. Y. Miyamura, H. Harada, S. Nakano, Shinichi Nishizawa, Koichi Kakimoto, Do thermal donors reduce the lifetimes of Czochralski-grown silicon crystals?, Journal of Crystal Growth, 10.1016/j.jcrysgro.2018.02.034, 489, 1-4, 2018.05, [URL], High-performance electronics require long carrier lifetimes within their silicon crystals. This paper reports the effects of thermal donors on the lifetimes of carriers in as-grown n-type silicon crystals grown by the Czochralski method. We grew silicon crystals with two different concentrations of thermal donors using the following two cooling processes: one was cooled with a 4-h halt after detaching the crystal from the melt, and the other was cooled continuously. The crystal grown with the cooling halt contained higher concentrations of thermal donors of the order of 1 × 1013 cm−3, while the crystal without the halt had no thermal donors. The measured bulk lifetimes were in the range of 15–18 ms. We concluded that thermal donors in Czochralski-grown silicon crystals do not act to reduce their lifetimes..
2. Akira Nakajima, Shunsuke Kubota, Kazuo Tsutsui, Kuniyuki Kakushima, Hitoshi Wakabayashi, Hiroshi Iwai, Shinichi Nishizawa, Hiromichi Ohashi, GaN-based complementary metal-oxide- semiconductor inverter with normally off Pch and Nch MOSFETs fabricated using polarisation-induced holes and electron channels, IET Power Electronics, 10.1049/iet-pel.2017.0376, 11, 4, 689-694, 2018.04, [URL], Gallium nitride (GaN)-based P-channel (Pch) and N-channel (Nch) metal-oxide-semiconductor field-effect transistors (MOSFETs) with normally off operations were realised. Both Pch and Nch MOSFETs were monolithically fabricated in a polarisation-junction platform wafer. The platform wafer was constructed with a GaN/aluminium GaN/GaN double heterostructure, which has both two-dimensional hole gas (2DHG) and 2D electron gas (2DEG). The drain currents of Pch and Nch MOSFETs flow through 2DHG and 2DEG, respectively. The threshold gate voltages of the fabricated Pch and Nch MOSFETs were -2.7 and 6.7 V, respectively. It was shown that the threshold voltage and the on-state resistance of the Pch MOSFET can be controlled by adjusting the 2DEG potential. Furthermore, using Pch and Nch MOSFETs, complementary MOS inverter operation was demonstrated..
3. Y. Miyamura, H. Harada, S. Nakano, Shinichi Nishizawa, Koichi Kakimoto, Relationship between carbon concentration and carrier lifetime in CZ-Si crystals, Journal of Crystal Growth, 10.1016/j.jcrysgro.2018.01.020, 486, 56-59, 2018.03, [URL], This paper aims to clarify the effect of carbon concentration on carrier lifetime in as-grown n-type and non-doped silicon crystals produced via the Czochralski (CZ) method. We grew n-type and non-doped silicon single crystals with 3-in. diameters along with different carbon and phosphorous contents. The resistivity, concentrations of oxygen and carbon, and lifetime were measured using four-point measurements, Fourier-transform infrared spectroscopy, and the eddy current method, respectively. The oxygen concentrations of the crystals were 6–8 × 1017 atoms/cm3, and the bulk lifetimes ranged from 10 to 20 ms. The carrier lifetime of CZ silicon crystals depended on dopant concentration but had no significant dependence on carbon concentration..
4. K. Tsutsui, K. Kakushima, T. Hoshii, A. Nakajima, Shinichi Nishizawa, H. Wakabayashi, I. Muneta, K. Sato, T. Matsudai, W. Saito, T. Saraya, K. Itou, M. Fukui, S. Suzuki, M. Kobayashi, T. Takakura, T. Hiramoto, A. Ogura, Y. Numasawa, I. Omura, H. Ohashi, H. Iwai, 3D scaling for insulated gate bipolar transistors (IGBTs) with low Vce(sat), 12th IEEE International Conference on Advanced Semiconductor Integrated Circuits, ASICON 2017
Proceedings - 2017 IEEE 12th International Conference on ASIC, ASICON 2017
, 10.1109/ASICON.2017.8252681, 2017-October, 1137-1140, 2018.01, [URL], Three dimensionally (3D) scaled IGBTs that have a scaling factor of 3 (k=3) with respect to current commercial products (k=1) were fabricated for the first time. The scaling was applied to the lateral and vertical dimensions as well as the gate voltage. A significant decrease in ON resistance, - Vce(sat) reduction from 1.70 to 1.26 V - was experimentally confirmed for the 3D scaled IGBTs..
5. Kazunori Hasegawa, Ichiro Omura, Shinichi Nishizawa, An evaluation circuit for DC-link capacitors used in a single- phase PWM inverter, 2017 International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management, PCIM Europe 2017
PCIM Europe 2017 - International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management
, 10.1109/SBMicro.2017.7990846, 2017.01, [URL], High-power conversion systems are based not only on three-phase inverters but also on singlephase inverters because modular multilevel cascade converters (MMCC) consist of half- or fullbridge single-phase converters. Their DC-link capacitors are a major constraint on the improvement of power density as well as of reliability. Evaluation of dc-link capacitors in terms of power loss, ageing, and failure rate will play an important role in the next-generation power converters. This paper presents an evaluation circuit for dc-link capacitors used in a high-power single-phase PWM inverter. The evaluation circuit produces a practical ripple current waveform and a dc bias voltage into a capacitor under test with a downscaled voltage-rating inverter, which is equivalent to those of the full-scale inverter. Theoretical analysis and experimental results verify the effectiveness of the evaluation circuit..
6. K. Kakushima, T. Hoshii, K. Tsutsui, A. Nakajima, Shinichi Nishizawa, H. Wakabayashi, I. Muneta, K. Sato, T. Matsudai, W. Saito, T. Saraya, K. Itou, M. Fukui, S. Suzuki, M. Kobayashi, T. Takakura, T. Hiramoto, A. Ogura, Y. Numasawa, I. Omura, H. Ohashi, H. Iwai, Experimental verification of a 3D scaling principle for low Vce(sat) IGBT, 62nd IEEE International Electron Devices Meeting, IEDM 2016
2016 IEEE International Electron Devices Meeting, IEDM 2016
, 10.1109/IEDM.2016.7838390, 10.6.1-10.6.4, 2017.01, [URL], Three dimensionally (3D) scaled IGBTs that have a scaling factor of 3 (k=3) with respect to current commercial products (k=1) were fabricated for the first time. The scaling was applied to the lateral and vertical dimensions as well as the gate voltage. A significant decrease in ON resistance, - Vce(sat) reduction from 1.70 to 1.26 V - was experimentally confirmed for the 3D scaled IGBTs..
7. K. Hasegawa, K. Kozuma, K. Tsuzaki, I. Omura, Shinichi Nishizawa, Temperature rise measurement for power-loss comparison of an aluminum electrolytic capacitor between sinusoidal and square-wave current injections, Microelectronics and Reliability, 10.1016/j.microrel.2016.07.049, 64, 98-100, 2016.09, [URL], DC-link capacitors are a major factor of degrading reliability of power electric converters because they usually have a shorter lifetime and higher failure rate than those of semiconductor devices or magnetic devices. Characteristics of the capacitors are usually evaluated by a single sinusoidal current waveform. However, actual current flowing out of the converter into the capacitor is a modulated square current waveform. This paper provides experimental comparison of the power loss dissipated in an aluminum electrolytic capacitor between sinusoidal and square-wave current injections. Power loss is estimated by temperature rise of the capacitor. Experimental results confirm that power losses of the square-wave current injection were always lower than those of the sinusoidal current injection by 10–20%. Moreover, the power losses of the square-wave current injection can be estimated by a synthesis of fundamental and harmonic currents based on the Fourier series expansion, which brings a high accuracy less than 1% when more than fifth harmonic current is introduced. This comparison will be useful for estimating power loss and life time of electrolytic capacitors..
8. Kazunori Hasegawa, Ichiro Omura, Shinichi Nishizawa, A new evaluation circuit with a low-voltage inverter intended for capacitors used in a high-power three-phase inverter, 31st Annual IEEE Applied Power Electronics Conference and Exposition, APEC 2016
2016 IEEE Applied Power Electronics Conference and Exposition, APEC 2016
, 10.1109/APEC.2016.7468295, 2016-May, 3032-3037, 2016.05, [URL], DC-link capacitors in power electronic converters are a major constraint on improvement of power density as well as reliability. Evaluation of the dc-link capacitors in terms of power loss, ageing, and failure rate will play an important role in design stages of the next-generation power converters. This paper proposes a new evaluation circuit for dc-link capacitors used in a high-power three-phase inverter, which is intended for testing power loss, failure rate, ageing, and so on. The evaluation circuit produces a practical ripple current waveform and a dc bias voltage into a capacitor under test, in which the ripple current is equivalent to that generated by the three-phase inverter on the dc link. The evaluation circuit employs a full-scale current-rating and downscaled voltage-rating inverter for producing the ripple current, so that the power rating of the evaluation circuit is much smaller than that of a full-scale current rating and full-scale voltage rating inverter..
9. Kazunori Hasegawa, Ichiro Omura, Shinichi Nishizawa, Design and Analysis of a New Evaluation Circuit for Capacitors Used in a High-Power Three-Phase Inverter, IEEE Transactions on Industrial Electronics, 10.1109/TIE.2015.2511097, 63, 5, 2679-2687, 2016.05, [URL], DC-link capacitors in power electronic converters are a major constraint on improvement of power density as well as reliability. Evaluation of the dc-link capacitors in terms of power loss, ageing, and failure rate will play an important role in design stages of the next-generation power converters. This paper proposes a new evaluation circuit for dc-link capacitors used in a high-power three-phase inverter, which is intended for testing power loss, failure rate, ageing, and so on. The evaluation circuit produces a practical ripple current waveform and a dc bias voltage into a capacitor under test, in which the ripple current is equivalent to that generated by the three-phase inverter on the dc link. The evaluation circuit employs a full-scale current-rating and downscaled voltage-rating inverter for producing the ripple current, so that the power rating of the evaluation circuit is much smaller than that of a full-scale current-rating and full-scale voltage-rating inverter. Theoretical analysis and simulated results verify the effectiveness of new evaluation circuit..
10. S. Araki, B. Gao, Shinichi Nishizawa, S. Nakano, Koichi Kakimoto, Total pressure-controlled PVT SiC growth for polytype stability during using 2D nucleation theory, Crystal Research and Technology, 10.1002/crat.201500344, 51, 5, 344-348, 2016.05, [URL], A total pressure-controlled physical vapor transport growth method that stabilizes SiC polytype is proposed. The supersaturation of carbon during SiC growth changed as a function of the growth time due to changes in the temperature difference between the surfaces of the source and the grown crystal. Supersaturation also varied as a function of the pressure inside the furnace. Therefore, modification of the pressure as a function of growth time allowed for constant supersaturation during growth. The supersaturation was calculated based on classical thermodynamic nucleation theory using data for heat and species of Si2C and SiC2 transfer in a furnace obtained from a global model. Based on this analysis, a method for polytype-stabilized SiC growth was proposed that involves decreasing the pressure as a function of growth time. The 4H-SiC prepared using this pressure-controlled method was more stable than that of 4H-SiC formed using the conventional constant-pressure method..
11. Shinichi Nishizawa, Wafer requirement for future power devices, 35th IEEE Region 10 Conference, TENCON 2015
TENCON 2015 - 2015 IEEE Region 10 Conference
, 10.1109/TENCON.2015.7372890, 2016-January, 2016.01, [URL], For the future power electronics system, one of the key issues is to achieve more high power densities. At the moment, several types of silicon devices are widely used from the low voltage to high voltage ranges. To achieve more high power densities by size reduction and more high operating temperature, not only the device but also wafer material should be improved. The current status and future prospect of PE system with silicon technology will be discussed. Then, the role of compound semiconductor and its devices will be also discussed by comparison with silicon PE system..
12. Akira Nakajima, Shinichi Nishizawa, Shunsuke Kubota, Rei Kayanuma, Kazuo Tsutsui, Hiromichi Ohashi, Kuniyuki Kakushima, Hitoshi Wakabayashi, Hiroshi Iwai, An Overview of GaN-Based Monolithic Power Integrated Circuit Technology on Polarization-Junction Platform, 37th IEEE International Symposium on Workload Characterization, IISWC 2015
2015 IEEE Compound Semiconductor Integrated Circuit Symposium, CSICS 2015
, 10.1109/CSICS.2015.7314489, 2015.10, [URL], This paper gives an overview of GaN-based polarization-junction (PJ) technologies. PJ platform wafers have both high-density 2D hole gas (2DHG) and 2D electron gas (2DEG) respectively induced by negative and positive polarization charges in undoped GaN/AlGaN/GaN double heterostructures. On the platform, monolithic operations of GaN-based n-channel transistors and p-channel transistors have been demonstrated. Because of temperature independent properties of the 2DHG and 2DEG, the GaN devices can operate in wide temperature range. In addition, high voltage transistors and diode are also available on the platform by using polarization-superjunction concept..
13. Sethavut Duangchan, Yusuke Uchikawa, Yusuke Koishikawa, Baba Akiyoshi, Kentaro Nakagawa, Satoshi Matsumoto, Masataka Hasegawa, Shinichi Nishizawa, The silicon on diamond structure by low-temperature bonding technique, 2015 65th IEEE Electronic Components and Technology Conference, ECTC 2015
2015 IEEE 65th Electronic Components and Technology Conference, ECTC 2015
, 10.1109/ECTC.2015.7159590, 2015-July, 187-192, 2015.07, [URL], We demonstrate fabrication a silicon on diamond structure at around room temperature using a plasma-activated bonding (PAB) method. Thin and flat silicon-dioxide (SiO2) film was used as an activation layer for PAB. The SiO2 film was prepared by a chemical vapor deposition and then a chemical mechanical polishing (CMP). The surface roughness after the CMP were average ∼1 nm rms at 300 nm thick. Thinning of the SiO2 film was carried out using 2.5%HF solution. We found that there are no significant change in the surface roughness after the thinning process. The roughness of SiO2 less than or equal to 1 nm is required for success bonding at low-temperature with vacuum environment. The scanning electron microscope has shown seamless at the bonding interface that proves to good bonding result..
14. Akira Nakajima, Shinichi Nishizawa, Hiromichi Ohashi, Rei Kayanuma, Kazuo Tsutsui, Shunsuke Kubota, Kuniyuki Kakushima, Hitoshi Wakabayashi, Hiroshi Iwai, GaN-based monolithic power integrated circuit technology with wide operating temperature on polarization-junction platform, 27th IEEE International Symposium on Power Semiconductor Devices and IC's, ISPSD 2015
2015 IEEE 27th International Symposium on Power Semiconductor Devices and IC's, ISPSD 2015
, 10.1109/ISPSD.2015.7123463, 2015-June, 357-360, 2015.06, [URL], Polarization junction platforms have high-density 2D hole gas (2DHG) and 2D electron gas (2DEG) respectively induced by negative and positive polarization charges in undoped GaN/AlGaN/GaN double heterostructures. Sheet resistance measurements in a wide temperature range (6-460 K) revealed that 2DHG and 2DEG resistances were monotonically enhanced with the temperature reduction. On the platform, monolithic operations of GaN-based devices including high-voltage n-channel (N-ch) transistors, N-ch Schottky diodes, low-voltage N-ch transistors and p-channel transistors has been demonstrated..
15. Akira Nakajima, Yosuke Tateishi, Hiroshi Murakami, Hidetomo Takahashi, Michiharu Ota, Ryoji Kosugi, Takeshi Mitani, Shinichi Nishizawa, Hiromichi Ohashi, High-speed dicing of SiC wafers by femtosecond pulsed laser, European Conference on Silicon Carbide and Related Materials, ECSCRM 2014
Silicon Carbide and Related Materials 2014
, 10.4028/www.scientific.net/MSF.821-823.524, 524-527, 2015.01, [URL], A novel dicing technology that utilizes femtosecond pulsed lasers (FSPLs) is demonstrated as a high-speed and cost-effective dicing process for SiC wafers. The developed dicing process consists of cleavage groove formation on a SiC wafer surface by the FSPL, followed by chip separation by pressing a cleavage blade. The effective FSPL scan speed on the SiC surfaces was 33 mm/s. Kerf loss is negligible in the developed FSPL dicing process. In addition, the residual lattice strain in the FSPL-diced SiC chips was comparably small to that of the conventional mechanical process using diamond saws, due to the absence of the lattice heating effect in femtosecond-laser processes..
16. Akira Nakajima, Wataru Saito, Shinichi Nishizawa, Hiromichi Ohashi, Theoretical Loss analysis of power converters with 1200 v class Si-IGBT and SiC-MOSFET, 2015 International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management, PCIM Europe 2015
PCIM Europe 2015; International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management; Proceedings of
, 2015, Power converter efficiencies of 1200-V class Si-IGBT/SiC-SBD hybrid pairs and a SiCMOSFET/ SiC-SBD pair were theoretically compared at a switching frequency above the limit of human hearing (20 kHz). Si-IGBT losses were simulated by TCAD. SiC device losses were calculated by analytical minimum loss models. Calculated efficiencies of the full-SiC pair were slightly higher than that of the hybrid pairs at conventional current densities less than 200 A/cm2. At a higher current density of 400 A/cm2, the hybrid pairs have a potential of high performance which is comparable with the full-SiC efficiency..
17. Akira Nakajima, Pucheng Liu, Masahiko Ogura, Toshiharu Makino, Kuniyuki Kakushima, Shinichi Nishizawa, Hiromichi Ohashi, Satoshi Yamasaki, Hiroshi Iwai, Generation and transportation mechanisms for two-dimensional hole gases in GaN/AlGaN/GaN double heterostructures, Journal of Applied Physics, 10.1063/1.4872242, 115, 15, 2014.04, [URL], The electrical properties of two-dimensional hole gases (2DHGs) in GaN/AlGaN/GaN double heterostructures were investigated. The layers were grown on sapphire substrates and a high-quality bulk GaN substrate. The coexistence of 2DHG and 2D electron gases on both sides of the AlGaN layer was confirmed by Hall effect measurements at 80-460 K. It was also verified that the 2DHGs were generated by negative polarization at the undoped GaN/AlGaN interface, which did not have a doped Mg acceptor. It was also demonstrated that the 2DHG density could be controlled by varying the AlGaN layer thickness and was inversely related to the 2DHG mobility. The measured relation indicated that the 2DHG mobility is mainly limited by phonon scatterings at around room temperature. As a result, the maximum 2DHG mobility of 16 cm/Vs at 300 K was achieved with a density of 1 × 10 cm-2..
18. Yuji Matsuoka, Keiji Wada, Mizuki Nakahara, Kazuto Takao, Kyungmin Sung, Hiromichi Ohashi, Shinichi Nishizawa, 2.5kV, 200kW bi-directional isolated DC/DC converter for medium-voltage applications, 7th International Power Electronics Conference, IPEC-Hiroshima - ECCE Asia 2014
2014 International Power Electronics Conference, IPEC-Hiroshima - ECCE Asia 2014
, 10.1109/IPEC.2014.6869671, 744-749, 2014.01, [URL], A bi-directional isolated DC/DC converter for medium-voltage applications have been discussed for the next-generation electrical grid, such as smart girds. To realize the DC/DC converter for installing on a power distribution system, it should be achieved higher-efficiency and lower-volume. Higher switching frequency enables to reduce the volume of the transformer. However, it is difficult to realize the both higher-efficiency and lower-volume of the DC/DC converter, because it could not operate higher frequency with high power devices such as medium-voltage IGBTs. This paper discusses switching losses and operation limitations for the actual medium-voltage isolated DC/DC converter, in the case of using Si-IGBT. In addition, the experimental system rated at 2.5 kV, 200 kW, and 5kH of the DC/DC converter with a medium-frequency transformer is designed, and the experimental results are shown..
19. Kentaro Nakagawa, Takuya Kodama, Satoshi Matsumoto, Takatoshi Yamada, Masataka Hasegawa, Shinichi Nishizawa, Impact of semiconductor on diamond structure for power supply on chip applications, Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes, 10.7567/JJAP.53.04EP16, 53, 4 SPEC. ISSUE, 2014.01, [URL], In this study, we assessed a semiconductor (silicon or GaN)-on-diamond (SeOD) structure and compared it with a conventional silicon on insulator (SOI) structure, i.e., diamond, for power-supply-on-chip (power-SoC) applications by numerical simulations. The SeOD structure has thermal advantages over the conventional SOI structure without degrading electrical characteristics even using a thin diamond film (0.3 μm)..
20. T. Shiramomo, B. Gao, F. Mercier, Shinichi Nishizawa, S. Nakano, Koichi Kakimoto, Study of the effect of doped impurities on polytype stability during PVT growth of SiC using 2D nucleation theory, Journal of Crystal Growth, 10.1016/j.jcrysgro.2013.03.036, 385, 95-99, 2014.01, [URL], The effect of nitrogen and aluminum as doped impurities on the stability of SiC polytypes (C- or Si-face 4H and 6H substrates) formed by physical vapor transport (PVT) was investigated. The stability of the polytypes was analyzed using classical thermodynamic nucleation theory with numerical results obtained from a global model including heat, mass and species transfer in a PVT furnace. The results reveal that the formation of 4H-SiC was more stable than that of 6H-SiC when a grown crystal was doped with nitrogen using C-face 4H- and 6H-SiC as seed crystals. In contrast, formation of 6H-SiC was favored over 4H-SiC when Si-face 4H- and 6H-SiC seed crystals were used. Meanwhile, the formation of 4H-SiC was more stable than that of 6H-SiC when aluminum was the dopant and C- and Si-faces of 6H-SiC were used as seed crystals. Formation of 6H-SiC occurred over that of 4H-SiC in the cases of C- and Si-faces of 4H-SiC as seed crystals..
21. Kentaro Nakagawa, Takuya Kodama, Satoshi Matsumoto, Takatoshi Yamada, Masataka Hasegawa, Shinichi Nishizawa, Erratum
Impact of semiconductor on diamond structure for power supply on chip applications (Japanese Journal of Applied Physics (2014) 53 (04EP16)), Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes, 10.7567/JJAP.53.069202, 53, 6, 2014, [URL].
22. S. Nakashima, T. Tomita, N. Kuwahara, T. Mitani, M. Tomobe, Shinichi Nishizawa, H. Okumura, Raman intensity profiles of zone-folded modes in SiC
Identification of stacking sequence of 10H-SiC, Journal of Applied Physics, 10.1063/1.4828996, 114, 19, 2013.11, [URL], Raman intensity profiles are measured for 10H-SiC crystals, for which various zone-folded phonon modes are observed. Raman intensity profiles are calculated based on a bond polarizability model assuming several stacking sequences for the 10H polytype using a linear chain model. Among several candidates for the stacking sequences, the 3322 stacking structure provides the best-fit profile for experimental spectral profiles. The hexagonality value of 0.4 predicted from the stacking sequence of this polytype is consistent with that derived from the frequency splitting between the experimental A1 and E-type transverse optical modes. This fact is consistent with an empirical rule that the value of the reduced wavevector for the strongest folded transverse acoustic and optical modes are equal to the hexagonality of the polytype. In the present analysis of the Raman intensity profiles, the calculated intensity profiles for specified folded transverse optical modes are found to be relatively strong and strikingly dependent on force-field parameters in α-SiC that consists of the mixture of the cubic and hexagonal stacking structures. These force-field parameters can reproduce well the experimental Raman intensity profiles of various SiC polytypes including 10H-SiC..
23. Akira Nakajima, Pucheng Liu, Masahiko Ogura, Toshiharu Makino, Shinichi Nishizawa, Satoshi Yamasaki, Hiromichi Ohashi, Kuniyuki Kakushima, Hiroshi Iwai, Temperature-independent two-dimensional hole gas confined at GaN/AlGaN heterointerface, Applied Physics Express, 10.7567/APEX.6.091002, 6, 9, 2013.09, [URL], The electrical properties at the GaN(0001)/AlGaN(0001) heterointerface have been investigated. Capacitance-voltage measurement and theoretical simulation results verify the existence of a two-dimensional (2D) hole gas, which is highly confined to the GaN/AlGaN interface with a peak carrier density of over 10 19 cm-3. Hall-effect measurements in the 80-460 K temperature range reveal that the 2D hole gas has zero activation energy and positive sheet-resistance temperature coefficients..
24. Frédéric Mercier, Shinichi Nishizawa, Comparative numerical study of the effects of rotating and traveling magnetic fields on the carbon transport in the solution growth of SiC crystals, Journal of Crystal Growth, 10.1016/j.jcrysgro.2011.11.019, 362, 1, 99-102, 2013.01, [URL], We present numerical simulations of the high temperature solution growth (HTSG) of silicon carbide (SiC) crystals. From a global simulation model, we investigate the influence of rotating magnetic fields (RMFs) and traveling magnetic fields (TMFs) on the crystal growth rate. The results reveal that heat and mass transfers are affected by magnetic fields. We show that direction of the solute flux must be controlled to increase the growth rate. For example, in presence of TMFs directed downwards the growth rate increases up to three times compared with the pure thermal HTSG. The proposed HTSG system coupled with magnetic fields has the same growth rate possibility as in the sublimation technique..
25. S. Leone, A. Henry, E. Janzén, Shinichi Nishizawa, Epitaxial growth of SiC with chlorinated precursors on different off-angle substrates, Journal of Crystal Growth, 10.1016/j.jcrysgro.2011.09.061, 362, 1, 170-173, 2013.01, [URL], This study focuses on the epitaxial growth of silicon carbide (SiC) epitaxial layers, adopting the chloride-based chemical-vapor-deposition (CVD) process, which allows to achieve ten times higher growth rate compared to the standard process based on the mixture of a silicon-containing gas and a hydrocarbon. In order to improve the material quality, substrates with different off-angles were used, since low off-angle substrates result in a reduction of killer defects for specific devices. Different growth mechanisms dominate for different substrate off-cut and an accurate set up of dedicated surface preparation procedures and tuning of growth parameters are needed. This study demonstrates that silicon-rich gas inputs are favorable for lower off-angle (nominally on-axis) substrates, while carbon-rich are beneficial for higher off-angles (usually 8° off-axis for 4H-SiC). Methyltrichlorosilane (MTS) is shown to be the best precursor to achieve the presented results..
26. Frédéric Mercier, Shinichi Nishizawa, Role of surface effects on silicon carbide polytype stability, Journal of Crystal Growth, 10.1016/j.jcrysgro.2011.11.052, 360, 1, 189-192, 2012.12, [URL], We investigated with ab initio calculations the energetics of the most common silicon carbide (SiC) polytypes. We considered the (0001) Si face and the (0001̄) C face of 3C-, 6H-, 4H- and 2H-SiC. Our investigation reveals that the energy differences among SiC polytypes are enhanced at the surface with respect to the bulk. We discuss the relevant role played by the surface for the crystal growth of SiC..
27. T. Shiramomo, B. Gao, F. Mercier, Shinichi Nishizawa, S. Nakano, Yoshihiro Kangawa, Koichi Kakimoto, Thermodynamical analysis of polytype stability during PVT growth of SiC using 2D nucleation theory, Journal of Crystal Growth, 10.1016/j.jcrysgro.2012.01.023, 352, 1, 177-180, 2012.08, [URL], We studied dependence of process parameters, such as temperature of a seed, pressure in a furnace and surface polarity of a substrate, on polytypes of SiC in a process of physical vapor transport. The analysis was based on a classical thermodynamic nucleation theory in conjunction with numerical results obtained from a global model. We investigated which polytype was more stable in the nucleation stage by a comparison of nucleation energies of each polytype. The results show that the formation of 4H-SiC was more stable than that of 6H-SiC when we used C-face SiC as a seed. Furthermore, the most stable polytype could change from 4H-SiC to 6H-SiC in a condition of higher supersaturation, with a condition of higher temperature of a seed and lower pressure in a furnace. Meanwhile, the formation of 6H-SiC was more stable than 4H-SiC when Si-face of a seed was used..
28. Henrik Pedersen, Stefano Leone, Olof Kordina, Anne Henry, Shinichi Nishizawa, Yaroslav Koshka, Erik Janzén, Chloride-based CVD growth of silicon carbide for electronic applications, Chemical Reviews, 10.1021/cr200257z, 112, 4, 2434-2453, 2012.04, [URL], A study was conducted to demonstrate chloride-based chemical vapor deposition (CVD) growth of silicon carbide (SiC) for electronic applications. SiC homoepitaxial growth was done using CVD with silane (SiH 4) as the silicon precursor and light hydrocarbons, including ethylene (C 2H 4) or propane (C 3H 8) as the carbon precursor. Simulation was an invaluable tool along with comparisons between different chlorinated chemistries to obtain a deeper understanding of the chloride-based process and provide more information. A number of approaches were selected to add chlorine to the gas mixture for the growth of SiC epitaxial layers using a chloride-based chemistry..
29. Stefano Leone, Olof Kordina, Anne Henry, Shinichi Nishizawa, Örjan Danielsson, Erik Janzén, Gas-phase modeling of chlorine-based chemical vapor deposition of silicon carbide, Crystal Growth and Design, 10.1021/cg201684e, 12, 4, 1977-1984, 2012.04, [URL], Kinetic calculations of the chemical phenomena occurring in the epitaxial growth of silicon carbide are performed in this study. The main process parameters analyzed are precursor types, growth temperature, Cl/Si ratio, and precursors' concentration. The analysis of the gas-phase reactions resulted in a model which could explain most of the already reported experimental results, performed in horizontal hot-wall reactors. The effect of using different carbon or silicon precursors is discussed, by comparing the gas-phase composition and the resulting C/Si ratio inside the hot reaction chamber. Chlorinated molecules with three chlorine atoms seem to be the most efficient and resulting in a uniform C/Si ratio along the susceptor coordinate. Further complexity in the process derives from the use of low temperatures, which affects not only the gas-phase composition but also the risk of gas-phase nucleation. The Cl/Si ratio is demonstrated to be crucial not only for the prevention of silicon clusters but also for the uniformity of the gas-phase composition..
30. Koichi Kakimoto, B. Gao, T. Shiramomo, S. Nakano, Shinichi Nishizawa, Thermodynamic analysis of SiC polytype growth by physical vapor transport method, Journal of Crystal Growth, 10.1016/j.jcrysgro.2011.03.059, 324, 1, 78-81, 2011.06, [URL], Crystal growth of a certain polytype of SiC in a process of physical vapor transport was studied on the basis of classical thermodynamic nucleation theory in conjunction with numerical results obtained from a global model. Formation of a certain polytype in the nucleation stage is determined by the energy balance among surface energy, formation energy and supersaturation. The preferential growth condition of a certain polytype was estimated. The value of supersaturation was estimated using a numerical model obtained by a global model that includes species transport as well as heat transport in a furnace. The results of calculation showed that 4H polytype is more stable than 15R, 6H and 3C polytypes. Free energy difference between 4H and 6H polytypes decreased when total pressure in the furnace decreased..
31. Frédéric Mercier, Shinichi Nishizawa, Numerical investigation of the growth rate enhancement of SiC crystal growth from silicon melts, Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes, 10.1143/JJAP.50.035603, 50, 3, 2011.03, [URL], Numerical study has been applied to analyze the high temperature solution growth process for bulk silicon carbide (SiC) crystal growth. A twodimensional axisymmetric model for 2-in. SiC crystal growth was used for this study. The purpose of this paper is to investigate the possible approaches to enhance the growth rate in this process. In particular, we studied the effect of an AC magnetic field on the carbon transport to the crystal growth interface. The results revealed that the carbon flux to the growing crystal is strongly affected by the coil position and the applied frequency. If these two process parameters are properly chosen, we show that the carbon flux at the growing front, and thus the growth rate of SiC, can be enhanced..
32. Frédéric Mercier, Shinichi Nishizawa, Solution growth of SiC from silicon melts
Influence of the alternative magnetic field on fluid dynamics, Journal of Crystal Growth, 10.1016/j.jcrysgro.2010.10.022, 318, 1, 385-388, 2011.03, [URL], We studied numerically the fluid dynamics of the silicon melt in the high temperature solution growth of silicon carbide (SiC) with the presence of alternative magnetic fields. A 2D-axisymmetric model for 2 in SiC crystal growth was used for this study. The results revealed that the melt convection is strongly affected by the coil position and the applied frequency. Results on the effect of electromagnetic convection in the presence of buoyancy convection are also given in this paper..
33. B. Gao, X. J. Chen, S. Nakano, Shinichi Nishizawa, Koichi Kakimoto, Analysis of SiC crystal sublimation growth by fully coupled compressible multi-phase flow simulation, Journal of Crystal Growth, 10.1016/j.jcrysgro.2010.08.032, 312, 22, 3349-3355, 2010.11, [URL], A fully coupled compressible multi-phase flow solver was developed to effectively design a large furnace for producing large-size SiC crystals. Compressible effect, convection and buoyancy effects, flow coupling between argon gas and species, and the Stefan effect are included. A small and experimental furnace is used to validate the solver. First, the essentiality of 2D flow calculation and the significance of incorporating buoyancy effect and gas convection, the Stefan effect, and flow interaction between argon gas and species were investigated by numerical results. Then the effects of argon gas on deposition rate, growth rate, graphitization on the powder source, and supersaturation and stoichiometry on the seed were analyzed. Finally, the advantages of an extra chamber design were explained, and improvement of growth rate was validated by the present solver..
34. Xuejiang Chen, Shinichi Nishizawa, Koichi Kakimoto, Numerical simulation of a new SiC growth system by the dual-directional sublimation method, Journal of Crystal Growth, 10.1016/j.jcrysgro.2010.02.027, 312, 10, 1697-1702, 2010.05, [URL], A new SiC growth system using the dual-directional sublimation method was investigated in this study. Induction heating and thermal conditions were computed and analyzed by using a global simulation model, and then the values of growth rate and shear stress in a growing crystal were calculated and compared with those in a conventional system. The results showed that the growth rate of SiC single crystals can be increased by twofold by using the dual-directional sublimation method with little increase in electrical power consumption and that thermal stresses can be reduced due to no constraint of the crucible lid and low temperature gradient in crystals..
35. Shinichi Nishizawa, Numerical modeling of SiC single crystal growth-sublimation and hot-wall epitaxy, Journal of Crystal Growth, 10.1016/j.jcrysgro.2008.09.105, 311, 3, 871-874, 2009.01, [URL], On the engineering point of view, it is important to develop a design technology of the furnace for SiC single crystal growth. In this point of view, the recent progress of modeling on both sublimation bulk growth and hot-wall epitaxy were presented. For the sublimation, the active control of grown crystal shape by modifying crucible geometry was demonstrated. The effect of nitrogen doping on the heat transfer in a growing crystal were also investigated. For the hot-wall epitaxy, growth rate, surface morphology, and doping concentration could be predicted qualitatively with taking account of the depositing surface conditions. Chlorine-containing system was supposed to provide more stable and uniform process than the common SiH4-based system..
36. K. Fukuda, S. Harada, J. Senzaki, M. Okamoto, Y. Tanaka, A. Kinoshita, R. Kosugi, K. Kojima, M. Kato, A. Shimozato, K. Suzuki, Y. Hayashi, K. Takao, T. Kato, Shinichi Nishizawa, T. Yatsuo, H. Okumura, H. Ohashi, K. Arai, Challenges of 4H-SiC MOSFETs on the C(000-1) face toward the achievement of ultra low on-resistance, Materials Science Forum, 10.4028/3-908453-11-9.907, 600-603, 907-912, 2009, [URL], The C(000-1) face of 4H-SiC has a lot of advantages for the power device fabrication such as the highest oxidation ratio and a smooth surface. However, the DMOS type power MOSFETs on the C(000-1) face have not been realized because of the difficulty of epitaxial growth and of high quality MOS interface formation. We have systematically investigated the device fabrication techniques for power MOSFETs on the C(000-1) face, and succeeded with the IEMOS which have blocking voltage of 660V and an on-resistance of 1.8mΩcm2 and excellent dynamic characteristics..
37. Michel Pons, Shinichi Nishizawa, Peter Wellmann, E. Blanquet, D. Chaussende, J. M. Dedulle, R. Madar, Silicon carbide growth
C/Si ratio evaluation and modeling, Materials Science Forum, 10.4028/3-908453-11-9, 600-603, 83-88, 2009, [URL], Modeling and simulation of the SiC growth processes, Physical Vapor Transport (PVT), Chemical Vapor Deposition (CVD), are sufficiently mature to help building new process equipment or up-scaling old ones. It is possible (i) to simulate accurately temperature and deposition distributions, as well as doping (ii) to quantify the limiting phenomena, (iii) to understand the important role of different precursors in CVD and hydrogen additions in PVT. The first conclusion of this paper is the importance of the "effective" C/Si ratio during CVD epitaxy in hot-wall reactors and its capability to explain the doping concentrations. The second conclusion is the influence of the C/Si ratio in alternative bulk growth technique involving gas additions..
38. Johji Nishio, Chiharu Ota, Tetsuo Hatakeyama, Takashi Shinohe, Kazutoshi Kojima, Shinichi Nishizawa, Hiromichi Ohashi, Ultralow-loss SiC floating junction Schottky barrier diodes (Super-SBDs), IEEE Transactions on Electron Devices, 10.1109/TED.2008.926666, 55, 8, 1954-1960, 2008.08, [URL], We have applied the floating junction (FJ) structure, which has been confirmed to be effective in reducing the on-resistance of Si power devices, to SiC FJ Schottky barrier diodes (SiC Super-SBDs). Optimization of the device parameters, which are derived by making improvements in the device simulator, and development of the fabrication process have enabled realization of Super-SBDs with a breakdown voltage of 2700 V and a specific on-resistance of 2.57 mΩ · cm2. These values correspond to the world record of 11.3 GW/cm2 for Baliga's figure-of-merit (BFOM} = 4Vbd2/Ron-sp)..
39. Akimasa Kinoshita, Kenji Suzuki, Junji Senzaki, Makoto Katou, Shinsuke Harada, Mitsuo Okamato, Shinichi Nishizawa, Kenji Fukuda, Fukuyoshi Morigasa, Tomoyoshi Endou, Takuo Isii, Teruyuki Yashima, Activation of p-type dopants in 4H-SiC using hybrid super-rapid thermal annealing equipment, Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes, 10.1143/JJAP.46.5342, 46, 8 A, 5342-5344, 2007.08, [URL], Rapid thermal annealing (RTA) on areas with diameters as large as 2 in. at high temperatures using hybrid super-RTA (HSRTA) equipment is performed in this study. The HS-RTA equipment consists of an infrared annealing unit and an RF induction annealing unit for uniform annealing over a 2-in.-φ susceptor. As a result of annealing using the HS-RTA equipment, temperature is elevated from RT to a high temperature (1600-1800°C) for less than 1 min. Using aluminum (Al)-implanted silicon carbide (SiC) samples, the performance of the HS-RTA equipment is evaluated. For Al-implanted samples annealed at 1700°C, the sheet resistance distribution on the 2-in.-φ susceptor is 8.0%..
40. Shinichi Nishizawa, Tomohisa Kato, Kazuo Arai, Effect of heat transfer on macroscopic and microscopic crystal quality in silicon carbide sublimation growth, Journal of Crystal Growth, 10.1016/j.jcrysgro.2006.12.022, 303, 1 SPEC. ISS., 342-344, 2007.05, [URL], Numerical simulation was applied to observe the phenomena inside a crucible in silicon carbide (SiC) sublimation growth. Numerical simulation results show that crystal quality as well as crystal shape strongly depends on the temperature distribution inside the crucible. Numerical simulation also suggested that it is important to reduce the residual stress in the crystal in order to avoid the generation of dislocations. From these results, SiC sublimation growth was controlled actively, and large and high quality SiC single crystal was grown..
41. Shinichi Nishizawa, Michel Pons, Numerical modeling of silicon carbide epitaxy in a horizontal hot-wall reactor, Journal of Crystal Growth, 10.1016/j.jcrysgro.2006.12.018, 303, 1 SPEC. ISS., 334-336, 2007.05, [URL], Numerical simulation was carried out on SiC-CVD in a horizontal hot-wall reactor. The growth and doping features of both Si- and C-terminated surfaces were analyzed by changing the inlet source gas conditions. The role of conditions at the growing surface on the growth feature was investigated. It was identified that the conditions at the growing surface are good parameters to explain the growth feature..
42. S. Nakashima, T. Mitani, T. Tomita, T. Kato, Shinichi Nishizawa, H. Okumura, H. Harima, Observation of surface polarity dependent phonons in SiC by deep ultraviolet Raman spectroscopy, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.75.115321, 75, 11, 2007.03, [URL], Backscattering Raman spectra of SiC polytype crystals with SiC{0001} polar faces have been measured using deep ultraviolet (DUV), UV, and visible (VIS) excitation sources. We have found that for DUV excitation the intensity profiles of zone-folded modes differ markedly for Si and C faces. This Raman spectral feature is attributed to the presence of nonpropagating phonon modes confined in the near-surface region. It is concluded that the surface-bound phonon modes created with DUV photon field extend over a region a few hundred nanometers in depth, and that the displacements of the phonon modes are anisotropic in the direction of the polar axis. This surface-orientation-dependent Raman spectrum can be used to identify the surface polarity of SiC polytypes..
43. Tomohisa Kato, Tomonori Miura, Keisuke Wada, Eiji Hozomi, Hiroyoshi Taniguchi, Shinichi Nishizawa, Kazuo Arai, Defect and growth analysis of SiC bulk single crystals with high nitrogen doping, Materials Science Forum, 556-557, 239-242, 2007, In this study, we report defect analysis in 4H-SiC crystals of high nitrogen doping grown by sublimation method, and we discuss key points for defect restraint. The growth was performed in two kinds of growth directions; c-axis and a-axis. In the c-axis grown crystal with carrier density greater than 1 × 10-19cm-3, defect propagation was confirmed in the vertical direction for a growth direction affected by the doping by x-ray topography. This phenomenon was not observed in the a-axis grown crystals. In sublimation growth, the quantity of impurities tends to increase as growth rate decreases. Therefore, in the c-axis growth of doped 4H-SiC bulk crystals, we have to be careful so that dopant does not increase too much without intention in grown layers with lower growth rate, for example at the beginning and end of the growth..
44. Tomohisa Kato, Keisuke Wada, Eiji Hozomi, Hiroyoshi Taniguchi, Tomonori Miura, Shinichi Nishizawa, Kazuo Arai, High throughput SiC wafer polishing with good surface morphology, Materials Science Forum, 556-557, 753-756, 2007, We report SiC wafer polishing study to achieve high throughput with extremely flat, smooth and damageless surface. The polishing consists of three process, wafer grinding, lapping and chemical mechanical polishing (CMP), which are completed in shortest about 200 minutes in total for 2 inch wafer. Specimens of 4H- and 6H-SiC were provided from slicing single crystal as wafers oriented (0001) with 0 or 8 degrees offset angle toward to 〈1120〉. By the first grinding using a diamond whetstone wheel, we realized flat surface on the wafers with small TTV error of 1 μm in 15 minutes. After second process of lapping, the wafers were finished by CMP using colloidal silica slurry. AFM observation showed not only scratch-free surface but also atomic steps on the wafers after CMP. Rms marks extremely flat value of 0.08 nm in 10 μm square area..
45. C. Ota, J. Nishio, T. Hatakeyama, T. Shinohe, K. Kojima, Shinichi Nishizawa, H. Ohashi, Simulation, fabrication and characterization of 4H-SiC floating junction schottky barrier diodes (Super-SBDs), Materials Science Forum, 556-557, 881-884, 2007, The calculation for 4H-SiC floating junction Schottky barrier diodes (Super-SBDs) was carried out by device simulation and the optimized device structure was fabricated. The best characteristics of the Super-SBDs were breakdown voltage of 2700V and the specific on-resistance of 2.57mΩcm 2. The world record of Bariga's Figure of Merit (BFOM) for SiC-SBD expressed by 4Vbd 2/Ron was improved to 11,354MW/cm2..
46. Shinichi Nishizawa, Michel Pons, Growth and doping modeling of SiC-CVD in a horizontal hot-wall reactor, Chemical Vapor Deposition, 10.1002/cvde.200606469, 12, 8-9, 516-522, 2006.08, [URL], Modeling and simulation of the SiC epitaxial growth, and doping in a horizontal hot-wall reactor from common precursors (SiH4; C 3H8 diluted in H2 for growth; N2 and Al(CH3)3 for n-type and p-type doping) are presented. The growth and doping features of SiC thin layers on both Si-terminated and C-terminated surfaces are analyzed as a function of various inlet source gas conditions, i.e., various C/Si ratios. The role of the actual surface mass fluxes of both Si-containing and C-containing species and their ratio is analyzed and compared to the inlet experimental parameters. It is demonstrated that the doping level resulting from lattice site competition effects can be quantified by the actual C/Si ratio calculated above the growing surface. Moreover, the surface morphology of the epitaxial layer is explained on the basis of the mass fluxes at the growing surface..
47. S. Nakashima, T. Kato, Shinichi Nishizawa, T. Mitani, H. Okumura, T. Yamamoto, Deep ultraviolet raman microspectroscopic characterization of polishing-induced surface damage in SiC crystals, Journal of the Electrochemical Society, 10.1149/1.2170546, 153, 4, 2006.04, [URL], We have investigated polishing-induced surface damage in nitrogen-doped {0001} 4H-SiC crystals with silicon and carbon faces through deep ultraviolet Raman microspectroscopy. The structural and electrical properties of the damaged layers were characterized as a function of the abrasive particle size, using pure phonon modes and a longitudinal-optical-phonon plasmon coupled mode as monitor bands. The degree of damage decreased with the size. Although abrasive polishing with finer particles enables the long-range order of the lattice to almost fully recover, the carrier density remains partly reduced in the polished surface layers. The number of defects that induces a reduction in the free carrier density differs between the Si and C faces of 4H-SiC crystals..
48. Shinichi Nishizawa, Michel Pons, Numerical modeling of SiC-CVD in a horizontal hot-wall reactor, Microelectronic Engineering, 10.1016/j.mee.2005.10.033, 83, 1 SPEC. ISS., 100-103, 2006.01, [URL], A numerical simulation was carried out on SiC-CVD in a horizontal hot-wall reactor. In order to explain the effect of surface polarity, Si-face and C-face, the surface reaction model was improved. Then, the growth processes and doping features of both Si-face and C-face were analyzed. The role of conditions at growing surface, such as surface mass flux of both Si-containing and C-containing species, surface concentration of Si-containing and C-containing species and their ratio, is investigated. Then, the deposition and etching rates, and doping concentration are analyzed as the function of those parameters. In addition, surface morphology of growing epitaxial layer is also investigated in connection with growing surface condition..
49. Tomohisa Kato, Shinichi Nishizawa, Hirotaka Yamaguchi, Kazuo Arai, In-situ observation of SiC bulk single crystal growth by XRD system, Journal of Rare Earths, 24, SUPPL., 49-53, 2006, In-situ analysis for SiC bulk single crystal growth was reported using vertical X-ray diffraction (XRD) system. A furnace for SiC sublimation growth combined with the XRD system which possessed three kinds of functions including topography, rocking curve measurement and crystal growth rate monitoring was developed. These functions could contribute as a powerful tool finding the optimum growth condition by dynamic observation in the crucible. The in-situ X-ray topography succeeded to capture dynamic elongation of defects and dislocation generated in the SiC growing crystals. The in-situ rocking curve measurement revealed appearance of mosaic structure in the SiC crystal grown with high growth rate. The in-situ growth rate monitoring also succeeded very precisely using the direct X-ray beam absorption. On the base of findings and facts obtained by the in-situ observations, the importance for the SiC growth was discussed..
50. K. Kojima, Shinichi Nishizawa, S. Kuroda, H. Okumura, K. Arai, Effect of growth condition on micropipe filling of 4H-SiC epitaxial layer, Journal of Crystal Growth, 10.1016/j.jcrysgro.2004.11.071, 275, 1-2, 2005.02, [URL], In this study, we studied the effect of growth conditions on micropipe (MP) filling during 4H-SiC epitaxial growth. We found that an MP in an on-axis substrate was filled during epitaxial growth and that this MP was filled with a spiral growth. The MP filling on on-axis substrates had a high probability and was independent of growth conditions. On the other hand, the probability of MP filling of 8° off-axis substrates showed a strong dependence on the growth pressure and the growth rate. The probability of MP filling increased with decreasing growth pressure or increasing growth rate. The probability of MP filling of the C-face was higher than that of the Si-face. From a comparison of a numerical simulation and experimental results, we found that the concentration of Si species just above the substrate was a crucial factor for MP filling..
51. Shinichi Nishizawa, Kazutoshi Kojima, Satoshi Kuroda, Kazuo Arai, Michel Pons, Modeling of SiC-CVD on Si-face/C-face in a horizontal hot-wall reactor, Journal of Crystal Growth, 10.1016/j.jcrysgro.2004.11.072, 275, 1-2, 2005.02, [URL], To explain the difference between SiC-CVD on the Si-face and C-face, a heterogeneous model was improved, in which the etching, growth and doping features of SiC-CVD in a horizontal hot-wall reactor were numerically analyzed. The improved model was able to explain the etching and growth features accurately. There was no difference between the Si- and C-face. In addition, we propose the surface C/Si ratio as the universal parameter of the SiC-CVD process. Concerning doping features, the improved model showed that nitrogen and aluminum doping incorporation could be explained by the site competition model, while taking into account the amount of surface silicon and surface carbon, respectively..
52. Y. Yamada, Shinichi Nishizawa, S. Nakashima, K. Arai, Continuous growth of SiC single crystal by the spray dried powder made of ultra-fine particle precursors, Materials Science Forum, 457-460, I, 131-134, 2004, We succeeded in continuous growth of SiC single crystal by CPD (Chemical Particle Deposition) method with the spray dried ultra-fine particle precursor powder. By the introduction of the spray dried spherical powder, we could clarify the mechanism of SiC single crystal growth by CPD method, through the observation of the deformation and the transformation of the spherical precursor powder into the epitaxial SiC grown layer on the seed crystal surface. The SiC layer grown by this method showed promising crystal quality indicated by the rocking curve with FWHM of 70-80 arcsec..
53. Y. Tanaka, T. Ohno, N. Oyanagi, Shinichi Nishizawa, T. Suzuki, K. Fukuda, T. Yatsuo, K. Arai, Fabrication of mesa-type pn diodes without forward degradation on ultara-high-quality 6H-SiC substrate, Materials Science Forum, 457-460, II, 1009-1012, 2004, Using our own substrate growth and epitaxial growth techniques, we fabricated a 1.4 kV mesa-type 6H-SiC pn diode with an ideal avalanche breakdown and without forward degradation. The 6H-SiC substrates were grown on Lely crystals with no micropipes and only minimal defects. A pn junction was fabricated by chemical vapor deposition (CVD) with p+/n epitaxial films. We obtained 1.4 kV breakdown voltage, consistent with the ideal breakdown voltage calculated from the thickness (10μm) and doping concentration (2×1016cm-3) of the drift layer. The application of 200 A/cm2 current stress in the forward direction produced no degradation, which is often observed with pn diodes on normal commercial substrates..
54. Shinichi Nishizawa, T. Kato, Y. Kitou, N. Oyanagi, F. Hirose, H. Yamaguchi, W. Bahng, K. Arai, High-quality SiC bulk single crystal growth based on simulation and experiment, Materials Science Forum, 457-460, I, 29-34, 2004, The numerical simulation and in-situ X-ray topography were applied to observe the phenomena inside a crucible. Numerical simulation pointed out that macroscopic grown crystal quality such as grown crystal shape strongly depends on the temperature distribution inside a crucible. In-situ X-ray topography revealed that when the defects were generated, and how the defects were propagated. Most of defects were generated at the initial growth stage. It is important to control the initial stage in order to obtain a high quality SiC single crystal. Numerical simulation also suggested that it is important reduce the residual stress in a grown crystal in order to avoid the dislocation occurrence. From these results based on numerical simulation and experiment, SiC sublimation growth was controlled actively, and the large and high quality SiC single crystal have been grown..
55. T. Kato, T. Ohno, F. Hirose, N. Oyanagi, Shinichi Nishizawa, K. Arai, Large diameter and long length growth of SiC single crystal, Materials Science Forum, 457-460, I, 99-102, 2004, We report 4H-SiC single crystal growth of 4inch in diameter and micropipe-free crystal growth. Large single crystals were grown along c-axis and enlarged from smaller (0001) seed crystals. We confirmed that the center part of grown crystals is different in defect elongation from the enlarged part by x-ray topography. Especially, micropipes (MP) and screw dislocations (SD) are converted to basal plane dislocations in the enlarged part. By using this phenomenon, we realized a high quality crystal in the enlarged part that has x-ray rocking curve FWHM as small as 10.6arcsec. We also confirmed the crystal did not have MP and SD in a region of about 1 inch in diameter using etch pits observation and synchrotron x-ray topography. We also report the long length growth along a-axis growth on a () seed crystal and succeeded in the MP-free crystal growth..
56. Yasunori Tanaka, Shinichi Nishizawa, Kenji Fukuda, Kazuo Arai, Toshiyuki Ohno, Naoki Oyanagi, Takaya Suzuki, Tsutomu Yatsuo, Fabrication of 1.4-kV mesa-type p+-n diodes with avalanche breakdown and without forward degradation on high-quality 6H-SiC substrate, Applied Physics Letters, 10.1063/1.1591062, 83, 2, 377-379, 2003.07, [URL], A 1.4-kV mesa-type 6H-SiC p+-n diode with an ideal avalanche breakdown and without forward degradation was fabricated. The 6H-SiC substrates were grown on Lely crystals with no micropipes and only minimal defects. The 1.4-kV breakdown voltage, consistent with the ideal breakdown voltage calculated from the thickness and doping concentration of the drift layer was obtained..
57. M. Nasir Khan, Shinichi Nishizawa, Kazuo Arai, Healing defects in SiC wafers by liquid-phase epitaxy in Si melts, Journal of Crystal Growth, 10.1016/S0022-0248(03)01177-1, 254, 1-2, 137-143, 2003.06, [URL], Silicon carbide epitaxial layers were grown by liquid-phase epitaxy on 6H-SiC-modified Lely crystals containing a high density of micropipes (MP) and other defects. The geometrical configuration of the seed crystals inside the crucible during LPE was such that the epitaxial layer growth occurred simultaneously on both the faces of the seed crystals. Layers in the range of 20-30μm thickness were deposited on both the faces of the crystals. X-ray diffraction, optical and scanning electron microscopy analyses were carried out to investigate these layers. It was found that dislocation and MP density in the substrate after LPE growth has been significantly reduced. Several MP were observed to decompose into non-hollow core dislocations. The growth centre associated with MP reduces in size in general and shifts to new centres due to the decomposition of the MP. As a result these new centres dominate the growing surfaces in LPE and make the healed MP completely invisible..
58. K. Jeganathan, M. Shimuzu, H. Okumura, F. Hirose, Shinichi Nishizawa, Initial stage of GaN nucleation on √3 × √3 R 30°-Ga reconstructed 4H-SiC(0 0 0 1)Si by molecular-beam epitaxy, Surface Science, 10.1016/S0039-6028(03)00016-5, 527, 1-3, 2003.03, [URL], Gallium nitride (GaN) initial nucleation kinetics by the influence of SiC(0 0 0 1)Si surface structure has been investigated. The Ga induced √3×√3R30° and 3×3 surface reconstructions are found to be more efficacious for GaN growth. During the initial stage of GaN/4H-SiC(0 0 0 1)Si growth by molecular-beam epitaxy, coherent polygon islands grow wide (∼40 nm) along the lateral directions. These coherent islands develop 2D growth through early coalescence as evidenced by the in situ reflection high-energy electron diffraction observation. The control of Ga-adatom migration and the adsorption of 1/3 and 1 monolayer (ML) of Ga-adatom govern the surface morphology of the GaN layers. The bulk surfaces of (1 × 1) and (2 × 1) introduces incoherent nuclei, resulting in a delay of GaN coalescence, exhibits rough growth front and poor surface morphology..
59. Shinichi Nishizawa, Masahiro Hirata, Numerical simulation of thermal transpiration in Capacitance diaphragm gauge, Journal of the Vacuum Society of Japan, 46, 3, 197-199, 2003, The Capacitance diaphragm gauge is one of the most important vacuum gauges in low and middle vacuum ranges. The difference of temperature between the sensor head and the vacuum chamber gives a non-linear sensitivity of the gauge due to thermal transpiration effect. This sensitivity depends not only on pressure but also gas species. It is supposed that under the same condition of gas-surface interaction, the sensitivity should be normalized by mean free path independently of gas species. However, the sensitivity normalized by mean free path also depends on gas species. In this study, by using DSMC method, the influence of gas-surface interaction on thermal transpiration was analyzed. In case of random reflection model, the sensitivity has non-linearity and depends on mean free path. On the other hand, in case of a perfectly elastic reflection model, the sensitivity is constant regardless mean free path. In case of complex reflection that is composed of random and elastic reflections, as increasing the elastic reflection component, the sensitivity decreases from that of random reflection to elastic reflection..
60. Norihiko Kiritani, Masakatsu Hoshi, Satoshi Tanimoto, Kazuhiro Adachi, Shinichi Nishizawa, Tsutomu Yatsuo, Hideyo Okushi, Kazuo Arai, Single Material Ohmic Contacts Simultaneously Formed on the Source/P-Well/Gate of 4H-SiC Vertical Mosfets, Materials Science Forum, 433-436, 669-672, 2003, We fabricated 4H-SiC vertical MOSFETs with contacts to the source, p-well and polycrystalline silicon (polysilicon) gate and these were simultaneously formed from a single material, using one deposition and a single contact annealing process. Typical specific contact resistances of 4.8×10 -5 Ωcm2 for the n+ source region, 1.5×10-6 Ωcm2 for the gate polysilicon and 5.2×10-4 Ωcm2 for the p-well contact region were obtained using Al/Ni (Al∼6%) as the contact metal. Also, the static characteristics of the vertical MOSFETs indicated that the MOS interface can withstand an even higher temperature process such as that used in ohmic-contact formation..
61. Shinichi Nishizawa, Yumi Michikawa, Tomohisa Kato, Fusao Hirose, Naoki Oyanagi, Kazuo Arai, Stress Analysis of SiC Bulk Single Crystal Growth by Sublimation Method, Materials Science Forum, 433-436, 13-16, 2003, In order to grow a high quality SiC bulk single crystal, it is important to moderate the residual stress in the grown crystal. The residual stress is affected by not only the temperature distribution, but also by the boundary conditions. In this study, firstly, the effect of the polycrystal and of the crucible walls on the stress distribution was numerically analyzed. The effect of the grown crystal shape on the residual stress was also analyzed and compared to the growth experiment. It is pointed out that in order to moderate the residual stress, it is necessary to take care not only of the temperature distribution but also of the boundary conditions..
62. M. Nasir Khan, Shinichi Nishizawa, Tomohisa Kato, Ryoji Kosugi, Kazuo Arai, Silicon carbide epitaxial layer growths on Acheson seed crystals from silicon melt, Materials Letters, 10.1016/S0167-577X(02)00783-8, 57, 2, 307-314, 2002.12, [URL], Silicon carbide epitaxial layer growth was carried out on both Si- and C-faces of the 6H-SiC Acheson seed crystals by a technique similar to that used for the sublimation growth of bulk single crystals of this material. High-resolution XRD (HRXRD) measurements showed the single crystalline structure for these grown layers. RHEED patterns confirmed that the grown layers are of single crystalline nature having (n×n) surface reconstruction. Raman spectroscopy showed the same polytype for the grown layers as that of the seed crystals. Atomic force and optical microscopy revealed smooth, uniform and mirror like surfaces for the grown layers. Step flow growth mechanism was observed on both Si- and C-faces of the seed crystal. The grown layers show similar characteristic features as that observed for the SiC layers grown by other techniques such as liquid phase epitaxy (LPE) and chemical vapor deposition (CVD). The technique used is a simple, viable and new alternative for growing SiC epitaxial layers..
63. Shinichi Nishizawa, Masahiro Hirata, DSMC analysis of thermal transpiration of capacitance diaphragm gauge, Vacuum, 10.1016/S0042-207X(02)00212-9, 67, 3-4, 301-306, 2002.09, [URL], The capacitance diaphragm gauge (CDG) is one of the most important vacuum gauges in low and medium vacuum ranges. CDG has a non-linear sensitivity below 100Pa because of the temperature difference between the sensor head and the vacuum chamber, which is called thermal transpiration. This sensitivity depends on gas species and pressure. In this study, by using a direct simulation Monte Carlo (DSMC) method, pressure distribution in the connecting tube of the gauge was obtained under the pressure range from molecular flow regime to viscous flow regime (10-2-102Pa) taking account of the temperature distribution along the connecting tube. Furthermore, the pressure dependence of the sensitivity of the CDG was derived from the pressure difference between the hot and cold ends, and found to be in good agreement with the pressure dependence of sensitivity obtained by an empirical equation. The influence of gas-surface interaction on the thermal transpiration was also analyzed..
64. Naoki Oyanagi, Shinichi Nishizawa, Kazuo Arai, A method of reducing micropipes in thin films by using sublimation growth, Materials Science Forum, 389-393, 1, 107-110, 2002, The micropipe termination by using a W or Pt thin film was demonstrated. A SiC single crystal was grown on the modified-Lely seed with W or Pt mask, by the sublimation method. The optical micrograph made it clear that a micropipe contained in seed crystal was terminated at the metal deposited interface. The effect of metal mask thickness on the termination efficiency was also studied..
65. Fusao Hirose, Yasuo Kitou, Naoki Oyanagi, Tomohisa Kato, Shinichi Nishizawa, Kazuo Arai, Characterization of inclusions in SiC bulk crystals grown by modified lely method, Materials Science Forum, 389-393, 1, 75-78, 2002, We have investigated two types of inclusions. They were a dendrite and a transparent inclusion, which have been seldom reported. The dendrite consisted of carbon, titanium and vanadium except for silicon. It should be carbide of titanium and vanadium. The titanium and vanadium might be incorporated from the source. By using a high purity source, it is possible to prevent generation of the dendrite. The transparent inclusion was observed in the SiC bulk crystal even in the growth with the chemical treated SiC source powder. A remarkable feature of the transparent inclusion was to have a small empty core. It was considered that the core might be a trace of Si droplet caused by C/Si ratio fluctuation of sublimated gas. Therefore, the C/Si ratio during the growth should be stabilized to prevent generation of the transparent inclusions..
66. Tomohisa Kato, Naoki Oyanagi, Yasuo Kitou, Shinichi Nishizawa, Kazuo Arai, Dislocation constraint by etch-back process of seed crystal in SiC bulk crystal growth, Materials Science Forum, 389-393, 1, 111-114, 2002, Dislocation constraint in the growth of SiC crystal by the modified Lely method was studied. In SiC single crystal growth, the dislocations and defects generally propagate from the seed crystal surface. However, when the etch back on the seed crystal surface in the sublimation process was performed prior to growth, defects and dislocations propagation in the interface between the seed crystal and the grown crystal were reasonably suppressed. The switchover from the etch back to the growth could be performed without changing the heating condition during the initial process. We noticed that the density of the hollow defects called as micropipes in the grown crystal were decreased to 1/10 compared to that of the seed crystal used. We consider the etch back process of the seed crystal is an effective method for constraining the defects in the SiC crystal growth..
67. Yasuo Kitou, Wook Bahng, Tomohisa Kato, Shinichi Nishizawa, Kazuo Arai, Flux-controlled sublimation growth by an inner guide-tube, Materials Science Forum, 389-393, 1, 83-86, 2002, The effect of the geometrical parameters of the inner structure of the crucible on the sublimation growth of bulk SiC was investigated. It was found that the gap between the seed crystal and the guide-tube was the important parameter for the single crystal growth separated from polycrystal. The growth rate ratio of the single/poly crystal increased up to 4. The broadening angle of the single crystal was controlled in the range of 0-30° by changing the taper angle of the guide-tube. The crystal quality in the periphery was improved compared with the crystal grown without the guide-tube. The growth process was discussed considering the flux flow of the sublimation gas..
68. Naoki Oyanagi, Hirotaka Yamaguchi, Tomohisa Kato, Shinichi Nishizawa, Kazuo Arai, Growth and evaluation of high quality SiC crystal by sublimation method, Materials Science Forum, 389-393, 1, 87-90, 2002, High quality 6H-SiC single crystal was grown using the modified Lely method under pressure-controlled low-growth-rate conditions. The grown crystal contained no micropipes and its etch pit density was 4 × 103 cm-2. The presence of a Pendellösung fringe as revealed by section topography showed that the grown crystal was composed of a single domain. The relation between etch pits and defects is also discussed..
69. M. Okamoto, R. Kosugi, Y. Tanaka, D. Takeuchi, S. Nakashima, Shinichi Nishizawa, K. Fukuda, H. Okushi, K. Arai, Homoepitaxial growth of 4H-SiC thin film below 1000°C by microwave plasma chemical vapor deposition, Materials Science Forum, 389-393, 1, 299-302, 2002, Microwave plasma chemical vapor deposition was performed aiming at low temperature homoepitaxial growth of 4H-SiC thin films. The growth rate of the deposited film depended strongly on the SiH4 flow rate, and a smooth surface could not obtained at high SiH4 flow rate. The surface morphology was also affected by the C/Si ratio. A high C/Si ratio was required to obtain smooth SiC films. Single crystalline 4H-SiC film growth has been achieved at a temperature as low as 970°C by growing under very high C/Si ratio (C/Si = 175) with a very low SiH4 flow rate (0.004sccm)..
70. Shinichi Nishizawa, Masahiro Hirata, Numerical simulation of thermal transpiration of capacitance diaphragm gauge, Journal of the Vacuum Society of Japan, 45, 3, 119-122, 2002, The Capacitance diaphragm gauge (CDG) is one of the most important vacuum gauges in low and middle vacuum ranges. The difference of temperature between the sensor head and the vacuum chamber gives a non-linear sensitivity of the gauge depending on the pressure due to thermal transpiration effect. Change in the sensitivity of about 4% between molecular flow regime and viscous flow regime is significant for metrological use of the gauge. In this study, by using a DSMC method, pressure distribution in the connecting tube of the gauge was obtained under the pressure range from molecular flow regime to viscous flow regime with taking account of temperature distribution along the connecting tube. It was in good agreement with the pressure dependence of sensitivity obtained by static expansion system experimentally. Furthermore, influence of gas-surface interaction on the thermal transpiration was also discussed..
71. Shinichi Nishizawa, Tomohisa Kato, Yasuo Kitou, Naoki Oyanagi, Kazuo Arai, Numerical simulation of heat and mass transfer in SiC sublimation growth, Materials Science Forum, 389-393, 1, 43-46, 2002, The numerical simulation of SiC sublimation growth was carried out. On the results of flux control growth with innerguide tube, the tube controls the thermal field inside a growth cavity, and makes it possible to grow single crystal without obstacle polycrystal. It was also cleared that the grown crystal shape strongly depends on the temperature distribution inside a growth cavity. On the results of in-process etching, it was pointed out that defects occurrence can be suppressed by taking care of temperature distribution on the initial growing surface. These results show that by controlling the thermal field inside a growth cavity, it is possible to control the macro and micro crystal quality such as grown crystal shape and defects, respectively..
72. Tomohisa Kato, Shinichi Nishizawa, Kazuo Arai, Dislocation constraint by etch back process of seed crystal in the SiC sublimation growth, Journal of Crystal Growth, 10.1016/S0022-0248(01)01578-0, 233, 1-2, 219-225, 2001.11, [URL], Dislocation constraint in the growth of SiC crystal by the modified Lely method was studied. In SiC single crystal growth, the dislocations and defects generally propagate from the seed crystal surface. However, when the etch back on the seed crystal surface in the sublimation process was performed prior to growth, defects and dislocation propagation in the interface between the seed crystal and the grown crystal were suppressed reasonably. The switchover from the etch back to the growth could be performed without changing heating condition during the initial process. We noticed that the density of the hollow defects called as micropipes in the grown crystal were decreased to 1/10 compared to that of the seed crystal used. We consider the etch back process of the seed crystal as an effective method for constraining the defects in the SiC crystal growth..
73. Shinichi Nishizawa, H. Yamaguchi, T. Kato, M. N. Khan, K. Arai, N. Oyanagi, Y. Kitou, W. Bahng, New crucible design for SiC single crystal growth by sublimation, Materials Research Society Symposium - Proceedings, 640, 2001.01, SiC bulk single crystal growth by sublimation was investigated. A new crucible design, double-walled crucible, was proposed, and its effect was confirmed numerically and experimentally. On the point of heat transfer in a growth cavity, double-walled crucible is better than conventional crucible. With a double-walled crucible, temperature of seed and source surfaces could be kept constant with better uniformity than that with a conventional crucible. It was deduced that a crystal growth rate could be kept constant with flat surface. Furthermore, in case of a double walled crucible, crystal enlarged rapidly with less inclusion. As the results, a double-walled crucible is useful to grow high quality SiC single crystal by sublimation..
74. T. Kato, N. Oyanagi, H. Yamaguchi, Shinichi Nishizawa, K. Arai, Defect analysis of SiC sublimation growth by the in-situ X-ray topography, Materials Science Forum, 353-356, 295-298, 2001, Silicon carbide (SiC) single crystal growth was studied by the in-situ observation using x-ray topographic technique. Occurrence and dynamics of defects, dislocations were observed in a real time display and captured as topographic images during sublimation growth (modified Lely method) of SiC crystals. From the analysis of these topographic images, high-density of dislocations and typical large defects, such as micropipes, domain boundaries and macrodefects were investigated. On the basis of our in-situ observation and analysis, we argue that dislocation and nucleation control on the seed crystal during initial growth are of prime importance for producing high quality SiC crystals..
75. Hirotaka Yamaguchi, Shinichi Nishizawa, Tomohisa Kato, Naoki Oyanagi, Sadafumi Yoshida, Kazuo Arai, In-situ X-ray topography on crystal growth of silicon carbide, Yosetsu Gakkai Shi/Journal of the Japan Welding Society, 70, 6, 23-27, 2001.
76. Tomohisa Kato, Naoki Oyanagi, Hirotaka Yamaguchi, Shinichi Nishizawa, M. Nasir Khan, Yasuo Kitou, Kazuo Arai, In-situ observation of silicon carbide sublimation growth by X-ray topography, Journal of Crystal Growth, 10.1016/S0022-0248(00)00958-1, 222, 3, 579-585, 2001, [URL], The crystal growth of silicon carbide (SiC) was studied by in-situ observation using X-ray topographic technique. The growth was performed by a sublimation method (the modified Lely method). The generation and evolution of defects and dislocations were observed as topographs in a real-time display. Defects and dislocations analyzed by the in-situ technique were compared with the postprocess observations using optical microscopy and X-ray topography. Dislocations in the initial growth layer and typical large defects, such as micropipes, macrodefects and domain boundaries, were investigated. We showed the possibility that large defects are induced by the accumulation of dislocations in the initial growth layer. Moreover, we observed that inhomogeneous growth starting in parts on the seed surface during the initial growth results in new defects in the growing crystal. We discuss the importance of dislocation and nucleation control on the SiC seed crystal during the initial growth, on the basis of facts and findings obtained by the in-situ as well as postprocess observations..
77. Shinichi Nishizawa, Hirotaka Yamaguchi, Tomohisa Kato, M. Nasir Khan, Kazuo Arai, Naoki Oyanagi, Yasuo Kitou, Wook Bahng, New crucible design for SiC single crystal growth by sublimation, Materials Research Society Symposium - Proceedings, 640, H1.5.1-H1.5.6, 2001, SiC bulk single crystal growth by sublimation was investigated. A new crucible design, double-walled crucible, was proposed, and its effect was confirmed numerically and experimentally. On the point of heat transfer in a growth cavity, double-walled crucible is better than conventional crucible. With a double-walled crucible, temperature of seed and source surfaces could be kept constant with better uniformity than that with a conventional crucible. It was deduced that a crystal growth rate could be kept constant with flat surface. Furthermore, in case of a double walled crucible, crystal enlarged rapidly with less inclusion. As the results, a double-walled crucible is useful to grow high quality SiC single crystal by sublimation..
78. M. Nasir Khan, Shinichi Nishizawa, Wook Bahng, Kazuo Arai, Liquid-phase epitaxy on 6H-SiC Acheson seed crystals in closed vessel, Journal of Crystal Growth, 10.1016/S0022-0248(00)00733-8, 220, 1-2, 75-81, 2000.11, [URL], SiC epitaxial growth was carried out on 6H-SiC Acheson seed crystals in liquid phase using a closed carbon crucible. The growth was carried out in the temperature range of 1500-1700 °C. The geometrical configurations of seed crystals inside the crucible were specific, such that epitaxial layer growth occurred simultaneously on both faces of the same seed crystal under the same growth conditions. Growth rates as high as 20-30 μm/h have been achieved. The growth rate and surface morphologies observed after the growth indicated that the temperature gradient on the seed crystal faces plays a dominant role for the growth mechanism of the epitaxial layers. The convection within the solvent caused by the temperature gradient enhances the mass transport to the growing layer faces enhancing the growth rate and also results in a better surface morphology of the epitaxial grown layers..
79. W. Bahng, Y. Kitou, Shinichi Nishizawa, H. Yamaguchi, M. Nasir Khan, N. Oyanagi, S. Nishino, K. Arai, Rapid enlargement of SiC single crystal using a cone-shaped platform, Journal of Crystal Growth, 10.1016/S0022-0248(99)00754-X, 209, 4, 767-772, 2000.02, [URL], We investigated the enlargement of SiC single crystal during physical vapor transport growth by modifying the shape of graphite lid. The single crystals grown on the cone-shaped platform were larger in diameter than those grown on the conventional one. The enlargement of ingot is discussed in terms of the dual role of polycrystals during crystal growth: (i) it provides a platform for single crystal and (ii) an obstacle to the lateral growth of ingot. The dependence of the broadening angle (β) of single crystal on the taper angle (θ) of the cone-shaped platform was also investigated and an optimum angle at a given growth condition found..
80. H. Yamaguchi, Shinichi Nishizawa, T. Kato, N. Oyanagi, W. Bahng, S. Yoshida, K. Arai, Y. Machitani, T. Kikuchi, In situ x-ray topography of silicon carbide during crystal growth by sublimation method, Review of Scientific Instruments, 10.1063/1.1150698, 71, 7, 2829-2832, 2000.01, [URL], We have developed an instrument for real-time observation of silicon carbide (SiC) crystal during the sublimation growth process by x-ray topography. It is constructed by combining an x-ray goniometer and a crystal-growth chamber. The vertical goniometer consists of three circles, an α circle for the x-ray source, a β circle for the detector, and a φ circle for the azimuthal rotation of the sample. The growing crystal boule is set at the center of the goniometer glued on a crucible lid. Transmission topographs are taken using an asymmetric (1011) reflection with the scattering angle 2 θB - β - α from the (0001)-oriented boule. A rotating-anode 18 kW generator with a molybdenum target is employed as the x-ray source. Topographs are observed by a direct imaging system using a charge-coupled device camera. Incident and scattered x-ray beams pass through beryllium windows mounted on the bottom and top flanges of the crystal-growth chamber, respectively. The crucibles are also designed for in situ measurements so that the x-ray beam path is separated from the source materials. The in situ topographs demonstrated the movement of micropipes and other defects during the crystal growth..
81. Tomohisa Kato, Naoki Oyanagi, Hirotaka Yamaguchi, Yukio Takano, Shinichi Nishizawa, Kazuo Arai, In-situ observation of SiC bulk single crystal growth by X-ray topography, Materials Science Forum, 338, 2000.01, We report here for the first time on the in-situ observations for SiC bulk single crystal growth by x-ray topographic technique. Occurrence and dynamic observation of the defects such as micropipes and domain boundaries during SiC crystal growth by the modified Lely method was investigated in a real time display. The in-situ observation was considered to contribute for optimizing the growth conditions and to interpret the mechanism of defects and dislocations formation..
82. H. Yamaguchi, N. Oyanagi, T. Kato, Y. Takano, Shinichi Nishizawa, W. Bahng, S. Yoshida, K. Arai, X-ray topographic study of SiC crystal at high temperature, Materials Science Forum, 338, 2000.01, We have developed an instrument for in-situ X-ray topography during crystal growth of silicon carbide (SiC). A vertical X-ray goniometer is combined with a furnace for sublimation growth. A high-power X-ray source and a TV imaging system using a CCD camera make possible to display the behaviors of defects in SiC crystal inside crucible. For a demonstration of the developed instrument, we show the topographs of a SiC crystal at high temperature. The topographs show distinct deformations developing with increasing temperature..
83. T. Henkel, Y. Tanaka, N. Kobayashi, Shinichi Nishizawa, S. Hishita, Beryllium implantation doping of silicon carbide, Materials Science Forum, 338, 2000, Structural properties of beryllium implanted silicon carbide have been investigated by secondary ion mass spectrometry, Rutherford backscattering/channeling, and Raman spectroscopy. Strong redistribution of beryllium has been found after a post-implantation anneal step at temperatures between 1300 °C and 1700 °C. The use of a pre-anneal process at 1000 °C before the high-temperature treatment as well as graphite as a surface encapsulant do not efficiently suppress redistribution of Be in the SiC lattice. The crystalline state of the implanted and annealed material is well recovered after annealing at temperatures above 1400 °C..
84. Wook Bahng, Yasuo Kitou, Shinichi Nishizawa, Hirotaka Yamaguchi, Muhammad Nasir Khan, Naoki Oyanagi, Kazuo Arai, Shigehiro Nishino, Enlargement of SiC single crystal
Enhancement of lateral growth using tapered graphite lid, Materials Science Forum, 338, 2000, We investigated the rapid enlargement of SiC single crystal during physical vapor transport growth by enhancing the lateral growth. The degrees of enlargement of the single crystals grown on the newly developed graphite lid were larger than those of crystals grown on the conventional one. Using a cone-shaped platform, the polycrystals grown around the single crystal could assist the lateral growth of single crystal. There was no significant difference between the central region and the enlarged region as far as the micropipe density was considered. The dependence of the broadening angle (β) of the single crystal on the taper angle (θ) of the cone-shaped platform was also investigated and an optimum angle at a given growth condition was found..
85. Yasuo Kitou, Wook Bahng, Shinichi Nishizawa, Shigehiro Nishino, Kazuo Arai, Pressure effect in sublimation growth of bulk SiC, Materials Science Forum, 338, 2000, The effect of growth pressure on the impurity incorporation as well as on the crystal quality was investigated in sublimation growth of a bulk SiC single crystal. The growth pressure was varied within the range of 1-100 Torr and it was found that many voids or precipitates were generated in the grown crystals and the Aluminum (Al) impurity incorporation increased during the Si-face growth when the growth pressure decreased. The pressure effect on the Al impurity incorporation with the change of step structure was discussed..
86. Shinichi Nishizawa, Yasuo Kitou, Wook Bahng, Naoki Oyanagi, Muhammad Nasir Khan, Kazuo Arai, Shape of SiC bulk single crystal grown by sublimation, Materials Science Forum, 338, 2000, Numerical simulation of sublimation SiC bulk single crystal growth was performed. Electromagnetic and temperature fields in a growth furnace were analyzed numerically. The relation between grown crystal shape and temperature distribution in a growth cavity was discussed. It is pointed out that the crystal shape has a close relationship with temperature distribution. By modifying the crucible design and temperature distribution in a growth cavity, it is possible to enhance the enlargement of grown crystal, and also possible to keep grown surface flat..
87. Naoki Oyanagi, Shinichi Nishizawa, Tomohisa Kato, Hirotaka Yamaguchi, Kazuo Arai, SiC single crystal growth rate measurement by in-situ observation using the transmission X-ray technique, Materials Science Forum, 338, 2000, The growth rate of SiC bulk single crystal by sublimation was measured in real time by using the transmission X-ray technique. The growth rate obtained by transmission X-ray intensity corresponds to the growth rate measured by nitrogen marker. The growth rate increases as the growth temperature increases and as pressure decreases. While taking into account the pressure balance between the surrounding pressure and vapor pressures of seed and source, the growth rate can be calculated by mass flux from source to seed..
88. T. Henkel, G. Ferro, Shinichi Nishizawa, H. Pressler, Y. Tanaka, H. Tanoue, N. Kobayashi, Structural characterization of silicon carbide etched by using a combination of ion implantation and wet chemical etching, Materials Science Forum, 338, 2000, Silicon carbide has been etched using a combination of high-dose ion implantation and wet chemical etching. Structural properties with respect to the remaining damage after etching have been studied using atomic force microscopy, Rutherford backscattering/channeling, and Raman spectroscopy. No significant deterioration of the crystal quality has been found after the etching process. Moreover, the as-etched surface is characterized by a lower roughness compared to virgin material. The results demonstrate that this etching method can be used for the fabrication of contacts on silicon carbide surfaces..
89. Muhammad Nasir Khan, Shinichi Nishizawa, Wook Bahng, Kazuo Arai, Temperature gradient effect on SiC epitaxy in liquid phase, Materials Science Forum, 338, 2000, Epitaxial growth of 6H-SiC was carried out on 6H-SiC Acheson seed crystals in the silicon melt using closed carbon crucible. The growth was carried out in the temperature range 1500-1700 °C. The geometry was specific one, such that epitaxial layer growth occurred simultaneously on both faces of seed crystal under similar growth conditions. We have measured the growth rate and studied the surface morphology of these grown layers. The temperature gradient within the melt seems to play a dominant role in the growth mechanism in liquid phase and also results in a better surface morphology of the grown layers..
90. H. Yamaguchi, Shinichi Nishizawa, W. Bahng, K. Fukuda, S. Yoshida, K. Arai, Y. Takano, Reflection and transmission X-ray topographic study of a SiC crystal and epitaxial wafer, Materials Science and Engineering B: Solid-State Materials for Advanced Technology, 61-62, 221-224, 1999.07, Defects in commercially available silicon carbide (SiC) wafers have been investigated by X-ray topography and optical microscopy. Dots appearing in transmission topographs are identified as a screw dislocation running through the [0001] direction by a comparative observation of the reflection topographs from the front and rear sides. In the peripheral region, these dots appear with high density and accompany large strain fields at the edge, which are related with the dislocations in the basal plane emanating from the dots and connecting them and large holes with diameters of about 10-20 μm opening at the epilayer surface. These large strain fields are considered to originate from the large Burgers vector associating with the screw dislocations..
91. Shinichi Nishizawa, H. Yamaguchi, T. Kato, N. Oyanagi, S. Yoshida, K. Arai, Development of X-ray topography system for in-situ observation of sublimation SiC single crystal growth, Denshi Gijutsu Sogo Kenkyusho Iho/Bulletin of the Electrotechnical Laboratory, 63, 8-9, 37-42, 1999, X-ray topography system for in-situ observation of sublimation SiC single crystal growth has been developed. The feature of SiC single crystal growth inside a closed carbon crucible over 2200°C is captured in real time. The system will clarify the SiC growth mechanism, such as growth rate, defect generation and stress distribution in a growing crystal, depending on temperature, pressure and other parameters. These results will be useful to develop a high quality and large SiC single crystal wafer, and enhance the hard electronics technology..
92. Taketoshi Hibiya, Shin Nakamura, Kusuhiro Mukai, Zheng Gang Niu, Nobuyuki Imaishi, Shinichi Nishizawa, Shin Ichi Yoda, Masato Koyama, Interfacial phenomena of molten silicon
Marangoni flow and surface tension, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 10.1098/rsta.1998.0195, 356, 1739, 899-909, 1998.04, [URL], Temperature oscillation due to the oscillatory Marangoni flow was measured for a molten half-zone silicon column (10 mm high and 10 mm in diameter with a temperature difference of 150 K between the upper and lower solid-liquid interfaces) under microgravity by using fine thermocouples. The flow is in a hypercritical condition; that is, the Marangoni number is estimated to be over 10 000. The structure of the Marangoni instability is two-fold symmetry for the small aspect ratio (height/radius) Γ of 1 and one-fold symmetry for the melt with Γ of 2. The surface tension of molten silicon was measured by a sessile drop method in carefully controlled ambient atmospheres with various oxygen partial pressures from 4 × 10-22 to 6 × 10-19 MPa. These measurements showed that the surface tension and its temperature coefficient showed a marked dependence on oxygen partial pressure. Accordingly the effect of oxygen partial pressure on the Marangoni flow should be made clear. Moreover, Marangoni flow at the flat surface, which corresponds to the flow for the Czochralski growth system, should also be studied..
93. Shin Nakamura, Taketoshi Hibiya, Koichi Kakimoto, Nobuyuki Imaishi, Shinichi Nishizawa, Akira Hirata, Kusuhiro Mukai, Shin Ichi Yoda, Tomoji S. Morita, Temperature fluctuations of the Marangoni flow in a liquid bridge of molten silicon under microgravity on board the TR-IA-4 rocket, Journal of Crystal Growth, 10.1016/S0022-0248(97)00440-5, 186, 1-2, 85-94, 1998.03, [URL], Temperature fluctuation measurements in a liquid bridge of molten silicon, which shows the Marangoni flow in highly super-critical condition, are performed in a half-zone configuration under microgravity on board a TR-IA-4 rocket and on the ground. In the microgravity experiment, two types of temperature oscillation are observed during the melting process of silicon and in the cylindrical half-zone melt. The former oscillation, which has a frequency of about 0.1 Hz during the melting process, has an antiphase correlation of temperature oscillation measured in thermocouples separated by 90° azimuthal angles. The latter oscillation in the cylindrical liquid bridge has no remarkable frequency; however, it tends to have the antiphase correlation in between thermocouples with 180° azimuthal angles. In the ground experiment, temperature fluctuations have a characteristic frequency of 0.2 Hz and there is an antiphase correlation of temperatures in thermocouples with 180° azimuthal angles by using the slender melt zone..
94. Y. L. Yao, J. Z. Shu, J. C. Xie, F. Liu, W. R. Hu, A. Hirata, Shinichi Nishizawa, M. Sakurai, Transition of oscillatory floating half zone convection from Earth's gravity to microgravity, International Journal of Heat and Mass Transfer, 10.1016/S0017-9310(96)00309-2, 40, 11, 2517-2524, 1997.01, [URL], Oscillatory features of floating half zone convection were experimentally studied by using the drop shaft facility of Japan Microgravity Center which supported microgravity period of 10 s. Coordinated measurements including free surface deformation and oscillation, temperature and flow pattern in both 1-g and micro-g environment were obtained. The oscillatory frequency and amplitude in micro-g condition were lower and larger than the ones in 1-g condition, respectively. The results gave, at first time, the oscillatory features such as free surface wave in micro-g, coordinated measurements of more than two physical quantities in the micro-g, and transition of thermocapillary oscillatory convection from 1-g to micro-g..
95. Wen Rui Hu, Akira Hirata, Shinichi Nishizawa, Diffusion dominated process for the crystal growth of a binary alloy, Journal of Crystal Growth, 10.1016/S0022-0248(96)00386-7, 169, 2, 380-392, 1996.11, [URL], The pure diffusion process has been often used to study the crystal growth of a binary alloy in the microgravity environment. In the present paper, a geometric parameter, the ratio of the maximum deviation distance of curved solidification and melting interfaces from the plane to the radius of the crystal rod, was adopted as a small parameter, and the analytical solution was obtained based on the perturbation theory. The radial segregation of a diffusion dominated process was obtained for cases of arbitrary Peclet number in a region of finite extension with both a curved solidification interface and a curved melting interface. Two types of boundary conditions at the melting interface were analyzed. Some special cases such as infinite extension in the longitudinal direction and special range of Peclet number were reduced from the general solution and discussed in detail..
96. Akira Hirata, Shinichi Nishizawa, Motoharu Noguchi, Masato Sakurai, Shouichi Yasuhiro, Nobuyuki Imaishi, Marangoni convection in a liquid bridge under microgravity conditions during parabolic flight, Journal of Chemical Engineering of Japan, 10.1252/jcej.27.65, 27, 1, 65-71, 1994.01, [URL], Marangoni convection in a silicone oil liquid bridge under microgravity conditions during parabolic flight was observed. The developing process of axis-symmetric laminar Marangoni convection was studied experimentally and numerically. To reach a steady state of the temperature field, much more time was needed than to reach a steady state of the velocity field. The effects of initial temperature conditions inside a liquid bridge, heat transfer across the free surface, and temperature dependence of viscosity on both velocity and temperature field were studied numerically..
97. Akira Hirata, Shinichi Nishizawa, Masato Sakurai, Nobuyuki Imaishi, Interfacial Contamination Caused by Water on Marangoni Convection in a Silicone Oil Liquid Bridge, Journal of Chemical Engineering of Japan, 10.1252/jcej.26.754, 26, 6, 754-756, 1993, [URL].
98. Shinichi Nishizawa, Aktra Hirata, Momentum, with Heat and Mass Transfer Through Gas-Liquid Interface with Accelerated Interfacial Velocity, Journal of Chemical Engineering of Japan, 10.1252/jcej.26.649, 26, 6, 649-655, 1993, [URL], Effects of interfacial velocity on interfacial momentum, heat and mass transfer through a gas-liquid interface were quantitatively investigated. The rates of momentum, heat and mass transfer depend not only on the magnitude of the interfacial velocity but also on the change of acceleration in interfacial velocity. In addition, by considering the interfacial tension gradient to be the cause of interfacial acceleration, the effect of the interfacial tension gradient on interfacial velocity, interfacial momentum, heat and mass transfer were also quantitatively investigated. Both the interfacial velocity and the rates of momentum, heat and mass transfer depend not only on the magnitude of the interfacial tension gradient but also on the change of interfacial tension..
99. A. Hirata, Shinichi Nishizawa, Y. Okano, Effect of interfacial tension gradient on momentum and mass transfer through a moving interface of single drops, Unknown Host Publication Title, 1992.01, The effect of interfacial tension gradient on interphase momentum and mass transfer through a moving interface of single drops was studied theoretically. Approximate equations for interfacial velocity, drag coefficient and Sherwood number were proposed with taking account of the interfacial tension gradient. The results of this theoretical study showed good agreement with previous experimental results. (A).
100. A. Hirata, Shinichi Nishizawa, Y. Okano, Effect of interfacial tension gradient on momentum and mass transfer through a moving interface of single drops, Unknown Host Publication Title, 1992, The effect of interfacial tension gradient on interphase momentum and mass transfer through a moving interface of single drops was studied theoretically. Approximate equations for interfacial velocity, drag coefficient and Sherwood number were proposed with taking account of the interfacial tension gradient. The results of this theoretical study showed good agreement with previous experimental results. (A).
101. Akira Hirata, Yasunori Okano, Shinichi Nishizawa, Numerical study of transport phenomena through fluid-liquid interfaces, Bulletin of Centre for Informatics (Waseda University), 8, 9-22, 1988.09, The effect of interfacial velocity and interfacial tension gradient on momentum, heat and mass transfer through fluid-liquid interfaces was studied numerically. The effect of interfacial tension gradient on the interfacial velocity was clarified. The results obtained in this study can explain quantitatively the previous experimental results on the interfacial acceleration due to the interfacial tension gradient. Interfacial contamination and interfacial turbulent phenomena were respectively related to the enhancement and suppression of the mobility of interface. Effects of physical properties, bulk flow speed and Prandtl number on starting points of interfacial turbulence were also discussed..

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