Shuu'ichirou Yamamoto, Tomotaka Watanabe and Shunri Oda

Research Center for Quantum Effect Electronics
Tokyo Institute of Technology 2-12-1, O-okayama, Meguro-ku, Tokyo 152, Japan
 

A novel method of Josephson junction formation using atomic force microscope (AFM) is presented. We have investigated surface modifications of YBaCuO films by using AFM with applied voltages. Ridge structures have been observed at the surface of YBaCuO applied voltages between 4V and 10V, the narrowest ridge line width fabricated being 150nm. Mechanism of the formation of the ridge structure is discussed. Current-voltage characteristics of AFM modified microbridges of YBaCuO thin films have been measured and analyzed.
 

Adv. Colloid and Interface Sci. in Press (1997) Preparation of Nanocrystalline Silicon Quantum Dot Structure by Pulsed Plasma Processes Shunri Oda Research Center for Quantum Effect Electronics Tokyo Institute of Technology O-Okayama, Meguro-ku, Tokyo 152, Japan Tel: 81-3-5734-3048, Fax 81-3-3720-9806, E-mail: soda@pe.titech.ac.jp Nanocrystalline silicon (nc-Si) is an ultrafine particle of silicon with dimensions less than 10 nm. Particles of this size exhibit quantum effects. We developed a novel method called digital plasma processing for the control of particle size, position, and surface electronic states of nc-Si. Preparation conditions for nc-Si were studied by using both solid-phase and vapor-phase nucleation methods. The ultrafine structure of nc-Si was investigated using high-resolution transmission electron microscopy. An atomic force microscope was used to image the surface morphology, to manipulate the position, and to measure the electrical properties of nc-Si. Quantum effect properties in nc-Si were investigated by using photoluminescence and Coulomb blockade characteristics. Monodispersed nc-Si with size variation of 8±1 nm were obtained by a pulsed plasma process. Cou lomb staircase was observed in current-voltage characteristics from nc-Si at room temperature. These results suggest that nc-Si is a promising candidate for the application of future ultra large scale integrated devices. Keywords nanocrystal, silicon, quantum dot, Coulomb blockade, single electron tunneling, atomic force microscopy, pulsed plasma process 1. Introduction 2. Preparation of nc-Si 　2.1 Solid phase nucleation 　2.2 Vapor phase nucleation 3. Position control of nc-Si 4. Properties of nc-Si 　4.1 Photoluminescence 　4.2 Single-electron tunneling 5. Conclusions Acknowledgment References
 

Japanese Journal of Applied Physics; 4031-4034 (1997) Fabrication of nanocrystalline silicon with small spread of grain size by pulsed gas plasma Toru Ifuku, Masanori Otobe, Akira Itoh, and Shunri Oda Department of Physical Electronics and Research Center for Quantum Effect Electronics Tokyo Institute of Technology Meguro-ku, Tokyo 152, Japan TEL +81-3-5734-2542, FAX +81-3-5734-2911 Nanocrystalline silicon (nc-Si) with a grain size of less than 10nm attracts considerable attention for quantum effect device applications. We have fabricated nc-Si in SiH4 plasma cell with very-high-frequency (VHF;144MHz) excitation[1,2]. In order to use nc-Si in future electron device successfully, it is required to control the grain size accurately. The SiH4 plasma cell is attached to a ultra-high-vacuum (UHV) chamber. Nano-crystalline silicon is formed in the gas phase of SiH4 plasma cell by coalescence of radicals. Photo-luminescence (PL) spectroscopy and electrical properties of nc-Si have been investigated. As-grown nc-Si sample shows infrared PL at room temperature (RT). After dry oxidation at 800℃, the peak energy shifts towards high-energy region and the sample shows red emission at RT. As for the electrical property, the Coulomb staircase was observed at RT in current-voltage characteristics. The next question is how to control the grain size of nc-Si with small spread. In this study, we use SiH4 plasma cell with a pulsed H2 gas supply for the purpose of making nc-Si with the small spread of grain size. When H2 gas is supplied, nucleation of nc-Si is enhanced. When H2 gas supply is off, nucleated nc-Si particles can grow larger in SiH4 plasma. In the next cycle of H2 gas supply, nc-Si particles grown in the previous cycle are pushed out of the cell into the deposition chamber, and nucleation for the next cycle takes place. Using this method, fabrication of nc-Si with a diameter around 8nm and a narrow spread (ｱ1nm) of grain size was realized successfully. REFERENCES [1] S. Oda and M. Otobe, Mat. Res. Soc. Symp. Proc. 358,721(1995). [2] M.Otobe, T. Kanai and S. Oda, Mat. Res. Soc. Symp. Proc. 377,51(1995).
 

Japanese Journal of Applied Physics; 4038-4041 (1997) Fabrication and Electrical Characteristics of Single Electron Tunneling Devices Based on Si Quantum dots Prepared by Plasma Processing Amit Dutta , Yoshiaki Honda, Masao Kimura, Masanori Otobe, Akira Itoh, Shunri Oda Department of Physical Electronics and Research Center for Quantum Effect Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152. Tel: +81-3-5734-2542, Fax: +81-3-5734-2911 Silicon based nanoelectronics have achieved considerable attention in recent years. We have fabricated single electron tunneling (SET) devices using a quasi one dimensional array of Si nano-particles. These Si nano-particles were prepared by very high frequency plasma processing developed by our group[1]. We have fabricated heavily doped poly-silicon electrodes on SiO2/Si substrates having very small gaps of 25~100 nm, by electron beam lithography and electron cyclotron resonance etching. Fabrication of electrode structure for SET transistors having two electrodes acting as source and drain electrodes and substrate as back gate and SET transistors having additional inplane electrodes acting as gate was performed. Si nano-particles were deposited on theses electrode structure. Although deposition of Si nano-particles are random, the electrical characteristics are determined by the lowest resistance tunneling path connecting the source and the drain electrodes. Hence, current flows through a quasi one dimensional array of quantum dots. I-V characteristics without application of gate voltage shows Coulomb staircase at room temperature. Inter-dot capacitance calculated form separation between steps of Coulomb staircase was in the range of 0.7 ~1.3 aF. Details of I-V characteristics and conductance vs. gate voltage characteristics will be reported. Reference: [1] S. Oda and M. Otobe, Mat. Res. Soc. Symp. Proc. 358, 721(1995).
 

応用物理; 66; 99-108; (1997) 単電子デバイス・回路の研究状況と今後の展望 田部道晴、小田俊理、平本俊郎、中里和郎、雨宮好仁 ひとつひとつの電子の動きを制御する，単電子デバイスと呼ばれる新しい概念が産声 をあげて約１０年が経った．この間，「単電子」を頭に冠するトランジスタ，メモリ ー，論理回路などさまざまなデバイス・回路が急ピッチで試作され，単電子物理に根 差した独特の振舞が次々と実証されている．さらに，単電子に適した新しい回路や情 報処理手法の研究も進展している．一方で，将来の集積化を念頭に置いたさまざまな 課題も浮き彫りにされつつある．ここでは，デバイス応用の立場から，単電子に関わ る研究状況と今後の展望について述べている．
 

Materials Research Society Symposium Proceedings; 452; 749-754 (1997) GRAIN-SIZE CONTROL OF NANOCRYSTALLINE SILICON BY PULSED GAS PLASMA PROCESS Toru Ifuku, Masanori Otobe, Akira Itoh and Shunri Oda Tokyo Institute of Technology Department of Physical Electronics and Research Center for Quantum Effect Electronics O-Okayama, Meguro-ku, Tokyo 152, Japan Recently, single electron charging effect has been demonstrated by using nanocrystalline Si (nc-Si) structures. We have fabricated nc-Si by SiH$_4 plasma cell with very-high-frequency (VHF) excitation [1,2]. However, it has been difficult to fabricate a uniform grain-sizestructure, which means the characteristics of single electron charging phenomena were not stable and were fluctuated. In this study, a new method using pulsed gas plasma technique to fabricate nc-Si is proposed in order to make a uniform grain-size structure. The grain size was controlled using pulsed gas supply of H$_2$ into SiH$_4$. The nucleation rate in the gas phase of SiH$_4$ plasma increasesduring supplying H$_2$, and the growth of nuclei can be suppressed. The next H$_2$ pulse effectively extract nc-Si out of plasma cell through the orifice to the substrates in the ultra-high-vacuum (UHV) chamber. With this method, fabrication of nc-Si with a diameter around 8nm and a narrow spread of grain size were realized successfully.
 

nt. Symp. Intrinsic Josephson Effect and THz Plasma Oscillations in High Tc Superconductors; (1997) Anomalous Current-Voltage Characteristics along the c-Axis in YBaCuO Thin Films Prepared by MOCVD and AFM Lithography Shuuﾕichirou Yamamoto, Atsushi Kawaguchi and Shunri Oda Research Center for Quantum Effect Electronics, Tokyo Institute of Technology, O-Okayama, Meguro-ku, Tokyo 152, Japan We have observed anomalous current-voltage characteristics related to intrinsic Josephson junction along c-axis of YBaCuO thin films. 180nm thick c-axis oriented YBaCuO films are deposited on SrTiO3(100) substrates by MOCVD at 650℃. To reduce Josephson coupling between CuO2 planes, these films are annealed at 500℃ and 2Torr of a gaseous mixture 1:1 oxygen and argon for 1hour. After annealing these films are immediately transferred to a metal evaporation system and 90nm-thick Au capping layer is deposited. Samples are coated with positive photo-resist and small mesa structures of 5μm x 5μm are fabricated by optical lithography. These mesas are re-shaped to 1.5μm x 1.7μm mesas by AFM lithography. Ar+ ion milling is performed to etch the Au capping layer and 35nm of YBaCuO layer. Further, these samples are coated with 300nm thick layer of photo-resist which serves as an insulator film. Au contact pads are fabricated by AFM lithography and Au deposition. Three terminal current-voltage measurement shows a clear hysteresis around 280μA with a maximum current height of 30μA. A number of steps could be observed both in the hysteresis loop and in the degenerate portion of current-voltage curve. The maximum width of each steps is about 2mV at 5K. These observations could be summarized as an effect of series connection of intrinsic Josephson junctions. Above 20K hysteresis disappeared. On the other hand, the steps could be observed up to 30K.
 

Silicon Nanoelectronics Workshop, Honolulu (1996) Coulomb staircase characteristics in silicon quantum dots fabricated by plasma processing S. Oda, M. Kimura and M. Otobe Research Center for Quantum Effect Electronics Tokyo Institute of Technology O-Okayama, Meguro-ku, Tokyo 152, Japan Silicon quantum dot structures with size less than 10nm have been fabricated in a glow-discharge plasma of silane and deposited on various substrates at room temperature. Lattice images in TEM observation have revealed that Si quantum dots are spherical in shape and form single domain crystal. The surface of Si quantum dots is covered by natural oxide 1.5nm thick. In an attempt to control diameter of Si quantum dots, we propose a method to separate nucleation and crystal growth. A short pulse of hydrogen gas was inserted in silane plasma to enhance nucleation. A monodispersed Si quantum dot structure with diameter of 8±1nm has been obtained. The effective size of the crystal region is 5nm. Si quantum dots have showed red photoluminescence at room temperature upon excitation by uv light from a He-Cd laser. When Si quantum dots are oxidized at 800oC, the peak of PL spectra shifts toward blue with increasing oxidation time. We have measured current-voltage characteristics from a quasi one dimensional array of Si quantum dots, deposited on ultrasmall (23nm apart) electrode patterns of heavily-doped polycrystalline Si fabricated using EB lithography and ECR etching. Structures due to Coulomb staircase have been observed in I-V curve at room temperature.
 

第43回応用物理学関係連合講演会, 26a-TC-1 (1996) ガスパルス供給プラズマを用いた粒径8nm±1nmの微結晶Siの作製 伊福徹、 乙部雅則、 小田俊理 　我々は、超高真空装置にSiH4プラズマセルを組み合わせ、粒径10nm前後の微結晶Si を作製している。1)今回核形成と核成長を時間的に分離して粒径を揃えることを目的 に、SiH4プラズマにH2をパルス供給することによる粒径制御を試みたのでその結果に ついて報告する。 　プラズマセルには、電源周波数としてVHF帯のものを用いた平行平板電極型を使っ た。SiH4プラズマ中にH2を1秒周期で0.5秒間パルス供給した。セルへH2を供給するこ とにより核形成速度が増加する。次にSiH4のみがセルに供給され、核の成長のみが起 きる。再度H2を供給することによりプラズマセル内のガス滞在時間が減少し、nc-Si をセルから押し出す。以上のような方法で作製したnc-Siの粒径は、SiH4のみで作製 した試料に比べ著しく揃っており、粒径8nm±1nmのnc-Siを成長することが出来た。 粒径はTEM像から求めた。 ［謝辞］TEM観察に御協力いただいた東工大理学部八木･山本研究室に感謝します。本 研究の一部は文部省科学研究費補助金･新技術事業団「さきがけ研究21」･倉田奨励金 の援助を受けた。 1) M. Otobe et al: Mater. Res. Soc. Symp. Proc. 377 (1995).(in press)
 

第57回応用物理学会学術講演会, 9a-B-8 (1996) シリコン量子ドットの表面酸化 伊福徹、 伊藤明、 小田俊理 　我々は、超高真空装置にSiH4プラズマセルを組み合わせ、粒径10nm前後のSi量子ド ット(SQD)を作製している1)。このようなSQDを用いた、将来の単一電子素子等への応 用を目的とし、その電気的特性を測定してきた2)。しかしSQD表面の自然酸化膜での リーク電流の発生や、基板表面に吸着している状態でSQDが簡単に動いてしまう等の 問題があった。その解決方法として、今回その場酸化を含む方法で、作製したnc-Si の酸化を試みた結果を報告する。 　Si基板をイオンミリングにより開けた穴の縁に堆積されたものを観察した。800℃ DryO2雰囲気で10分間酸化することにより、nc-Siの周囲に3nm程度と、自然酸化膜厚 (1.5nm)より厚い酸化膜が形成できたことが分かる。またnc-Siと基板のSiO2が癒着し ている様子から、nc-Siを熱酸化させることにより、基板に固定できると考えられ る。 ［謝辞］TEM観察に御協力いただいた東工大理学部八木･山本研究室に感謝します。本 研究の一部は文部省科学研究費補助金の援助を受けた。 1) M. Otobe et al: Mater. Res. Soc. Symp. Proc. 377 (1995). 2) 乙部、木村他、第43回春季応物講演予稿集No.1、26pZA1,2 (1995年)、p60,61