Fons, Paul



Faculty of Science and Technology, Department of Electronics and Electrical Engineering (Yagami)




Books 【 Display / hide

  • Phase change materials : science and applications

    Raoux, Simone, Wutting, Matthias, Springer, 2009,  Page: xxii, 446 p.

Papers 【 Display / hide

  • Dimensional transformation of chemical bonding during crystallization in a layered chalcogenide material

    Yuta Saito, Shogo Hatayama, Yi Shuang, Paul Fons, Alexander V. Kolobov, Yuji Sutou

    Scientific Reports (NATURE RESEARCH)  11 ( 1 ) 4782 - 4782 2021.12

    Research paper (scientific journal), Joint Work, Except for reviews,  ISSN  2045-2322

     View Summary

    Two-dimensional (2D) van der Waals (vdW) materials possess a crystal structure in which a covalently-bonded few atomic-layer motif forms a single unit with individual motifs being weakly bound to each other by vdW forces. Cr Ge Te is known as a 2D vdW ferromagnetic insulator as well as a potential phase change material for non-volatile memory applications. Here, we provide evidence for a dimensional transformation in the chemical bonding from a randomly bonded three-dimensional (3D) disordered amorphous phase to a 2D bonded vdW crystalline phase. A counterintuitive metastable “quasi-layered” state during crystallization that exhibits both “long-range order and short-range disorder” with respect to atomic alignment clearly distinguishes the system from conventional materials. This unusual behavior is thought to originate from the 2D nature of the crystalline phase. These observations provide insight into the crystallization mechanism of layered materials in general, and consequently, will be useful for the realization of 2D vdW material-based functional nanoelectronic device applications. 2 2 6

  • Dielectric relaxation in amorphous and crystalline Sb2Te3 thin films

    A. A. Kononov, R. A. Castro, Y. Saito, P. Fons, G. A. Bordovsky, N. I. Anisimova, A. V. Kolobov

    Journal of Materials Science: Materials in Electronics  2021.05

    Research paper (scientific journal), Joint Work, Except for reviews

  • Electric Fields and Interfacial Phase-Change Memory Structures

    Fons Paul, Kolobov Alexander V., Saito Yuta


    Research paper (scientific journal), Joint Work, Except for reviews,  ISSN  1862-6254

     View Summary

    Phase-change memory (PCM), a nonvolatile electrical memory based upon the local structure of chalcogenide compounds such as (Formula presented.), is playing an ever increasing role in society. In PCM, information is stored via structure, specifically crystalline or amorphous phases. As generation of the amorphous state requires a melt-quench process, the energy required for switching is relatively large. Interfacial PCM (iPCM) is a form of PCM that greatly improves energy efficiency consisting of a short-period (Formula presented.) superlattice. Unlike conventional PCM, iPCM is believed to switch between two crystalline states. The large energy reduction to switch iPCM and the absence of a melt-quenched phase has led to speculation that electrical fields may play a role in the switching process. Herein, ab initio molecular dynamics is employed to explore electric field effects on proposed iPCM structures. Structures are terminated by van der Waals (vdW)-bonded layers to avoid dangling bonds. Unlike previous speculation in the literature, the effect of electrical fields on Ge atoms is minimal with Te–Te vdW gaps experiencing the largest change. The electric-field-induced rearrangements vary; however, for all structures, a dilation in the vdW gap is observed possibly facilitating the proposed switching mechanisms.

  • Crystalline Sb<inf>2</inf>Te<inf>3</inf>: Side Surfaces and Disappearance of Dirac Cones

    Alexander V. Kolobov, Paul Fons, Yuta Saito

    Physica Status Solidi - Rapid Research Letters (WILEY-V C H VERLAG GMBH)  15 ( 3 )  2021.03

    Research paper (scientific journal), Joint Work, Except for reviews,  ISSN  1862-6254

     View Summary

    Sb Te is an end point of quasibinary GeTe–Sb Te phase-change alloys and also a prototypical topological insulator (TI). TIs are materials that behave like insulators in their interior but whose surfaces are characterized by metallic states with linear dispersion, the so-called Dirac cones. Such surface states are symmetry protected, robust, and are maintained even in the presence of surface defects. It has been tacitly implied that any surfaces of a TI possess this property. Herein, using ab initio simulations, it is demonstrated that cleaving Sb Te along certain side surfaces may lead to the disappearance of Dirac surface states. In particular, it is shown that the (110) surface of the typical TI Sb Te is slightly gapped, whereas the ((Formula presented.)) surface is metallic. The significance and potential benefits of the obtained results for practical applications in planar devices and memory cells are discussed. 2 3 2 3 2 3 2 3

  • Chalcogenide Materials Engineering for Phase-Change Memory and Future Electronics Applications: From Sb-Te to Bi-Te

    Saito Yuta, Mitrofanov Kirill V., Makino Kotaro, Fons Paul, Kolobov Alexander V., Tominaga Junji, Uesugi Fumihiko, Takeguchi Masaki


    Research paper (scientific journal), Joint Work, Accepted,  ISSN  1862-6254

     View Summary

    Chalcogenide materials play essential roles in modern nonvolatile memory technology in the form of both phase-change memory (PCM) and selector devices. Herein, Bi–Te binary alloys are explored as an alternative candidate for superlattice (SL) or interfacial PCM (iPCM). GeTe/Bi Te (GT/BT) SL exhibits similar structural features to conventional GeTe/Sb Te (GT/ST) SL, such as highly oriented crystal grains and intermixing. Furthermore, preliminary device measurements show that Ge–Bi–Te (GBT) SL switches in a similar manner to conventional Ge–Sb–Te (GST), suggesting that they may be a promising candidate for memory applications. In addition, Bi Te /Sb Te (BT/ST) heterostructure films have been successfully fabricated and show clear interface stacking at the atomic level. Although the BT/ST heterostructure is ostensibly a p–n junction, rectifying behavior is not observed in current (I)–voltage (V) measurements due to the existence of a large number of carriers in both layers. Finally, density functional theory (DFT)-based simulations suggest that an ideal BT/ST heterostructure may possess intriguing topological properties that can enable novel functional devices. The Bi–Te binary alloys offer promising potential for optimizing PCM performance as well as for the realization of novel functional electronic devices. 4 3 2 3 2 3 2 3

display all >>

Reviews, Commentaries, etc. 【 Display / hide

  • Erratum: Effects of RbF postdeposition treatment and heat-light soaking on the metastable acceptor activation of CuInSe<inf>2</inf> thin film photovoltaic devices (Applied Physics Letters (2018) 113 (063901) DOI: 10.1063/1.5031898)

    Ishizuka S., Shibata H., Nishinaga J., Kamikawa Y., Fons P.

    Applied Physics Letters (Applied Physics Letters)  113 ( 11 )  2018.09

    ISSN  00036951

     View Summary

    © 2018 Author(s). We have found an error in Fig. 3(d) in the original published version of our paper.1 The annotations -6V and +3.0V are incorrect and should be +0.6V and -30 V, respectively. Figure 1 is a corrected version for the original Fig. 3(d). (Figure Presented).

  • Ultrafast Dynamics of Electron-phonon Coupling in Transition-metal Dichalcogenides

    Kotaro Makino, Yuta Saito, Shuuto Horii, Paul Fons, Alexander V. Kolobov, Atsushi Ando, Keiji Ueno, Richarj Mondal, Muneaki Hase


    Institution technical report and pre-print, etc., Joint Work

     View Summary

    Time-domain femtosecond laser spectroscopic measurements of the ultrafast
    lattice dynamics in 2H-MoTe2 bulk crystals were carried out to understand the
    carrier-phonon interactions that govern electronic transport properties. An
    unusually long lifetime coherent A1g phonon mode was observed even in the
    presence of very large density of photo-excited carriers at room temperature.
    The decay rate was observed to decrease with increasing excitation laser
    fluence. Based on the laser fluence dependence including the inducement of
    significant phonon softening and a peculiar decrease in phonon decay rate, we
    attribute the long lifetime lattice dynamics to weak anharmonic phonon-phonon
    coupling and a carrier-density-dependent deformation potential electron-phonon

  • Resistive switching characteristics of interfacial phase-change memory at elevated temperature

    Kirill V. Mitrofanov, Yuta Saito, Noriyuki Miyata, Paul Fons, Alexander V. Kolobov, Junji Tominaga

    Japanese Journal of Applied Physics 57 ( 4 ) 04FE06.1‐04FE06.4 2018.04

    Other article, Joint Work,  ISSN  0021-4922

     View Summary

    © 2018 The Japan Society of Applied Physics. Interfacial phase-change memory (iPCM) devices were fabricated using W and TiN for the bottom and top contacts, respectively, and the effect of operation temperature on the resistive switching was examined over the range between room temperature and 200 °C. It was found that the highresistance (RESET) state in an iPCM device drops sharply at around 150°C to a low-resistance (SET) state, which differs by ∼400Ω from the SET state obtained by electric-field-induced switching. The iPCM device SET state resistance recovered during the cooling process and remained at nearly the same value for the RESET state. These resistance characteristics greatly differ from those of the conventional Ge-Sb-Te (GST) alloy phase-change memory device, underscoring the fundamentally different switching nature of iPCM devices. From the thermal stability measurements of iPCM devices, their optimal temperature operation was concluded to be less than 100 °C.

  • Switching effects in iPCM beyond standard operation

    MITROFANOV Kirill V, SAITO Yuta, MIYATA Noriyuki, FONS Paul, KOLOBOV Alexander V, TOMINAGA Junji

    Proceedings of the Symposium on Phase Change Oriented Science 30th   18‐20 2018

    Other article, Joint Work

  • Electric Field Effects in GeTe/Sb&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt; interfacial phase change memory

    FONS Paul, FONS Paul, KOLOBOV Alexander V, SAITO Yuta, MITROFANOV Kirill V, MAKINO Kotaro, TOMINAGA Junji

    Proceedings of the Symposium on Phase Change Oriented Science 30th   28‐31 2018

    Other article, Joint Work

display all >>

Research Projects of Competitive Funds, etc. 【 Display / hide

  • Understanding the crystallization mechanism of amorphous van der Waals layered materials


    MEXT,JSPS, Grant-in-Aid for Scientific Research, Fons JamesPaul, Grant-in-Aid for Scientific Research (B), Principal Investigator


Courses Taught 【 Display / hide











display all >>