Shogo Hatayama
@aist.go.jp
National Institute of Advanced Industrial Science and Technology (AIST)
Scopus Publications
Scopus Publications
Yi Shuang, Yuta Saito, Shogo Hatayama, Paul Fons, Ando Daisuke, and Yuji Sutou
Elsevier BV
Wen Hsin Chang, Shogo Hatayama, Yuta Saito, Naoya Okada, Takahiko Endo, Yasumitsu Miyata, and Toshifumi Irisawa
Springer Science and Business Media LLC
Shogo Hatayama, Kotaro Makino, and Yuta Saito
Springer Science and Business Media LLC
AbstractGe2Sb2Te5 (GST), the most mature phase-change materials (PCM), functions as a recoding layer in nonvolatile memory and optical discs by contrasting the physical properties upon phase transition between amorphous and crystalline phases. However, GST faces challenges such as a large extinction coefficient (k) and low thermal stability of the amorphous phase. In this study, we introduce RuSbTe as a new PCM to address the GST concerns. Notably, the crystallization temperature of the amorphous RuSbTe is approximately 350 °C, significantly higher than GST. A one-order-of-magnitude increase in the resistivity contrast was observed upon phase transition. The crystalline (0.35–0.50 eV) and amorphous (0.26–0.37 eV) phases exhibit relatively small band gap values, resulting in substantial k. Although RuSbTe demonstrates a k difference of approximately 1 upon crystallization at the telecommunications C-band, the refractive index (n) difference is negligible. Unlike GST, which induces both phase retardation and amplitude modulation in its optical switch device, RuSbTe exhibits amplitude-only modulation. This study suggests that RuSbTe has the potential to enable new photonic computing devices that can independently control the phase and amplitude. Combining RuSbTe with phase-only modulators could open avenues for advanced applications.
Kentaro Saito, Shogo Hatayama, and Yuta Saito
Wiley
A critical element within the 3D XPoint architecture is the Ovonic threshold switch (OTS) material, which serves a crucial role as a selector. The development of novel OTS materials devoid of hazardous elements such as As and Se is imperative for mitigating environmental impact. The Si–Te binary telluride is a representative As/Se‐free OTS material, demonstrating stable switching. However, its thermal stability is insufficient for enduring annealing processes in semiconductor manufacturing. To address this challenge, this study proposes the incorporation of Mn into the Si–Te alloy. While the introduction of transition metals into chalcogenide glass typically reduces the electrical resistivity, potentially compromising the ON/OFF ratio, the OFF current for the device containing 26 at% Mn is observed to be lower than that for the undoped Si–Te device. Furthermore, the thermal stability of the Mn–Si–Te film surpasses that of its pristine counterpart. X‐ray photoelectron spectroscopy and density‐functional theory simulations provide evidence of Mn–Te bonding formation in the Mn–Si–Te amorphous alloy, thus suggesting the role of Mn–Te bonding in enhancing thermal stability. In these findings, a promising avenue for the advancement of novel OTS materials is provided.
Yi Shuang, Shunsuke Mori, Takuya Yamamoto, Shogo Hatayama, Yuta Saito, Paul J. Fons, Yun-Heub Song, Jin-Pyo Hong, Daisuke Ando, and Yuji Sutou
American Chemical Society (ACS)
Phase-change materials such as Ge-Sb-Te (GST) exhibiting amorphous and crystalline phases can be used for phase-change random-access memory (PCRAM). GST-based PCRAM has been applied as a storage-class memory; however, its relatively low ON/OFF ratio and the large Joule heating energy required for the RESET process (amorphization) significantly limit the storage density. This study proposes a phase-change nitride, CrN, with a much wider programming window (ON/OFF ratio more than 105) and lower RESET energy (one order of magnitude reduction from GST). High-resolution transmission electron microscopy revealed a phase-change from the low-resistance cubic CrN phase into the highly resistive hexagonal CrN2 phase induced by the Soret-effect. The proposed phase-change nitride could greatly expand the scope of conventional phase-change chalcogenides and offer a strategy for the next-generation of PCRAM, enabling a large ON/OFF ratio (∼105), low switching energy (∼100 pJ), and fast operation (∼30 ns).
Misako Morota, Shogo Hatayama, Yi Shuang, Shunsuke Mori, Yuji Sutou, Paul Fons, and Yuta Saito
Elsevier BV
Naoya Okada, Wen Hsin Chang, Shogo Hatayama, Yuta Saito, and Toshifumi Irisawa
IOP Publishing
Abstract We investigated the electrical junction properties of the layered Sb2Te3 film formed on Si substrates. The current−voltage characteristics of the Sb2Te3/n-Si heterojunction showed an ohmic properties, whereas the Sb2Te3/p-Si heterojunction exhibited rectifying properties with a high barrier height of 0.77 eV. The capacitance−voltage characteristics of MOS capacitors with the Sb2Te3 electrode indicated an effective work function of 4.44 eV for the Sb2Te3 film. These findings suggest that the Sb2Te3/Si heterostructure possesses a low conduction band offset, as inferred from the temperature dependence of the current−voltage characteristics of the Sb2Te3/n-Si.
Shogo Hatayama, Shunsuke Mori, Yuta Saito, Paul J. Fons, Yi Shuang, and Yuji Sutou
American Chemical Society (ACS)
The burgeoning field of optoelectronic devices necessitates a mechanism that gives rise to a large contrast in the electrical and optical properties. A SmTe film with a NaCl-type structure demonstrates significant differences in resistivity (over 105) and band gap (approximately 1.45 eV) between as-deposited and annealed films, even in the absence of a structural transition. The change in the electronic structure and accompanying physical properties is attributed to a rigid-band shift triggered by a valence transition (VT) between Sm2+ and Sm3+. The stress field within the SmTe film appears closely tied to the mixed valence state of Sm, suggesting that stress is a driving force in this VT. By mixing the valence states, the formation energy of the low-resistive state decreases, providing nonvolatility. Moreover, the valence state of Sm can be regulated through annealing and device-operation processes, such as applying voltage and current pulses. This investigation introduces an approach to developing semiconductor materials for optoelectrical applications.
Kotaro Makino, Verdad C. Agulto, Shogo Hatayama, Kosaku Kato, Toshiyuki Iwamoto, and Makoto Nakajima
IEEE
Dielectric constants of materials are a key parameter for device design. We report on the results of terahertz time-domain ellipsometry (THz-TDE) for thick and thin metal films and found that the difference between the experimental and fitting results based on the Drude model becomes significant with increasing the thickness of the metal films. It was found that the conductivity of the metal film decreases with decreasing the film thickness due to the size effect. We discuss a possible origin of the error and demonstrate that THz-TDE is capable of dielectric constant evaluation for thin metal films.
Shogo Hatayama, Yuta Saito, Paul Fons, Yi Shuang, Mihyeon Kim, and Yuji Sutou
Elsevier BV
Yi Shuang, Qian Chen, Mihyeon Kim, Yinli Wang, Yuta Saito, Shogo Hatayama, Paul Fons, Daisuke Ando, Momoji Kubo, and Yuji Sutou
Wiley
Abstract2D van der Waals (vdW) transition metal di‐chalcogenides (TMDs) have garnered significant attention in the nonvolatile memory field for their tunable electrical properties, scalability, and potential for phase engineering. However, their complex switching mechanism and complicated fabrication methods pose challenges for mass production. Sputtering is a promising technique for large‐area 2D vdW TMD fabrication, but the high melting point (typically Tm > 1000 °C) of TMDs requires elevated temperatures for good crystallinity. This study focuses on the low‐Tm 2D vdW TM tetra‐chalcogenides and identifies NbTe4 as a promising candidate with an ultra‐low Tm of around 447 °C (onset temperature). As‐grown NbTe4 forms an amorphous phase upon deposition that can be crystallized by annealing at temperatures above 272 °C. The simultaneous presence of a low Tm and a high crystallization temperature Tc can resolve important issues facing current phase‐change memory compounds, such as high Reset energies and poor thermal stability of the amorphous phase. Therefore, NbTe4 holds great promise as a potential solution to these issues.
Yuta Saito, Shogo Hatayama, Wen Hsin Chang, Naoya Okada, Toshifumi Irisawa, Fumihiko Uesugi, Masaki Takeguchi, Yuji Sutou, and Paul Fons
Royal Society of Chemistry (RSC)
Here, we report on the growth of GeTe2 thin films, a metastable phase. The GeTe2 film was found to be a semiconductor with a layered structure.
Wen Hsin Chang, Shogo Hatayama, Yuta Saito, Naoya Okada, Takahiko Endo, Yasumitsu Miyata, and Toshifumi Irisawa
Wiley
Bokusui Nakayama, Hikaru Nagase, Hiromori Takahashi, Yuta Saito, Shogo Hatayama, Kotaro Makino, Eiji Yamamoto, and Toshiharu Saiki
Proceedings of the National Academy of Sciences
Indirect interactions via shared memory deposited on the field (“field memory”) play an essential role in collective motions. Some motile species, such as ants and bacteria, use attractive pheromones to complete many tasks. Mimicking these kinds of collective behavior at the laboratory scale, we present a pheromone-based autonomous agent system with tunable interactions. In this system, colloidal particles leave phase-change trails reminiscent of the process of pheromone deposition by individual ants, and the trails attract other particles and themselves. To implement this, we combine two physical phenomena: the phase change of a Ge 2 Sb 2 Te 5 (GST) substrate by self-propelled Janus particles (pheromone deposition) and the AC (alternating current) electroosmotic (ACEO) flow generated by this phase change (pheromone attraction). Laser irradiation causes the GST layer to crystalize locally beneath the Janus particles, owing to the lens heating effect. Under AC field application, the high conductivity of the crystalline trail causes a field concentration and generates ACEO flow, and we introduce this flow as an attractive interaction between the Janus particles and the crystalline trail. By changing the AC frequency and voltage, we can tune the attractive flow, i.e., the sensitivity of the Janus particles to the trail, and the isolated particles undergo diverse states of motion, from self-caging to directional motion. A swarm of Janus particles also shows different states of collective motion, including colony formation and line formation. This tunability enables a reconfigurable system driven by a pheromone-like memory field.
Yi Shuang, Shogo Hatayama, Daisuke Ando, and Yuji Sutou
Elsevier BV
Shogo Hatayama, Takuya Yamamoto, Shunsuke Mori, Yun-Heub Song, and Yuji Sutou
American Chemical Society (ACS)
Data recording based on the phase transition between amorphous and crystalline phases in a phase-change material (PCM) generally consumes a large amount of operation energy. Heat confinement and scaling down of the contact area between the PCM and electrode are effective strategies for reducing the operation energy in the memory device. Contrary to conventional PCM, such as Ge-Sb-Te compounds (GST), Cr2Ge2Te6 (CrGT) exhibits low thermal conductivity and low-energy memory operation characteristics even in a relatively large contact area. Herein, we show that the operation energy of the CrGT-based memory device is greatly reduced by scaling down. Based on the present results, an operation energy at subpico J order, which was achieved using carbon nanotubes or graphene nanoribbon in the GST-based device, can be realized in the contact area comparable to the product level in the CrGT-based device. The numerical simulation suggests that small thermal and electrical conductivities enhance the thermal efficiency, resulting in a small operation energy for amorphization. It was also found that the residual metastable phase after the amorphization process increased the operation energy for crystallization by the simulation. In other words, these results indicate that further small operation energy can be realized in the CrGT-based device by reducing the metastable phase volume.
Mihyeon Kim, Shunsuke Mori, Yi Shuang, Shogo Hatayama, Daisuke Ando, and Yuji Sutou
Wiley
Shogo Hatayama, Yuta Saito, Kotaro Makino, Noriyuki Uchida, Yi Shuang, Shunsuke Mori, Yuji Sutou, Milos Krbal, and Paul Fons
Royal Society of Chemistry (RSC)
The crystallization mechanism of sputter-deposited amorphous Mo–Te film is revealed enabling the large-area growth of 2D materials.
Milos Krbal, Vit Prokop, Jan Prikryl, Jhonatan Rodriguez Pereira, Igor Pis, Alexander V. Kolobov, Paul J. Fons, Yuta Saito, Shogo Hatayama, and Yuji Sutou
American Chemical Society (ACS)
Takuya Yamamoto, Shogo Hatayama, and Yuji Sutou
Elsevier BV
Leonid Bolotov, Shinji Migita, Ryouta Fujio, Manabu Ishimaru, Shogo Hatayama, and Noriyuki Uchida
Elsevier BV
Shogo Hatayama, Yuta Saito, and Noriyuki Uchida
AIP Publishing
Yuta Saito, Shogo Hatayama, Yi Shuang, Paul Fons, Alexander V. Kolobov, and Yuji Sutou
Springer Science and Business Media LLC
AbstractTwo-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. Cr2Ge2Te6 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.
Takuya Yamamoto, Shogo Hatayama, Yun-Heub Song, and Yuji Sutou
IOP Publishing
To evaluate the Thomson effect on the temperature increase in Ge2Sb2Te5 (GST)-based phase-change random access memory (PCRAM), we created new dimensionless numbers based on Buckingham’s П theorem. The influence of the Thomson effect on the temperature increase depends on the dominant factor of electrical resistance in a PCRAM cell. When the effect is dominated by the volumetric resistance of the phase-change material C = ρ c Δ x / σ ≪ O ( 1 ) , the dimensionless evaluation number is B = μ T σ Δ ϕ k , where ρ c is the contact resistance, Δx is the thickness of PCM, σ and k are the electrical and thermal conductivities, μ T is the Thomson coefficient, and Δϕ is the voltage. When the contact resistance cannot be ignored, the evaluation number is B/(1 + C). The characteristics of hexagonal-type crystalline GST in a PCRAM cell were numerically investigated using the defined dimensionless parameters. Although the contact resistance of GST exceeded the volumetric resistance across the temperature range, the ratio of contact resistance to the whole resistance reduced with increasing temperature. Moreover, increasing the temperature of GST enhanced the influence of the Thomson effect on the temperature distribution. At high temperatures, the Thomson effect suppressed the temperature increase by approximately 10%–20%.
Shogo Hatayama, Yun-Heub Song, and Yuji Sutou
Elsevier BV