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National Institute of Advanced Industrial Science and Technology (AIST)
Takuya Yamamoto, Shogo Hatayama, and Yuji Sutou
Materials and Design, ISSN: 02641275, eISSN: 18734197, Volume: 216, Published: April 2022 Elsevier BV
Leonid Bolotov, Shinji Migita, Ryouta Fujio, Manabu Ishimaru, Shogo Hatayama, and Noriyuki Uchida
Microelectronic Engineering, ISSN: 01679317, Volume: 258, Published: 1 April 2022 Elsevier BV
Mihyeon Kim, Shunsuke Mori, Yi Shuang, Shogo Hatayama, Daisuke Ando, and Yuji Sutou
Physica Status Solidi - Rapid Research Letters, ISSN: 18626254, eISSN: 18626270, Published: 2022 Wiley
Shogo Hatayama, Yuta Saito, and Noriyuki Uchida
APL Materials, eISSN: 2166532X, Published: 1 January 2022 AIP Publishing
Yuta Saito, Shogo Hatayama, Yi Shuang, Paul Fons, Alexander V. Kolobov, and Yuji Sutou
Scientific Reports, eISSN: 20452322, Published: December 2021 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
Materials Research Express, eISSN: 20531591, Published: November 2021 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
Materials Science in Semiconductor Processing, ISSN: 13698001, Volume: 133, Published: October 2021 Elsevier BV
Milos Krbal, Vit Prokop, Alexey A. Kononov, Jhonatan Rodriguez Pereira, Jan Mistrik, Alexander V. Kolobov, Paul J. Fons, Yuta Saito, Shogo Hatayama, Yi Shuang, Yuji Sutou, Stepan A. Rozhkov, Jens R. Stellhorn, Shinjiro Hayakawa, Igor Pis, and Federica Bondino
ACS Applied Nano Materials, eISSN: 25740970, Pages: 8834-8844, Published: 24 September 2021 American Chemical Society (ACS)
Shogo Hatayama, Keisuke Kobayashi, Yuta Saito, Paul Fons, Yi Shuang, Shunsuke Mori, Alexander V. Kolobov, and Yuji Sutou
Physical Review Materials, eISSN: 24759953, Published: August 2021 American Physical Society (APS)
Yi Shuang, Shogo Hatayama, Yuta Saito, Paul Fons, Alexander V. Kolobov, Daisuke Ando, and Yuji Sutou
Applied Surface Science, ISSN: 01694332, Volume: 556, Published: 1 August 2021 Elsevier BV
Shunsuke Mori, Shogo Hatayama, Daisuke Ando, and Yuji Sutou
Japanese Journal of Applied Physics, ISSN: 00214922, eISSN: 13474065, Published: April 2021 IOP Publishing
Yi Shuang, Shogo Hatayama, Daisuke Ando, and Yuji Sutou
Physica Status Solidi - Rapid Research Letters, ISSN: 18626254, eISSN: 18626270, Published: March 2021 Wiley
Shogo Hatayama, Yasunori Abe, Daisuke Ando, and Yuji Sutou
Physica Status Solidi - Rapid Research Letters, ISSN: 18626254, eISSN: 18626270, Published: March 2021 Wiley
Milos Krbal, Vit Prokop, Jan Prikryl, Jhonatan Rodriguez Pereira, Igor Pis, Alexander V. Kolobov, Paul J. Fons, Yuta Saito, Shogo Hatayama, and Yuji Sutou
Crystal Growth and Design, ISSN: 15287483, eISSN: 15287505, Published: 2021 American Chemical Society (ACS)
Shogo Hatayama, Takashi Yagi, and Yuji Sutou
Results in Materials, eISSN: 2590048X, Published: December 2020 Elsevier BV
Shunsuke Mori, Shogo Hatayama, Yi Shuang, Daisuke Ando, and Yuji Sutou
Nature Communications, eISSN: 20411723, Published: 1 December 2020 Springer Science and Business Media LLC
AbstractDisplacive transformation is a diffusionless transition through shearing and shuffling of atoms. Diffusionless displacive transition with modifications in physical properties can help manufacture fast semiconducting devices for applications such as data storage and switching. MnTe is known as a polymorphic compound. Here we show that a MnTe semiconductor film exhibits a reversible displacive transformation based on an atomic-plane shuffling mechanism, which results in large electrical and optical contrasts. We found that MnTe polycrystalline films show reversible resistive switching via fast Joule heating and enable nonvolatile memory with lower energy and faster operation compared with conventional phase-change materials showing diffusional amorphous-to-crystalline transition. We also found that the optical reflectance of MnTe films can be reversibly changed by laser heating. The present findings offer new insights into developing low power consumption and fast-operation electronic and photonic phase-change devices.
Satoshi Shindo, Yi Shuang, Shogo Hatayama, Yuta Saito, Paul Fons, Alexander V. Kolobov, Keisuke Kobayashi, and Yuji Sutou
Journal of Applied Physics, ISSN: 00218979, eISSN: 10897550, Volume: 128, Published: 28 October 2020 AIP Publishing
Cu2GeTe3 (CGT) is a promising phase change material for phase change random access memory (PCRAM) applications because of its high thermal stability in the amorphous phase and its capability to undergo rapid phase change. In this paper, the electrical conduction mechanism of a CGT memory device fabricated using W electrodes (W/CGT) was investigated using current–voltage (I–V) measurements and angle resolved hard x-ray photoelectron spectroscopy (AR-HAXPES). The I–V characteristics of the W/CGT memory device were found to display non-linear behavior in the RESET (amorphous) state, while linear behavior was observed in the SET (crystalline) state, indicating that the W/CGT memory device exhibited Schottky conduction in the RESET state, but Ohmic conduction in the SET state. The effective Schottky barrier height was found to increase linearly as the ideality factor decreased to unity with the ideal W/CGT Schottky barrier height in the RESET state estimated to be 0.33 eV, a value in good agreement with the directly measured Schottky barrier height of 0.35 eV between W and amorphous CGT by AR-HAXPES measurements. These results suggest that the interface between the metal electrode and the phase change material plays an important role in PCRAM devices, and its comprehensive understanding is necessary for future application development.
Yuta Saito, Paul Fons, Alexander V Kolobov, Kirill V Mitrofanov, Kotaro Makino, Junji Tominaga, Shogo Hatayama, Yuji Sutou, Muneaki Hase, and John Robertson
Journal of Physics D: Applied Physics, ISSN: 00223727, eISSN: 13616463, Published: 8 July 2020 IOP Publishing
Yi Shuang, Shogo Hatayama, Hiroshi Tanimura, Daisuke Ando, Tetsu Ichitsubo, and Yuji Sutou
Materials Advances, eISSN: 26335409, Pages: 2426-2432, Published: 2020 Royal Society of Chemistry (RSC)
This work investigated the phase change mechanism of a nitrogen-doped Cr2Ge2Te6 phase change material via Raman scattering.
Yi Shuang, Shogo Hatayama, Junseop An, Jinpyo Hong, Daisuke Ando, Yunheub Song, and Yuji Sutou
Scientific Reports, eISSN: 20452322, Published: 1 December 2019 Springer Science and Business Media LLC
AbstractThree-dimensional crossbar technology has been of great significance for realizing high density and multiple terabytes of data storage in memory devices. However, to further scale down the size of memory devices, a selector exhibiting nonlinear electrical properties should be in series with a memory layer in case of unwanted sneak current disturbance. Conventional selectors usually utilize a complicated multilayer structure to realize the high nonlinearity of current, which might be incompatible with certain manufacturing processes or limit the scalability of memory. Herein, we propose a simple heterojunction diode using an n-type oxide semiconductor, specifically, InGaZnO4 (IGZO), and a p-type phase change material (PCM), specifically, N-doped Cr2Ge2Te6 (NCrGT), to realize self-selective performance. The electrode/IGZO/NCrGT/plug-electrode structure with an IGZO/NCrGT pn diode and NCrGT/plug-electrode Schottky diode can realize bidirectional, self-selective phase change random access memory (PCRAM) for either amorphous or crystalline NCrGT. The approximate equilibrium energy band diagrams for the IGZO/NCrGT pn junction and the IGZO/NCrGT/W hybrid junction were proposed to explain the possible conduction mechanism. We demonstrated that hybrid diode-type PCM memory exhibits both selectivity and resistive switching characteristics. The present findings offer new insight into selector technology for PCM.
Shogo Hatayama, Yi Shuang, Paul Fons, Yuta Saito, Alexander V. Kolobov, Keisuke Kobayashi, Satoshi Shindo, Daisuke Ando, and Yuji Sutou
ACS Applied Materials and Interfaces, ISSN: 19448244, eISSN: 19448252, Pages: 43320-43329, Published: 20 November 2019 American Chemical Society (ACS)
Cr2Ge2Te6 (CrGT) is a phase change material with higher resistivity in the crystalline phase than in the amorphous phase. CrGT exhibits an ultralow operation energy for amorphization. In this study, the origin of the increased resistance in crystalline CrGT compared to amorphous CrGT and the underlying phase change mechanism were investigated in terms of both local structural change and the associated change in electronic state. The density of states at the Fermi level in crystalline CrGT decreased with increasing annealing temperature and became negligible upon annealing at 380°C. Simultaneously, the Fermi level shifted from the vicinity of the valence band to the bandgap center, leading to an increase in resistance. The phase change from amorphous to crystalline CrGT occurred through a metastable crystalline phase with a local structure similar to that of the amorphous phase. Cr nanoclusters were confirmed to exist in both the amorphous and crystalline phases. The presence of Cr nanoclusters induced Cr vacancies in the crystalline phase. These Cr vacancies generated hole carriers, leading to p-type conduction. Photoelectron spectroscopy of the Cr 2s core level clearly indicated a decrease in the fraction of Cr-Cr bonds and an increase in the fraction of Cr-Te bonds in crystalline CrGT upon annealing. Meanwhile, the coordination number of the Cr nanoclusters decreased as the number of Cr-Cr bonds was reduced. Together, these results imply that the origin of the increased resistance in crystalline CrGT is the filling of Cr vacancies by Cr atoms diffusing from Cr nanoclusters.
Shogo Hatayama, Daisuke Ando, and Yuji Sutou
Journal of Physics D: Applied Physics, ISSN: 00223727, eISSN: 13616463, Published: 13 June 2019 IOP Publishing
In this study, the relation between density contrast and optical contrast for Cr2Ge2Te6 (CrGT) was studied. The CrGT showed negative density contrast between its as-deposited amorphous and crystalline phases. We confirmed that the CrGT satisfies the linear relation between the density contrast and optical contrast at wavelength of 630 nm, consistent with the behaviors of reported phase-change materials (PCMs). In CrGT films annealed at temperatures below 350 °C, the films exhibited a negative density contrast and negative optical contrast at 630 nm, consistent with the Clausius–Mossotti relationship. However, the 380 °C-annealed film showed a positive optical contrast at 630 nm due to a drastic increase in reflectance even with the negative density contrast. The absorption coefficient measurements as a function of wavelength indicated that there is no electron delocalization in the crystalline CrGT phase. It was found that the crystalline CrGT exhibited a drastic increase in the refractive index upon annealing at 380 °C at a wavelength range of visible light, causing an increase in the reflectance. Similar behavior was also observed in ultraviolet (330 nm) and infrared light (1000 nm) ranges. Such a large difference in refractive index in a wide wavelength range covering from ultraviolet to infrared light ranges introduces the possibility to realize the CrGT-based metasurface.
Yuta Saito, Shogo Hatayama, Yi Shuang, Satoshi Shindo, Paul Fons, Alexander V. Kolobov, Keisuke Kobayashi, and Yuji Sutou
Applied Physics Express, ISSN: 18820778, eISSN: 18820786, Published: 1 May 2019 IOP Publishing
Shogo Hatayama, Yuji Sutou, Daisuke Ando, Junichi Koike, and Keisuke Kobayashi
Journal of Physics D: Applied Physics, ISSN: 00223727, eISSN: 13616463, Published: 9 January 2019 IOP Publishing
A Cr2Ge2Te6 (CrGT) phase change material (PCM) was studied. Different from conventional PCMs, it shows an inverse resistance change between a low-resistance amorphous phase and a high-resistance crystalline phase. Moreover, the anomalous low resistivity in the amorphous CrGT is considered to be due to a large carrier density, but the mechanism of electrical transport is still not clear. In this study, the electrical transport mechanism of the amorphous CrGT was discussed based on the temperature dependence of the resistivity, carrier density, mobility, and current–voltage characteristics. Above 300 K, the conduction mechanism of the amorphous CrGT was thermally activated band conduction, which is different from the conventional Ge–Sb–Te PCMs that show Poole–Frenkel conduction in the amorphous phase. Below 300 K, the amorphous CrGT shows hopping conduction, changing from variable range hopping (Mott VRH) to Efros–Shklovskii variable range hopping (ES-VRH) with decreasing temperature. The crossover from Mott VRH to ES-VRH was observed at around 200 K. Furthermore, the Fermi level was not pinned at the center of bandgap; instead, it was located near the valence band.
S. Hatayama, Y. Sutou, D. Ando, and J. Koike
MRS Communications, ISSN: 21596859, eISSN: 21596867, Pages: 1167-1172, Published: 1 September 2018 Springer Science and Business Media LLC
The crystallization mechanism and kinetics of Cr 2 Ge 2 Te 6 (CrGT) films were investigated by differential scanning calorimetry. The average Avrami exponent ( n a ) analysis indicated that CrGT exhibits a growth-dominant crystallization in the range of heating rate ( β ) of 10–50°C/min. In comparison, Ge 2 Sb 2 Te 5 (GST) showed a nucleation-dominant crystallization. The n a of CrGT was about 3, and was majorly independent of β . The n a of GST decreased with an increasing β , which asymptotically approached a value of around 3. The kinetic constant of CrGT was evaluated to be almost the same with that of GST, indicating that CrGT undergoes fast crystallization.