Residual oxygen-driven p–n conversion and thermoelectric properties in CrN films Yi Shuang, Yuta Saito, Shogo Hatayama, Mihyeon Kim, Paul Fons, Yuji Sutou Journal of Materials Chemistry A, 2026 Oxygen-controlled defect engineering enables p–n conversion in CrN thin films for homojunction and flexible thermoelectric applications.
First Demonstration of (111) GeOI nMOSFETs utilizing Novel Ge-Bi-Te/Ge vdW S/D Contacts Wen Hsin Chang, Shogo Hatayama, Fang Jui Chu, Naoya Okada, Toshifumi Irisawa, Yao Jen Lee, Yeong Her Wang, Tatsuro Maeda, Yuta Saito 10th IEEE Electron Devices Technology and Manufacturing Conference Emerging Semiconductor Devices and Manufacturing Technologies Edtm 2026, 2026 The feasibility of Ge-Bi-Te(GBT)/Ge(111) van der Waals (vdW) interface as source/drain (S/D) contacts has been systematically investigated. Through appropriate thermal treatment, Bi2Te3 crystallizes on Ge(111), resulting in the formation of a vdW interface. This process effectively mitigates Fermi-level pinning in n-type Ge. A low specific contact resistivity of 6.54× 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> Ω-cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was achieved through refined transmission line method (RTLM) characterization, and a reduction in parasitic resistance was also attained by GeOI(111) nMOSFET utilizing GBT/Ge S/D contacts, compared to those with conventional W/Ge S/D counterparts. These findings indicate that GBT is a viable option for contact applications in Ge-based devices, particularly for nMOSFETs.
Structural and Electric Characterization of Sputtered Pt/WSe2Contacts toward High-Performance 2D p-FETs Ryuichi Nakajima, Tomonori Nishimura, Kaito Kanahashi, Shogo Hatayama, Wen Hsin Chang, Yuta Saito, Toshifumi Irisawa, Keiji Ueno, Yasumitsu Miyata, Takashi Taniguchi, Kenji Watanabe, Kosuke Nagashio ACS Omega, 2025 For high-performance p-type field-effect transistors (FETs) based on two-dimensional (2D) materials, the use of Pt as the contact metal, with its high work function, is advantageous for effective hole injection into the 2D channel. However, the high-energy sputtering process required to deposit Pt, due to its high melting point, often induces significant damage to the 2D materials. Recently, the achievement of nearly ideal van der Waals contacts in Sb2Te3/MoS2 via sputtering has motivated us to investigate WSe2 p-FETs with sputtered Pt electrodes. Notably, reasonable p-FET performance was observed even in monolayer WSe2. However, various characterizations revealed that the crystal structure of WSe2 was no longer preserved, suggesting the formation of a quasi-edge contact between Pt-sputtered WSe2 and the WSe2 channel. Moreover, from the perspective of sputtering applicability, the relationship between deposition methods, deposited materials, and the resulting extent of damage was systematically examined.
Low-loss Sb2S3 optical phase shifter enabled by optimizing sputtering conditions Yuto Miyatake, Tomohiro Anda, Yosuke Wakita, Kotaro Makino, Shogo Hatayama, Makoto Okano, Kasidit Toprasertpong, Shinichi Takagi, Mitsuru Takenaka Optical Materials Express, 2025 By quantitatively evaluating the atomic concentrations of sputtered Sb2S3 films with different sputtering powers and Ar flows, we reveal that a sputtered Sb2S3 film becomes close to the stoichiometric composition as the sputtering power and Ar flow decrease. We characterize the optical properties of Sb2S3 and show that the lower sputtering power leads to a better figure of merit of Sb2S3 as an optical phase shifter in the near infrared (NIR) range. Based on these results, we achieve a loss per phase shift of 0.33 dB/π at a wavelength of 1.55 μm, one of the lowest losses among Sb2S3 phase shifters in the NIR range.
Realization of ideal Ohmic contact to n-Ge: The key roles of Ge-Bi-Te for quasi-van der Waals interface formation Wen Hsin Chang, Shogo Hatayama, Naoya Okada, Toshifumi Irisawa, Yuta Saito APL Materials, 2025 The feasibility of Bi2Te3 as S/D contact for n-Ge has been investigated. After thermal treatment at 400 °C, the formation of Ge1−xBi2+xTe4 (GBT) ternary compound and the realization of the quasi-van der Waals interface are verified from x-ray diffraction analysis and scanning transmission electron microscope observation at the GBT/Ge interface. According to the junction diode characteristics of GBT/Ge, exhibiting Ohmic and rectified behaviors on n-Ge and p-Ge, respectively, it is found that strong Fermi-level pinning commonly observed between metal and n-Ge is mitigated and the Fermi level of Ge moves toward the conduction band side. In addition, the interfacial layer between GBT and Ge even disappeared after 400 °C annealing, leading to ideal Ohmic behavior compared to its Bi2Te3 counterpart. GBT/Ge hetero-structure possesses high thermal stability, which shows tolerance for the back-end-of-the-line process. Through the comprehensive evaluation, GBT shows exceptional potential for achieving low contact resistance in Ge nMOSFETs.
Composition-Dependent Functionality of Threshold Switching Behavior in Amorphous Ge-Te Binary System Shogo Hatayama, Kentaro Saito, Jun Usami ACS Applied Electronic Materials, 2025 Amorphous chalcogenides, including Ge–Te binary system, exhibit distinct threshold switching behaviors essential for memory and selector applications. This study explores the threshold switching mechanisms in Te-rich Ge–Te compositions, systematically identifying key factors differentiating ovonic threshold switching (OTS) from phase change memory (PCM) functionality. Experimental results reveal that Te-rich compositions exhibit OTS behavior, characterized by reversible resistive switching at threshold voltages ranging from 1.2 to 1.6 V. In contrast, compositions near stoichiometric (GeTe) display PCM behavior, undergoing irreversible crystallization at a higher threshold voltage (∼5.0 V). Optical and electrical measurements correlate these behaviors with variations in band gap and trap depth. Our findings indicate that the difference in functionality is reflected in the relative position of the Fermi level with respect to the conduction band minimum: OTS materials exhibit deeper trap states, while PCM compositions have shallower trap states. This insight into the electronic structure provides a foundation for optimizing material properties to enhance the performance of next-generation memory and selector devices.
Unveiling the Significant Role of Schottky Interfaces for Threshold Voltage in Ovonic Threshold Switching Shogo Hatayama, Keisuke Hamano, Yi Shuang, Mihyeon Kim, Paul Fons, Yuta Saito ACS Applied Electronic Materials, 2025 High Resolution Image Download MS PowerPoint Slide Ovonic threshold switching (OTS) is a type of volatile resistive switching primarily observed in amorphous chalcogenides. The switching process involves an abrupt transition from a high-resistance state to a low-resistance state when a voltage above a specific threshold ( V th ) is applied. OTS materials serve as selectors in nonvolatile memories with 3D XPoint-type structures, in combination with phase-change materials (PCMs), which exhibit threshold-type nonvolatile resistive switching. Despite the existence of transport models that can explain the OTS behavior, the role of the metal–OTS interface has been underexplored. This study employs angle-resolved hard X-ray photoelectron spectroscopy to investigate the interfacial electronic structure of Ge–Te-based OTS materials with different metal electrodes. The results indicate that V th varies with the work function of the contact metal because the onset voltage for impact ionization is affected by band bending at the interface. Our findings reveal that interfacial properties significantly influence OTS behavior, offering a novel method for controlling V th . This study underscores the importance of selecting appropriate metal contacts for optimizing the performance of OTS devices.
Amorphous-to-crystalline transition-induced two-step thin film growth of quasi-one-dimensional penta-telluride ZrTe5 Yi Shuang, Yuta Saito, Shogo Hatayama, Paul Fons, Ando Daisuke, Yuji Sutou Journal of Materials Science and Technology, 2025 Quasi-one-dimensional (quasi-1D) van der Waals (vdWs) materials, such as ZrTe5, exhibit unique electrical properties and quantum phenomena, making them attractive for advanced electronic applications. However, large-scale growth of ZrTe5 thin films presents challenges. We address this by employing sputtering, a common semiconductor industry technique. The as-deposited ZrTe5 film is amorphous, and post-annealing induces a crystallization process akin to transition-metal dichalcogenides. Our study investigates the electrical and optical properties during this amorphous-to-crystalline transition, revealing insights into the underlying mechanism. This work contributes to the fundamental understanding of quasi-1D materials and introduces a scalable fabrication method for ZrTe5 which offers the possibility of fabricating unique future electronic and optical devices.
Tunable pheromone interactions among microswimmers Bokusui Nakayama, Hikaru Nagase, Hiromori Takahashi, Yuta Saito, Shogo Hatayama, Kotaro Makino, Eiji Yamamoto, Toshiharu Saiki Proceedings of the National Academy of Sciences of the United States of America, 2023
