Yuta Saito

National Institute of Advanced Industrial Science and Technology · Device Technology Research Institute

Highly oriented SbTe films were successfully deposited by RF-magnetron sputtering on both crystalline and amorphous substrates. A novel deposition mechanism and method are proposed based on van der Waals epitaxy. Due to the selective reactivity of the top surface atoms of the substrate with sputtered atoms, a Te monolayer is the first layer formed...

The electronic structure of the as-deposited amorphous and crystalline phases of the transition-metal based Cu2GeTe3 phase-change material has been systematically investigated using hard-X-ray photoemission spectroscopy and density-functional theory simulations. We shed light on Cu d electrons and reveal that participation of d electrons in the bon...

Phase-change memories based on reversible amorphous-crystal transformations in pseudobinary GeTe-Sb2Te3 alloys are one of the most promising nonvolatile memory technologies. The recently proposed superlattice-based memory, or interfacial phase-change memory (iPCM), is characterized by significantly faster switching, lower energy consumption, and be...

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 2 Ge 2 Te 6 is known as a 2D vdW ferromagnetic insulator as well as a potential phase change material for non-volatile memory...

This paper reviews recent developments in the sputter growth of chalcogenide thin films for phase-change memory applications. We focus primarily on the growth of highly oriented Sb2Te3, which is a layered material. Sb2Te3 is an end point compound of the GeTe–Sb2Te3 pseudobinary tie-line and an important component of heterogeneously structured phase...

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 wit...

Two‐dimensional transition metal dichalcogenides (TMDCs) demonstrate great potential in nanoelectronics devices owing to their high carrier mobility in the atomically thin channel regime. However, high contact resistance between source/drain electrodes and TMDC channels hinders the TMDCs applications in the very‐large‐scale integration (VLSI) field...

A NaCl-type SmTe film showed remarkable electrical contrast (> 10 ⁵ ) between as-deposited and annealed films in the absence of a transition in the crystal structure. A rigid-band shift induced by a valency transition between Sm ²⁺ and Sm ³⁺ was found to give rise to significant change in the electronic structure along with a concomitant resistivit...

Trigonal selenium is a prototypical one-dimensional (1D) van der Waals (vdW) solid, where covalently bonded helical chains are held together by weaker vdW forces. In this work, we have studied structural transformation from a three-dimensional amorphous phase of non-interacting Se chains into a 1D vdW crystal using x-ray absorption spectroscopy. Th...

The resistive switching of an Ovonic threshold switch (OTS) material is characterized by Poole-Frenkel (PF) conduction and atomic rearrangement originating from chalcogen defects; thus, most OTS materials contain Se and/or Te. In addition to these chalcogen elements, As is included to form rigid amorphous networks. However, since As and Se are toxi...

We report on the coherent phonon spectroscopy of polymorphic MoTe2 single crystals using a femtosecond-pulsed laser to investigate the relationship between structural phase transitions and photo-thermal effects induced by high-density laser excitation. Even when a femtosecond pulsed laser was used, which generally induces fewer heat accumulation ef...

We report on the coherent phonon spectroscopy of polymorphic MoTe2 single crystals using a femtosecond-pulsed laser to investigate the relationship between structural phase transitions and photo-thermal effects induced by high-density laser excitation. Even when a femtosecond pulsed laser was used, which generally induces fewer heat accumulation ef...

The crystallization mechanism of sputter-deposited amorphous Mo–Te film is revealed enabling the large-area growth of 2D materials.

We explore non-equilibrium states in a two-dimensional transition-metal dichalcogenide under periodical phonon driving. A time-frequency analysis reveals strong coupling between electrons and a long-lived coherent phonon, leading to phonon-combination modes in the early time region.

Decreasing the thickness of semiconductors to the few monolayer limit often results in structural relaxation and the appearance of new properties. In this work, we demonstrate the bi-stability of 2ML thick CdTe, where the zinc blende structure of the bulk phase is metastable and the stable (ground) state is represented by an inverted structure with...

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have demonstrated a very strong application potential. In order to realize it, the synthesis of stoichiometric 2D TMDCs on a large scale is crucial. Here, we consider a typical TMDC representative, MoS2, and present an approach for the fabrication of well-ordered crystalline films via th...

Different from the prototypical elemental semiconductors such as Si and Ge, chalcogenide-based phase-change materials (PCMs) generally show very high resistivity contrast between the amorphous and crystalline phases. In contrast to conventional PCMs, such as Ge-Sb-Te alloys, where the amorphous phase possesses higher resistivity, Cr 2 Ge 2 Te 6 (Cr...

Sb2Te3 is well known as a phase-change material, and is expected to exhibit high spin–charge current conversion because of its strong spin–orbit interactions. Reversible phase-change characteristics enable manipulation of physical properties depending on phase. Therefore, it is anticipated that its spin properties can also be tuned by controlling t...

Sb2Te3 is an endpoint of the GeTe-Sb2Te3 quasi-binary tie-line that represents phase-change alloys widely used in optical and non-volatile phase-change memory devices. In the crystalline form it is also a prototypical topological insulator with a layered structure where covalently bonded quintuple layers are held together by weak van der Waals forc...

Phase-change memory based upon resistive switching due to phase transitions between the amorphous and crystalline phases of chalcogenide-based phase-change materials (PCMs) has been widely studied for the next generation non-volatile memory (NVM). Unlike traditional PCMs such as Ge2Sb2Te5, nitrogen doped Cr2Ge2Te6 (NCrGT) was reported to not exhibi...

Sb 2 Te 3 is an end-point of the GeTe-Sb 2 Te 3 quasibinary tie-line that represents phase-change alloys widely used in optical and non-volatile phase-change memory devices. In the crystalline form it is also a prototypical topological insulator with a layered structure where covalently bonded quintuple layers are held together by weak van der Waal...

We investigate ultrafast phonon dynamics in the Bi$_{1-x}$Sb$_{x}$ alloy system for various compositions $x$ using a reflective femtosecond pump-probe technique. The coherent optical phonons corresponding to the A$_{1g}$ local vibrational modes of Bi-Bi, Bi-Sb, and Sb-Sb are generated and observed in the time domain with a few picoseconds dephasing...

We investigate ultrafast phonon dynamics in the Bi1−x Sb x alloy system for various compositions x using a reflective femtosecond pump-probe technique. The coherent optical phonons corresponding to the A1g local vibrational modes of Bi–Bi, Bi–Sb, and Sb–Sb are generated and observed in the time domain with a few picoseconds dephasing time. The freq...

Sb2Te3 is an end point of quasibinary GeTe‐ Sb2Te3 phase‐change alloys and also a prototypical topological insulator. Topological insulators 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...

The results of a study on dielectric relaxation in thin layers of amorphous MoTe2 using dielectric spectroscopy are presented. Dipole-relaxation polarization has been observed. The activation energy of the relaxation process was calculated to be Ea = (0.44±0.02) eV. The dispersion of the dielectric constant and the presence of a maximum of dielectr...

Dielectric relaxation in thin layers of amorphous and crystalline GeSb2Te4 was studied. A relaxation process associated with the manifestation of dipole-relaxation polarization is found. The appearance of dipoles is thought to be caused by the off-center location of Ge and Sb in cubic fragments of GeSb2Te4. The activation energies for relaxation pr...

Due to their electronic configuration, chalcogenides (materials that contain sulphur, selenium or tellurium) can form different bonding geometries with the creation of metastable phases. Thus, the presence of lone‐pair electrons in chalcogenide glasses results in their ability to undergo reversible photostructural changes, while resonant (metavalen...

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...

The dephasing of coherent longitudinal optical (LO) phonons in ion-irradiated GaP has been investigated with a femtosecond pump-probe technique based on electro-optic sampling. The dephasing time of the coherent LO phonon is found to be dramatically prolonged by the introduction of a small amount of defects by means of Ga-ion irradiation. The maxim...

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/Bi4Te3 (GT/BT) SL exhibits similar structural features to conventional GeTe...

Phase‐change memory (PCM), a nonvolatile electrical memory based upon the local structure of chalcogenide compounds such as (Ge2Sb2Te5), 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 en...

The dephasing of coherent longitudinal optical (LO) phonons in ion-irradiated GaP has been investigated with a femtosecond pump-probe technique based on electro-optic sampling. The dephasing time of the coherent LO phonon is found to be dramatically prolonged by the introduction of a small amount of defects by means of Ga-ion irradiation. The maxim...

The results of a study on dielectric relaxation and charge transfer in thin layers of amorphous MoS2 using dielectric spectroscopy are presented. Both dipole‐relaxation polarization and hopping charge transfer have been observed. The activation energies of the relaxation process Ea and conductivity Eσ have been calculated and found to be approximat...

This paper reviews metastability in three different classes of chalcogenides – chalcogenide glasses, phase‐change alloys, and transition‐metal dichalcogenides – with the focus on the relationship between structural metastability and the nature of chemical bonding. While the presence of lone‐pair electrons in chalcogenide glasses enables a variety o...

Interfacial phase change memory devices based on chalcogenide superlattices show a remarkable performance improvement over traditional phase change memory devices. Here, we report on the effects of the resistive switching of Ge–Te/Sb–Te superlattices in the presence of an external magnetic field at elevated temperature. In addition to the unique th...

We report on the dynamics of coherent phonons in semimetal 1T′-MoTe2 using femtosecond pump-probe spectroscopy. On an ultrafast sub-picosecond timescale at room temperature, a low frequency and long-lifetime shear phonon mode was observed at 0.39 THz, which was previously reported in the form of a characteristic phonon only in the low temperature T...

We report on the dynamics of coherent phonons in semimetal 1T'-MoTe2 using femtosecond pump-probe spectroscopy. On an ultrafast sub-picosecond time scale at room temperature, a low frequency and long-lifetime shear phonon mode was observed at 0.39 THz, which was previously reported in the form of a characteristic phonon only in the low temperature...

2D van der Waals chalcogenides such as topological insulators and transition-metal dichalcogenides and their heterostructures are now at the forefront of semiconductor research. In this paper, we discuss the fundamental features and advantages of van der Waals bonded superlattices over conventional superlattices made of 3D materials and describe in...

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 te...

Chalcogenide superlattices (SL) consist of alternate stacking of GeTe and Sb$_{2}$Te$_{3}$ layers. The structure can become a 3D topological insulator depending on the constituent layer thicknesses, making the design of the SL period a central issue for advancing chalcogenide SL as potential candidates for spin devices as well as for optimization o...

We report on pump-probe based helicity dependent time-resolved Kerr measurements of chalcogenide superlattices, consisting of alternately stacked GeTe and Sb$_{2}$Te$_{3}$ layers under infrared excitation. The Kerr rotation signal consists of the specular Inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE), both of which are f...

Ge–Sb–Te (GST) phase change materials exhibit a metal-to-insulator transition and therefore are expected to be useful for a variety of photonics applications in addition to their primary application in optical and electrical memory devices. Here, we demonstrate that the phase change causes drastic changes in the optical and dielectric properties of...

We report on pump-probe based helicity dependent time-resolved Kerr measurements under infrared excitation of chalcogenide superlattices, consisting of alternately stacked GeTe and Sb₂Te₃ layers. The Kerr rotation signal consists of the specular Inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE), both of...

Ge–Sb–Te alloy based phase‐change memory devices have recently been shown to be switchable in a bipolar mode. However, to date bipolar switching has only been demonstrated at relatively low speeds (on the order of tens of µs), and with endurance limited to 10⁴ switching cycles. In this work, in lieu of a Ge–Sb–Te alloy, fast and durable bipolar swi...

Multilayered structures of GeTe and Sb2Te3 phase change material, also referred to as interfacial phase change memory (iPCM), provide superior performance for nonvolatile electrical memory technology in which the atomically controlled structure plays an important role in memory operation. Here, we report on terahertz (THz) wave generation measureme...

We report upon observation of a transient Fano resonance, a quantum interference between coherent phonon and Dirac plasmons, that persisted for up to 1 picosecond in the topological insulator Sb2Te3 investigated using coherent phonon spectroscopy.

We have systematically investigated the spatial and temporal dynamics of crystallization that occurs in the phase-change material Ge2Sb2Te5 upon irradiation with an intense THz pulse. THz pump and optical probe spectroscopy revealed that Zener tunneling induces a nonlinear increase in the conductivity of the crystalline phase. This result indicates...

Chalcogenide superlattices (SLs) consist of the alternate stacking of GeTe and Sb2Te3 layers. The structure can act as a 3D topological insulator depending on the constituent layer thicknesses, making the design of the SL period a central issue for advancing chalcogenide SLs as potential candidates for spin devices as well as for the optimization o...

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...

Ge-Sb-Te ternary alloys are key materials for phase-change random access memory (PCRAM). In PCRAM, data recording relies on reversible switching between amorphous and crystalline phases by electrical pulse induced Joule heating. We have proposed and developed GeTe/Sb 2 Te 3 superlattice phase change memory, which is known as interfacial phase chang...

Chalcogenide superlattices (SL) consist of alternately stacked GeTe and Sb2Te3 layers. The structure can act as a 3D topological insulator depending on the constituent layer thicknesses, making the design of the SL period a central issue for advancing chalcogenide SL as potential candidates for spin devices as well as for optimization of the curren...

GeTe/Sb 2 Te 3 superlattices, also known as interfacial phase-change memory (iPCM), exhibit significantly faster switching and are characterized by much lower power consumption and longer data retention compared to devices based on alloyed materials. In early work, the superior performance of iPCM was linked to a crystal-crystal transition between...

Cu2GeTe3 (CGT) phase-change material, a promising candidate for advanced fast nonvolatile random-access-memory devices, has a chalcopyritelike structure with sp3 bonding in the crystalline phase; thus, the phase-change (PC) mechanism is considered to be essentially different from that of the standard PC materials (e.g., Ge-Sb-Te) with threefold to...

We explore the ultrafast reflectivity response from photo-generated coupled phonon-surface Dirac plasmons in Sb2Te3 topological insulators several quintuple layers thick. The transient coherent phonon spectra obtained at different time frames exhibit a Fano-like asymmetric line shape of the A1g2 mode, which is attributed to quantum interference bet...

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 high-resistance (RESET) state in an iPCM device drops sharply at aroun...

The ternary transition-metal chalcogenide HfGeTe4 is found to have a direct band gap of around 1.3 eV in its monolayer form, while it has an indirect band gap of about a half of that value in the bulk form. In contrast to the two-dimensional flat layers seen in most van der Waals solids, HfGeTe4 has zigzag shaped layers. The zigzag shape can allow...

Topological insulators (TIs) are characterized by possessing metallic (gapless) surface states and a finite band-gap state in the bulk. As the thickness of a TI layer decreases down to a few nanometer, hybridization between the top and bottom surfaces takes place due to quantum tunneling, consequently at a critical thickness a crossover from a 3D-...

Temperature-dependent variations in electric switching and transverse resistance of phase-change [(GeTe)2(Sb2Te3)]n (n=4 and 8) chalcogenide superlattice (CSL) films were studied using conductive scanning probe microscopy (SPM). Three temperature regions with different electric transport properties were recognized in point current-voltage (I-V) spe...

With rapid advances occurring in terahertz (THz) radiation generation techniques, the interaction between matter and intense THz fields has become an important research topic. Among different types of THz radiation sources, the free electron laser (FEL) is a promising experimental tool that is expected to pave the way for new forms of material proc...