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Phase-change materials such as Ge-Sb-Te (GST) exhibiting amorphous and crystalline phases can be used for phase-change random access memories (PCRAM). GST-based PCRAM has been applied as a storage-class memory; however, its relatively low ON/OFF ratio and large Joule heat energy for the RESET process (amorphization) significantly limit the storage density. This study proposes a novel phase-change nitride, CrN, with a much wider programming window (ON/OFF ratio more than 105) and lower RESET energy (a 90 % reduction from GST). High-resolution transmission electron microscopy revealed a unique phase-change from low resistive cubic CrN into high resistive hexagonal CrN2 induced by the Soret-effect. The proposed phase-change nitride could greatly expand the scope of conventional phase-change chalcogenides and offer a new strategy for next-generation PCRAM, enabling a large ON/OFF ratio, low energy, and fast operation.

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.

A NaCl-type SmTe film showed remarkable electrical contrast (> 105) 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 Sm2+ and Sm3+ was found to give rise to significant change in the electronic structure along with a concomitant resistivity change. Moreover, operation of a SmTe-based memory device was demonstrated by a standard measurement scheme with switching driven by electrical pulse-induced Joule heating. Since SmTe remains crystalline in both resistive states, resistance drift is suppressed offering improved readout reliability. Furthermore, the SmTe films were found to exhibit much better data retention, maintaining data for 10 years at 262 °C in contrast to conventional nonvolatile recording material such as Ge-Sb-Te (85-110 °C). The use of a valency transition reveals a new approach to the development of nonvolatile high performance resistive change materials.

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. 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.

Displacive 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., Designing low power consumption and fast operation electronic phase-change devices remains a challenge. Here, the authors demonstrate the reversible displacive transformation in polycrystalline MnTe films to enable resistive switching via fast Joule heating for fast nonvolatile memory applications.

Fast and reversible switching utilizing the resistance contrast upon phase transition of phase change materials (PCMs) has been widely studied for next generation nonvolatile memory (NVM). Cr2Ge2Te6 (CrGT) and N-doped CrGT (NCrGT) PCMs have been demonstrated to show enhanced memory performance compared to the traditional Ge2Sb2Te5 (GST) PCM. We investigated here the crystallization behavior of Cr2Ge2Te6 (CrGT) and the effect of nitrogen (N) doping on it. We revealed that the Ge- or Cr-centered defective octahedral structure dominated in the amorphous phase of CrGT and the evolution of the c-axis-aligned Ge–Ge dimer and Cr-centered octahedral structure marked the beginning of crystallization. The further formation of new Cr–Te bonds resulted in a dramatic decline of carrier density in the crystalline phase, which explained the inverse resistance change between the amorphous and crystalline phases of CrGT. The N atom can form stronger bonds with Ge or Cr than Cr–Te, restraining the shift of Cr to the center of the octahedron to form the new Cr–Te bonds, resulting in a constant carrier density change during crystallization. The thermal stability and data retention properties also show an improvement by N doping.

Corresponding to the principles of biological synapses, an essential prerequisite for hardware neural networks using electronics devices is the continuous regulation of conductance. We implemented artificial synaptic characteristics in a (GeTe/Sb2Te3)16 iPCM with a superlattice structure under optimized identical pulse trains. By atomically controlling the Ge switch in the phase transition that appears in the GeTe/Sb2Te3 superlattice structure, multiple conductance states were implemented by applying the appropriate electrical pulses. Furthermore, we found that the bidirectional switching behavior of a (GeTe/Sb2Te3)16 iPCM can achieve a desired resistance level by using the pulse width. Therefore, we fabricated a Ge2Sb2Te5 PCM and designed a pulse scheme, which was based on the phase transition mechanism, to compare to the (GeTe/Sb2Te3)16 iPCM. We also designed an identical pulse scheme that implements both linear and symmetrical LTP and LTD, based on the iPCM mechanism. As a result, the (GeTe/Sb2Te3)16 iPCM showed relatively excellent synaptic characteristics by implementing a gradual conductance modulation, a nonlinearity value of 0.32, and 40 LTP/LTD conductance states by using identical pulse trains. Our results demonstrate the general applicability of the artificial synaptic device for potential use in neuro-inspired computing and next-generation, non-volatile memory.

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, ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CrGT) exhibits the opposite dependence. Namely, the amorphous phase is characterized by a lower resistivity than the crystalline phase. Although density functional theory calculations suggest that Cr clusters are responsible for the low resistivity of amorphous CrGT, the effects of composition on the electrical properties have yet to be investigated. In this work, the dependence of the electrical properties on Cr content and the role of the Cr clusters were investigated experimentally using Hall effect, hard x-ray photoelectron spectroscopy (HAXPES), and optical property measurements. The electrical properties were found to be dependent on the Cr content. From a HAXPES core-level spectra analysis, it was found that the increased carrier density correlated with the extent of Cr clusters, indicating that the hole carriers present likely originated from Cr clusters. The increased concentration of Cr clusters was also found to lead to a shift of the valence band edge toward the Fermi level as well as to a decrease in the optical band gap. It has been suggested that the Cr clusters may induce the formation of new energy states close to the valence band edge. These results indicate that the Cr clusters play an essential role in determining the electrical properties of amorphous CrGT, and that tuning the film composition is an effective way to optimize device properties for nonvolatile memory applications.

Three-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.

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 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 band gap 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.

To realize a superelastic Mg-Sc lightweight alloy at room temperature, we investigated the stress–strain behavior of Mg-18.7 at.% Sc alloy sheets at room temperature with martensitic transformation starting temperature of approximately −90 °C. We found that the superelasticity of the alloy strongly depends on the grain size and a distinct room temperature superelasticity (~3% superelastic strain) was obtained in the alloy with a large grain size. The obtained maximum superelastic strain was smaller than that expected from the orientation dependence of the transformation strain. This result suggests that the stress-induced martensite phase was stabilized because of the introduction of slip defects.

In a Mg-Sc system, a bcc (β) phase exists in a high-temperature region, and the quenched β shows drastic age-hardening and shape-memory properties. In addition to the disordered β phase, the ordered β (B2) phase is reported to exist in a low-temperature region. We report that the quenched β phase can be ordered through aging treatment at 573 K. This finding indicates that the mechanical characteristics and shape-memory functionality of β-type Mg-Sc alloys can be controlled by ordering the β phase.

A Cr2Ge2Te6 (CrGT) film is known to be a phase change material showing low-energy phase change between amorphous and crystalline phases. A crystalline Cr2Ge2Te6 (CrGT) film obtained from an amorphous phase via metastable crystalline phase has been reported to possess different local structure from the CrGT bulk material. The bulk CrGT is known to show a band conduction, while the conduction mechanism of the CrGT film has not been clear yet. In this study, the conduction mechanism of the CrGT film was discussed based on the temperature dependence of the conductivity (σ), Seebeck coefficient (S), and Hall properties. It was found that the CrGT film shows a variable range hopping conduction below 170 ​K, while it shows a mixed-conduction of band-like conduction and nearest-neighbor hopping conduction above 300 ​K. Such the mixed-conduction mechanism was found to cause the positive temperature dependence of the σ and S in the CrGT film. Furthermore, the thermal conductivity of the CrGT film was measured to be 0.16 ​W/mK, which is much lower than that of the bulk material. The mixed-conduction mechanism and the low thermal conductivity introduced by nanostructuring through sputtering method is suggested to promising strategy to enhance the thermoelectric properties of the CrGT material.

MnTe films exhibit a remarkable change in physical properties upon polymorphic transformation from β (wurtzite-type) to α (nickeline-type) phase. In this study, the mechanism of the β to α transformation in MnTe films capped with a tungsten layer during isothermal annealing at 500 °C was investigated. X-ray diffraction analyses revealed that the out-of-plane c-axis orientation was maintained during the polymorphic transformation. High-resolution transmission electron microscopy revealed that the β-phase transformed to the α-phase via a β′-phase which has a wurtzite-type structure as the β-phase but with a slight difference in the coordinates of the Te atoms. The β → β′ transformation was induced by the puckering process during which Mn- and Te-atomic planes shift in opposite directions along the c-axis, whereas the β′ → α transformation was induced by a buckling process during which pairs of Mn- and Te-atomic planes alternately move to the opposite directions along the 210 direction. Meanwhile, in the MnTe film without the tungsten capping layer, the out-of-plane c-axis orientation was not retained during the polymorphic transformation. These results imply that the sequential two-stage polymorphic transformation maintaining the out-of-plane c-axis orientation is caused by the constraint on the MnTe film induced by the tungsten capping layer.

Cu 2 Mg intermetallic compound was selected as a candidate material for liner-barrier free interconnects for advanced LSI interconnects. The blanket films of Cu 2 Mg showed an excellent resistivity scaling with resistivity value of only 25 μΩcm at a film thickness of 5 nm. However, a thick MgO layer was formed within the underlying SiO 2 layer by interface reaction between Cu 2 Mg and SiO 2 . Progressive growth of MgO made it unfavorable for Cu 2 Mg to be used as liner-barrier free interconnects.

The structure of amorphous ${\mathrm{Cu}}_{2}{\mathrm{GeTe}}_{3}$ is investigated by a combination of anomalous x-ray scattering and extended x-ray absorption fine-structure experiments. The experimental data are analyzed with reverse Monte Carlo modeling, and they are interpreted in terms of short-range-order parameters as well as by using ring statistics and persistent homology to study the intermediate-range order. Based on this information, the structural relationship of the amorphous phase to the corresponding crystal is discussed. It is found that the amorphous network can be rationalized by small atomic displacements of the crystal structure, directed toward the intrinsic void regions. This structural similarity establishes the possibility of a fast phase-change process. On the other hand, the atomic rearrangements also lead to the formation of new chemical bonds and to distortions on the intermediate-range-order level. These are realized by a collapse and contraction of the strict hexagonal ring arrangements of the crystal and by the formation of small, triangular rings as well as Cu cluster configurations. These structural features allow for a new understanding of the phase-change property contrast of this material, especially concerning the density change and the optical contrast.

The crystallization mechanism and kinetics of Cr2Ge2Te6 (CrGT) films were investigated by differential scanning calorimetry. The average Avrami exponent (na) analysis indicated that CrGT exhibits a growth-dominant crystallization in the range of heating rate (β) of 102–50°C/min. In comparison, Ge2Sb2Te5 (GST) showed a nucleation-dominant crystallization. The na of CrGT was about 3, and was majorly independent of β. The na of GST decreased with an increasing p, 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.

In our previous research, we reported that Mg-Sc alloys with a bcc (β) phase showed significant age hardening by the formation of fine hcp (α) precipitates in the β phase. In this study, we investigated the kinetics of the aging precipitation in the α+β two-phase Mg-Sc alloy. The incubation time for age hardening became shorter with increasing aging temperature. The relationship between the volume fraction of α precipitates and aging time was better described by the Austin-Rickett equation than by the Johnson-Mehl-Avrami-Kolmogorov equation, and the obtained n values are 1.2–1.4. According to the n values and the morphology of fine α precipitates, the mechanism of α precipitation in the Mg-Sc alloy with an α+β two-phase was proposed to be a diffusion-controlled reaction, including the one-dimensional growth of α precipitates at a constant or decreasing nucleation rate. In addition, the activation energy for α precipitation was calculated to be 83.5 kJ mol−1. This value is much smaller than that of self-diffusion in Mg, while it is almost the same as that of grain boundary diffusion in Mg, namely interface diffusion. This reveals that the growth of α precipitates is dominated by interface diffusion along the boundaries between the α precipitate and the β phase.