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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.

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.

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.

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.

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.

Purpose To define the median endometrial thickness (ET) in office gynecology is thought to be important for clinical practice. However, there are few reports about ET that have included the general female population on a large scale. The median ET was determined prospectively in premenopausal women who attended office gynecology for cervical cancer screening. Methods In total, 849 women were enrolled. The median ET was determined by using transvaginal ultrasound and the relationships between the ET and various clinical factors were analyzed. Results The participants' median age was 38.5 years. The median ET was 8.6 mm (90% and 95% quantiles: 13.8 and 15.8 mm). The ET was not related to their age, symptoms, obstetric history, geographical location, or risk factors for endometrial cancer. In the women with a menstrual cycle length of 28–30 days, the ET was 7 mm on days 1–6, but it increased from 5.4 mm immediately after menstruation (day 7 or 8) to 9.2 mm on days 13–14. Subsequently, the ET increased further to 11.1 mm on day 18. Conclusion In all the women, the upper limit of the ET was 13.8 mm and 15.8 mm in the 90% and 95% quantile, respectively, in office gynecology.

This paper investigates the impact of contact resistance on the memory window in phase-change random access memories (PCRAMs) using $$\hbox {Ge}_{2}\hbox {Se}_{2}\hbox {Te}_{5}$$Ge2Se2Te5 (GST). We discuss the increase of contact resistance, as device is scaled down to a nanometer size and the effects of contact resistivity changes with respect to the resistance window between the set and reset states. In a contact area of $$400 \,\hbox {nm}^{2}$$400nm2, the contact resistance in the set state occupies more than 80 % of the total resistance, and the occupied area increases as the contact area is scaled upward. The memory window is significantly degraded as the set resistance increases because of the increasing contact resistance. To maintain the memory window with more than two orders of magnitude of the resistance in a $$100 \,\hbox {nm}^{2}$$100nm2 area, the contact resistance should be decreased to less than 60 % of that of a $$400\, \hbox {nm}^{2}$$400nm2 area by reducing contact resistivity or by some other method. We examine the reduction of contact resistance achieved by adopting a three-dimensional contact structure, and we propose this structure as a candidate for the scaled PCRAM.

${\mathrm{Cu}}_{2}{\mathrm{GeTe}}_{3}$ (CGT) phase-change material, a promising candidate for advanced fast nonvolatile random-access-memory devices, has a chalcopyritelike structure with $s{p}^{3}$ 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 sixfold $p$-like bonding. In order to reveal the PC mechanism of CGT, the electronic structure change due to PC has been investigated by laboratory hard x-ray photoelectron spectroscopy and combined first-principles density-functional theory molecular-dynamics simulations. The valence-band spectra, in both crystalline and amorphous phases, are well simulated by the calculations. An inherent tendency of Te $5s$ lone-pair formation and an enhanced participation of Cu $3d$ orbitals in the bonding are found to play dominant roles in the PC mechanism. The electrical conductivity of as-deposited films and its change during the PC process is investigated in connection with valence-band spectral changes near the Fermi level. The results are successfully analyzed, based on a model proposed by Davis and Mott for chalcogenide amorphous semiconductors. The results suggest that robustness of the defect-band states against thermal stress is a key to the practical application of this material for memory devices.

The structure of crystal GeCu2Te3 was investigated by X-ray absorption fine structure (XAFS) measurement. We found that the Ge–Te interatomic distances obtained from XAFS are larger than those obtained from X-ray diffraction, and the Cu–Te distances are smaller. The averaged Ge–Te and Cu–Te distances obtained from XAFS are almost equal to the corresponding interatomic distances in amorphous GeCu2Te3. Therefore both crystal and amorphous GeCu2Te3 seem to be built up of the same local configurations of GeTe4 and CuTe4 tetrahedrons. This would be the reason why the phase change in GeCu2Te3 occurs very fast.

An amorphous manganese oxide layers formed by chemical vapor deposition have been studied as a copper diffusion barrier. The thermal stability of the barrier layer was assessed by annealing Cu/MnSixOy/SiO2/Si samples at 400 °C for various times up to 10 h. Transmission electron microscopy, energy-dispersive X-ray spectroscopy (EDX), secondary ion mass spectroscopy (SIMS), capacitance-voltage and current–voltage measurements were performed. Failure of the barrier property is marked by observing the copper peak appearing in EDX and SIMS spectra data from the SiO2 region. Amorphous MnSixOy barrier with a thickness of 1.2 nm has failed in preventing Cu diffusion into SiO2 substrate after anneal at 400°C in vacuum for 1h, as proven by the presence of Cu in the dielectric (SiO2) layer. However, the amorphous MnSixOy with the thickness of 2.0 nm barrier was thermally stable and could prevent Cu from inter-diffusion to the SiO2 substrate after annealing at 400 °C even up to 10 h.

The gradual erasing operation from reset state to set state adjusting pulse amplitude, duration time and falling time respectively in phase change device using Ge1Cu2Te3 is investigated. For this procedure, a relatively high voltage and increased falling time, which was able to produce both long-term potential and long-term depression in the time interval between pre-spike and post-spike is choosing. The results suggested that the presence of synaptic behaviour was due to controlled falling time rather than pulse amplitude.

This article examines the feasibility of Cu-Al-Mn superelastic alloy bars as possible self-sensor components, taking electrical resistance measurement as a feedback. Superelastic alloy bars change their crystallographic structure with phase transformation, as well as electrical resistance during loading–unloading process at ambient temperature. This work studies the relationship between strain and electrical resistance measurements of superelastic alloys at room temperature. Such relationship can be used in determining the state of a shape memory alloy–based structure effectively, without separate sensors, by appropriately measuring the changes in electrical resistance during and after structure’s loading history. Quasi-static cyclic tensile tests are conducted in this article to investigate the relationship between electrical resistance and strain for a 4-mm-diameter Cu-Al-Mn superelastic alloy bar. It was demonstrated that linear relationship with little hysteresis can be achieved up to 10% strain. The test observations support the feasibility of newly developed Cu-Al-Mn superelastic alloy bars, characterized by low material cost and high machinability, as a multifunctional material for both structural and sensing elements.

Moreover, the resistivity was found to decrease by more than 3 orders after annealing at 673K. Such changes in the resistivity of the films may be due to the change of crystal structure of the films, but was not clear. Because the XRD patterns of the as-deposited films showed only one vague peak, it would not be appropriate to identify the crystal structure of the as-deposited films from the X-ray diffraction (XRD) analysis. Therefore, in this study, we systematically investigated the electrical properties of Fe-O-N films deposited by reactive sputtering by widely changing O2 (0.05­0.25sccm) and N2 (0­5sccm) gas flow rate, and the relationships between electrical properties and crystal structure of the Fe-O-N films were discussed based on not only XRD measurements but also transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) analysis and temperature dependence of the resistivity of the films.

We measured the impact of the thermoelectric effect, especially the Peltier effect, upon the operation of phase change memory (PCM) in which the contact resistance between the phase change material and the electrode dominates the total resistance. A PCM device having a pillar structure of diameter 500 nm was fabricated using GeCu2Te3 (GCT) material. During read operation, the set state (with crystalline PCM) showed ohmic contact between the PCM and the electrode, whereas the reset state (with amorphous PCM) showed Schottky contact. The Schottky contact between the amorphous GCT and the electrode showed a bias polarity dependence of set operation owing to the Peltier effect, which is one of the thermoelectric effects. As PCM devices scale down to the nanometre scale, research on contact resistance and various related effects will become more important.

New interconnect materials that have a low line resistivity are required to address issues associated with the increased resistivity due to the aggressive downscaling of future semiconductor devices. In this work, CuAl2 thin films are investigated as a potential material for liner- and barrier-free interconnect applications. The results show that CuAl2 blanket films adhere well to and do not undergo interdiffusion with SiO2, as well as having a favorable size effect of resistivity. Furthermore, the filling of CuAl2 in narrow low-k trenches is investigated, and an excellent gap-filling performance is registered. These features suggest that CuAl2 is a promising alternative to Cu that does not require any additional liner or barrier layers for feature sizes less than 10 nm.

The effects of grain size on the stress–strain characteristics of Cu–Al–Mn-based pseudoelastic (PE) sheet specimens with various relative grain sizes d/t (d = grain size, t = sheet thickness) were investigated using cyclic tensile testing. The stress–strain characteristics such as the yield stress σy, work-hardening rate dσPE/de and stress hysteresis of pseudoelasticity Δσ decreased, and the obtained maximum PE strain e PE MAX increased with increasing d/t. The σy, dσPE/de, Δσ and e PE MAX plots as a function of d/t were divided into three regions. The critical points dividing the plots into the three regions were located at d/t ≈ 1 and d/t ≈ 2. These results can be explained from the standpoint of constraints between grains, depending on grain structure such as a fine equiaxed grain structure, a columnar-like grain structure and a fully columnar grain structure.

A selective switching device utilizing a phase-change material was investigated. In this work, we present a new concept to realize serially a selective switching and memory operation in a multiple phase-change memory with only phase-change materials without any semiconductor switching device. A phase-change material for selective switching can be expected to have a higher resistance amorphous phase and to show lower melting and crystallization temperatures than a phase-change material for a memory. Here, we present a structural method to obtain the above requirements. In addition, we confirm the switching operation by a selective current pulse for multiple phase-change materials from the experiment. From these results, we expected that one of the multiple phasechange materials can be replaced in a switching device without the need for an additional selective device, and that such a device would be feasible for 3-dimensional PCM architecture.

The electrical and optical properties of Fe­O­N films were investigated in order to find their possibilities for solar cell application. Fe­O­N thin films were deposited on glass substrates by RF magnetron sputtering using an Ar­N2­O2 reactive gas. Under optimum flow rates of nitrogen and oxygen, the Fe­O­N films showed equivalent electrical properties to amorphous Si that has been conventionally used for thin film solar cells. Bandgap narrowing was also observed from 2.0 to 1.9 eV. The observed results were considered to be due to the formation of hematitemagnetite mixed phase, and the introduction of oxygen vacancies and/or nitrogen interstitials. [doi:10.2320/matertrans.M2013170]

The crystallization kinetics of amorphous Ge 1 Cu 2 Te 3 (GCT) films prepared by sputter deposition were investigated by differential scanning calorimetry under non-isothermal conditions. An exothermic peak due to crystallization was observed in the temperature range 230–270 °C. It was found that the local activation energy for crystallization is almost constant when the crystallization fraction is less than about 0.15 and then monotonically decreases with increasing crystallization fraction, which indicates that the crystallization of amorphous GCT films is a multi-step mechanism. The local Avrami exponent decreased from more than 5 to 1.7 with increasing crystallization fraction. It was demonstrated by the Ozawa method that GCT amorphous films show a higher thermal stability than Ge 2 Sb 2 Te 5 amorphous films, with an estimated failure time of over 70 years at 125 °C, which is well beyond the data retention requirements of the International Technology Roadmap for Semiconductors. In addition, the thickness change in GCT amorphous films accompanying crystallization was measured by atomic force microscopy. The GCT amorphous film was found to show a thickness increase of only 2.0% on crystallization, which is desirable to enhance the endurance of phase change random access memory devices.

Rolled sheets of AZ31 Mg alloys were deformed at room temperature under tension along the rolling direction. A number of large surface steps were observed in the region near the fractured edge. Transmission electron microscopy showed the presence of twins under these steps. Crystallographic analysis indicated that the twins are of the { 1 0 1 ¯ 1 } – { 1 0 1 ¯ 2 } double twin type with their basal planes tilted by 37° with respect to the basal planes of the matrix, which is considered to make basal dislocation slip highly active within the twin. The localized deformation within the twins may lead to the formation of crack-like faults and to failure in a macroscopically brittle manner.