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Ternary Ge-Sb-Se glass which is one the prototypical chalcogenide glasses features a quite broad glass forming region mainly covering Se-sufficient compositions. However, as far as moldability issue is concerned, only a limited area belonging to Se-deficient compositions has been verified to be more adequate for use as molded lens transparent up to the long-wavelength infrared range, i.e., 8–12 μm. Commercially available Ge27.5Sb12.5Se60 (at%) glass well exemplifies practicality of the Se-deficient Ge-Sb-Se glasses, which are categorized commonly as a low refractive index dispersion glass. We discover that partial substitution of selenium with sulfur in this glass augments its thermal and mechanical properties, and at the same time its refractive index becomes more dispersive over wavelength. Here, EXAFS technique employed in this study to elucidate local structural evolutions of each constituent in quaternary Ge27.5Sb12.5SxSe60-x glasses confirms that 1) the empirical 8 − N rule is preserved for each atom; 2) both homopolar Ge–Sb and Ch-Ch (Ch = S or Sb) bonds are hardly found; 3) Ge atom prefers S to Se as its nearest neighbor, whereas such a preference is absent for Sb; 4) homopolar Ge–Ge bonds abound more than estimation derived from preservation of chemical order, and are formed at the expense of Ge–Se bonds rather than Ge–S bonds. These findings suggest that the present sulfur-selenide glasses are supposed to possess an identical short-range topological order, i.e., their mean coordination number is unaltered, but their chemical order is broken especially in the vicinity of Ge atom. The violated chemical order related with Ge atoms should be taken into consideration when interpreting their composition-dependent properties and performing compositional optimization with regard to molded lens applications.

Electrical phase change device using the Ge-doped eutectic Sb-Te (e.g., Ge1Sb8Te2) film is known to exhibit improved energy efficiency thanks to lowered threshold voltage as well as decreased power consumption for the reset operation, as compared with Ge2Sb2Te5 film. Ge K-edge EXAFS analysis is employed in this study in an effort to elucidate such merits of Ge1Sb8Te2 film in connection with its local atomic arrangements. It is then verified that a Ge atom is four-fold coordinated in its nearest-neighboring shell both in the as-deposited and in the annealed films. It needs to be highlighted that approximately two Sb atoms constitute the Ge tetrahedral units in its amorphous state; however, after being crystallized, heteropolar Ge-Sb bonds hardly exist in this Ge1Sb8Te2 film. It has been known that crystallization temperature and activation energy for crystallization of this Ge1Sb8Te2 composition are greater than those of Ge2Sb2Te5 composition. In addition, these two phase change materials exhibit distinctly different crystallization mechanisms, i.e., nucleation-dominant for Ge2Sb2Te5 film but growth-dominant for Ge1Sb8Te2 film. These discrepancies in the crystallization-related properties are delineated in terms of the local structural changes verified from the present EXAFS analysis.

In an effort to verify whether the Ovonic threshold switching in amorphous chalcogenide film is concomitant with any noticeable atomic rearrangements, we analyze EXAFS spectra of amorphous chalcogenide Ge60Se40 (at%) film measured in situ under DC electric field. The in situ EXAFS measurements are performed with specifically prepared specimens, in which 100-nm-thick amorphous Ge60Se40 layer is sandwiched between two metallic electrode layers, all the while a constant DC electric bias being applied. Our EXAFS analysis indicates that only Debye-Waller factor undergoes a noticeable change upon the Ovonic threshold switching, keeping both coordination number and interatomic distance unaltered. The changes in Debye-Waller factors of Ge-Se and Ge-Ge pairs appear to be reversible during the ON/OFF cycle; however, heteropolar Ge-Se pair experiences a conspicuously greater change in its Debye-Waller factor than homopolar Ge-Ge counterpart. Increase of thermal disorder due to Joule heating is insufficient to support this observation, which instead is caused by a preferential increase of static disorder associated with heteropolar Ge-Se bonds in the ON state. The heteropolar Ge-Se pair is supposed to feel the electrical wind force (momentum transfer by conducting electrons) as well as the Coulombic force more strongly than the homopolar Ge-Ge pair.

EXAFS analysis is performed on equiatomic GeSe and GeTe films to elucidate changes in the local atomic environments induced by crystallization. It reveals for both the films that the 4:2 configuration dominates in their as-deposited amorphous state. In their crystallized state, however, the 3:3 configuration prevails together with quasi-crystalline Ge clusters. The two chalcogenide films exhibit very similar characteristics such as hidden glass transition phenomenon during heating, dominance of the 3:3 configuration in their melt state, decreased optical bandgap upon crystallization, and reduced temperature dependence of electrical conduction accompanied with crystallization. These common properties are partly attributed to the structural changes identified through our EXAFS analysis. In particular, their previously reported non-ohmic current-voltage behaviors associated with the phase switching and the threshold switching are explained in terms of the local structural information. The special phase-change properties of the equiatomic GeSe film is explicated by its amorphous structure.

Sulfur-based chalcogenide Li–Ge–Ga–S powders for use in solid electrolyte of Li + ion batteries have been successfully synthesized via a low-temperature solution-based process. Their Li + ion conductivity turns out to be ~ 7 × 10 − 4 S/cm at room temperature which is quite comparable with that of melt-quenched or mechanically-alloyed analogues. It is revealed that their microstructure becomes amorphized appropriately at a specific Ga/Ge ratio where the ionic conductivity is maximized. A structural model is proposed, which emphasizes correlations between gallium and lithium inside the amorphous structures.

As for the molded lens applications of ternary Ge–Sb–Se glass, Ge-rich compositions are normally preferable since thermal and mechanical properties are better in these compositions. In an effort to explicate thermal properties of such glasses suitable for practical molding process, we have evaluated their compositional dependence of glass transition temperature (T g ) and softening point temperature (T s ). Among three constituent atoms, Ge turns out to exert a most conspicuous correlation with T g and T s . This phenomenological behavior is then explained in connection with structural evolution induced by Ge. Based on analysis of mean coordination number and average bond energy, it is inferred that the glass structure becomes energetically more stable and topologically more connected with increasing Ge content.

Chalcogenide glass in the ternary Ge-Sb-Se system is inherently moldable, thus being considered as a strong candidate material for use in infrared-transmitting lens applications from the viewpoint of thermal and mechanical stability. In an effort to experimentally determine compositional region suitable for the molded lens applications, we evaluate its compositional dependence of hardness. Among the constituent atoms, Ge content turns out to exert a most conspicuous correlation with hardness. This phenomenological behavior is then explained in connection with the structural evolution that Ge brings about.

In an effort to evaluate the practicability of an imprinting technique for amorphous chalcogenide film in Ge-based compositions, we investigate the deformation behavior of the surface of amorphous GeSe₂ film deposited via a thermal evaporation route according to varying static loads applied at elevated temperatures. We observe that, under these static loading conditions, crystallization tends to occur on its surface relatively more easily than in As-based As₂Se₃ films. As for the present GeSe₂ film, higher processing temperatures are required in order to make its surface reflect the given stamp patterns well; however, in this case, its surface becomes partially crystallized in the monoclinic GeSe₂ phase. The increased vulnerability of this amorphous GeSe₂ film toward surface crystallization under static loading, when compared with the As₂Se₃ counterpart, is explained in terms of the topological aspects of its amorphous structure.

EXAFS spectroscopic analysis is employed to elucidate the amorphous structure of evaporation-deposited Ge x Se 100 − x ( x = 30, 49 or 69 in at%) films. The difficulty in the determination of local structure resulting from the similar backscattering amplitude of Ge and Se atoms has been overcome through comparative analysis of film specimens subjected to controlled heat treatment. Our results indicate that the 4(Ge):2(Se) structural model, which satisfies the (8 − N ) rule, is valid for all of the compositions studied. However, the nearly equiatomic Ge 49 Se 51 composition with 4(Ge):2(Se) amorphous structure crystallizes into orthorhombic GeSe crystalline phase with 3(Ge):3(Se) structural arrangements and quasi-crystalline Ge clusters. This significant difference in the local structure of amorphous and crystalline states is a distinctive signature of phase change compositions.

We employ EXAFS spectroscopy to refine the local atomic arrangements of evaporation-deposited equiatomic GeSe film. Amorphous structure of the as-deposited GeSe film turns out to satisfy mainly the 4:2 structural model. The crystallized GeSe film, however, consists of the orthorhombic GeSe crystals with the 3:3 atomic arrangements and quasi-crystalline Ge clusters. Its temperature-dependent electrical resistance is explained in connection with this structural difference, which exemplifies significance of the chemical environments on the electrical switching phenomena observed from amorphous chalcogenide solids.

We studied GexSe1−x for the potential application in the Ovonic Threshold Switching (OTS) device. We found that, as Ge concentration increased, the thermal stability was deteriorated while the device performances were improved. In addition, using Spectroscopic Ellipsometry (SE) technique, the energy gap (Eg) and the Urbach energy (EU) were found to show non-monotonic dependences, with their minimum of about 1.0 eV of Eg for Ge0.6Se0.4 and 40 meV of EU for Ge0.5Se0.5. These changes are consistent with the changes in device characteristics, which might be explained in terms of the change in the number of Se-Se bondings.

We performed a study on the scalability of Ovonic Threshold Switching (OTS) devices using an amorphous chalcogenide material, Ge0.4Se0.6. As the cell size decreased, the maximum driving current was estimated to be over 3 × 10 7 A/cm 2 , surpassing the state of the art devices based on crystalline Si. However, the threshold voltage (VTH), the holding voltage (VH), and the holding current (IH) were observed to increase laying challenges to be resolved for developing non-destructive and low-power consuming selector devices. VTH was found to be reduced by decreasing the thickness of GeSe film until 40 nm, below which it started to saturate. This might be associated with the Schottky barrier formed at the interface between the amorphous semiconductor and the metal electrode.

The Ovonic Threshold Switch (OTS) based on an amorphous chalcogenide material has attracted much interest as a promising candidate for a high-performance thin-film switching device enabling 3D-stacking of memory devices. In this work, we studied on the electronic structure of amorphous Sb-doped Ge0.6Se0.4 (in atomic mole fraction) film and its characteristics as to OTS devices. From the optical absorption spectroscopy measurement, the band gap (E-g) was found to decrease with increasing Sb content. In addition, as Sb content increased, the activation energy (E-a) for electrical conduction was found to decrease down to about one third of E-g from a half. As to the device characteristics, we found that the threshold switching voltage (V-th) drastically decreased with the Sb content. These results, being accountable in terms of the changes in the bonding configuration of constituent atoms as well as in the electronic structure such as the energy gap and trap states, advance an effective method of compositional adjustment to modulate V-th of an OTS device for various applications.

To improve the quality of chest compression (CC), we developed the assistant-push method, whereby the second rescuer pushes the back of the chest compressor during CC. We investigated the effectiveness and feasibility of assistant push in achieving and maintaining the CC quality.This was a randomized crossover trial in which 41 subjects randomly performed both of standard CC (single-rescuer group) and CC with instructor-driven assistant push (assistant-push group) in different order. Each session of CC was performed for 2 minutes using a manikin. Subjects were also assigned to both roles of chest compressor and assistant and together performed CC with subject-driven assistant push. Depth of CC, compression to recoil ratio, duty cycle, and rate of incomplete recoil were quantified.The mean depth of CC (57.0 [56.0-59.0] vs 55.0 [49.5-57.5], P.001) was significantly deeper, and the compression force (33.8 [29.3-36.4] vs 23.3 [20.4-25.3], P.001) was stronger in the assistant-push group. The ratio of compression to recoil, duty cycle, and rate of incomplete chest recoil were comparable between the 2 groups. The CC depth in the single-rescuer group decreased significantly every 30 seconds, whereas in the assistant-push group, it was comparable at 60- and 90-second time points (P = .004). The subject assistant-push group performed CCs at a depth comparable with that of the instructor assistant-push group.The assistant-push method improved the depth of CC and attenuated its decline, eventually helping maintain adequate CC depth over time. Subjects were able to feasibly learn assistant push and performed effectively.

Switching devices based on Ovonic Threshold Switching (OTS) have been considered as a solution to overcoming limitations of Si-based electronic devices, but the reduction of switching voltage is a major challenge. Here, we investigated the effect of Bi-doping in Ge0.5Se0.5 thin films on their thermal, optical, electrical properties, as well as on the characteristics of OTS devices. As Bi increased, it was found that both of the optical energy gap (Egopt) and the depth of trap states decreased resulting in a drastic reduction of the threshold voltage (Vth) by over 50%. In addition, Egopt was found to be about three times of the conduction activation energy for each composition. These results are explained in terms of the Mott delocalization effect by doping Bi.

We investigated the effect of nitrogen (N) doping on the threshold voltage of an ovonic threshold switching device using amorphous GeSe. Using the spectroscopic ellipsometry, we found that the addition of N brought about significant changes in electronic structure of GeSe, such as the density of localized states and the band gap energy. Besides, it was observed that the characteristics of OTS devices strongly depended on the doping of N, which could be attributed to those changes in electronic structure suggesting a method to modulate the threshold voltage of the device.