The film with a composition close to Ge2Sb2Te5 was fabricated by the supercycle atomic layer deposition (ALD) of GeTe and SbTe, followed by tellurization annealing. Supercycle processes are widely used for thin film deposition of multicomponent materials and often exhibit non-ideal growth behavior. Since only in situ analysis can reveal the substrate-dependent growth behavior, we used in situ quartz crystal microbalance (QCM) to study the growth mechanism during ALD supercycle processes at 85 degree celsius. GeTe grown on SbTe was more Te-deficient than continuously grown GeTe film. As a result, more Te-deficient Ge-Sb-Te films were formed than expected. By annealing in a Te ambient at 250 degree celsius, the Te-deficient Ge-Sb-Te film was converted to the Ge0.23Sb0.23Te0.54 close to Ge2Sb2Te5 film, which had a high density equivalent to 95 % of the FCC structure of Ge2Sb2Te5. The film showed excellent conformality and uniform composition in a trench pattern, suggesting a uniform crystallization temperature of 118 degree celsius at all locations.

AbstractThreshold‐switching devices based on amorphous chalcogenides are considered for use as selector devices in 3D crossbar memories. However, the fundamental understanding of amorphous chalcogenide is hindered owing to the complexity of the local structures and difficulties in the trap analysis of multinary compounds. Furthermore, after threshold switching, the local structures gradually evolve to more stable energy states owing to the unstable homopolar bonds. Herein, based on trap analysis, DFT simulations, and operando XPS analysis, it is determined that the threshold switching mechanism is deeply related to the charged state of Se–Se homopolar defects. A threshold switching device is demonstrated with an excellent performance through the modification of the local structure via the addition of alloying elements and investigating the time‐dependent trap evolution. The results concerning the trap dynamics of local atomic structures in threshold switching phenomena may be used to improve the design of amorphous chalcogenides.

Considering the three-dimensional vertical phase-change random access memory device application, we studied the deposition process to prepare conformal crystalline GeSbTe thin films.

Correction for ‘Composition control of conformal crystalline GeSbTe films by atomic layer deposition supercycles and tellurization annealing’ by Yewon Kim et al., J. Mater. Chem. C, 2022, 10, 9691–9698, DOI: https://doi.org/10.1039/D2TC00784C.

Recently, phase change memory is one of highly developed memory products; however, further research is needed to manage intrinsic variations in switching voltage and resistance drift, which is crucial for high density memory development. Current–voltage characteristics, their temperature dependence, and voltage‐dependent resistance switching characteristics at various programming times are investigated. These data provide an indirect characterization of microstructural phase distribution in the phase change material during memory operations. Subthreshold current–voltage symmetry in phase change memory cells can be used for identifying filamentary or non‐filamentary conducting paths, both of which are controllably formed by adjusting the programming time. This work may contribute to an improved operation method to mitigate intrinsic variations in phase change memories.

As shown during the 2009 pandemic H1N1 (A(H1N1)pdm09) outbreak, egg-based influenza vaccine production technology is insufficient to meet global demands during an influenza pandemic. Therefore, there is a need to adapt cell culture-derived vaccine technology using suspended cell lines for more rapid and larger-scale vaccine production. In this study, we attempted to generate a high-growth influenza vaccine strain in MDCK cells using an A/Puerto/8/1934 (H1N1) vaccine seed strain. Following 48 serial passages with four rounds of virus plaque purification in MDCK cells, we were able to select several MDCK-adapted plaques that could grow over 10⁸ PFU/ml. Genetic characterization revealed that these viruses mainly had amino acid substitutions in internal genes and exhibited higher polymerase activities. By using a series of Rg viruses, we demonstrated the essential residues of each gene and identified a set of high-growth strains in MDCK cells (PB1D153N, M1A137T, and NS1N176S). In addition, we confirmed that in the context of the high-growth A/PR/8/34 backbone, A/California/7/2009 (H1N1), A/Perth/16/2009 (H3N2), and A/environment/Korea/deltaW150/2006 (H5N1) also showed significantly enhanced growth properties (more than 10⁷ PFU/ml) in both attached- and suspended-MDCK cells compared with each representative virus and the original PR8 vaccine strain. Taken together, this study demonstrates the feasibility of a cell culture-derived approach to produce seed viruses for influenza vaccines that are cheap and can be grown promptly and vigorously as a substitute for egg-based vaccines. Thus, our results suggest that MDCK cell-based vaccine production is a feasible option for producing large-scale vaccines in case of pandemic outbreaks.

Phase change memory (PCM) that is operated on resistance changes caused by joule heating has been suggested as the next-generation memory for scaling since its programming current scales linearly. We propose a U-shaped cell design to further reduce the reset current in PCM devices, which enables easier and more efficient scaling than conventional PCMs. Simulation studies of heat transfer demonstrated that our U-shaped design with a dashed heater has a higher thermal efficiency of 4.97K/μA compared to 3.36K/μA in a lance cell with a ring heater for the same storage node. The reset current can be better scaled proportionate to k2.0 in which the exponent is higher than the lance cell of k1.5 in non-isotropic scaling. This better scalability is attributed to the small programming volume of the U-shaped cell, which was verified by transmission electron microscopy analysis. Furthermore, the cyclic endurance of the U-shaped cell was enhanced by 1 order of magnitude compared to a lance cell and the thinner CGeSbTe films reduced the reset current further. Our results show that a U-shaped cell is a highly promising design to scale reset current in next-generation PCM devices.

As the cell size of phase change memory devices decreases to less than 100nm, the dry etch used for cell patterning becomes extremely critical because of its impact on the properties of memory cells. HBr gas has been known as the etchant that can minimize surface etching damage to GeSbTe-based phase change materials. However, the findings reported herein show that the HBr etch of CGeSbTe (CGST) films causes voids after annealing at temperatures below 400°C due to the vaporization of volatile bromides (GeBr4, SbBr2, and TeBr2) that form when bromine diffuses during the etch. In this investigation, we report on the use of NH3 etchants to suppress the formation of volatile compounds and thereby eliminate void formation in CGST materials. The properties of NH3 etchants were compared to those of HBr etchants as a function of both etch rate and profiles. The effects of void suppression as observed in transmission electron microscopy images of as-etched CGST film after annealing indicate that phase change memory devices etched using an NH3 exhibited enhanced set cycles that were over 108 superior to results of HBr etchants by 2 orders of magnitude.

Phase Change Memory (PCM) has the potential for use as the flash memories for the next generation due to its scalability, long endurance, high speed, and the possibility of random access. To successfully integrate phase change materials of a GeSbTe (GST) alloy in the device fabrications, the presence of a Ti layer is required to obtain proper contact resistance characteristics and also to improve the adhesion property between the GST alloys and the top TiN electrode. The findings reported herein indicate that Ti can readily diffuse into the GST layer and reduce the set resistance when the subsequent device fabrication involves the heating process. As a result, the reset current significantly increases and the endurance cycles of the PCM device are significantly degraded. We proposed the use of a Highly Nitrogen doped GST (HNGST) layer as a barrier to Ti diffusion, in which the presence of the barrier was able to restore the reset current of the PCM devices down to the level of as-etched PCM cell by the use of a 9nm thick HNGST barrier. In addition, endurance cycles were significantly enhanced from 106 to over 108 by blocking the diffusion of Ti into the program volume of the cell with the HNGST barrier layer.

The lower cyclic endurance of Phase Change Memory (PCM) devices limits the spread of its applications for reliable memory. The findings reported here show that micro-voids and excess vacancies that are produced during the deposition process and the subsequent growth in sputtered carbon-doped GeSbTe films is one of the major causes of device failure in PCM with cycling. We found that the size of voids in C15(Ge21Sb36Te43) films increased with increasing annealing temperature and the activation energy for the growth rate of voids was determined to be 2.22 eV. The film density, which is closely related to voids, varies with the deposition temperature and sputtering power used. The lower heat of vaporization of elemental Sb and Te compared to that for elemental Ge and C is a major cause of the low density of the film. It was possible to suppress void formation to a considerable extent by optimizing the deposition conditions, which leads to a dramatic enhancement in cyclic endurance by 2 orders of magnitude in PCM devices prepared at 300oC-300W compared to one prepared at 240oC-500W without change of compositions.

Phase Change Memory (PCM) has been proposed for use as a substitute for flash memory to satisfy the huge demands for high performance and reliability that promise to come in the next generation. In spite of its high scalability, reliability, and simple structure, high writing current, e.g., RESET current, has been a significant obstacle to achieving a high density in storage applications and the low power consumption required for use in mobile applications. We report herein on an attempt to determine the level of carbon incorporated into a GeSbTe (GST) film that is needed to reduce the RESET current of PCM devices. The crystal structure of the film was transformed into an amorphous phase by carbon doping, the stability of which was enhanced with increasing carbon content. This was verified by the small grain size and large band gap that are typically associated with carbon. The increased level of C-Ge covalent bonding is responsible for these enhancements. Thus, the resistance of the carbon doped Ge2Sb2Te5 film was higher than that for an undoped GST film by a factor of 2 orders of magnitude after producing a stable face-centered cubic phase by annealing. As a consequence, the PCM devices showed a significant reduction in RESET current as low as 23% when the carbon content was increased to 11.8 at. %. This can be attributed to the elevated SET resistance, which is proportional to the dynamic resistance of the PCM device, caused by the high resistance due to a carbon doped GST film. (C) 2015 AIP Publishing LLC.

This article studies the responses of students who read The Lion, the Witch and the Wardrobe to see how they differ from reading the Christian message in the Bible. This study employs qualitative methods to understand readers’ literary experiences, reflections on faith, realizations and resolutions. Response journals, interviews, and surveys were used to collect the data. The findings revealed that a literary work can turn difficult creeds into interesting stories that are easier to understand. It is also easier to empathize with the characters in literary works than with those of the Bible. As a result, fictional works can allow readers to compare their own experiences with the creeds and help them make new resolutions. Literary works and the Bible have a complementary relationship in Christian education. Christian educators should make the most use out of literary works as an important means to help students discover the Christian truth.

In this study, we propose a nitrogen-incorporated GeBiTe ternary phase of N7.9(Ge46.9Bi7.2Te45.9) as a phase change material for reliable PCM (Phase Change Memory) with high speed operation. We found that the N7.9(Ge46.9Bi7.2Te45.9) film shows the resistance value of 40 kΩ after annealing at 440oC for 10 minutes, which is much higher than the value of 3.4 kΩ in the case of conventional N7.0(Ge22.0Sb22.0Te56.0) films. A set operation time of 14 nsec was achieved in the devices due to the increased probability of the nucleation by the addition of the elemental Bi. The long data retention time of 10 years at 85oC on the base of 1% failure was obtained as the result of higher activation energy of 2.52 eV for the crystallization compared to the case of N7.0(Ge22.0Sb22.0Te56.0) film, in which the activation energy is 2.1 eV. In addition, a reset current reduction of 27% and longer cycles of endurance as much as 2 order of magnitude compared to the case of N7.0(Ge22.0Sb22.0Te56.0) were observed at a set operation time of 14 nsec. Our results show that N7.9(Ge46.9Bi7.2Te45.9) is highly promising for use as a phase change material in reliable PCMs with high performance and also in forthcoming storage class memory applications, too.

Low voltage programming of multi-level-cell phase change random access memory (MLC PRAM) is important for future low power high density applications of PRAM devices [1]. We have characterized voltage dependent resistance switching characteristics of PRAM devices with various voltage pulses, based on which a dual-pulse programming method is systematically proposed to demonstrate some principles of low voltage MLC programming. A microstructural model is also introduced to provide a comprehensive explanation of resistance swiching for various voltage pulses.

The failure mechanism of reversibility in the Ge2Sb2Te5 (GST) film was analyzed microscopically based on transmission electron microscopy and ab initio density functional theory (DFT). In this study, the crystallization region was limited to the range of 500-600 degrees C under fast ramping rates, enabling the GST amorphous phase to approach a supercooled region above the glass transition temperature. The densification accompanying phase transformation under fast ramping rates induces two deformation behaviors: phase separation and void formation. In the disorder-order transition, the disordered domain of GST is dominated by low viscosity while approaching the supercooled region, which induces phase separation to compensate for the densification. However, coexisting cubic and hexagonal phases show void formation around the interfaces with some phase separation. The DFT calculation shows that the polymorphic transition, a fast, martensitic transformation between the cubic and hexagonal phases, induces a vacancy cluster at the interface. In the tensile stress state, void growth can be easily driven from the vacancy cluster as the nuclei for compensating for densification.

Needs for the performance improvement of memory subsystem in big data and clouding computing era begin to open new markets for emerging memories such as phase change memory, spin-torque-transfer magnetic memory, and metal oxide memory. To fulfill these needs, a cost-effective and high-speed phase change memory cell scheme was introduced at 19nm technology node, which is directly scalable down to 1y or 1z nm nodes and can be extendable to stacked array for higher density. Here, key technologies such as self-aligned cell patterning and vertical poly-Si diode switch on metal word line were adopted. In addition, damascene Ge-Sb-Te technologies were optimized to improve programming speed and to show excellent cell performances.

Based on field induced atomic migration, one of the cycling endurance failure mechanisms in phase change memories, we propose an electrical treatment method to reduce atomic migration. The electric treatment is performed by applying compensation voltage after set or reset voltage applications, and we have a substantial enhancement of cycling endurance characteristics. The polarity of the compensation voltage is negative with respect to the programming voltages. To further investigate the effect of the electrical treatment, we introduced a parameter named melting voltage, which steadily increases as the number of set/reset cycling increases. This observation is consistent with the increase of resistance at the reset voltage, which supports void formation in the active region of Ge2Sb2Te5 with cycling stress.