Phase Separation in Ge-Rich GeSbTe at Different Length Scales: Melt-Quenched Bulk versus Annealed Thin Films

Integration of the prototypical GeSbTe (GST) ternary alloys, especially on the GeTe-Sb2Te3 tie-line, into non-volatile memory and nanophotonic devices is a relatively mature field of study. Nevertheless, the search for the next best active material with outstanding properties is still ongoing. This search is relatively crucial for embedded memory applications where the crystallization temperature of the active material has to be higher to surpass the soldering threshold. Increasing the Ge content in the GST alloys seems promising due to the associated higher crystallization temperatures. However, homogeneous Ge-rich GST in the as-deposited condition is thermodynamically unstable, and phase separation upon annealing is unavoidable. This phase separation reduces endurance and is detrimental in fully integrating the alloys into active memory devices. This work investigated the phase separation of Ge-rich GST alloys, specifically Ge5Sb2Te3 or GST523, into multiple (meta)stable phases at different length scales in melt-quenched bulk and annealed thin film. Electron microscopy-based techniques were used in our work for chemical mapping and elemental composition analysis to show the formation of multiple phases. Our results show the formation of alloys such as GST213 and GST324 in all length scales. Furthermore, the alloy compositions and the observed phase separation pathways agree to a large extent with theoretical results from density functional theory calculations

Thick Does the Trick: Genesis of Ferroelectricity in 2D GeTe-Rich (GeTe)m (Sb2 Te3 )n Lamellae

The possibility to engineer (GeTe)(m)(Sb2Te3)n phase-change materials to co-host ferroelectricity is extremely attractive. The combination of these functionalities holds great technological impact, potentially enabling the design of novel multifunctional devices. Here an experimental and theoretical study of epitaxial (GeTe)(m)(Sb2Te3)n with GeTe-rich composition is presented. These layered films feature a tunable distribution of (GeTe)m(Sb2Te3)(1) blocks of different sizes. Breakthrough evidence of ferroelectric displacement in thick (GeTe)m(Sb2Te3)(1) lamellae is provided. The density functional theory calculations suggest the formation of a tilted (GeTe)m slab sandwiched in GeTe-rich blocks. That is, the net ferroelectric polarization is confined almost in-plane, representing an unprecedented case between 2D and bulk ferroelectric materials. The ferroelectric behavior is confirmed by piezoresponse force microscopy and electroresistive measurements. The resilience of the quasi van der Waals character of the films, regardless of their composition, is also demonstrated. Hence, the material developed hereby gathers in a unique 2D platform the phase-change and ferroelectric switching properties, paving the way for the conception of innovative device architectures

Mesenchymal Stem Cells for Regenerative Medicine for Duchenne Muscular Dystrophy

Mesenchymal stem cells (MSCs) are multipotent stem cells that can be isolated from both foetal and adult tissues. Several groups demonstrated that transplantation of MSCs promoted the regeneration of skeletal muscle and ameliorated muscular dystrophy in animal models. Mesenchymal stem cells in skeletal muscle, also known as fibro-adipogenic progenitors (FAPs), are essential for the maintenance of skeletal muscle. Importantly, they contribute to fibrosis and fat accumulation in dystrophic muscle. Therefore, MSCs in muscle are a pharmacological target for the treatment of muscular dystrophies. In this chapter, we briefly update the knowledge on mesenchymal stem/progenitor cells and discuss their therapeutic potential as a regenerative medicine treatment of Duchenne muscular dystrophy

Interface formation during the growth of phase change material heterostructures based on Ge-Rich Ge-Sb-Te alloys

In this study, we present a full characterization of the electronic properties of phase change material (PCM) double-layered heterostructures deposited on silicon substrates. Thin films of amorphous Ge-rich Ge-Sb-Te (GGST) alloys were grown by physical vapor deposition on Sb2Te3 and on Ge2Sb2Te5 layers. The two heterostructures were characterized in situ by X-ray and ultraviolet photoemission spectroscopies (XPS and UPS) during the formation of the interface between the first and the second layer (top GGST film). The evolution of the composition across the heterostructure interface and information on interdiffusion were obtained. We found that, for both cases, the final composition of the GGST layer was close to Ge2SbTe2 (GST212), which is a thermodynamically favorable off-stoichiometry GeSbTe alloy in the Sb-GeTe pseudobinary of the ternary phase diagram. Density functional theory calculations allowed us to calculate the density of states for the valence band of the amorphous phase of GST212, which was in good agreement with the experimental valence bands measured in situ by UPS. The same heterostructures were characterized by X-ray diffraction as a function of the annealing temperature. Differences in the crystallization process are discussed on the basis of the photoemission results

High-Throughput Calculations on the Decomposition Reactions of Off-Stoichiometry GeSbTe Alloys for Embedded Memories

Chalcogenide GeSbTe (GST) alloys are exploited as phase change materials in a variety of applications ranging from electronic non-volatile memories to neuromorphic and photonic devices. In most applications, the prototypical Ge2Sb2Te5 compound along the GeTe-Sb2Te3 pseudobinary line is used. Ge-rich GST alloys, off the pseudobinary tie-line with a crystallization temperature higher than that of Ge2Sb2Te5, are currently explored for embedded phase-change memories of interest for automotive applications. During crystallization, Ge-rich GST alloys undergo a phase separation into pure Ge and less Ge-rich alloys. The detailed mechanisms underlying this transformation are, however, largely unknown. In this work, we performed high-throughput calculations based on Density Functional Theory (DFT) to uncover the most favorable decomposition pathways of Ge-rich GST alloys. The knowledge of the DFT formation energy of all GST alloys in the central part of the Ge-Sb-Te ternary phase diagram allowed us to identify the cubic crystalline phases that are more likely to form during the crystallization of a generic GST alloy. This scheme is exemplified by drawing a decomposition map for alloys on the Ge-Ge1Sb2Te4 tie-line. A map of decomposition propensity is also constructed, which suggests a possible strategy to minimize phase separation by still keeping a high crystallization temperature

Unraveling the crystallization kinetics of the Ge2Sb2Te5 phase change compound with a machine-learned interatomic potential

Abstract The phase change compound Ge2Sb2Te5 (GST225) is exploited in advanced non-volatile electronic memories and in neuromorphic devices which both rely on a fast and reversible transition between the crystalline and amorphous phases induced by Joule heating. The crystallization kinetics of GST225 is a key functional feature for the operation of these devices. We report here on the development of a machine-learned interatomic potential for GST225 that allowed us to perform large scale molecular dynamics simulations (over 10,000 atoms for over 100 ns) to uncover the details of the crystallization kinetics in a wide range of temperatures of interest for the programming of the devices. The potential is obtained by fitting with a deep neural network (NN) scheme a large quantum-mechanical database generated within density functional theory. The availability of a highly efficient and yet highly accurate NN potential opens the possibility to simulate phase change materials at the length and time scales of the real devices

Exploring Optimal Tillage Improved Soil Characteristics and Productivity of Wheat Irrigated with Different Water Qualities

Irrigation with low water quality can adversely affect soil characteristics, optimal moisture for tillage, and crop productivity, particularly in arid and semi-arid regions. We determined the optimal moisture for tillage processing and the effects of optimal and wet tillage on physical and chemical soil characteristics and wheat productivity after irrigation with different water qualities (waste, saline, and highly saline water). We used the Atterberg limit to determine the suitable moisture content for tillage. Tillage at optimal moisture content improved soil characteristics by reducing soil salinity, sodicity, bulk density, shear strength, compaction, and increasing hydraulic conductivity compared to that of wet tillage. It also enhanced growth and productivity of wheat grown with low quality of water (i.e., fresh and waste water), resulting in higher grain yield and root weight at different growth stages than that of saline and highly saline water. In conclusion, tillage at optimal moisture content alleviates the impact of salinity through improving soil physical and chemical characteristics. Optimum tillage can be applied at 20 and 24 days from the previous irrigation in saline and highly saline soils, respectively. Irrigation with waste water resulted in a higher wheat grain yield than saline and highly saline water

Crystallization and Electrical Properties of Ge-Rich GeSbTe Alloys

Enrichment of GeSbTe alloys with germanium has been proposed as a valid approach to increase the crystallization temperature and therefore to address high-temperature applications of non-volatile phase change memories, such as embedded or automotive applications. However, the tendency of Ge-rich GeSbTe alloys to decompose with the segregation of pure Ge still calls for investigations on the basic mechanisms leading to element diffusion and compositional variations. With the purpose of identifying some possible routes to limit the Ge segregation, in this study, we investigate Ge-rich Sb2Te3 and Ge-rich Ge2Sb2Te5 with low (40 at %) amounts of Ge. The formation of the crystalline phases has been followed as a function of annealing temperature by X-ray diffraction. The temperature dependence of electrical properties has been evaluated by in situ resistance measurements upon annealing up to 300 °C. The segregation and decomposition processes have been studied by scanning transmission electron microscopy (STEM) and discussed on the basis of density functional theory calculations. Among the studied compositions, Ge-rich Ge2Sb2Te5 is found to be less prone to decompose with Ge segregation