Thickness-Dependent Crystallization of Ultrathin Antimony Thin Films for Monatomic Multilevel Reflectance and Phase Change Memory Designs

[Image: see text] Phase change materials, with more than one reflectance and resistance states, have been a subject of interest in the fields of phase change memories and nanophotonics. Although most current research focuses on rather complex phase change alloys, e.g., Ge2Sb2Te5, recently, monatomic antimony thin films have aroused a lot of interest. One prominent attractive feature is its simplicity, giving fewer reliability issues like segregation and phase separation. However, phase transformation and crystallization properties of ultrathin Sb thin films must be understood to fully incorporate them into future memory and nanophotonics devices. Here, we studied the thickness-dependent crystallization behavior of pulsed laser-deposited ultrathin Sb thin films by employing dynamic ellipsometry. We show that the crystallization temperature and phase transformation speed of as-deposited amorphous Sb thin films are thickness-dependent and can be precisely tuned by controlling the film thickness. Thus, crystallization temperature tuning by thickness can be applied to future memory and nanophotonic devices. As a proof of principle, we designed a heterostructure device with three Sb layers of varying thicknesses with distinct crystallization temperatures. Measurements and simulation results show that it is possible to address these layers individually and produce distinct and multiple reflectance profiles in a single device. In addition, we show that the immiscible nature of Sb and GaSb could open up possible heterostructure device designs with high stability after melt-quench and increased crystallization temperature. Our results demonstrate that the thickness-dependent phase transformation and crystallization dynamics of ultrathin Sb thin films have attractive features for future memory and nanophotonic devices

Chalcogenides by Design:Functionality through Metavalent Bonding and Confinement

A unified picture of different application areas for incipient metals is presented. This unconventional material class includes several main‐group chalcogenides, such as GeTe, PbTe, Sb2Te3, Bi2Se3, AgSbTe2 and Ge2Sb2Te5. These compounds and related materials show a unique portfolio of physical properties. A novel map is discussed, which helps to explain these properties and separates the different fundamental bonding mechanisms (e.g., ionic, metallic, and covalent). The map also provides evidence for an unconventional, new bonding mechanism, coined metavalent bonding (MVB). Incipient metals, employing this bonding mechanism, also show a special bond breaking mechanism. MVB differs considerably from resonant bonding encountered in benzene or graphite. The concept of MVB is employed to explain the unique properties of materials utilizing it. Then, the link is made from fundamental insights to application‐relevant properties, crucial for the use of these materials as thermoelectrics, phase change materials, topological insulators or as active photonic components. The close relationship of the materials' properties and their application potential provides optimization schemes for different applications. Finally, evidence will be presented that for metavalently bonded materials interesting effects arise in reduced dimensions. In particular, the consequences for the crystallization kinetics of thin films and nanoparticles will be discussed in detail

High Resolution Imaging of Chalcogenide Superlattices for Data Storage Applications:Progress and Prospects

Phase-change materials (PCMs) based on Ge–Sb–Te alloys are a strong contender for next-generation memory technology. Recently, PCMs in the form of GeTe–Sb 2 Te 3 superlattices (CSLs) have shown superior performance compared to ordinary PCM memory, which relies on switching between amorphous and crystalline phases. Although detailed atomic structure switching models have been developed with the help of ab-initio simulations, there is still fierce scientific debate concerning the experimental verification of the actual crystal structures pertaining to the two CSL memory states. One of the strongest techniques to provide this information is (scanning) transmission electron microscopy ((S)TEM). The present article reviews the analyses of CSLs using TEM-based techniques published during the last seven years since the seminal 2011 Nature Nanotechnology paper of Simpson et al., showing the superior performance of the CSL memory. It is critically reviewed what relevant information can be extracted from the (S)TEM results, also showing the impressive progress that has been achieved in a relatively short time frame. Finally, an outlook is given including several open questions. Although debate on actual switching mechanism in CSL memory is clearly not settled, still there is consensus in this field that CSL research has a bright future

Resolving hydrogen atoms at metal-metal hydride interfaces

Hydrogen as a fuel can be stored safely with high volumetric density in metals. It can, however, also be detrimental to metals causing embrittlement. Understanding fundamental behavior of hydrogen at atomic scale is key to improve the properties of metal-metal hydride systems. However, currently, there is no robust technique capable of visualizing hydrogen atoms. Here, we demonstrate that hydrogen atoms can be imaged unprecedentedly with integrated differential phase contrast, a recently developed technique performed in a scanning transmission electron microscope. Images of the titanium-titanium monohydride interface reveal remarkable stability of the hydride phase, originating from the interplay between compressive stress and interfacial coherence. We also uncovered, thirty years after three models were proposed, which one describes the position of the hydrogen atoms with respect to the interface. Our work enables novel research on hydrides and is extendable to all materials containing light and heavy elements, including oxides, nitrides, carbides and borides

Van der Waals Epitaxy of Pulsed Laser Deposited Antimony Thin Films on Lattice-matched and Amorphous Substrates

Monatomic antimony thin films have recently attracted attention for applications in phase change memory, nanophotonics, and 2D materials. Although some promising results have been reported, the true potential of Sb thin films is still hindered by the scalability issue and the lack of reliable bottom-up production. Here we demonstrate the growth of Sb thin films on a lattice-matching and amorphous substrates using pulsed laser deposition (PLD). C-axis out-of-plane textured Sb thin films were successfully deposited on Sb2Te3 and SiO2/Si3N4 substrates. In the case of growth on Sb2Te3, we show that an intermediate phase is formed at the Sb2Te3-Sb interface playing a crucial role in forming a solid coupling and thus maintaining epitaxy leading to the production of high-quality Sb thin films. A 3 - 4 nm amorphous Sb seed layer was used to induce texture and suitable surface termination for the growth of Sb thin films on amorphous substrates. The deposition parameters were fine-tuned, and the growth was monitored in situ by a Reflective High Energy Electron Diffraction (RHEED). Scanning/Transmission Electron Microscopy (S/TEM) unveiled the local structure of produced films showing the formation of ꞵ-phase Sb thin films. Our results demonstrate the feasibility to produce very smooth high-quality antimony thin films with uniform coverage, from few layers to large thicknesses, using pulsed laser deposition. We believe the results of our work on scalable and controllable Sb growth have the potential to open up research on phase-change materials and optoelectronics research

Radiation damage and defect dynamics in 2D WS₂:A low-voltage scanning transmission electron microscopy study

Modern low-voltage scanning transmission electron microscopes (STEMs) have been invaluable for the atomic scale characterization of two-dimensional (2D) materials. Nevertheless, the observation of intrinsic structures of semiconducting and insulating 2D materials with 60 kV-microscopes has remained problematic due to electron radiation damage. In recent years, ultralow-voltage microscopes have been developed with the prospects of minimizing radiation damage of such 2D materials, however, to date only ultralow-voltage TEM investigations of semiconducting and insulating 2D materials have been reported, but similar results using STEM, despite being more widely adopted, are still missing. Here we report a quantitative analysis of radiation damage and beam-induced defect dynamics in semiconducting 2D WS2 during 30 kV and 60 kV-STEM imaging, particularly by recording atomic resolution electrostatic potential movies using integrated differential phase contrast to visualize both the light sulfur and heavy tungsten atoms. Our results demonstrate that electron radiation damage of 2D WS2 aggravates by a factor of two when halving the electron beam energy from 60 keV to 30 keV, from which we conclude electronic excitation and ionization to be the dominant mechanism inducing defects and damage during low-voltage STEM imaging of semiconducting 2D materials

Polarity-dependent reversible resistance switching in Ge–Sb–Te phase-change thin films

In this paper, we demonstrate reversible resistance switching in a capacitorlike cell using a Ge–Sb–Te film that does not rely on amorphous-crystalline phase change. The polarity of the applied electric field switches the cell resistance between lower- and higher-resistance states, as was observed in current-voltage characteristics. Moreover, voltage pulses less than 1.25 V showed this switching within time scales of microseconds with more than 40% contrast between the resistance states. The latter are found to be nonvolatile for months. The switching could also be achieved at nanoscales with atomic force microscopy with a better resistance contrast of three orders of magnitude.

L1(0) ordering and magnetic interactions in FePt nanoparticles embedded in MgO and SiO2 shell matrices

FePt nanoparticles have been encapsulated in insulating and protective MgO shells, using a two step chemical process, in order to prevent sintering during the heat-treatment process required for the L1(0) ordering. The FePt nanoparticles were initially prepared using a standard polyol process and then dispersed in a magnesium oxide solution. As a basis for comparison FePt/SiO2 nanocomposites have been also synthesized using a modified aqueous sol-gel route as the second step. The magnetic and microstructural properties of FePt/MgO and FePt/SiO2 nanocomposites are compared with those of FePt nanoparticles. The presence of oxide matrices leads to more homogeneous microstructures and better magnetic properties. While higher coercivity values have been obtained in FePt/SiO2, the MgO matrix is proven to provide better physical and magnetic isolations of the FePt nanoparticles. However, for FePt:MgO molar ratios exceeding 1:20 no L1(0) ordering has been achieved

Additive manufacturing of interstitial-strengthened high entropy alloy: Scanning strategy dependent anisotropic mechanical properties

A non-equiatomic interstitial-strengthened high entropy alloy (iHEA), Fe49.5Mn30Co10Cr10C0.5 (at.%), is manufactured by laser powder-bed fusion (LPBF) with stripe and chessboard scanning strategies. The present study highlights the correlation between the laser scanning strategies with resulting microstructure, textures, and anisotropic mechanical properties in as-built iHEA. The results show that the LPBF processed iHEA exhibits an excellent strength-ductility synergy due to the combined deformation mechanisms of dislocation slip, martensite phase transformation- and nano twinning-induced plasticity. The samples printed by the stripe scanning strategy show more evident mechanical anisotropy than that of the chessboard-scanned samples. The difference in the degree of mechanical anisotropy is mainly attributed to the heterogeneous grain morphology and crystallographic texture resulted from different scanning strategies