In situ observations of the reversible vacancy ordering process in van der Waals-bonded Ge-Sb-Te thin films and GeTe-Sb2Te3 superlattices

In situ characterization of vacancy ordering in Ge-Sb-Te phase-change memory alloys

Tailoring the degree of structural disorder in Ge-Sb-Te alloys is important for the development of non-volatile phase-change memory and neuro-inspired computing. Upon crystallization from the amorphous phase, these alloys form a cubic rocksalt-like structure with a high content of intrinsic vacancies. Further thermal annealing results in a gradual structural transition towards a layered structure and an insulator-to-metal transition. In this work, we elucidate the atomic-level details of the structural transition in crystalline GeSb2Te4 by in situ high-resolution transmission electron microscopy experiments and ab initio density functional theory calculations, providing a comprehensive real-time and real-space view of the vacancy ordering process. We also discuss the impact of vacancy ordering on altering the electronic and optical properties of GeSb2Te4, which is relevant to multilevel storage applications. The phase evolution paths in Ge-Sb-Te alloys and Sb2Te3 are illustrated using a summary diagram, which serves as a guide for designing phase-change memory devices.

Reversible Amorphous-Crystalline Phase Transformation in an Ultrathin van der Waals FeTe System

Searching for new phase-change materials for memory and neuromorphic device applications and further understanding the phase transformation mechanism are attracting wide attention. Phase transformation from the amorphous phase to the crystal phase has been unraveled in iron telluride (FeTe) bulk film deposited by pulsed laser deposition, recently. However, the van der Waals-layered feature of FeTe in the crystal form was not noted, which will benefit the scaling of the memory devices and shine light on phase-change heterostructures or interfacial phase-change materials. Moreover, the demonstration of advanced memory or neuromorphic device applications is lacking. Here, we investigate the phase transformation of FeTe starting from mechanically exfoliated van der Waals layers from a bulk single crystal. Surficial amorphization is revealed at the surface layers of FeTe flakes after exfoliation under ambient conditions, which could be transformed back to the crystalline phase with laser irradiation or heating. The conductance drop of the flake devices near 400 K verifies the phase transformation electrically. Memristor behavior of the amorphous surface in FeTe has been further demonstrated, proving the reversibility of the phase transformation and shining light on the possible applications of neuromorphic devices.

Advances in Versatile GeTe Thermoelectrics from Materials to Devices

Driven by the intensive efforts in the development of high-performance GeTe thermoelectrics for mass-market application in power generation and refrigeration, GeTe-based materials display a high figure of merit of >2.0 and an energy conversion efficiency beyond 10%. However, a comprehensive review on GeTe, from fundamentals to devices, is still needed. In this regard, the latest progress on the state-of-the-art GeTe is timely reviewed. The phase transition, intrinsic high carrier concentration, and multiple band edges of GeTe are fundamentally analyzed from the perspectives of the native atomic orbital, chemical bonding, and lattice defects. Then, the fabrication methods are summarized with a focus on large-scale production. Afterward, the strategies for enhancing electronic transports of GeTe by energy filtering effect, resonance doping, band convergence, and Rashba band splitting, and the methods for strengthening phonon scatterings via nanoprecipitates, planar vacancies, and superlattices, are comprehensively reviewed. Besides, the device assembly and performance are highlighted. In the end, future research directions are concluded and proposed, which enlighten the development of broader thermoelectric materials.

Strain-induced van der Waals gaps in GeTe revealed by in situ nanobeam diffraction

Ordered germanium vacancies in germanium telluride thermoelectric material are called van der Waals (vdW) gaps, and they are beneficial for the thermoelectric performance of the material. The vdW gaps have been observed by atomic resolution scanning transmission electron microscopy, but their origin remains unclear, which prevents their extensive application in other materials systems. Here, we report that the occurrence of vdW gaps in germanium telluride is mainly driven by strain from the cubic-to-rhombohedral martensitic transition. Direct strain and structural evidence are given here by in situ nanobeam diffraction and in situ transmission electron microscopy observation. Dislocation theory is used to discuss the origin of vdW gaps. Our work here paves the way for self-assembling two-dimensional ordered vacancies, which establishes a previously unidentified degree of freedom to adjust their electronic and thermal properties.

Materials Screening for Disorder-Controlled Chalcogenide Crystals for Phase-Change Memory Applications

Tailoring the degree of disorder in chalcogenide phase-change materials (PCMs) plays an essential role in nonvolatile memory devices and neuro-inspired computing. Upon rapid crystallization from the amorphous phase, the flagship Ge-Sb-Te PCMs form metastable rocksalt-like structures with an unconventionally high concentration of vacancies, which results in disordered crystals exhibiting Anderson-insulating transport behavior. Here, ab initio simulations and transport experiments are combined to extend these concepts to the parent compound of Ge-Sb-Te alloys, viz., binary Sb2 Te3 , in the metastable rocksalt-type modification. Then a systematic computational screening over a wide range of homologous, binary and ternary chalcogenides, elucidating the critical factors that affect the stability of the rocksalt structure is carried out. The findings vastly expand the family of disorder-controlled main-group chalcogenides toward many more compositions with a tunable bandgap size for demanding phase-change applications, as well as a varying strength of spin-orbit interaction for the exploration of potential topological Anderson insulators.

A first-principles study of the switching mechanism in GeTe/InSbTe superlattices

Interfacial Phase Change Memories (iPCMs) based on (GeTe)₂/Sb₂Te₃ superlattices have been proposed as an alternative candidate to conventional PCMs for the realization of memory devices with superior switching properties. The switching mechanism was proposed to involve a crystalline-to-crystalline structural transition associated with a rearrangement of the stacking sequence of the GeTe bilayers. Density functional theory (DFT) calculations showed that such rearrangement could be achieved by means of a two-step process with an activation barrier for the flipping of Ge and Te atoms which is sensitive to the biaxial strain acting on GeTe bilayers. Within this picture, strain-engineering of GeTe bilayers in the GeTe-chalcogenide superlattice can be exploited to further improve the iPCM switching performance. In this work, we study GeTe-InSbTe superlattices with different compositions by means of DFT, aiming at exploiting the large mismatch (3.8%) in the in-plane lattice parameter between GeTe and In₃SbTe₂ to reduce the activation barrier for the switching with respect to the (GeTe)₂-Sb₂Te₃ superlattice.

"Stickier"-Surface Sb2Te3 Templates Enable Fast Memory Switching of Phase Change Material GeSb2Te4 with Growth-Dominated Crystallization

Ge-Sb-Te (GST)-based phase-change memory (PCM) excels in the switching performance but remains insufficient of the operating speed to replace cache memory (the fastest memory in a computer). In this work, a novel approach using Sb₂Te₃ templates is proposed to boost the crystallization speed of GST by five times faster. This is because such a GST/Sb₂Te₃ heterostructure changes the crystallizing mode of GST from the nucleation-dominated to the faster growth-dominated one, as confirmed by high-resolution transmission electron microscopy, which captures the interface-induced epitaxial growth of GST on Sb₂Te₃ templates in devices. Ab initio molecular dynamic simulations reveal that Sb₂Te₃ templates can render GST sublayers faster crystallization speed because Sb₂Te₃'s "sticky" surface contains lots of unpaired electrons that may attract Ge atoms with less antibonding interactions. Our work not only proposes a template-assisted PCM with fast speed but also uncovers the hidden mechanism of Sb₂Te₃'s sticky surface, which can be used for future material selection.

Effect of vacancy disorder in phase-change materials

Ge-Sb-Te-based phase-change materials (PCMs) exhibit contrasting electrical and optical properties upon change in atomic structures, which contain the octahedral p -orbital bonding and also substantial disordered vacancies. While extensive studies have been carried out, there is little detailed analysis of how the vacancy distribution and bonding nature are inter-correlated to affect the physical properties. We studied the effect of vacancy distribution on the octahedral p -bonding network in PCMs using a simple tight-binding model and ab initio calculations. We showed that the octahedral p -bonding network can be described as a collection of independent linear chains and that the vacancy disorders are rephrased as a distribution of atomic chain pieces. This finding enables to link the vacancy distribution to various aspects of materials properties such as total energy, structural distortions, and charge localization.

A layered Ge2Sb2Te5 phase change material

In this study, a universal Ge₂Sb₂Te₅ phase change material was sputtered to obtain a layered structure. The crystalline phase of this material was prepared by annealing. SEM (scanning electron microscopy) and HRTEM (high-resolution transmission electron microscopy) images give confirmed that the sputtered Ge₂Sb₂Te₅ thin film in crystalline phase has multiple layers. The layers can be exfoliated by acetone. The thicknesses of acetone-exfoliated crystalline and amorphous flakes are approx. 10-60 nm.

Phase change thin films for non-volatile memory applications

The rapid development of Internet of Things devices requires real time processing of a huge amount of digital data, creating a new demand for computing technology. Phase change memory technology based on chalcogenide phase change materials meets many requirements of the emerging memory applications since it is fast, scalable and non-volatile. In addition, phase change memory offers multilevel data storage and can be applied both in neuro-inspired and all-photonic in-memory computing. Furthermore, phase change alloys represent an outstanding class of functional materials having a tremendous variety of industrially relevant characteristics and exceptional material properties. Many efforts have been devoted to understanding these properties with the particular aim to design universal memory. This paper reviews materials science aspects of chalcogenide-based phase change thin films relevant for non-volatile memory applications. Particular emphasis is put on local structure, control of disorder and its impact on material properties, order-disorder transitions and interfacial transformations.