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 Sb2Te3, 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.
Metal–insulator transition (MIT) is one of the most essential topics in condensed matter physics and materials science. The accompanied drastic change in electrical resistance can be exploited in electronic devices, such as data storage and memory technology. It is generally accepted that the underlying mechanism of most MITs is an interplay of electron correlation effects (Mott type) and disorder effects (Anderson type), and to disentangle the two effects is difficult. Recent progress on the crystalline Ge1Sb2Te4 (GST) compound provides compelling evidence for a disorder-driven MIT. In this work, we discuss the presence of strong disorder in GST, and elucidate its effects on electron localization and transport properties. We also show how the degree of disorder in GST can be reduced via thermal annealing, triggering a disorder-driven metal–insulator transition. The resistance switching by disorder tuning in crystalline GST may enable novel multilevel data storage devices.
cutting-edge 3D cross-point memory technology. [20,21] Crystallization of GST at elevated temperatures (500-700 K) occurs via rapid formation and growth of crystallites. In PCRAM cells, the amorphous PCM region is typically surrounded by a crystalline PCM matrix, for example, in the form of a "mushroom head"; therefore, crystal growth at the amorphous-crystalline boundaries is another important ingredient for memory programming.Since full crystallization occurs on the nanosecond time scale for nanometer-sized amorphous marks, the simulation of this process is within reach of ab initio molecular dynamics (AIMD) methods based on density functional theory (DFT). Indeed, quite a few first-principles studies have focused on the crystallization kinetics of Ge 2 Sb 2 Te 5 . [22][23][24][25][26][27][28][29][30][31][32] The system sizes ranged from several tens of atoms up to 900 atoms. In some of these simulations, crystallization was facilitated by constructing 2D crystalline templates, [23] by manually inserting a crystalline seed inside the amorphous network, [26] or by using the metadynamics [33] method. [29,30] These works have shed light on the microscopic mechanisms responsible for the fast crystallization, which proved to be crucial for designing a novel PCM, Sc 0.2 Sb 2 Te 3 , with subnanosecond writing speed. [34][35][36][37][38][39] Furthermore, AIMD simulations indicated that the growth velocity of Ge 2 Sb 2 Te 5 is of the order of a meter per second at ≈600 K, [29,30] in fair agreement with experimental data. [40] The electronic structure of the recrystallized phase is of high interest, since it determines the properties of the SET state of memory cells. The stable crystalline phase of GST is trigonal and consists of layers of Ge, Sb, and Te atoms. [41][42][43] However, upon fast crystallization of amorphous GST, a metastable cubic rocksalt-like phase is obtained: Te atoms occupy one sublattice, whereas Ge, Sb, and vacancies appear to be arranged in a random fashion on the second sublattice. [44][45][46][47][48] AIMD crystallization simulations have corroborated this picture. [24][25][26][27][28][29][30] The large amount of disorder in the rocksalt-like phase has a substantial impact on the electronic states-and thus on transport properties. At room temperature, GST shows relatively high p-type conductivity, due to self-doping. However, low-temperature transport measurements revealed that GST and related chalcogenide crystals are in fact Anderson insulators and that an insulator-metal transition can be induced by thermal annealing. [49][50][51][52] Our previous DFT simulations of GST crystals indicated that vacancy clusters induce localization of the electronic states near the edge of the valence band, and that the ordering of vacancies into layers reduces the total energy of the system and drives both a rocksalt-to-trigonal structural transition and an insulatorto-metal phase transition. [53][54][55] In these computational works, models of disordered GST were created using a quasi-random number generator an...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.