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The disorder‐to‐order (crystallization) process in phase‐change materials determines the speed and storage polymorphism of phase‐change memory devices. Only by clarifying the fine‐structure variation can the devices be insightfully designed, and encode and store information. As essential phase‐change parent materials, the crystallized Sb–Te binary system is generally considered to have the cationic/anionic site occupied by Sb/Te atoms. Here, direct atomic identification and simulation demonstrate that the ultrafast crystallization speed of Sb–Te materials is due to the random nature of lattice site occupation by different classes of atoms with the resulting octahedral motifs having high similarity to the amorphous state. It is further proved that after atomic ordering with disordered chemical occupation, chemical ordering takes place, which results in different storage states with different resistance values. These new insights into the complicated route from disorder to order will play an essential role in designing neuromorphic devices with varying polymorphisms. [ABSTRACT FROM AUTHOR]

Reversible switching of room‐temperature electrical resistivity due to crystal‐amorphous transition is demonstrated in various chalcogenides for development of non‐volatile phase change memory. However, such reversible thermal switching of room‐temperature electrical resistivity has not reported in transition metal oxides so far, despite their enormous studies on the electrical conduction like metal‐insulator transition and colossal magnetoresistance effect. In this study, a thermally reversible switching of room‐temperature electrical resistivity is reported with gigantic variation in a layered nickelate Sr2.5Bi0.5NiO5 (1201‐SBNO) composed of (Sr1.5Bi0.5)O2 rock‐salt and SrNiO3 perovskite layers via unique crystalline phase changes between the conducting 1201‐SBNO with ordered (O‐1201), disordered Sr/Bi arrangements in the (Sr1.5Bi0.5)O2 layer (D‐1201), and insulating oxygen‐deficient double perovskite Sr2BiNiO4.5 (d‐perovskite). The O‐1201 is reentrant by high‐temperature annealing of ≈1000 °C through crystalline phase change into the D‐1201 and d‐perovskite, resulting in the thermally reversible switching of room‐temperature electrical resistivity with 102‐ and 109‐fold variation, respectively. The 1201‐SBNO is the first oxide to show the thermally reversible switching of room‐temperature electrical resistivity via the crystalline phase changes, providing a new perspective on the electrical conduction for transition metal oxides. [ABSTRACT FROM AUTHOR]

Phase‐change random‐access memory (PCRAM) devices suffer from pronounced resistance drift originating from considerable structural relaxation of phase‐change materials (PCMs), which hinders current developments of high‐capacity memory and high‐parallelism computing that both need reliable multibit programming. This work realizes that compositional simplification and geometrical miniaturization of traditional GeSbTe‐like PCMs are feasible routes to suppress relaxation. While to date, the aging mechanisms of the simplest PCM, Sb, at nanoscale, have not yet been unveiled. Here, this work demonstrates that in an optimal thickness of only 4 nm, the thin Sb film can enable a precise multilevel programming with ultralow resistance drift coefficients, in a regime of ≈10−4–10−3. This advancement is mainly owed to the slightly changed Peierls distortion in Sb and the less‐distorted octahedral‐like atomic configurations across the Sb/SiO2 interfaces. This work highlights a new indispensable approach, interfacial regulation of nanoscale PCMs, for pursuing ultimately reliable resistance control in aggressively‐miniaturized PCRAM devices, to boost the storage and computing efficiencies substantially. [ABSTRACT FROM AUTHOR]

Metavalent bonding (MVB) is characterized by the competition between electron delocalization as in metallic bonding and electron localization as in covalent or ionic bonding, serving as an essential ingredient in phase‐change materials for advanced memory applications. The crystalline phase‐change materials exhibits MVB, which stems from the highly aligned p orbitals and results in large dielectric constants. Breaking the alignment of these chemical bonds leads to a drastic reduction in dielectric constants. In this work, it is clarified how MVB develops across the so‐called van der Waals‐like gaps in layered Sb2Te3 and Ge–Sb–Te alloys, where coupling of p orbitals is significantly reduced. A type of extended defect involving such gaps in thin films of trigonal Sb2Te3 is identified by atomic imaging experiments and ab initio simulations. It is shown that this defect has an impact on the structural and optical properties, which is consistent with the presence of non‐negligible electron sharing in the gaps. Furthermore, the degree of MVB across the gaps is tailored by applying uniaxial strain, which results in a large variation of dielectric function and reflectivity in the trigonal phase. At last, design strategies are provided for applications utilizing the trigonal phase. [ABSTRACT FROM AUTHOR]

Phase‐change semiconductor is one of the best candidates for designing nonvolatile memory, but it has never been realized in organic semiconductors until now. Here, a phase‐changeable and high‐mobility organic semiconductor (3,6‐DATT) is first synthesized. Benefiting from the introduction of electrostatic hydrogen bond (S···H), the molecular conformation of 3,6‐DATT crystals can be reversibly modulated by the electric field and ultraviolet irradiation. Through experimental and theoretical verification, the tiny difference in molecular conformation leads to crystalline polymorphisms and dramatically distinct charge transport properties, based on which a high‐performance organic phase‐change memory transistor (OPCMT) is constructed. The OPCMT exhibits a quick programming/erasing rate (about 3 s), long retention time (more than 2 h), and large memory window (i.e., large threshold voltage shift over 30 V). This work presents a new molecule design concept for organic semiconductors with reversible molecular conformation transition and opens a novel avenue for memory devices and other functional applications. [ABSTRACT FROM AUTHOR]

Nonvolatile phase‐change random access memory (PCRAM) is regarded as one of the promising candidates for emerging mass storage in the era of Big Data. However, relatively high programming energy hurdles the further reduction of power consumption in PCRAM. Utilizing narrow edge‐contact of graphene can effectively reduce the active volume of phase change material in each cell, and therefore realize low‐power operation. Here, it demonstrates that the power consumption can be reduced to ≈53.7 fJ in a cell with ≈3 nm‐wide graphene nanoribbon (GNR) as edge‐contact, whose cross‐sectional area is only ≈1 nm2. It is found that the polarity of the bias pulse determines its cycle endurance in the asymmetric structure. If a positive bias is applied to the graphene electrode, the endurance can be extended at least one order longer than the case with a reversal of polarity. In addition, the introduction of the hexagonal boron nitride (h‐BN) multilayer leads to a low resistance drift and a high programming speed in a memory cell. The work represents a great technological advance for the low‐power PCRAM and can benefit in‐memory computing in the future. [ABSTRACT FROM AUTHOR]

One central task of developing nonvolatile phase change memory (PCM) is to improve its scalability for high‐density data integration. In this work, by first‐principles molecular dynamics, to date the thinnest PCM material possible (0.8 nm), namely, a monolayer Sb2Te3, is proposed. Importantly, its SET (crystallization) process is a fast one‐step transition from amorphous to hexagonal phase without the usual intermediate cubic phase. An increased spatial localization of electrons due to geometrical confinement is found to be beneficial for keeping the data nonvolatile in the amorphous phase at the 2D limit. The substrate and superstrate can be utilized to control the phase change behavior: e.g., with passivated SiO2 (001) surfaces or hexagonal Boron Nitride, the monolayer Sb2Te3 can reach SET recrystallization in 0.54 ns or even as fast as 0.12 ns, but with unpassivated SiO2 (001), this would not be possible. Besides, working with small volume PCM materials is also a natural way to lower power consumption. Therefore, the proposed PCM working process at the 2D limit will be an important potential strategy of scaling the current PCM materials for ultrahigh‐density data storage. [ABSTRACT FROM AUTHOR]

One central task of developing nonvolatile phase change memory (PCM) is to improve its scalability for high‐density data integration. In this work, by first‐principles molecular dynamics, to date the thinnest PCM material possible (0.8 nm), namely, a monolayer Sb2Te3, is proposed. Importantly, its SET (crystallization) process is a fast one‐step transition from amorphous to hexagonal phase without the usual intermediate cubic phase. An increased spatial localization of electrons due to geometrical confinement is found to be beneficial for keeping the data nonvolatile in the amorphous phase at the 2D limit. The substrate and superstrate can be utilized to control the phase change behavior: e.g., with passivated SiO2 (001) surfaces or hexagonal Boron Nitride, the monolayer Sb2Te3 can reach SET recrystallization in 0.54 ns or even as fast as 0.12 ns, but with unpassivated SiO2 (001), this would not be possible. Besides, working with small volume PCM materials is also a natural way to lower power consumption. Therefore, the proposed PCM working process at the 2D limit will be an important potential strategy of scaling the current PCM materials for ultrahigh‐density data storage. [ABSTRACT FROM AUTHOR]

B Metavalent Bonding b In article number 2300901, Wei Zhang, Matthias Wuttig, and co-workers elucidate how metavalent bonding (MVB) develops across the so called van der Waals (vdW)-like gaps in layered Sb2Te3 and Ge-Sb-Te alloys. Metavalent Bonding in Layered Phase-Change Memory Materials (Adv. Sci. 15/2023) By tailoring the degree of MVB across the gaps via uniaxial strain, a large variation of optical reflectivity can be obtained, enabling potential photonic applications. [Extracted from the article]

B Phase Change Cells b In article number 2202222 by Haomin Wang, Zhitang Song, and co-workers, edge of graphene ribbons is used to address phase change materials. Minimizing the Programming Power of Phase Change Memory by Using Graphene Nanoribbon Edge-Contact (Adv. Sci. 25/2022) The highly scaled memory cells approach the extreme scaling limits of phase change materials technology, achieving ultralow power consumption, high programming speeds, outstanding low/high resistance ratios, and nice stability/endurance at a few-nanometer bit dimensions. [Extracted from the article]