Suppressing Structural Relaxation in Nanoscale Antimony to Enable Ultralow-Drift Phase-Change Memory Applications

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.

Reversible switching in bicontinuous structure for phase change random access memory application

SiSbTe phase change materials (PCMs) have excellent thermal stabilities. Their properties and microstructures are strongly affected by their Si content. Si3.3Sb2Te3 (SST) gives the best electrical performance, at Si contents of around 40%. Here, use of a combination of an advanced three-dimensional (3D) tomography technique and transmission electron microscopy clearly showed that a crystallized SST film has a uniform equiaxed-structure in 3D space, and consists of a reversible Sb2Te3 (ST) phase and an amorphous (a-) Si phase, which are well nested with each other. The a-Si nest localizes structure switching and diffusion of the host element in the nano-area. The most innovative aspect is significant retention of the metastable face-centered cubic (f-) ST phase, even above 370 °C, in this bicontinuous system. Specifically, the a-Si frame is stable and the ST phase switches between a- and f-structures under external stimulation. This promotes faster SET speed and low-power RESET consumption. Our results give new insights into PCM systems. They suggest that bicontinuous structures are potential candidates for use in phase-change random access memory devices, especially in automotive electronics applications that require a high data retention ability.

Structural and electronic properties of liquid, amorphous, and supercooled liquid phases of In2Te5 from first-principles

In₂Te₅ is a stoichiometric compound in the In-Te system of interest for applications in phase change electronic memories and thermoelectrics. Here, we perform a computational study of the structural, dynamical, and electronic properties of the liquid, supercooled liquid, and amorphous phases of this compound by means of density functional molecular dynamics simulations. Models of the supercooled liquid and amorphous phases have been generated by quenching from the melt. The structure of the liquid phase is characterized by a mixture of defective octahedral and tetrahedral local environments of In atoms, while the amorphous phase displays a mostly tetrahedral local geometry for In atoms with corner and edge sharing tetrahedra similar to those found in the crystalline phases of the In₂Te₅, InTe, and In₂Te₃ compounds. Comparison with our previous results on liquid and amorphous In₂Te₃ and further data on the structural properties of liquid In₂Te₃ are also discussed. The analysis of the electronic properties highlights the opening of a mobility gap in In₂Te₅ at about 150 K below the liquidus temperature.

Understanding the Structure and Properties of Sesqui-Chalcogenides (i.e., V2 VI3 or Pn2 Ch3 (Pn = Pnictogen, Ch = Chalcogen) Compounds) from a Bonding Perspective

A number of sesqui-chalcogenides show remarkable properties, which make them attractive for applications as thermoelectrics, topological insulators, and phase-change materials. To see if these properties can be related to a special bonding mechanism, seven sesqui-chalcogenides (Bi₂ Te₃ , Bi₂ Se₃ , Bi₂ S₃ , Sb₂ Te₃ , Sb₂ Se₃ , Sb₂ S₃ , and β-As₂ Te₃ ) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui-chalcogenides (Bi₂ Te₃ , Bi₂ Se₃ , Sb₂ Te₃ , and β-As₂ Te₃ ) show an unconventional bond-breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal-like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui-chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum-chemistry-based bonding analysis employing charge partitioning, which reveals that the four sesqui-chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.

The low-temperature heat capacity of the Sb2Te3-x Se x solid solution from experiment and theory

The lattice dynamics of Sb₂Te_3-x Se _x (x  =  0, 0.6, 1.2, 1.8, 3) mixed crystals have been studied by a combination of low-temperature heat-capacity measurements between 2-300 K and first-principles calculations. The results from the experimental and theoretical investigations are in excellent agreement. While Sb₂Se₃ can be considered as a harmonic lattice oscillator in this temperature range, for the isostructural compounds Sb₂Te₃, Sb₂Se_0.6Te_2.4, Sb₂Se_1.2Te_1.8 and Sb₂Se_1.8Te_1.2 (tetradymite structure type; R [Formula: see text] m) a small anharmonic contribution to the total heat capacity has to be taken into account at temperatures above 250 K. For the compounds which crystallize in the tetradymite structure type the experimental and theoretical data show unambiguously that the exchange of Te by Se leads to an increase of the bonding polarity and consequently to a hardening of the bonding which is reflected in an increase of the Debye temperatures with increasing Se contents. In addition, our studies clearly demonstrate that the mixed crystals in the stability field of the tetradymite structure type are characterized by a strong non-ideal mixing behavior.