Fast phase change with no preconditions: Random access memory (RAM) devices that rely on phase changes are primarily limited by the speed of crystallization. Rao et al. combined theory with a simple set of selection criteria to isolate a scandium-doped antimony telluride (SST) with a subnanosecond crystallization speed (see the Perspective by Akola and Jones). They synthesized SST and constructed a RAM device with a 700-picosecond writing speed. This is an order of magnitude faster than previous phase-change memory devices and competitive with consumer dynamic access, static random access, and flash memory. Science , this issue p. 1423 ; see also p. 1386

Memory devices with high speed and high density are highly desired to address the ‘memory wall’ issue. Here we demonstrated a highly scalable, three-dimensional stackable ferroelectric diode, with its rectifying polarity modulated by the polarization reversal of Hf_0.5Zr_0.5O₂ films. By visualizing the hafnium/zirconium lattice order and oxygen lattice order with atomic-resolution spherical aberration-corrected STEM, we revealed the correlation between the spontaneous polarization of Hf_0.5Zr_0.5O₂ film and the displacement of oxygen atom, thus unambiguously identified the non-centrosymmetric Pca2₁ orthorhombic phase in Hf_0.5Zr_0.5O₂ film. We further implemented this ferroelectric diode in an 8 layers 3D array. Operation speed as high as 20 ns and robust endurance of more than 10⁹ were demonstrated. The built-in nonlinearity of more than 100 guarantees its self-selective property that eliminates the need for external selectors to suppress the leakage current in large array. This work opens up new opportunities for future memory hierarchy evolution.

Although phase change memory technology has developed drastically in the past two decades, the cognition of the key switching materials still ignores an important member, the face-centered cubic Sb₂Te₃. Apart from the well-known equilibrium hexagonal Sb₂Te₃ crystal, we prove the metastable face-centered cubic Sb₂Te₃ phase does exist. Such a metastable crystal contains a large concentration of vacancies randomly occupying the cationic lattice sites. The face-centered cubic to hexagonal phase transformation of Sb₂Te₃, accompanied by vacancy aggregation, occurs at a quite lower temperature compared to that of Ge₂Sb₂Te₅ alloy. We prove that the covalent-like bonds prevail in the metastable Sb₂Te₃ crystal, deviating from the ideal resonant features. If a proper doping technique is adopted, the metastable Sb₂Te₃ phase could be promising for realizing reversibly swift and low-energy phase change memory applications. Our study may offer a new insight into commercialized Ge–Sb–Te systems and help in the design of novel phase change materials to boost the performances of the phase change memory device.

To bridge the gap of access time between memories and storage systems, the concept of storage class memory has been put forward based on emerging nonvolatile memory technologies. For all the nonvolatile memory candidates, the unpleasant tradeoff between operation speed and retention seems to be inevitable. To promote both the write speed and the retention of phase change memory (PCM), Sc doped Ge₂Sb₂Te₅ (SGST) has been proposed as the storage medium. Octahedral Sc-Te motifs, acting as crystallization precursors to shorten the nucleation incubation period, are the possible reason for the high write speed of 6 ns in PCM cells, five-times faster than that of Ge₂Sb₂Te₅ (GST) cells. Meanwhile, an enhanced 10-year data retention of 119 °C has been achieved. Benefiting from both the increased crystalline resistance and the inhibited formation of the hexagonal phase, the SGST cell has a 77% reduction in power consumption compared to the GST cell. Adhesion of the SGST/SiO₂ interface has been strengthened, attributed to the reduced stress by forming smaller grains during crystallization, guaranteeing the reliability of the device. These improvements have made the SGST material a promising candidate for PCM application.

Here, Zr-doped Sb₂Te alloy is proposed for phase change memory (PCM). Zr-doping enhances the crystallization temperature and thermal stability of Sb₂Te alloy effectively. Crystalline Zr₂(Sb₂Te)₉₈ film is manifested as a single phase without phase separation and the growth of crystal grain is dramatically suppressed. The density change of Zr₂(Sb₂Te)₉₈ material between amorphous and crystalline is ∼2.65 ± 0.03%, which is much smaller than that of Ge₂Sb₂Te₅ (6.5%). Phase change memory cells based on Zr₂(Sb₂Te)₉₈ material can be reversibly switched by applying 40–400 ns width voltage pulses, and the reset current is relatively small when comparing with the prototypical Ge₂Sb₂Sb₅ material. The resistance ON-OFF ratio of about 1.3 orders of magnitude is enough for figuring “0” and “1” out. Besides, endurance up to 4.1 × 10⁴ cycles makes Zr-doped Sb₂Te alloy a potential candidate for PCM.