Phase-change memory picks up speed

If you’ve ever watched a movie on Blu-ray Disc or listened to a rewritable CD, you’ve used phase-change memory, a system of digital storage that writes and erases bits by switching an alloy material between crystalline and amorphous-solid states. With its exceptional stability, durability, and scalability, phase-change memory is a leading contender to replace capacitor-based random-access memory (RAM) as the workhorse memory of next-generation computers. And unlike conventional RAM, phase-change alloys retain their logic states even if there’s no power supply.

But if phase-change memory is to be used for computationally laborious cache operations, it will need to be faster. Computers typically perform those operations in fractions of a nanosecond; writing a bit to phase-change memory currently takes tens or hundreds of nanoseconds.

Now a team from Xi’an Jiaotong University and the Shanghai Institute of Microsystem and Information Technology, both in China, has closed that time gap. The key was to speed up the amorphous-to-crystalline transition, the bottleneck in the write process. To that end, the team doped an ordinary phase-change material—a blend of antimony and tellurium—with scandium. The Sc atoms bind strongly with Te, at just the right angles and distances to stabilize four-atom rings that can serve as building blocks for larger crystals. (The accompanying illustration, from a simulation, shows a few such rings amid an otherwise disordered jumble of atoms; Sc, Sb, and Te are red, yellow, and blue, respectively.) With those building blocks at the ready, the alloy transitions from amorphous to crystalline quickly enough for a bit to be written in just 700 ps. Importantly, the Sc-doped bit is just as stable as its undoped counterpart: Even at 85 °C, it can faithfully store a logic state for more than a decade. (F. Rao et al., Science, in press, doi:10.1126/science.aao3212 .)