Researchers have introduced a novel technique to alter the crystal structure of semiconductors using significantly less power, potentially one billion times lower power density. This breakthrough could expand the use of phase-change memory (PCM), a cutting-edge memory technology that has the potential to revolutionize data storage in devices ranging from smartphones to computers.
Atoms in amorphous solids, like glass, lack a defined structure and are arranged randomly, similar to grains of sand scattered on a beach. Typically, the process of making materials amorphous, termed amorphization, demands high energy levels. The prevalent method is the melt-quench process, where a material is heated until it becomes liquid, and then swiftly cooled to prevent the atoms from organizing into a crystal structure.
Researchers from the University of Pennsylvania’s School of Engineering and Applied Science, the Indian Institute of Science, and the Massachusetts Institute of Technology have devised a new technique for amorphizing indium selenide wires. This approach uses up to one billion times less power density, as outlined in a recent article in Nature. This significant progress could broaden the application of phase-change memory (PCM), a promising technology that might transform data storage on everything from phones to computers.
In phase-change memory, data is stored by toggling the material between amorphous and crystalline states, functioning akin to a switch. However, the high energy requirements for these transformations have hindered large-scale commercial use. “One of the key reasons phase-change memory devices haven’t become widely adopted is the energy they consume,” stated Ritesh Agarwal, a distinguished scholar and professor in Materials Science and Engineering at Penn Engineering, who is also a co-author of the study.
Agarwal’s team has explored alternatives to the melt-quench method for over ten years, building on their 2012 discovery that electrical pulses can amorphize certain alloys without the need for melting.
Years later, Gaurav Modi, one of the lead authors and a former doctoral student, began working with indium selenide, a semiconductor known for its unique characteristics: it’s ferroelectric, meaning it can spontaneously polarize, and piezoelectric, meaning mechanical stress causes it to generate electric power.
Modi stumbled upon the new approach unintentionally. While running a current through In2Se3 wires, the electricity stopped flowing unexpectedly. A closer look revealed that significant portions of the wires had turned amorphous. “This was quite surprising,” said Modi. “I even thought I might have damaged the wires. Typically, electrical pulses are needed to induce amorphization, but here, a continuous current had altered the crystalline structure, which was unexpected.”
The process of understanding this unusual phenomenon took nearly three years. Agarwal sent samples of the wires to Pavan Nukala, a former graduate student who is now an Assistant Professor at IISc and another senior author of the paper. “Over recent years, we’ve developed a variety of in situ microscopy tools at IISc. It was time to utilize them — we needed to examine this process very carefully,” Nukala noted. “We discovered that different properties of In2Se3 — including its two-dimensional nature, ferroelectricity, and piezoelectricity — integrated to create this ultra-low energy pathway for amorphization through shocks.”
Ultimately, the researchers found the process resembles an avalanche or an earthquake. Initially, tiny segments within the In2Se3 wires start to amorphize as the electric current deforms them. Thanks to the piezoelectric properties and layered structure of the wires, the current pushes parts of these layers into unstable configurations, similar to the minor shifts of snow atop a mountain.
Once a critical threshold is reached, this movement triggers a swift propagation of deformation throughout the wire. The distorted segments collide, generating a sound wave that travels through the material, akin to seismic waves permeating the earth’s crust during an earthquake.
This sound wave, referred to as an “acoustic jerk,” stimulates additional deformation, merging numerous small amorphous zones into a larger one measuring in micrometers — thousands of times bigger than the original areas — similar to an avalanche gaining momentum rolling down a slope. “It’s incredible to see all these phenomena interacting across various scales simultaneously,” remarked Shubham Parate, a doctoral student at IISc and co-first author of the study.
The collaborative investigation has paved the way for future discoveries. “This creates a new area of research on structural transformations in materials when diverse properties align. The potential implications for designing low-power memory devices are immense,” Agarwal remarked.
The research was conducted at the University of Pennsylvania School of Engineering and Applied Science, the Indian Institute of Science, and the Massachusetts Institute of Technology, supported by various agencies including the U.S. Office of Naval Research, the U.S. National Science Foundation, and others, along with facilities at CeNSE and the Advanced Facility for Microscopy and Microanalysis (AFMM) at IISc.
