Stanford-led research shows that an emerging memory technology, based on a new class of semiconductor materials, could deliver the best of both worlds (large and fast memory), storing data permanently while allowing certain operations to occur up to a thousand times faster than today’s memory devices. The new approach may also be more energy efficient.
“This work is fundamental but promising,” said Aaron Lindenberg, an associate professor of materials science and engineering at Stanford and of photon science at the SLAC National Accelerator Laboratory. “A thousandfold increase in speed coupled with lower energy use suggests a path toward future memory technologies that could far outperform anything previously demonstrated.”
Lindenberg led a 19-member team, including researchers at SLAC, who detailed their experiments in Physical Review Letters.
Their findings provide new insights into the experimental technology of phase-change memory.
This animation shows how data is stored using phase-change memory technology. Phase-change materials can exist in two atomic structures, disordered or ordered. An electric jolt flip-flops these structures back and forth to form the zeroes and ones of digital software. (Image credit: Tricia Seibold)
Researchers have developed ways to flip-flop the structural and electronic states of these materials – changing their phase from one to zero and back again – by applying short bursts of heat, supplied electrically or optically.
Phase-change materials are attractive as a memory technology because they retain whichever electronic state conforms to their structure. Once their atoms flip or flop to form a one or a zero, the material stores that data until another energy jolt causes it to change. This ability to retain stored data makes phase-change memory nonvolatile just like the silicon-based flash memory in smartphones.
But permanent storage is only one desired attribute. A next-generation memory technology also needs to perform certain operations faster than today’s chips. By using extremely precise measurements and instrumentation, the researchers sought to demonstrate the speed and energy potential of phase-change technology – and what they found was encouraging.
The Stanford researchers jolted a small sample of amorphous material with an electrical field comparable in strength to a lightning strike. Their instrumentation detected that the amorphous-on state – initiating the flip from zero to one – occurred less than a picosecond after they applied the jolt.
To comprehend the brevity of a picosecond, it’s roughly the time it would take for a beam of light, traveling at 186,000 miles per second, to pass through two pieces of paper.
Many chalcogenide glasses undergo a breakdown in electronic resistance above a critical field strength. Known as threshold switching, this mechanism enables field-induced crystallization in emerging phase-change memory. Purely electronic as well as crystal nucleation assisted models have been employed to explain the electronic breakdown. Here, picosecond electric pulses are used to excite amorphous Ag4In3Sb67Te26. Field-dependent reversible changes in conductivity and pulse-driven crystallization are observed. The present results show that threshold switching can take place within the electric pulse on sub-picosecond time-scales – faster than crystals can nucleate. This supports purely electronic models of threshold switching and reveals potential applications as an ultrafast electronic switch.
They have demonstrated that an amorphous PCM crystallizes under repetitive excitation with single-cycle THz pulses of few picosecond duration and sufficient field strength. We provide experimental evidence for most of the characteristic features of threshold switching: The formation of conducting filaments through highly non-linear conduction mechanisms, a threshold field that is in reasonable agreement with literature values and the reversibility at above threshold, but sub-crystallization conditions. In the initially amorphous device, THz pulses induce TS during the picosecond excitation. TS increases the conductivity and the resulting higher current heats the material locally above the crystallization temperature. Upon repetitive excitation, atomic rearrangements toward the crystalline phase can take place. Due to the thermodynamically irreversible nature of crystallization, these modifications are retained after the material has cooled down by heat transport into the substrate. The observation of picosecond TS is particularly important for ovonic switches, sometimes used as preselectors in memory devices which then allow subpicosecond access to the specific memory cell. In this sense, TS allows the design of ultrafast electronic switches.
SOURCES – Stanford University, Arxiv