The Nobel-prize-winning wonder material graphene has been broadly touted as a more-than-promising replacement for silicon in VLSI CMOS chips. Interest levels are so high that there are whole conferences dedicated to the 2D wonder material. But, somewhat quizzically, electronic devices that actually use the single-molecule-thick, highly-conducting material have been slow in arriving.

The main problem is that graphene is not a natural semiconductor—and it's very hard to make it behave like one. There have been various attempts to make graphene switch on and off like a regular semiconductor, but so far nothing has really panned out—especially not on a commercial, mass-producible scale.

Carbon is one of the periodic table's most multi-talented elements, however, and graphene is far from the end of the story as far as carbon's uses in microelectronics goes.

Last month, at its Swiss research lab in Rüschlikon on the shores of Lake Zurich, IBM Research revealed two VLSI (very-large-scale integration) breakthroughs with non-graphene carbon-based materials: first, it has cured phase-change memory (PCM) chips of two of their biggest problems, signal drift and noise—perhaps accelerating the arrival of microprocessors that work more like the human brain. Second, it has successfully developed a way to apply electrical contacts to 10-nanometre carbon nanotubes, turning them into efficient transistors for the first time.

Resistance is useful

Phase-change memory is not a new concept. Re-writeable CDs and DVDs use a phase-change alloy called GST (germanium-antimony-tellurium) to store binary zeroes and ones. When heat is applied via the data-writing laser, the GST changes phase from a crystalline state to an amorphous state. These states have different reflectance, allowing data to be read from the disc by another, weaker laser.

Alternatively, instead of using a laser to meddle with a phase-change material, an electric current can be used to switch the state of a PCM cell. "And instead of reflectivity you measure electrical resistance, which is different in the amorphous and crystalline states," explained Evangelos Eleftheriou, one of IBM's top specialists in memory technology.

The electronics industry is not short of ways to store one bit of data in a memory cell, though. The CMOS field effect transistors commonly used in dynamic RAM and non-volatile flash memories have served us very well so far, in terms of scaling, performance, and cost.

PCM, however, offers something altogether more tantalising: the ability to store more bits per cell. This is possible because multiple resistance levels can be stored by changing the phase of different proportions of the phase change material. While two resistance levels allow one bit to be stored, setting four resistance levels stores two bits, and eight levels lets three bits be stored per cell. To the industry, the economics are compelling: it means an ostensible 1 gigabit memory chip would actually store 2 gigabits (at two bits/cell) or 3 gigabits (at three bits/cell). Those are attractive multipliers for anybody making flash SSDs for servers, PCs, mobile phones, and USB drives.

The real target, said IBM, is getting a PCM cell to store multiple resistance levels in a memory that can compete economically with incumbent technologies like NAND flash. "Our focus is this multi-bit functionality," said Eleftheriou.

But it has been proving far from easy. "It might sound trivial to do this, but it is absolutely not trivial because of resistance drift," Eleftheriou explained. "A few microseconds after programming a cell the resistances go all over the place. And that's a serious problem." That's why, so far, they have not been able to make multi-bit PCM commercially available.

IBM has been trying to work out how to keep those resistances rock steady—and using novel memory cell architectures and, more recently, developing carbon-based phase-change materials, they appear to be getting somewhere.

No place like ohm

It all started to look promising one Saturday in December 2013, at Wabe Koelmans' apartment. The IBM Zurich researcher was monitoring an experiment he and colleague Abu Sebastian had left running at the lab on Friday night. Messaging each other as they watched a data acquisition system display lab data on their laptops, it became ever more apparent that the answer to the drift problem was materialising before their eyes.

Over the weekend, the pair ran 60 experiments to confirm that they had, in fact, virtually eliminated resistance drift over time. Their trick? They had refined a design for a memory cell that separates the writing current from the reading current, a move that seems to reduce drift. The technology was published on September 3 in an open access paper in the journal Nature Communications.

IBM Research

IBM Research

The idea, basically, is to surround the phase-change material with titanium nitride (the blue region in the diagram above). Titanium nitride provides a path for the read current to flow around the amorphous region rather than going right through it—an effect which, for reasons of some pretty arcane electrochemistry, keeps the resistance of the cell from drifting. "The titanium nitride is slightly conductive but not too much," explained Koelmans. "It stabilises our resistances. And the measurement noise is much lower too."

But the resistance drift and noise performance could be better still, Koelmans said. Now the research group's focus is shifting towards carbon as the basis of a new type of high-speed phase-change material: "There are more materials that switch between different resistance states and one we found interesting in our research is carbon because it is quite cheap and can be very fast at switching," he explained.

After bit of trial and error they hit on the idea of adding oxygen molecules to carbon to make a large polymer-like molecule that forms gangly, nanoscale filaments when different currents are passed through it. "We started with amorphous carbon, oxygenated it, and took advantage of its formation and disruption of these graphitic filaments to change resistance values," explained Eleftheriou. "It's a completely different method of phase switching."

Even at this early stage, the phase-changing oxygenated carbon is already able to switch states in 10 nanoseconds and cycle through 10,000 write cycles, the researchers report in a research paper published on October 23 in Nature Communications.

When the nascent technology is refined further it should be able to handle millions of write cycles. "PCM is 100 times faster than [NAND] flash. Consumer grade flash does about 3,000 cycles, and enterprise-grade flash 30,000 cycles but PCM should sustain 10 million," said IBM engineer Nikolaos Papandreou.

Perhaps most importantly, the carbon-based phase-change material can be easily laid down with existing CMOS processes, too, through regular vapour deposition.

In May, IBM completed a three-year PCM development program with memory maker SK Hynix of South Korea—though it remains to be seen when, or if commercialised products will actually appear. "IBM has demonstrated that PCM is technically feasible," explained Eleftheriou. "Now the manufacturers need to determine if they can produce it cost competitively compared to flash."

Neuromorphism

Whatever happens in that commodity memory space, however, there is a far more exciting application of phase-change memory waiting in the wings: neuromorphic computer chips, which process data in a much more brain-like way. The idea is that PCM's multiple resistance values strongly mirror the way signal "weights" build up on a biological synapse—one of many inputs to a mammalian neuron. As the weight of evidence for a logical condition builds up on a collection of synapses they can reach a point where the neuron "fires" a voltage, called an action potential.

Paul Marks

Paul Marks

Paul Marks



Papandreou and his colleagues at IBM Research in the US have built a neuromorphic development computer, into which a variety of multi-bit PCM chips—varying in capacity from just 100 multi-bit cells to 4 million—can be plugged.

Having a flexible development platform allows researchers to focus on optimising their neuromorphic algorithms. But with some of the circuitry implemented in FPGAs—field-programmable gate arrays; software-reconfigurable logic circuits—even the hardware can be modified if need be, explained Papandreou. "It lets us work out how long the device holds the resistance levels in the face of temperature changes. So we can optimise our readout schemes so that they are resilient to any temperature variations."

In one of the IBM Zurich labs, Angeliki Pantazi is using the neuromorphic processor to divine images hidden in a sea of noise—without any programming at all. As I watch, a chaotic field of white dots on a blue background play out on a screen. No image is discernible, at least not to my eyes. On a neuromorphic output screen, however, we can see the pixels that have been activated more than once and latched resistances (neural weights) in two neurons and 40,000 synapses modelled in the processor. And out of that visual noisefield comes—surprise, surprise!—the IBM logo. "There is no computation happening. The pixel data is just getting into the synapses and the system is recognising the pattern by itself," Pantazi said.

The way that logo appeared, plucked from noise, and sans programming, was both magical and the sign of something powerful in the making, almost like something from science fiction. Indeed, it reminded me of the kind of nascent device a robot from the future might travel back in time to destroy, Terminator 2 style. In that movie, the robot's tried to destroy a chip that ultimately allowed an artificial general intelligence, Skynet, to dominate humanity... but I digress.

Back in the real world, IBM's aim is to create brainlike pattern-spotting computers able to better identify malignant cancer patterns in MRI and CT scans, or the glimmer of exploding suns in a super-dim starfield—and make intelligent human-like decisions when it does. In short, it sounds a bit like IBM is aiming for something like Watson, but faster and at far lower power consumption.

And it's not only happening in Zurich—in the US, IBM has developed brain-like computing tech alongside HP, Hughes Research Labs, and DARPA, the Pentagon's research arm, in their joint SyNAPSE project. The firm is seeking partners for its homegrown 1-million-neuron brain chip, TrueNorth, too.

Listing image by Paul Marks