Even Gamma Rays Can't Stop This Memory

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In space, high-energy gamma radiation can change the properties of semiconductors, altering how they work or rendering them completely unusable. Finding devices that can withstand radiation is important not just to keep astronauts safe but also to ensure that a spacecraft lasts the many years of its mission. Constructing a device that can easily measure radiation exposure is just as valuable. Now, a globe-spanning group of researchers has found that a type of memristor, a device that stores data as resistance even in the absence of a power supply, can not only measure gamma radiation but also heal itself after being exposed to it.

Memristors have demonstrated the ability to self-heal under radiation before, says Firman Simanjuntak, a professor of materials science and engineering at the University of Southampton whose team developed this memristor. But until recently, no one really understood how they healed—or how best to apply the devices. Recently, there’s been “a new space race,” he says, with more satellites in orbit and more deep-space missions on the launchpad, so “everyone wants to make their devices … tolerant towards radiation.” Simanjuntak’s team has been exploring the properties of different types of memristors since 2019, but now wanted to test how their devices change when exposed to blasts of gamma radiation.

Normally, memristors set their resistance according to their exposure to high-enough voltage. One voltage boosts the resistance, which then remains at that level when subject to lower voltages. The opposite voltage decreases the resistance, resetting the device. The relationship between voltage and resistance depends on the previous voltage, which is why the devices are said to have a memory.

The hafnium oxide memristor used by Simanjuntak is a type of memristor that cannot be reset, called a WORM (write once, read many times) device, suitable for permanent storage. Once it is set with a negative or positive voltage, the opposing voltage does not change the device. It consists of several layers of material: first conductive platinum, then aluminum doped hafnium oxide (an insulator), then a layer of titanium, then a layer of conductive silver at the top.

When voltage is applied to these memristors, a bridge of silver ions forms in the hafnium oxide, which allows the current to flow through, setting its conductance value. Unlike in other memristors, this device’s silver bridge is stable and fixes in place, which is why once the device is set, it usually can’t be returned to a rest state.

That is, unless radiation is involved. The first discovery the researchers made was that under gamma radiation, the device acts as a resettable switch. They believe that the gamma rays break the bond between the hafnium and oxygen atoms, causing a layer of titanium oxide to form at the top of the memristor, and a layer of platinum oxide to form at the bottom. The titanium oxide layer creates an extra barrier for the silver ions to cross, so a weaker bridge is formed, one that can be broken and reset by a new voltage.

The extra platinum oxide layer caused by the gamma rays also serves as a barrier to incoming electrons. This means a higher voltage is required to set the memristor. Using this knowledge, the researchers were able to create a simple circuit that measured amounts of radiation by checking the voltage that was required to set the memristor. A higher voltage meant the device had encountered more radiation.

From a regular state, the memristor forms a stable conductive bridge. Under radiation, a thicker layer of titanium oxide creates a slower-forming, weaker conductive bridge.OM Kumar et al./IEEE Electron Device Letters

But the true marvel of these hafnium oxide memristors is their ability to self-heal after a big dose of radiation. The researchers treated the memristor with 5 megarads of radiation—500 times more than a lethal dose in humans. Once the gamma radiation was removed, the titanium oxide and platinum oxide layers gradually dissipated, the oxygen atoms returning to form hafnium oxide again. After 30 days, instead of still requiring a higher-than-normal voltage to form, the devices that were exposed to radiation required the same voltage to form as untouched devices.

“It’s quite exciting what they’re doing,” says Pavel Borisov, a researcher at Loughborough University in the UK who studies how to use memristors to mimic the synapses in the human brain. His team conducted similar experiments with a silicon oxide based memristor, and also found that radiation changed the behavior of the device. In Borisov’s experiments, however, the memristors did not heal after the radiation.

Memristors are simple, lightweight, and low power, which already makes them ideal for use in space applications. In the future, Simanjuntak hopes to use memristors to develop radiation-proof memory devices that would enable satellites in space to do onboard calculations. “You can use a memristor for data storage, but also you can use it for computation,” he says, “So you could make everything simpler, and reduce the costs as well.”

This research was accepted for publication in a future issue of Electron Device Letters.

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