“Nanodevices (traditionally based on inorganic semiconductors) are usually fabricated via lithography,” explains team member and lead author of this study Giovanni Ligorio. “Here, we have shown that we can make devices with a nanometric footprint using a different technique, GLAD, which allows us to produce memory devices with a density of roughly 1 GB/cm2 employing novel materials, that is, organic semiconductors.”

With the ever increasing demand for logic memory technology with higher density and speed and low power consumption, researchers are looking into new materials and architectures. Although traditional memory, like dynamic random access memory (DRAM), which is commonly used in central processing units (CPUs), is very fast, it consumes a lot of energy when refreshing logic elements. It is also expensive to make. Flash memory, which, for its part is often used in mobile data storage, suffers from low operation speed and low endurance.

A simple, two-terminal device

One technology that shows promise for overcoming the limitations of current memories is resistive switching non-volatile memory (R-NVM). A R-NVM is a simple, two-terminal device formed by two electrodes separated by an insulating or semiconducting material.

In their work, Ligorio and colleagues focused on bistable devices in which the resistance of the device can be reversibly switched between a high resistance state and a low resistance one. These two states represent the ON and OFF states of the device. Each state can be read non-destructively and no power is needed to maintain the state. This is why it is called non-volatile.

Organic materials could be better for making R-NVMs

The layer between the electrodes in R-NVMs can be made from organic materials (such as small molecules or polymers) as well as inorganic metal oxides. Organic materials are better in many ways here – for example they can be processed at room temperature, or near to it, can be implemented on flexible substrates and printed too. They are also cheaper to produce than their inorganic counterparts. However, to make high-density memory modules, organic R-NVM devices need to be reduced in size.

One way to make such devices without resorting to cumbersome lithography techniques is to use GLAD, say the researchers. “This technique allows us to tailor nanostructured morphologies through physical vapour deposition while controlling the orientation of the substrate on which the device is grown with respect to the vapour source direction,” explains Ligorio. “When we deposit thin films of the material onto a substrate at an oblique angle, the vapour flux is non-perpendicular to the substrate surface and an inclined nanostructure is produced.

Fabricating the devices in structured arrays

“When we apply the proper bias between the two electrodes of the memory device, we are able to form a conductive path (or filament). This filament shorts the electrodes and drastically changes the resistivity characteristics of the device,” he tells nanotechweb.org. “We can thus use the applied current to switch the device from its original high resistivity state to a low resistivity one (the ON and OFF states of the memory).”

Spurred on by the results, the team, led by Norbert Koch, says that it is now busy trying to fabricate devices in structured arrays using its technique. “In the present study, the nanodevices were not ordered in an array but were randomly distributed. Ordering them will allow us to connect them via crossbar electrodes that we will synthesize using printing technology,” says Ligorio.

The research is detailed in Nano Letters DOI: 10.1021/acs.nanolett.6b04794