28 Jan 2013

Lithography technique opens door to 2D electronics

US researchers are edging closer to two-dimensional electronics after "merging" graphene with hexagonal boron nitride.

Rice University scientists have combined the two materials into atom-thick sheets and fabricated a variety of patterns at nano-scale dimensions.

Rice materials scientist Pulickel Ajayan and colleagues first introduced a technique to stitch the identically structured materials together nearly three years ago.

Then late last year, researchers from Cornell University scientists reported a technique to fabricate atomic-layer hetero-structures through sequential growth schemes.

Now, the Rice researchers' latest lithography technique offers the possibility of shrinking electronic devices into even smaller packages.

The team can write features to 100nm resolution, so as Ajayn says, the only real limits are those defined by modern lithographic techniques.

"It should be possible to make fully functional devices with circuits 30, even 20 nanometres wide, all in two dimensions," says Ajayn's colleague and fellow Rice researcher Jun Lou. "That would make circuits on about the same scale as in current semiconductor fabrication."

Graphene has been touted as a wonder material since its discovery in the last decade. Even at one atom thick, the hexagonal array of carbon atoms has proven its potential as a fascinating electronic material.

But to build a working device, conductors alone will not do. Graphene-based electronics require similar, compatible 2-D materials for other components, and researchers have found hexagonal boron nitride (h-BN) works nicely as an insulator.

H-BN looks like graphene, with the same chicken-wire atomic array. The earlier work at Rice showed that merging graphene and h-BN via chemical vapor deposition (CVD) created sheets with pools of the two that afforded some control of the material's electronic properties.

However, in their latest work finely detailed patterns of graphene have been laced into gaps created in sheets of h-BN.

Combs, bars, concentric rings and even microscopic Rice Owls were laid down through a lithographic process.

The interface between elements, seen clearly in scanning transmission electron microscope images taken at Oak Ridge National Laboratories, shows a razor-sharp transition from graphene to h-BN along a subnanometer line.

"This is very precisely engineered. We can control the domain sizes and the domain shapes, both of which are necessary to make electronic devices," says Lou.

The new technique also began with CVD. Zheng Liu, fellow Rice research scientist, and his colleagues first laid down a sheet of h-BN. Laser-cut photoresistant masks were placed over the h-BN, and exposed material was etched away with argon gas. A focused ion beam system was later used to create even finer patterns, down to 100-nanometer resolution, without masks.

After the masks were washed away, graphene was grown via CVD in the open spaces, where it bonded edge-to-edge with the h-BN. The hybrid layer could then be picked up and placed on any substrate.

While there's much work ahead to characterize the atomic bonds where graphene and h-BN domains meet and to analyze potential defects along the boundaries, Liu's electrical measurements proved the components' qualities remain intact.

"By doing all kinds of growth, then etching, then regrowth, the intrinsic properties of these two materials are still not affected," Lou says. "Insulators stay insulators; they're not doped by the carbon. And the graphene still looks very good. That's important, because we want to be sure what we're growing is exactly what we want."

Liu explains the next step is to place a third element, a semiconductor, into the 2-D fabric. "We're trying very hard to integrate this into the platform," he says. "If we can do that, we can build truly integrated in-plane devices."

"This process is robust, repeatable and creates materials with very nice properties and with dimensions that are at the limit of what is possible," he concludes.

• Research is published in Nature Nanotechnology

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