System Bits: Nov. 5

Silicon Photonics And Graphene
The industry is looking towards silicon photonics that will increase the rate at which electronic systems can communicate with each other and reduce power consumption. Researchers at MIT, Columbia University and IBM’s T. J. Watson Research Center are already a few steps beyond the traditional attempts to build optical components using materials such as Gallium Nitride (GaN) on a silicon substrate. They are looking toward graphene as a photodetector. Graphene consists of atom-thick sheets of carbon atoms arranged hexagonally.

Graphene is responsive to a wider range of light frequencies than the materials typically used in photodetectors, so graphene-based optoelectronic chips conceivably could use a broader-band optical signal, enabling them to move data more efficiently. “A two-micron photon just flies straight through a germanium photodetector,” says Dirk Englund, Jamieson Career Development Assistant Professor of Electrical Engineering and Computer Science at MIT, “but it is absorbed and leads to measurable current in grapheme.”

The problem with graphene as a photodetector has traditionally been its low responsivity. A sheet of graphene will convert only about 2% of the light passing through it into an electrical current. That’s actually quite high for a material only an atom thick, but it’s still too low to be useful.

In a new graphene-on-silicon photodetector, electrodes (gold) are deposited, slightly asymmetrically, on either side of a silicon waveguide (purple). The asymmetry causes electrons kicked free by incoming light to escape the layer of graphene (hexagons) as an electrical current.
GRAPHIC COURTESY OF THE RESEARCHERS

When light strikes a photoelectric material such as germanium or graphene, it kicks electrons orbiting atoms of the material into a higher energy state, where they’re free to flow in an electrical current. If they don’t immediately begin to move, however, they’ll usually drop back down into the lower energy state. So one standard trick for increasing a photodetector’s responsivity is to “bias” it — to apply a voltage across it that causes the electrons to flow before they lose energy.

The problem is that the voltage will inevitably induce a slight background current that adds “noise” to the detector’s readings, making them less reliable. So Englund, his student Ren-Jye Shiue, Columbia’s Xuetao Gan and their collaborators instead used a photodetector design developed by Fengnian Xia and his colleagues at IBM, which produces a slight bias without the application of a voltage.

In the new design, light enters the detector through a silicon channel—a “waveguide”—etched into the surface of a chip. The layer of graphene is deposited on top of and perpendicular to the waveguide. On either side of the graphene layer is a gold electrode. But the electrodes’ placement is asymmetrical: One of them is closer to the waveguide than the other.

“There’s a mismatch between the energy of electrons in the metal contact and in graphene,” Englund says, “and this creates an electric field near the electrode.” When electrons are kicked up by photons in the waveguide, the electric field pulls them to the electrode, creating a current.

In experiments, the researchers found that, unbiased, their detector would generate 16 milliamps of current for each watt of incoming light. Its detection frequency was 20 gigahertz—already competitive with germanium. With the application of a slight bias, the detector could get up to 100 milliamps per watt, a responsitivity commensurate with that of germanium.

Faster Switches
Along with faster communication, there is a desire for a new type of electronic switch. The planar transistor that has served the industry for several decades is almost a relic of the past. FinFETs are being accepted as the new devices, but many researchers already see the point at which FinFETs also have problems and new materials are going to be required. IBM and SLAC scientists have been looking at a new material that can flip from being an electrical conductor to an insulator.

Led by Stuart Parkin of IBM, the team worked with vanadium dioxide, a material whose metal-to-insulator transition behavior has been studied for decades because of its potential for use in transistors and other nanoelectronic switches. It has two big advantages. First, it switches much faster than traditional transistor materials. And second, it does so at near-room temperature.

Until now, however, researchers did not know how to control the temperature at which this occurs, which is essential to its widespread use.

The IBM/SLAC team was able to control the temperature by growing thin films of vanadium dioxide on a substrate in a way that compressed or stretched the film’s crystalline structure, or lattice. As a result, the distance between vanadium atoms in the lattice ranged from 0.5 percent shorter to 1.5 percent longer than normal. This, in turn, slightly changed the arrangement of electrons around the nuclei of vanadium atoms in the lattice.

This image shows how straining vanadium dioxide causes the vanadium atoms (gold) to move (arrows), altering the arrangement of electrons in the material and changing its metal-insulator transition temperature. (Nagaphani Aetukuri/IBM-Stanford Spintronic Science and Applications Center)

The researchers found that over the 2% change in lattice spacing, the material’s metal-insulator switching temperature decreased by more than 100 degrees Fahrenheit, from 152 to 44 degrees Fahrenheit.
X-ray experiments at Lawrence Berkeley National Laboratory showed a direct correlation between the amount of lattice strain and changes in both the arrangement of electrons and the metal-insulator switching temperature.

As follow-on research, the IBM/SLAC teams will next use SLAC’s Linac Coherent Light Source to learn why the material switches so quickly.