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October 15, 2009

Resonant interband tunneling diodes made with Chemical Vapor Deposition

Researchers at Ohio State University have discovered a way to make quantum devices using technology common to the chip-making industry today. The team fabricated a device called a tunneling diode using the most common chip-making technique, called chemical vapor deposition. Manufacturers could potentially fabricate quantum devices directly on a silicon chip, side-by-side with their regular circuits and switches. Resonant interband tunneling diodes (RITDs) could be used for ultra-low-power computer chips operating with small voltages and producing less wasted heat. They could also be used for imaging applications.

The quantum device in question is a resonant interband tunneling diode (RITD) -- a device that enables large amounts of current to be regulated through a circuit, but at very low voltages. That means that such devices run on very little power.

RITDs have been difficult to manufacture because they contain dopants -- chemical elements -- that don’t easily fit within a silicon crystal.

Atoms of the RITD dopants antimony or phosphorus, for example, are large compared to atoms of silicon. Because they don’t fit into the natural openings inside a silicon crystal, the dopants tend to collect on the surface of a chip.

They discovered that RITD dopants could be added during chemical vapor deposition, in which a gas carries the chemical elements to the surface of a wafer many layers at a time. The key was determining the right reactor conditions to deliver the dopants to the silicon.

“One key is hydrogen,” he said. “It binds to the silicon surface and keeps the dopants from clumping. So you don’t have to grow chips at 320 degrees Celsius [approximately 600 degrees Fahrenheit] like you do when using molecular beam epitaxy. You can actually grow them at a higher temperature like 600 degrees Celsius [more than 1100 degrees Fahrenheit] at a lower cost, and with fewer crystal defects.”

Tunneling diodes are so named because they exploit a quantum mechanical effect known as tunneling, which lets electrons pass through thin barriers unhindered.

In theory, interband tunneling diodes could form very dense, very efficient micro-circuits in computer chips. A large amount of data could be stored in a small area on a chip with very little energy required.




Researchers judge the usefulness of tunneling diodes by the abrupt change in the current densities they carry, a characteristic known as “peak-to-valley ratio.” Different ratios are appropriate for different kinds of devices. Logic circuits such as those on a computer chip are best suited by a ratio of about 2.

The RITDs that Berger’s team fabricated had a ratio of 1.85. “We’re close, and I’m sure we can do better,” he said.

He envisions his RITDs being used for ultra-low-power computer chips operating with small voltages and producing less wasted heat.

RITDs could form high-resolution detectors for imaging devices called focal plane arrays. These arrays operate at wavelengths beyond the human eye and can permit detection of concealed weapons and improvised explosive devices. They can also provide vision through rain, snow, fog, and even mild dust storms, for improved airplane and automobile safety, Berger said. Medical imaging of cancerous tumors is another potential application.


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