Painting a semiconductor

Researchers at the University of Toronto have created a semiconductor which is faster than conventional chips. They just painted a liquid onto a piece of glass. The semiconductor films produced with this technique are so sensitive that one of the first use of these new chips could be in night vision cameras.

Making electronic chips and other devices is a dirty and expensive process. Building them from chemical solutions have been tried for a while because of the low costs. But the resulting chips were not really good performers. Now, researchers at the University of Toronto have created a semiconductor which is faster than conventional chips. They just painted a liquid onto a piece of glass. And as an example, at room temperature, the semiconductor films produced with this technique are "about ten times more sensitive to infrared rays than the sensors that are currently used in military night-vision and biomedical imaging." So one of the first use of these new chips could be in night vision cameras.

Here are some excerpts from this University of Toronto news release.

"Traditional ways of making computer chips, fibre-optic lasers, digital camera image sensors – the building blocks of the information age – are costly in time, money, and energy," says Professor Ted Sargent [who was the leader] of the research group. Conventional semiconductors have produced spectacular results -- the personal computer, the Internet, digital photography -- but they rely on growing atomically-perfect crystals at 1,000 degrees Celsius and above, he explains.

In "Light sensors from a test tube", an article by Jessica Ebert which appeared in Chemistry World, a publication of the Royal Society of Chemistry (RSC), UK, gives some details about the fabrication process.

The scientists started with a piece of glass patterned with gold electrodes, and painted it with a solution containing semiconductor nanocrystals. In a process called spin-coating, the solution was forced to spread over the glass. After soaking the device in methanol, a thin, uniform film of light-sensitive nanoparticles remained.
By attaching a battery to the gold electrodes and shining a beam of infrared light on the semiconductor film, Sargent and colleagues could measure its sensitivity. "At room temperature, these films were about 10 times more sensitive to infrared rays than the sensors that are currently used in military night-vision and biomedical imaging," he told Chemistry World.

Here is an illustration showing how this painting process works (Credit: Ted Sargent).

Paint-on semiconductor outperforms chips

Besides night vision cameras, this new manufacturing process could be used for other applications, in communications for example.

And the value of this work didn't escape to Nature which published these findings under the title "Ultrasensitive solution-cast quantum dot photodetectors" (Volume 442, Number 7099, Pages 180-183, July 13, 2006). Here are two links to the abstract and to the Editor's summary, "A practical solution."

The best electronic and optoelectronic devices are built via semiconductor crystal growth on a single-crystal substrate. Over 100 papers have been published in recent years in Nature on alternative devices, produced instead from the solution phase. They have some advantages over conventional crystalline semiconductor devices: ease of fabrication, physical flexibility and -- most important -- low cost. The problem was the poor electronic performance of solution-processed devices, compared with single-crystal counterparts. But that could change now: a team from the University of Toronto reports that one such system -- colloidal quantum dots of lead sulphide -- can actually outperform the state-of-the-art crystalline alternative.

With the billions of dollars invested in current factories, I don't think that the semiconductor industry will adopt this new chemical process in the near future.

Sources: University of Toronto news release, via EurekAlert!, July 12, 2006; and various web sites

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