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  • Paint-on Photodetectors Are Devised

Photonics Spectra
Sep 2006
Using quantum dots offers alternative to traditional semiconductor devices.

Anne L. Fischer

Because of their use in digital cameras and in night-vision and security systems — all growing markets — and in fiber optic communications, photodetectors are in hot demand. It’s no wonder that researchers are looking for ways to simplify the process by which they are produced. A group at the University of Toronto has developed a manufacturing process that paints a solution of quantum dots onto gold electrodes.

Painting a liquid containing quantum dots onto gold electrodes has simplified the process of developing photodetectors.

The investigators placed gold electrodes on a glass slide and painted a solution that contained light-sensitive colloidal quantum-dot nanoparticles onto the slide by spin-coating. The solution spread across the surface, and the particles remained after the solvent had evaporated, leaving a smooth semiconductor film. An electrical source was then wired to the gold electrodes.

When a beam of infrared radiation from a 975-nm laser from BWTek of Newark, Del. (or, to acquire responsivity spectra, from a lamp coupled through filters and a monochromator) was shone onto the light-sensitive film, the nanoparticles absorbed the light, energizing the electrons that flowed between the gold electrodes. The researchers found that the current was proportional to the brightness of the light striking the photodetector, showing that the device was photoconductive.

Traditional infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs. In comparison, the new method is more flexible, costs less, and offers a larger device area and convenient material integration. After fabricating solution-cast quantum-dot photodetectors from several synthesis batches, the researchers determined that these photodetectors exhibit better detectivity than high-quality epitaxially grown devices operating at room temperature. Furthermore, when they are stored in air for two weeks, their performance does not change by more than 20 percent and, when stored in a nitrogen glove box, their performance does not change more than 20 percent over a period of months.

The next research steps involve coating a readout integrated circuit with the ultrasensitive photodetecting material to produce an imaging chip or a focal plane array, sensitized into the short-wavelength infrared.

Nature, July 13, 2006, pp. 180-183.

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