Want to catch the colours of a rainbow? Then cameras need to ditch their colander-like image sensors. That鈥檚 the basic idea behind a new sensor which claims to capture unparalleled amounts of light and colour.
Nanoengineer of the University of Toronto, Canada, realised that the image sensors currently used in digital cameras and cellphones waste most of the light that hits them. The sensors, or complementary metal-oxide semiconductor (CMOS) photodiodes, are built in such a way that metal tracks have to criss-cross their surface. These tracks, which carry signals from the photodiode, block much of the light, so just a fraction hits the pixels. That means the resulting picture is nowhere near as sharp and colourful as it could be.
鈥淐MOS isn鈥檛 a logical platform for light sensing,鈥� Sargent says. 鈥淚t doesn鈥檛 treat every photon as precious.鈥� It would be better to have the sensing area above the connectors, he says.
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Quantum film
This week, InVisage of Menlo Park, California, where Sargent is chief technology officer, revealed a prototype 2-megapixel 鈥渜uantum film鈥� sensor, whose entire surface senses light 鈥� with the troublesome tracks hidden safely away beneath it.
The sensing layer is a film of quantum dots 鈥� crystals of a semiconductor material just 2 nanometres wide. It was created by dispersing the crystals in a solution, then layering them onto the surface of a chip using a technique called spin coating. 鈥淲e get a quantum film many hundreds of quantum dots deep, with millions of dots per pixel,鈥� says Sargent.
A quantum dot nanocrystal confines electrons to a region so small that they no longer behave like electrons in a regular semiconductor. Usually, only an incoming photon of a certain wavelength can excite an electron to jump energy level, allowing light to be sensed at that wavelength. But confinement in a quantum dot artificially limits the energy levels an electron can jump between, allowing the crystal to sense photons of particular wavelengths.
Tuned nanocrystals
The nanocrystal鈥檚 light-sensing properties can be tuned simply by changing its dimensions. 鈥淲e have tuned ours to absorb the entire visible spectrum and we generate an electron for every photon absorbed,鈥� says Sargent.
The electron then flows into a conventional CMOS image-sensing circuit beneath it. The result is a chip that, for an equivalent size, has a megapixel resolution four times as great.
But InVisage faces 鈥渟ignificant but surmountable鈥� challenges competing with existing silicon technology on sensing speeds and cost, says Seth Coe-Sullivan of in Watertown, Massachusetts, which makes quantum dot displays. The biggest challenge, however, is probably going to be demonstrating long-term reliability against CMOS, he predicts.
Although quantum film鈥檚 colour sensitivity has 鈥済reat potential鈥�, it will not be economically viable unless its developers can show the system works seamlessly with the other standard components used in imaging, says , an imaging expert at the Technical University of Delft in the Netherlands.
