Most of today's digital cameras achieve color by using red, green, and blue Bayer color filters through which light passes on its way to the camera's image sensors, which then convert the light into electrical signals.
In a new study, researchers successfully fabricated 100 x 100 arrays of vertical nanowires with radii of 80, 100, 120, and 140 nm, allowing the nanowires to absorb different wavelengths of light. The scientists demonstrated that these nanowire-based photodetectors can photograph color images of test scenes and the Macbeth ColorChecker card with a quality that is very similar to that obtained with a conventional camera.
The new filter-free color imaging technique has some key advantages compared with the conventional filter technique, with perhaps the most important being a higher absorption efficiency that allows for higher pixel densities and higher resolution. The researchers predict that adding a bottom photodetector to the nanowire array would make it possible, in principle, for the device to absorb all incoming light and convert it into photocurrent. Such a device has the potential for extremely high photon efficiencies compared to filter-based devices, which by their nature absorb approximately half of the incoming light before it reaches the image sensor. The greater efficiency would then pave the way for cameras with higher resolutions. In addition to an improved efficiency, this approach simplifies the fabrication process. As the researchers explain, the pixels with different color responses can be defined at the same time through a single lithography step.
Furthermore, the nanowire-based photodetectors also offer the opportunity for multispectral imaging. Cameras use multispectral imaging to capture light at different frequencies of the spectrum, including frequencies beyond the visible light range. With the new method, different parts of the spectrum can be targeted for absorption by fabricating nanowires with specific radii, a relatively simple process compared to fabricating filters and other methods. The researchers plan to work on further improving the photodetectors in the future.
"We are currently working on incorporating substrate photodetectors to increase the efficiency as we mentioned above," said coauthor Hyunsung Park at Harvard University. "In addition, we are developing elliptical nanowire-based photodetectors for polarization-resolved imaging. The major hurdle for commercialization is the higher dark current level of these devices, due to the fact they are produced by etching. This comes from the fact that there are many surface states, due to the large surface-to-volume ratio of the nanowires and damage to the silicon crystal structure from dry etching. We believe that this will be resolved in the future through alternative fabrication process or by adding passivation layers."