Photodetectors

Without special illumination present day commercial cameras see only in the light of day. Is this a fundamental limit, or only a blip in time: in the future, with the right technology, could each of us see in the dark?
 
Light is produced at all times - including at night - in the earth's atmosphere. This is the result of light emission from hydroxyl radicals (OH) known as hydroxyl airglow, or nightglow.
 
As a result, there is a surprising amount of light in the infrared wavelength range 1.6-1.8 µm.
 
Cameras that are sensitive in the nightglow region can therefore see at night. Such cameras cannot be made using low-cost silicon processing, because by virtue of its fixed bandgap silicon is insensitive to wavelengths longer than 1.1 µm. Experimental cameras that are sensitive to considerably longer wavelengths - in particular the short-wavelength infrared (SWIR) wavelengths beyond 1-2 µm - have already been shown to produce a clear image under unilluminated outdoor conditions at a range from 15 feet for visible cameras to 450 feet for SWIR cameras.
 
Extending camera sensitivity into the infrared thus has the potential to bring tremendous improvements to security systems and consumers electronics applications. However, existing infrared imaging technologies are very expensive - they cost tens of thousands of dollars per camera - to penetrate high-volume civilian security markets. The procedure is costly because it involves epitaxial growth of compound semiconductors; and because it employs a manual assembly that has poor yield. Because of their complexity, cost, and low yield, SWIR cameras are used today primarily in military markets, severely curtailing their total addressable market.
 
We are working instead within a completely different fabrication process, one based on spin- coating of pre-fabricated semiconductor colloidal quantum dots onto a silicon chip [1, 2]. Spin-coating is a straightforward, large-area technology fully compatible with monolithic integration with silicon - indeed it is already used repeatedly in the fabrication of any CMOS silicon chip, since photoresist for lithography and patterning is applied using spin-coating [3, 4]. We spin-coat ~ 5 nm-diameter PbS nanocrystals onto a chip. Our method also allows exceptional device performance to be achieved at a fraction of a cost of an epitaxy-based sensors.
 
We are improving further the sensitivity of our photodetector technology [5, 6, 7]; and to integrate it with a pre-fabricated silicon read-out integrated circuit to realize a SWIR-sensitive focal arrays [8, 9].

[1]  G. Konstantatos, L. Levina, J. Tang, E. H. Sargent, “Sensitive Solution-Processed Bi2S3 Nanocrystalline Photodetectors,” Nano Letters,vol. 8, pp.  4002-4006, 2008.

[2] G. Konstantatos, J. Clifford, L. Levina, E. H. Sargent, “Sensitive solution-processed visible-wavelength photodetectors,” Nature Photonics, vol. 1, pp. 531-534, 2007.

[3] S. Hinds, L. Levina, E. J. D. Klem, G. Konstantatos, V. Sukhovatkin, E. H. Sargent, “Smooth-Morphology Ultrasensitive Solution-Processed Photodetectors,” Advanced Materials, vol. 20, pp. 1-5, 2008.

[4] S. A. McDonald, G. Konstantatos, S. Zhang, P. W. Cyr, E. J. D. Klem, L. Levina, E. H. Sargent, “Solution-processed PbS quantum dot infrared photodetectors and photovoltaics,” Nature Materials, vol. 4, no. 2, pp. 138-142,  2005.

[5] G. Konstantatos, L. Levina, A. Fischer, E. H. Sargent, “Engineering the temporal response of photoconductive photodetectors via selective introduction of surface trap states, Nano Letters, vol. 8, pp. 1446-50, 2008.

[6] G.  Konstantatos, E. H. Sargent, “PbS colloidal quantum dot photoconductive photodetectors: Transport, traps, and gain,” Applied Physics Letters, vol. 91, p. 173505, 2007.

[7] G. Konstantatos, I. Howard, A. Fischer, S. Hoogland, J. Clifford, E. Klem, L. Levina, E. H. Sargent, “Ultrasensitive solution-cast quantum dot photodetectors,“ Nature, vol. 442, pp. 180-183, 2006.

[8] S. Hoogland, G. Konstantatos, E. H. Sargent, ”Paint-on Optoelectronics,” Optics and Photonics News, vol. 17, pp. 18-23, 2006.

[9] J. Clifford,, G. Konstantatos, K. W. Johnston, S. Hoogland, L. Levina, E. H. Sargent, “Fast, sensitive and spectrally tuneable colloidal-quantum-dot photodetectors,” Nature Nanotechnology, 2008.