Solar-Blind UV Photodetectors

October 28, 2013

Most residential fire alarms operate as smoke detectors, but how do you detect a fire that produces no smoke? This is a problem when you're working with hydrogen, since the sole combustion product of burning hydrogen is water. There is no smoke, and a hydrogen flame is actually dim, since most of its energy is not contained in the visible spectrum of light. This is a problem even when hydrogen is stored cryogenically, and hydrogen is more easily ignited than most fuels.

A hydrogen flame does emit considerable ultraviolet light (UV), and that's the basis of most hydrogen flame detectors, which detect UV light below a wavelength of 300 nm. The 300 nm wavelength cutoff is critical, since ultraviolet radiation from the Sun would trigger these detectors in the absence of a flame (see figure).

The solar spectrum at sea level is shaped by absorption of the atmosphere. (Modified Wikimedia Commons image.)

UV detectors designed to operate in daylight are called "solar-blind." Solar blind UV detectors are useful for purposes other than hydrogen flame detection. There are other short wavelength UV ("deep" UV) emitters in the environment worth detecting, the most notable being corona discharges in the air. Corona discharges can occur on high voltage electrical transmission lines, where they cause considerable radio frequency interference, degrade insulation, and produce ozone.

Detectors can be made solar-blind by adding a suitable filter, although this will attenuate some of the UV radiation. Modern technology allows the inexpensive fabrication of dichroic filters with bandpass and bandstop characteristics. These are built from alternating layers of transparent materials with different refractive index. Filters can also be made from an electrical conductor nanoscale grid, one example of which has a 27% transmission at 290 nm wavelength and a 100 nm bandwidth.[1]

A better solar-blind option than using a broadband UV detector with a filter is to make a detector that responds only to the intended wavelengths. As shown in the figure, the threshold wavelength for optical absorption can be tuned in some materials such that the only response will be at solar-blind wavelengths.

Response of Al(x)Ga(1-x)N Schottky diodes. Data from Muñoz, et al.,[2] referenced in fig. 26.2 of ref. 3.[3] (Illustration by the author using Inkscape.)

Scientists at Ishinomaki Senshu University (Ishinomaki, Japan) have been developing a particular type of solar-blind photodiode with a heterojunction.[4-5] In a standard diode, n- and p-doped versions of the same material (e.g., silicon) are used to make the diode junction. In a heterojunction diode, the junction is between dissimilar materials with different band gaps. Heterojunctions are used to advantage in solid state optical devices and high speed transistors, and the 2000 Nobel Prize in Physics was awarded to Herbert Kroemer and Zhores Alferov for heterojunction research.

The hetereojunction of the Japanese device was formed between beta-gallium oxide (β-Ga₂O₃) and silicon carbide (6H-SiC). The device was formed by evaporation of gallium in an oxygen plasma onto a substrate of p-type 6H-SiC.[4] The heterostructure solved the problem that it's not possible to make p-type gallium oxide.[5] The band gap of the β-Ga₂O₃, 4.9 eV, and that of the 6H-SiC, 3.02 eV, gave a linear current response with UV illumination under a reverse-bias condition. The response time was a few milliseconds.[4]

Solar-blind UV β-Ga2O3/SiC heterojunction photodiode developed at Ishinomaki Senshu University. (Ishinomaki/S.Nakagomi, Ishinomaki Senshu University.)[5]

References:

1. Wen-Di Li and Stephen Y. Chou, "Solar-blind deep-UV band-pass filter (250 - 350 nm) consisting of a metal nano-grid fabricated by nanoimprint lithography," Optics Express, vol. 18, no. 2 (January 18, 2010), pp. 931-937. Available as a PDF file, here.
2. E. Muñoz, E. Monroy, J.L. Pau, F Call, F Omnès and P Gibart, "III nitrides and UV detection," J Physics Condensed Matter, vol. 13, no. 32 (August 13, 2001) pp.7115-7137.
3. U. Schühle and J.-F. Hochedez, "Solar-blind UV detectors based on wide band gap semiconductors," Chapter 26 of Observing Photons in Space, M. C. E. Huber, A. Pauluhn, J. L. Culhane, J. G. Timothy, K. Wilhelm, and A. Zehnder, Eds.), pp. 429–439 (PDF File).
4. Shinji Nakagomi, Toshihiro Momo, Syuhei Takahashi and Yoshihiro Kokubun, "Deep ultraviolet photodiodes based on β-Ga2O3/SiC heterojunction, Appl. Phys. Lett., vol. 103, no. 7 (August 12, 2013), Document No. 072105 (4 pages).
5. A better device to detect ultraviolet light, American Institute of Physics Press Release, October 4, 2013.