The majority of the solar radiation that reaches the earth lies close to the visible region of the spectrum. The intensity falls off from there slowly into the infrared and more quickly into the ultraviolet (UV). Recently, as the growth of high Al composition AlGaN has matured, interest has focused on detectors operating in the solar-blind region of the UV spectrum. The solar-blind region corresponds to the strong atmospheric absorption of solar UV in the range of 240-290 nm. This creates a natural low background window for detection of man-made UV sources. The development of UV photodetectors has been driven by numerous applications in the defence, commercial, and scientific arenas. These include covert space-to-space communications, secure non-line-of-sight communications, early missile threat detection, UV spectroscopy, chemical and biological threat detection, flame detection and monitoring, UV environmental monitoring, and UV astronomy. Many organic and inorganic compounds have absorption lines or florescence lines in the UV region of the spectrum. If a number of different cut-off wavelength UV photodetectors are used, it is possible to determine the presence of individual spectral lines and attempt to identify the presence of specific chemicals. This potential for compact solid-state florescence spectroscopy is one of the most interesting applications.
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