NIST MEASURES THE DUV RESPONSIVITY OF GaN AND AlGaN PHOTODIODES FOR SOLAR-BLIND DETECTORS.

NIST researchers, in collaboration with colleagues from private industry, and the University of Minnesota, recently have measured the deep ultraviolet (DUV) responsivity of GaN and AlGaN photodiodes for use as solar-blind photodetectors. A solar-blind photo-detector has high sensitivity to ultraviolet radiation, but no appreciable response to visible radiation. These devices are important tools in many scientific and industrial applications in which one wants to measure ultraviolet light in the presence of high intensity visible light; for example, observing solar ultraviolet intensity in environmental monitoring systems, and measuring the wafer plane dose in deep ultraviolet lithography processes. Presently, however, simple, solar-blind detectors are not available.

The measurements were made from the infrared to the DUV portion of the electromagnetic spectrum (1500 nm to 116 nm). In spite of the relatively large band gap exhibited by these materials, the visible-light rejection ratio of the photodiodes remains far short of that needed for most applications. The observed response to visible light of the detectors can be accounted for by modeling the effect of line dislocation defects in the detectors crystal structure. The defects create regions of high electric field strength, the existence of which enables photoresponse in the visible spectrum. Both materials are known to be highly susceptible to such defects, and the researchers are developing techniques to reduce the defect density by a factor of 200, which will increase the visible-light rejection ratio to [10.sup.6].