Design and Physical Characterization of CaS Solar-blind UV Detector

Abstract

Ultraviolet (UV) photodetectors working in solar-blind spectrum ranging from 220 to 280 nm with very high sensitivity are of importance for versatile applications, such as flame detection, missile plume sensing, chemical/biological agents detection, air and water purification, space-to-space communications, etc. A truly solar-blind UV photodetector requires strong out-of-band radiation rejection. In this thesis, the research work on a novel solid-state solar-blind UV detector is presented.

 

CaS, with a direct bandgap of 5.38 eV, should hold a cut-off wavelength at around 230 nm. Thus, it is expected to be a strong candidate as the active-layer for high performance solar-blind UV photodiodes. In this study, a seed-layer-assisted growth approach via molecular beam epitaxy was developed, which was demonstrated to yield rocksalt CaS thin films with high crystalline quality on zinc-blende GaAs substrates. The developed Au/CaS/GaAs solar-blind photodiodes were demonstrated to show superior performance in its visible rejection power of over 5 orders and quantum efficiency as high as 19%. The key contents of this thesis include the basic working principle of a Schottky-barrier photodiode, the seed-layer-assisted growth approach and the studies of the photoresponse of the developed CaS solar-blind UV detector system. Also, an optical beam splitting method with a compact quartz-based optical setup was designed and tested. Such a diffraction splitting is demonstrated to enjoy close to 3 orders of rejection power. Finally, the potential of combining the CaS solar-blind UV detector combined with a UV band-pass filter and the compact optical setup as a novel flame sensing unit is addressed.