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Delta-doped Imaging Detectors for UV Astronomy
Date: Monday, June 7, 2010
Time: 4:00 PM
Location: 125 Steele, Caltech
Speaker: Michael Hoenk
Surface passivation of back-illuminated silicon imaging detectors has recently attracted considerable attention in the astronomical community because of the critical importance of photometric stability and the recent discovery of quantum efficiency hysteresis in the Wide Field Camera 3 detectors. Delta-doped charge-coupled devices were invented at JPL’s Microdevices Laboratory in 1992 to solve a key problem with scientific imaging detectors that were being developed for the Hubble Space Telescope. Using molecular beam epitaxy to achieve sub-nanometer control of the surface composition, delta doping achieves nearly 100% internal quantum efficiency, and eliminates quantum efficiency hysteresis in back-illuminated CCDs and complimentary metal-oxide semiconductor (CMOS) imaging arrays. This degree of control is essential to achieving the required performance. Quantization of electron and hole states in the delta-doped surface results in extremely high electric fields and uniquely effective isolation of photoelectrons from the surface. Experimental data affirm the efficiency and stability of delta-doped detectors, even under intense illumination by Lyman-alpha radiation at 121.6 nm. We have recently demonstrated delta-doped photon counting CCDs with antireflection coatings capable of achieving quantum efficiencies greater than 50% in the vacuum ultraviolet. Finally, JPL has recently acquired and installed an 8-inch silicon molecular beam epitaxy in order to enable high capacity delta doping for back illumination of scientific imagers of CMOS, CCD or hybrid design.