CQNl_" RESPONSE TO 100% INTERNAL QUANTUM EFFICIENCY SILICON PHOTODIODES TO LOW ENERGY ELECTRONS AND IONS

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1 I ' 4 46 Title: CQNl_" RESPONSE TO 100% INTERNAL QUANTUM EFFICIENCY SILICON PHOTODIODES TO LOW ENERGY ELECTRONS AND IONS A uthor(s): H. 0. Funsten D. M. Suszcynsky R. Korde S. M. Ritzau Submitted to: I EEE Nuclear Science Symposium and Medical Imaging Conference Anaheim, California November 2-9, 1996 Los Alamos National Laboratory, an affirmative actiordequai opportunity employer, is operated by the University of California for the US. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Government retains a flonexclusive, royalty-free to publish or reproduce the published form of this contribution, or to allow others to do so, for US. Government purposes. The Los Akmos National Laboratory requests that the publisher identify this article as work performed under the auspices of the US. Department of Energy. Form No. 836 A5 ST 2629 iob1

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4 RESPONSE OF 100% INTERNAL QUANTUM EFFICIENCY SI PHOTODIODES TO LOW ENERGY ELECTRONS AND IONS H.O. Funsten and D.M. Suszcynsky, Los Alamos National Lab. R. Korde, International Radiation Detectors S. M. Ritzau University of Virginia Silicon photodiodes with only a 60 A Si02 front window are used in fusion and space research for detection of X W photons with high quantum efficiency. In these environments, plasma ions and electrons can deposit energy in the active layer of the photodiode and influence the XUV measurement. Here, we characterize the response of these photodiodes to bombardment of 1-20 kev electrons and 30 kev H, Hz, Ne, and At. For electrons, the responsivity is 0.24 AN, inferring an electron-hole pair creation energy of 4.2 ev. The measured responsivity to 30 kev H2+ is approximately 0.2 NW, corresponding to an electronhole pair creation energy of 5 ev. Photodiode damage due to ion bombardment is observed though an exponential decrease of the responsivity with ion dose. The decrease in responsivity is more rapid with increasing ion mass. Annealing of damage induced by heavy ions is observed by subsequent bombardment with protons. Corresponding Author: Herbert Funsten MS 0466 Los Alamos National Laboratory Los Alamos, NM Ph: (505) FAX: (505) hfunsten@lanl.gov

5 Response of 100% Internal Quantum Efficiency Silicon Photodiodes to Low Energy Electrons and Ions Herbert 0. Funsten and David M. Suszcynsky Los Alamos National Laboratory, Los Alamos, IVM Raj Korde International Radiation Detectors. Inc., Torrance, CA Stephen M. Ritzau University of Virginia, Charlottesville, VA Introduction Silicon p-n junction photodiodes with 100% internal quantum detection efficiency, developed by International Radiation Detectors, can be used to measure XUV photons. These diodes do not have a doped dead region that is common to p-n junction diodes and have zero surface recombination resulting in near theoretical quantum detection efficiencies for XUV photons. These AXUV photodiodes can be used, for example, in fusion and space research, in which the photodiode is also exposed to plasma ions and electrons that can transit the thin (-60 A SO*) window and can interfere with the XUV photon measurement. Alternately, detection of ions and electrons allows these photodiodes to be employed as an ion or electron detector. Here, we characterize the photovoltaic response of AXUV photodiodes to incident 1-40 kev electrons and kev protons, H;, 0.25 Ne' and Ar'. Low Energy Electron Irradiation Electrons incident at an energy E B E Aranging,~ from 1 kev to 40 kev are created using a scanning electron microscope. The electron beam is measured using a current I B ~ M Faraday cup both before and after the beam strikes the photodiode to ensure that the beam current did not change while the photodiode current I p was being measured. The responsivity of the photodiode is simply R = Electron Beam Energy, EBw (kev) Fig. 1. Responsivity of AXUV photodiodes to electron bornbmdment. M I BEAMEBEAM. Page 1 m

6 The responsivity of the photodiode was observed to be independent of both the electron beam current and the beam current density. The responsivity as a function of the electron beam energy is shown in Fig. 1. The average measured responsivity over the energy range 1-40 kev is 0.24 * A/W. This corresponds to an electron-hole pair creation energy of 4.2 ev and compares to the electron-hole pair creation energy of 3.6 ev for Si. The difference can be attributed to non-ionizing interactions that constitute a significant fraction of energy-loss events at these energies. The responsivity was not observed to change with total electron dose, so damage to the photodiode by 1-40 kev electron bombardment is minimal. Low Energy Ion Irradiation Photodiodes were exposed to H, He, Ne. and Ar ion beams at energies ranging from 10 to 30 kev. The ion beam current was measured before and after irradiation of the photodiode to ensure the beam current had not drifted. Figure 2 shows the photodiode responsivity as a function of the ion beam dose for 30 kev Hz+. The extrapolate valued of R at zero fluence is 0.2 A/W, corresponding to an electron-hole pair creation energy of 5 ev. This value is greater than that of electrons at an equivalent energy and results from the large component of eiastic 10' collisions of the ions as they penetrate the active region of the photodiode. The responsivity decreases with increasing ion flux, indicating damage to the photodiode. As shown in Fig. 2, irradiation by heavy ions, e.g., Ar, show a rapid decrease toward a minimum responsivity of approximately Damage by heavy ions, measured using R, is observed to be stable over several tens of minutes. Subsequent H irradiation increases the responsivity, indicating that partial annealing of the heavy ion damage. Page 2