Modeling of Beta Conductivity in Tritiated Amorphous Silicon
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Modeling of Beta Conductivity in Tritiated Amorphous Silicon Stefan Costea1, Franco Gaspari1, Tome Kosteski1, Stefan Zukotynski1, Nazir P. Kherani2 and Walter T. Shmayda2 1 Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario M5S 3G4, Canada; 2 Ontario Power Technologies, Toronto, Ontario, M8Z 5S4, Canada
ABSTRACT The change with time in the electrical conductivity of a hydrogenated-tritiated amorphous silicon film (a-Si:H:T) has been studied. The conductivity decreased with time after deposition. A model is developed to account for the decrease. The radioactive decay of tritium into helium produces energetic beta particles. Each β particle creates over 1500 electron-hole pairs in the film thereby increasing the conductivity of the film. The 3He atoms diffuse away leaving dangling bonds behind. We find that neutral dangling bonds (D0) are responsible for the decrease in conductivity by acting as recombination centers in the material. INTRODUCTION Tritiated-hydrogenated amorphous silicon (a-Si:H:T) is a novel thin film material where tritium, a radioactive isotope of hydrogen, is bonded with silicon in the amorphous network. Tritium decays into helium. The half-life for the process is 12.3 years. The decay produces a β particle with a mean energy of 5.7 keV. The effect of the tritium decay on the amorphous network is two-fold: The tritium atom decays into a helium atom, which leaves the bond site thus creating a dangling bond (DB). The β particle generates over 1500 electron-hole pairs as it loses its energy through collisions. These effects can be used to shed light on the physics of DBs in amorphous silicon. In particular, one can use the fact that defects are being created at a known rate, which is governed by a nuclear decay process, to relate the electrical and optical properties of the material directly to the change in the density of DBs. In an earlier paper [1] we reported a decrease in the conductivity with time during the first week after deposition. We attributed the decrease to an increase in the neutral dangling bonds (D0) concentration. In this paper we focus on the effect of the increase in density of DBs on the conductivity during the first week, and we propose a model that correlates the changes in DBs with the decrease in conductivity.
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EXPERIMENTAL Hydrogenated-tritiated amorphous silicon films were deposited using the dc saddlefield deposition system. The details of the deposition system and of the films are described elsewhere [1]. Glass substrates (Corning 7059) were used for conductivity measurements. Ohmic coplanar Ni-Cr pads with a gap of 0.1 cm were evaporated onto the film to provide electrical contacts. Teflon insulated wire was silver-pasted onto each pad. A bias of 10 V was applied between the contacts and the current was measured with an electrometer, while the temperature was changed in steps of 5 oC. Figure 1 shows the behavior of the conductivity vs. temperature during the first week following deposition. The low temperature condu
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