Recombination in Tritiated Amorphous Silicon
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RECOMBINATION IN TRITIATED AMORPHOUS SILICON Tome Kosteski1, Franco Gaspari1, David Hum1, Stefan Costea1, 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 Tritiated amorphous silicon was used for the intrinsic layer of a p-i-n hydrogenated amorphous silicon diode. Current versus voltage measurements were carried out on the diode over time under dark and illuminated conditions. There was a decrease in the forward characteristic of the diode when measured under dark conditions and there was a decrease in photovoltaic power. These changes can be explained by the creation of dangling bonds when bonded tritium atoms decay. By annealing the diode at 125 °C, most of its photovoltaic properties could be recovered. It was also found that luminescence could be recovered in tritiated amorphous silicon by thermal annealing. INTRODUCTION The presence of metastable defects in hydrogenated amorphous silicon (a-Si:H) affects the performance of photovoltaic cells and thin film transistors. Stabler and Wronski [1] found that defects can be created by illuminating a-Si:H. The defects decreased the dark conductivity. Further, the dark conductivity could be restored by annealing the sample at 150 °C. Annealing has been used to restore photovoltaic power and luminescence of degraded a-Si:H [2,3]. Using tritium rather than hydrogen permits the study of dangling bonds unlike any other method. Tritium is a radioactive isotope of hydrogen and decays by the following reaction: T →3He + + β − + ν , where ν is a antineutrino and β − is a beta particle. The half-life of tritium is 12.3 years and the mean energy of the beta particle is 5.69 keV. Chemically, tritium behaves like hydrogen and thereby can be used as a tracer to study hydrogen behaviour. The energy of the β particle and helium recoil (~ 3 eV) are insufficient to cause damage to the structure. Replacing hydrogen with tritium in a-Si provides an opportunity to directly observe the effects of dangling bonds produced by removing bonded hydrogen from the structure without otherwise changing the matrix. Previous methods of creating dangling bonds, other than the use of light, have relied on high energy particle bombardment or hydrogen removal through heating of a-Si:H. Particle
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bombardment will inflict serious damage to the structure of the material and high temperature hydrogen effusion will also change the matrix. The use of tritium ensures that after tritium decays the Si-T bond becomes a silicon dangling bond while the rest of the matrix remains relatively unchanged. Considering that the decay constant of tritium is 1.78x10-9 s-1 and that there can be over 10 atomic percent bonded tritium in the film, the density of dangling bonds will increase at a rate of over 1017 cm-3 per day. Dangling bonds are expected to give rise to states deep in the band gap of the material. Photo-luminescen
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