Self-catalyzed Tritium Incorporation in Amorphous and Crystalline
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Self-catalyzed Tritium Incorporation in Amorphous and Crystalline Silicon Baojun Liu1, Nazir P. Kherani2, Kevin P. Chen1, Tome Kosteski2, Keith Leong2 and Stefan Zukotynski2 1 Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, PA 15261, U.S.A. 2 Department of Electrical and Computer Engineering, University of Toronto, Toronto, ON M5S 3G4, Canada
ABSTRACT Tritiated amorphous and crystalline silicon is prepared by exposing silicon samples to tritium gas (T2) at various pressures and temperatures. Total tritium content and tritium concentration depth profiles in the tritiated samples are obtained using thermal effusion and Secondary Ion Mass Spectroscopy (SIMS) measurements. The results indicate that tritium incorporation is a function of the material microstructure rather than the tritium exposure condition. The highest tritium concentration attained in the amorphous silicon is about 20 at.% on average with a penetration depth of about 50 nm. In contrast, the tritium occluded in the c-Si is about 4 at.% with a penetration depth of about 10 nm. The tritium concentration observed in aSi:H and c-Si is higher than reported results from post-hydrogenation experiments. The beta irradiation appears to catalyze the tritiation process and enhance the tritium dissolution in silicon material. INTRODUCTION Tritiated hydrogenated amorphous silicon (a-Si:H:T) is a candidate nuclear fuel for chipscale radioisotope micropower sources (RIMS) [1]. The preparation of a-Si:H:T was previously attained by introducing tritium gas in a silane glow discharge DC saddle field plasma enhanced chemical vapor deposition (PECVD) process [2], and more recently using a simple and versatile method where both hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) were tritiated through direct exposure to tritium gas (T2) [3]. In this paper, we report the tritium effusion and SIMS (Secondary Ion Mass Spectroscopy) study of tritiated silicon through the post-tritiation process of both amorphous and crystalline silicon, and further clarify the tritiation mechanism. EXPERIMENT Three difference amorphous silicon films were deposited on c-Si substrates, at various substrate temperatures, using the dc saddle-field PECVD technique. The substrate temperature, thickness of the film and hydrogen concentration of the a-Si:H films are listed in Table I. The aSi:H samples, as well as the c-Si samples, were exposed to tritium gas at pressures up to 120 bar and temperatures up to 250 oC. Thermal effusion measurements were conducted in order to determine the total tritium content, and hence the atomic concentration, and its bonding characteristics [4]. SIMS (Secondary Ion Mass Spectroscopy) was carried out to determine the tritium depth profiles.
Table I. Substrate temperature, thickness and hydrogen concentration in the a-Si:H films. Sample Name Substrate Temperature (oC) Thickness of Film (µ µm) Hydrogen Concentration (at.%)
aSiH1 115 0.6 35
aSiH2 200 0.3 20
aSiH3 315 2.0 17
RESULTS
Tritium Ev
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