Tritium Induced Defects in Amorphous Silicon
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TRITIUM INDUCED DEFECTS IN AMORPHOUS SILICON J . Whitakera, J. Vinera, S. Zukotynskib, E. Johnsonb, P.C. Taylora, and P. Stradinsc a
Department of Physics, University of Utah, Salt Lake City, UT 84112 Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada M5S 1A1 c National Renewable Energy Laboratory, Golden, CO 80401 b
ABSTRACT We report the growth of tritium induced defects in tritium doped hydrogenated amorphous silicon (a-Si:H,T) as measured by electron spin resonance (ESR) and photothermal deflection spectroscopy (PDS). The measurements allow one to examine the accumulation of defects in a-Si:H,T where the defect production mechanism is known. Defects produced by tritium decay are found to be much less numerous than the number of decayed tritium atoms and they are metastable like Staebler-Wronski defects. These results provide new insight into the metastable defect creation and the role of hydrogen motion. INTRODUCTION The appearance of optically or electrically induced defects in hydrogenated amorphous silicon (a-Si:H), especially those that contribute to the Staebler-Wronski (SW) effect [1], has been the topic of numerous studies [1,2], yet the mechanism of defect creation and annealing is far from clarified. We have investigated the intrinsic growth of defects in tritiated, hydrogenated amorphous silicon (a-Si:H,T) as an alternative approach to inducing defects optically. Tritium decays to He3, emitting a beta particle (average energy of 5.7 keV) and an antineutrino. This reaction has a half-life of 12.5 years; therefore, in these tritium-doped samples each beta decay can, in principle, create a defect by converting a bonded tritium to an interstitial helium, leaving behind a silicon dangling bond. We have tracked these Si dangling bond defects through electron spin resonance (ESR) and photothermal deflection spectroscopy (PDS). Like the optically induced defects, the defects created by tritium decay also anneal out at approximately 150 C, confirming the earlier findings by luminescence [3]. Preliminary results indicated that their number is much less than the number of decayed tritium atoms [4]. This similarity to Staebler-Wronski effect is by no means obvious. For beta-decay the defect production rate and mechanism are known and are primarily governed by the unbinding of tritium from Si due to its conversion to He. The mechanisms have been proposed for the production of defects that mediate the SW effect, the breaking of “weak” Si-Si bonds with subsequent hydrogen mediated migration [5] and the detachment of H by breaking Si-H bonds with subsequent H diffusion and trapping [6,7]. In addition, due to the potential effects of annealing, the rate of production of defects by recombination of photocarriers is not well known. In this work we perform a systematic study of the tritium-decay-induced accumulation kinetics and thermal stability, to obtain new insight into the defect creation and annealing mechanisms in a-Si:H.
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EXPERIMENTAL PROCEDURES Th
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