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0989-A02-04

Metastable Defects in Tritiated Amorphous Silicon Tong Ju1, Janica Whitaker2, Stefan Zukotynski3, Nazir Kherani3, P. Craig Taylor4, and Paul Stradins5 1 Physics, University of Utah, Salt Lake City, UT, 84112 2 ATK Thiokol Hazard Analysis, Brigham City, UT, 84302 3 University of Toronto, Toronto, M5S 3G4, Canada 4 Colorado School of Mines, Golden, CO, 80401 5 National Renewable Energy Laboratory, Golden, CO, 80401 ABSTRACT We have observed the growth of defects caused by tritium decay in tritiated a-Si: H instead of inducing defects optically. We kept the samples in liquid nitrogen for two years. After two years the ESR signal reached 1019 cm-3 with no evidence of saturation. However, the density is still less than the density of tritium that has decayed. We step-wise annealed (isochronally annealed) one sample up to 200 ∫C, where all of the defects were annealed out. Another sample was isothermally annealed at 300 K for several months. At this temperature, the defects anneal slowly.

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INTRODUCTION The appearance of optically or electrically induced defects in hydrogenated amorphous silicon (a-Si: H), especially those that contribute to the Staebler-Wronski effect [1], has been the topic of numerous studies, yet the mechanism of defect creation and annealing is far from clarified. This paper presents another method to induce silicon dangling-bond defects by replacing some of the hydrogen, 1H, with tritium, 3H. Tritium decays to 3He, emitting a beta particle (average energy of 5.7 keV) and an antineutrino. This reaction has a half ñlife of 12.5 years. The samples discussed in this paper contain approximately 7 and 10.4 at. % tritium. In these tritium-doped a-Si: H samples each beta decay will create a defect by converting a tritium, which is bonded to silicon, to an interstitial helium, leaving behind a silicon dangling bond. We have tracked these defects through electron spin resonance (ESR) and photothermal deflection spectroscopy (PDS) [2]. The densities we measured at room temperature were smaller by orders of magnitude ñ only about 5x1017cm-3. Therefore, there should exist a mechanism of defect annealing that is capable of healing ~ 1020cm-3 defects at room temperature. In the present work, we extend these studies to 77K, in order to establish the saturation behavior at this temperature and the thermal stability of the Si dangling bond defects introduced by tritium decay. EXPERIMENTAL Both samples studied were made at the University of Toronto in 1996. The tritium gas was mixed with SiH4, and samples were deposited using a DC glow discharge deposition system at various substrate temperatures. The samples used in this experiment were deposited on glass substrates at temperatures of 423 K (150 ∫C) (further referred to as G181) and 498 K (225 ∫C)

(referred to as G83). The G181 sample used in this study was 1.5 um thick and the G83 sample used in this study was 0.26 um thick. Shortly after deposition, high temperature tritium effusion experiments determined the tritium concentration to be approximate