Observation of a Hydrogen Doublet Site in High Defect Density As-Grown a-Si:H by 1 H NMR
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A13.5.1
Observation of a Hydrogen Doublet Site in High Defect Density As-Grown a-Si:H by 1H NMR
D. Bobela, T. Su, P. C. Taylor, and G. Ganguly1 Department of Physics, University of Utah Salt Lake City, Utah, U.S.A. 1
United Solar Ovonics Corp Troy, Michigan, U.S.A ABSTRACT 1
H NMR studies of hydrogenated amorphous silicon (a-Si:H) with ~1017 cm-3 defects grown by PECVD with a rate of 5 Å/s show the existence of a hydrogen doublet for both asgrown and light-soaked samples. We observe the doublet over the temperature range from 5 to 20 K in a sample where no light soaking has occurred. The doublet line shapes display no narrowing over this temperature range. Vibrational modes characteristic of SiH2 wagging and scissor modes are seen from infrared spectroscopy. These results suggest that the doublet is due to SiH2 that occurs at a density of approximately 1 at. % in this sample. From line shape analysis, we estimate a lower limit of 1.8 Å for the hydrogen-to-hydrogen separation.
INTRODUCTION Since the discovery of the Staebler Wronski effect (SWE), many studies have contributed to our understanding of the role of hydrogen in producing this metastable effect in hydrogenated amorphous silicon (a-Si:H). Models of this metastability, such as those of Stutzmann et al. [1] and Branz [2], have the common component that hydrogen must play a role in forming or stabilizing a metastable site. Recent 1H NMR studies of device quality aSi:H have found a paired hydrogen site (H-H) with an H-H distance of 2.3 ± 0.2 Å [3]. This previous study indicates that the H-H site is indeed metastable; however, the study does not identify the specific H-H bonding configuration. A natural candidate for the bonding configuration is SiH2, which has an H-H separation of 2.4 Å, assuming a tetrahedral structure with a Si-H bond length of 1.5 Å. Furthermore, a recent calculation by Tasfaye et al. demonstrates the existence of SiH2 in the amorphous network with an H-H separation of approximately 2.4 Å [4]. Although these results are suggestive, no conclusive measurements of the H-H distance for SiH2 have been performed in the a-Si:H system. The difficulty with assuming that SiH2 is the metastable site derives from a stability argument. Annealing studies have shown little change in SiH2 concentrations for temperatures up to 600 K [5], and yet it is known that defect annealing of metastable defects occurs at temperatures of ~ 200° C. From these disparate temperature dependences it seems unlikely that SiH2 is created or destroyed during light soaking and subsequent annealing. It is possible that the metastable doublet observed by Su et al. somehow becomes observable in
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the NMR experiments after light soaking, but there are some compelling arguments to suggest that this is not the case [3]. These considerations motivate the current work. In particular, by combining NMR and far infrared spectroscopic evidence, we positively identify a specific NMR lineshape as due to SiH2 and present the first detailed 1H NMR study of stable SiH2 existing in a
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