Voids in Hydrogenated Amorphous Silicon: A Comparison of ab initio Simulations and Proton NMR Studies
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Voids in Hydrogenated Amorphous Silicon: A Comparison of ab initio Simulations and Proton NMR Studies Sudeshna Chakraborty1, David C Bobela2, P C Taylor3, and D. A. Drabold1 1 Department of Physics and Astronomy, Ohio University, Athens, OH, 45701 2 Department of Physics, University of Utah, Salt Lake City, UT, 84112 3 Department of Physics, Colorado School of Mines, Golden, CO, 80401 ABSTRACT Recently, a new hydrogen NMR signal has been observed in a number of PECVD prepared hydrogenated amorphous silicon (a-Si:H) films of varying quality. It is speculated that the signal is the consequence of a dipolar-coupled hydrogen pair separated, on average, by 1.8 ± 0.1 Å. To elucidate the possible bonding configurations responsible for the NMR data of ref. [1], we have used ab initio simulation methods to determine a set of relaxed structures of a-Si:H with varying void sizes and H-concentrations. Models containing two isolated hydrogen atoms indicate a preferred H-H distance of approximately 1.8 Å when the two atoms bond to nearest neighbor silicon atoms. This separation also occurs for models containing small, hydrogenated voids, but the configurations giving rise to this H-H distance do not appear to be unique. For larger voids, a proton separation of about 2.4Å is seen, as noted previously [2]. There appears to be consistency between the computed structures and the NMR data for configurations consisting of isolated hydrogen pairs or for clusters of an even number of hydrogen atoms with the constraint that the average H-H distance is 1.8 Å. In this paper, we will discuss the most probable bonding configurations of clustered hydrogen based upon the extent of the NMR data and simulated structures. INTRODUCTION The microstructure of hydrogenated voids in a-Si:H, and their potential role in the Staebler-Wronski effect, are not well understood. Early nuclear magnetic resonance (NMR) studies on films prepared by plasma enhanced chemical vapor deposition (PECVD) revealed that hydrogen occurs in isolated and clustered environments with a small fraction of H2 molecules existing in larger voids [3]. The intuitive idea of a hydrogenated divacancy gained support when multiple quantum NMR studies on PECVD films showed evidence of clusters of six hydrogen atoms [4]. Recently, a new hydrogen NMR signal has been observed by Bobela et al. in a number of PECVD prepared a-Si:H films [1]. It is speculated that the signal is the consequence of a dipolar-coupled hydrogen pair separated, on average, by 1.8 ± 0.1 Å. However, more complex hydrogen configurations, such as those resulting from hydrogenated voids or polysilane like chains, (SiH2)n can also explain the sometimes unusual dipolar-dipole powder patterns that have consistently emerged in a number of a-Si:H films. To elucidate the possible bonding configurations responsible for the NMR data of Ref. [1], we have used ab initio simulation methods to determine an ensemble of structures of a-Si:H with varying void sizes and Hconcentrations. Studies are still underway as to wheth
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