Voids in Hydrogenated Amorphous Silicon: A Comparison of ab initio Simulations and Proton NMR Studies

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 angstrom. 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 angstrom 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 2.4 angstrom 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 angstrom. 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.