Microscopic origin of light-induced ESR centers in undoped hydrogenated amorphous silicon

29Si hyperfine (hf) structures of light-induced electron-spin-resonance (LESR) centers of g=2.004 and 2.01 have been investigated in undoped hydrogenated amorphous silicon (a-Si:H) with different 29Si content (1.6, 4.7,9.1 at. %) by means of pulsed and multifrequency (3,11,34 GHz) ESR techniques. We have experimentally deconvoluted overlapping LESR signals using the difference in the spin-lattice relaxation time between the two signals. The deconvoluted 29Si hf structure of g=2.004 indicates that the wave function of the g=2.004 center spreads mainly over two Si atoms. Accordingly, we propose that the origin of g=2.004 is electrons trapped in antibonding states of weak Si-Si bonds rather than those trapped at positively charged dangling bonds. The isotropic hf splittings were estimated to be around 7 mT for g=2.004 and below 3 mT for g=2.01, which are in good agreement with characteristics of the antibonding and bonding states of the weak Si-Si bond. We suggest, from our 29Si hf data and other experimental findings, that the g=2.004 center is localized spatially more than conduction-band-tail electrons detected by photoluminescence