The diffusive motion of H was used to investigate H-trapping in H-depleted amorphous samples, and to determine an approximate H-diffusion density of states. The diffusion profiles revealed clear evidence of deep traps which were separate from shallow traps, and the results could be explained in terms of a simple division of the H states into deep traps, shallow traps, and transport states. The concentration of deep traps was between 8 x 1019 and 2 x 1020/cm3; of which about 30% could be identified with dangling bonds. The energy of the deep traps was at least 1.9eV below the transport states. The diffusion was dispersive, with a power-law time dependence, and could be characterized by an exponential distribution of hopping barriers with a width of about 0.09eV. The shallow traps were identified with clustered H pairs which determined the H chemical potential at high H concentrations. It was suggested that the results were consistent with various possibilities. One extreme was the case where H was bonded mainly at void surfaces and the transport energy was very different in amorphous material, as compared with crystalline material. The other extreme was the case where H resided mainly in platelet structures, and the transport energy was essentially the same as in crystalline material. The actual situation depended upon the deposition conditions.

W.B.Jackson, C.C.Tsai: Physical Review B, 1992, 45[12], 6564-80