Hydrogen transport in amorphous silicon

Abstract

The diffusive transport of hydrogen is used to investigate H trapping in hydrogen-depleted amorphous Si (a-Si) samples and to determine a rough H-diffusion density of states. The diffusion profiles show clear evidence of deep traps separated from shallow traps, and the results are well explained by a simple division of the H states into deep traps, shallow traps, and transport states. The concentration of deep traps is about (0.8--2)\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, of which about 30% can be identified with dangling bonds. The energy of the deep traps is at least 1.9 eV below the transport states. The diffusion is dispersive with a power-law time dependence and can be characterized by an exponential distribution of hopping barriers with a width of roughly 0.09 eV. The shallow traps are identified with clustered H pairs which determine the H chemical potential at high H concentrations. The results are compared with calculations and other recent ideas on H bonding energetics. The results are consistent with a range of possibilities. One extreme is the case in which H is predominately bonded on void surfaces and the transport energy is substantially different in a-Si than in crystalline Si (c-Si); the other extreme is the case in which H predominately resides in platelet structures and the transport energy is roughly the same as in c-Si. The actual case depends on the deposition conditions.