TRAP-LIMITED HYDROGEN DIFFUSION IN A-SI-H

Hydrogen transport in a-Si:H was investigated by deuterium diffusion experiments where the deuterium source was either a remote atomic deuterium plasma or a deuterated amorphous silicon layer. An enhanced hydrogen diffusion and a considerably lower diffusion activation energy (0.5 eV instead of 1.2-1.5 eV) is observed for diffusion from a plasma. This behavior is attributed to saturation of hydrogen configurations with high binding energy, which act as deep hydrogen traps and control hydrogen transport in diffusion experiments from a deuterated layer, by deuterium atoms injected from the plasma. Diffusion from a plasma is then dominated by hopping through states with low binding energy. A density-of-states distribution is derived for the hydrogen configurations controlling diffusion consisting of (i) a shallow state where hydrogen is weakly bonded and can diffuse with a small activation energy of 0.5 eV and (ii) deep states with binding energy larger than 1.2 eV and a total density of approximately 10(22) cm-3. In glow discharge a-Si:H, hydrogen is bonded to deep states and only a relatively small fraction (20-60%) of these states are empty.