Hydrogen diffusion ina-Si:H: Solution of the tracer equations including capture by exchange

We propose a model of trap-controlled diffusion in which diffusing atoms can be captured either by trapping at an empty trap or by exchange with chemically identical trapped atoms of different isotope number. We solve the model's equations describing tracer diffusion experiments, and compare the predictions in the cases of dominant exchange and of dominant trapping. Comparison with experiments in a-Si:H shows that exchange is the dominant capture mechanism in D tracer diffusion measurements. This allows us to understand the relatively short distance ({~}200 {}) D travels before trapping. It also reconciles the smoothness of the D tracer diffusion profiles at long times with the existance of deeply bound (>2.0 eV) H in a-Si:H.