Reduced Surface and Grain Boundary Recombination

A simple model to explain these results has been proposed (38) that invokes splitting of surface or grain boundary states resulting from a strong interaction of these surface states with chemi-sorped species. The split states lie outside the band gap, and hence are inactive as recombination centers. Ions which do not 

Strongly chemisorb (such as Bi3+), do not split the surface states sufficiently and do not effect recombination velocities. 

Similar passivation effects of atomic hydrogen on grain boundaries in polycrystalline silicon have also been reported (39). However, the explanation here is that the surface or grain boundary states are filled upon interaction with covalently bonding atoms, and these filled states then become inactive as recombination centers. Further work is required to establish the correct model. 

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