SEMICONDUCTOR SURFACE EFFECT DURING RECOMBINATION

In any real application, the semiconductor material isn't sufficiently large and therefore surfaces exist between the semiconductor and an adjacent medium. When a semiconductor is abruptly terminated, the perfect periodic nature of the idealized single-crystal  lattice ends abruptly at the surface. The disruption of the periodic potential function results in allowed electronic energy states within the energy bandgap. 

The Shockley-Read-Hall recombination theory shows that the excess minority carrier lifetime is inversely proportional to the density of trap states. So we may argue that since the density of trap states at the surface is larger than in the bulk, the excess minority carrier life time at the surface will be smaller than the corresponding life time in the bulk material. That means recombination process is faster in surface than bulk of the material.