Diffusion-drift-reaction model for positron trapping and annihilation at interfaces with space charge in bulk materials

The exact solution of a diffusion-drift-reaction model for the trapping and annihilation of positrons at interfaces with adjacent space-charges in bulk materials is presented. Closed-form expressions are obtained for the mean positron lifetime and for the intensity of the positron lifetime component associated with trapping at interfaces. The exact solutions can be conveniently applied for the analysis of experimental data. Compared to available thin-film models, the present approach takes into account that drift usually occurs only in a part of the diffusion zone. The model clearly reveals that for typical drift-zone parameters, positron drift has a substantial impact on the positron annihilation characteristics, that is, disregarding existing drift could lead to misjudgement of experimental positron annihilation data.