COMPUTER-SIMULATION OF PHOTOCONDUCTIVITY DECAY IN AGBR MICROCRYSTALS - A 2-CENTER MODEL

A model for photoconductivity decay in which initial trapping occurs a t surface traps, followed by deep trapping at volume traps, is studied . The Monte Carlo simulations follow the random walk of the electron a nd hole following excitation. The volume traps are assumed to be rando mly dispersed, but their effect on electron motion is simulated by tra pping at the boundaries of concentric cubes that span the volume of th e microcrystal. The probability of trapping at the boundaries is a fun ction of the radius and concentration of traps. Although this model is a necessary component in any model to study doping effects in AgBr mi crocrystals, it fails to produce the correct dependence of decay time on microcrystal edge length observed in undoped microcrystals. This fa ilure arises because the lower surface-to-volume ratio in large microc rystals causes electrons to spend less time at surface traps, which ma kes them more prone to volume trapping. Experimentalists have determin ed trapping radii of dopants by plotting trapping Fates against dopant concentration and extracting the trapping radius from the slope of th e linear relationship. A similar linear relationship was seen in the s imulation results, but the trapping radius required to reproduce the e xperimental slopes was a factor of two to three larger than that deter mined experimentally. This discrepancy is most probably due to neglect of electron-hole recombination in deriving equations used to calculat e the experimental trapping radius.