  We investigate the impact of threading dislocations with an edge component (a
or a+c-type) on carrier recombination and diffusion in GaN(0001) layers close
to the surface as well as in the bulk. To this end, we utilize
cathodoluminescence imaging of the top surface of a GaN(0001) layer with a
deeply buried (In,Ga)N quantum well. Varying the acceleration voltage of the
primary electrons and comparing the signal from the layer and the quantum well
enables us to probe carrier recombination at depths ranging from the close
vicinity of the surface to the position of the quantum well. Our experiments
are accompanied by fully three-dimensional Monte Carlo simulations of carrier
drift, diffusion, and recombination in the presence of the surface, the quantum
well, and the dislocation, taking into account the dislocation strain field and
the resulting piezoelectric field at the dislocation outcrop. Near the surface,
this field establishes an exciton dead zone around the dislocation, the extent
of which is not related to the carrier diffusion length. However, reliable
values of the carrier diffusion length can be extracted from the dipole-like
energy shift observed in hyperspectral cathodoluminescence maps recorded around
the dislocation outcrop at low acceleration voltages. For high acceleration
voltages, allowing us to probe a depth where carrier recombination is
unaffected by surface effects, we observe a much stronger contrast than
expected from the piezoelectric field alone. This finding provides unambiguous
experimental evidence for the strong nonradiative activity of edge threading
dislocations in bulk GaN and hence also in buried heterostructures.
