  In this letter, we introduce a geometric model to explain the origin of the
observed shallow levels in semiconductors threaded by a dislocation density. We
show that a uniform distribution of screw dislocations acts as an effective
uniform magnetic field which yields bound states for a spin-half quantum
particle, even in the presence of a repulsive Coulomb-like potential. This
introduces energy levels within the band gap, increasing the carrier
concentration in the region threaded by the dislocation density and adding
additional recombination paths other than the near band-edge recombination.
