  We investigated the origin of the high reverse leakage current in light
emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by
molecular beam epitaxy on Si substrates. To this end, capacitance deep level
transient spectroscopy (DLTS) and temperature-dependent current-voltage (I-V)
measurements were performed on a fully processed NW-LED. The DLTS measurements
reveal the presence of two distinct electron traps with high concentrations in
the depletion region of the p-i-n junction. These band gap states are located
at energies of $570\pm20$ and $840\pm30$ meV below the conduction band minimum.
The physical origin of these deep level states is discussed. The
temperature-dependent I-V characteristics, acquired between 83 and 403 K, show
that different conduction mechanisms cause the observed leakage current. On the
basis of all these results, we developed a quantitative physical model for
charge transport in the reverse bias regime. By taking into account the mutual
interaction of variable range hopping and electron emission from Coulombic trap
states, with the latter being described by phonon-assisted tunnelling and the
Poole-Frenkel effect, we can model the experimental I-V curves in the entire
range of temperatures with a consistent set of parameters. Our model should be
applicable to planar GaN-based LEDs as well. Furthermore, possible approaches
to decrease the leakage current in NW-LEDs are proposed.
