  We show that GdN nanoislands can enhance inter-band tunneling in GaN PN
junctions by several orders of magnitude, enabling low optical absorption
low-resistance tunnel junctions (specific resistivity 1.3 X 10-3 {\Omega}-cm2)
for various optoelectronic applications. We exploit the ability to overgrow
high quality GaN over GdN nanoislands to create new nanoscale heterostructure
designs that are not feasible in planar epitaxy. GdN nanoisland assisted
inter-band tunneling was found to enhance tunneling in both of the polar
orientations of GaN. Tunnel injection of holes was confirmed by low temperature
operation of GaN p-n junction with a tunneling contact layer, showing strong
electroluminescence down to 20K. The availability of tunnel junctions with
negligible absorption could not only improve the efficiency of existing
optoelectronic devices significantly, but also enable new electronic and
optical devices based on wide band gap materials.
