  High-quality N-polar GaN p-n diodes are realized on single-crystal N-polar
GaN bulk substrate by plasma-assisted molecular beam epitaxy. The
room-temperature current-voltage characteristics reveal a high on/off current
ratio of 10^11 at 4 V and an ideality factor of 1.6. As the temperature
increases to 200 C, the apparent ideality factor gradually approaches 2. At
such high temperatures, Shockley-Read-Hall recombination times of 0.32-0.46 ns
are estimated. The measured electroluminescence spectrum is dominated by a
strong near-band edge emission, while deep level and acceptor-related
luminescence is greatly suppressed. A relatively high reverse breakdown field
of 2.4 MV/cm without field-plates is achieved. This work indicates that the
quality of N-polar GaN diodes is now approaching to that of their
state-of-the-art Ga-polar counterparts.
