  In this letter, we report on unipolar vertical transport characteristics in
c-plane GaN/AlGaN/GaN heterostructures. Vertical current in heterostructures
with random alloy barriers was found to be independent of dislocation density
and heterostructure barrier height, and significantly higher than theoretical
estimates. Percolation-based transport due to random alloy fluctuations in the
ternary AlGaN is suggested as the dominant transport mechanism, and confirmed
through experiments showing that non-random or digital AlGaN alloys and
polarization-engineered binary GaN barriers can eliminate percolation transport
and reduce leakage significantly. The understanding of vertical transport and
methods for effective control proposed here will greatly impact III-nitride
unipolar vertical devices.
