  The phenomenon of resonant tunneling transport through polar double-barrier
heterostructures is systematically investigated using a combined experimental
and theoretical approach. On the experimental side, GaN/AlN RTDs are grown by
MBE. In-situ electron diffraction is employed to monitor the number of
monolayers incorporated into each tunneling barrier. Using this precise
epitaxial control at the monolayer level, we demonstrate exponential modulation
of the resonant tunneling current as a function of barrier thickness. Both the
peak voltage and characteristic threshold bias exhibit a dependence on barrier
thickness as a result of the intense electric fields present in the polar
heterostructures. To get further insight into the asymmetric tunneling
injection, we present an analytical theory for tunneling transport across polar
heterostructures. A general expression for the resonant tunneling current with
contributions from coherent and sequential tunneling processes is introduced.
After applying this theory to the case of GaN/AlN RTDs, their experimental
current-voltage characteristics are reproduced over both bias polarities, with
tunneling currents spanning several orders of magnitude. This agreement allows
us to elucidate the role played by the internal polarization fields on the
magnitude of the tunneling current and broadening of the resonant line shape.
Under reverse bias, we identify new tunneling features originating from highly
attenuated resonant tunneling phenomena, which are completely captured by our
model. Our analytical model, provides a simple expression which reveals the
connection between the polar RTD design parameters and its current-voltage
characteristics. This new theory paves the way for the design of polar resonant
tunneling devices exhibiting efficient resonant current injection and enhanced
tunneling dynamics, as required in various practical applications.
