  We report on tunnel-injected deep ultraviolet light emitting diodes (UV LEDs)
configured with a polarization engineered Al0.75Ga0.25N/ In0.2Ga0.8N tunnel
junction structure. Tunnel-injected UV LED structure enables n-type contacts
for both bottom and top contact layers. However, achieving Ohmic contact to
wide bandgap n-AlGaN layers is challenging and typically requires high
temperature contact metal annealing. In this work, we adopted a compositionally
graded top contact layer for non-alloyed metal contact, and obtained a low
contact resistance of Rc=4.8x10-5 Ohm cm2 on n-Al0.75Ga0.25N. We also observed
a significant reduction in the forward operation voltage from 30.9 V to 19.2 V
at 1 kA/cm2 by increasing the Mg doping concentration from 6.2x1018 cm-3 to
1.5x1019 cm-3. Non-equilibrium hole injection into wide bandgap Al0.75Ga0.25N
with Eg>5.2 eV was confirmed by light emission at 257 nm. This work
demonstrates the feasibility of tunneling hole injection into deep UV LEDs, and
provides a novel structural design towards high power deep-UV emitters.
