  Ultra violet light emitting diodes (UV LEDs) face critical limitations in
both the injection efficiency and light extraction efficiency due to the
resistive and absorbing p-type contact layers. In this work, we investigate the
design and application of polarization engineered tunnel junctions for
ultra-wide bandgap AlGaN (Al mole fraction higher than 50%) materials towards
highly efficient UV LEDs. We demonstrate that polarization-induced 3D charge is
beneficial in reducing tunneling barriers especially for high composition AlGaN
tunnel junctions. The design of graded tunnel junction structures could lead to
low tunneling resistance below 10-3 Ohm cm2 and low voltage consumption below 1
V (at 1 kA/cm2) for high composition AlGaN tunnel junctions. Experimental
demonstration of 292 nm emission was achieved through non-equilibrium hole
injection into wide bandgap materials with bandgap energy larger than 4.7 eV,
and detailed modeling of tunnel junctions shows that they can be engineered to
have low resistance, and can enable efficient emitters in the UV-C wavelength
range.
