  In this work, we investigate the use of nanoscale polarization engineering to
achieve efficient hole injection from metals to ultra-wide band gap AlGaN, and
we show that UV-reflective aluminum (Al) layers can be used for hole injection
into p-AlGaN. The dependence of tunneling on the work function of the metal was
investigated, and it was found that highly reflective Al metal layers can
enable efficient hole injection into p-AlGaN, despite the relatively low work
function of Al. Efficient tunneling hole injection was confirmed by light
emission at 326 nm with on-wafer peak external quantum efficiency and wall-plug
efficiency of 2.65% and 1.55%, respectively. A high power density of 83.7 W/cm2
was measured at 1200 kA/cm2. The metal/semiconductor tunnel junction structure
demonstrated here could provide significant advantages for efficient and
manufacturable device topologies for high power UV emitters.
