Inverse-Tapered p-Waveguide for Vertical Hole Transport in High-[Al] AlGaN Emitters

We report a high-aluminum-containing ([Al] $\,\,{\sim }\,\,0.6$ ) AlGaN multiple-quantum well (MQW) double-heterojunction (DH) emitter employing an inverse-tapered-composition AlGaN:Mg p-type waveguide grown on a $c$ plane Al-polar AlN bulk substrate. Using numerical simulations, we have determined that the inverse-tapered p-type waveguide design is necessary for high [Al] containing p-n junction devices as any valence band discontinuity at the junction will limit the vertical hole transport and induce a larger voltage-drop across the structure. The fabricated ultraviolet MQW DH emitter can sustain a DC current of at least 500 mA and a pulsed current of at least 1.07 A, which corresponds to a current density of 10 and 18 kA/cm $^{2}$ at maximum measured voltage of 15 and 20 V with the measured series resistance of 15 and 11 $\Omega $ , respectively.