Polarization engineering of III-Nitride materials in the nitrogen polarity has allowed for improved 2DEG confinement in GaN/AlGaN HEMTs and reduction in efficiency droop in MQW LEDs. Achieving p-type material through Mg doping continues to be a challenge in the nitrogen polarity compared to the gallium polarity with lower stability for substitutional replacement of Mg for Ga under nitrogen-rich conditions as calculated by density functional theory  and shown experimentally . High conductivity p-type material in the nitrogen polar orientation is needed for improved efficiency of a variety of III-nitride emitters, detectors and high-hole-mobility transistors (HHMTs).
In this work, GaN:Mg was overgrown on N-polar GaN templates on on-axis sapphire substrates by MOCVD. The underlying GaN template growths were optimized to achieve control over hillock density. Low and high hillock density templates were then used for overgrowth of GaN:Mg to study dependency of Mg incorporation efficiency on hillock density. Similar N-polar uGaN/GaN:Mg were epitaxially grown on high hillock density and low hillock density N-polar GaN/sapphire to form Cs-free photocathode detector devices. Photocathodes with high hillock density produced a quantum efficiency of 24.4% whereas low hillock density device produced an efficiency of only 0.8%, larger than 30X difference. To understand the origin of this difference, material quality and composition of these devices were studied. Secondary ion mass spectroscopy (SIMS) showed Mg concentration of 4x10^22 atoms/cm3 and 5x10^19 atoms/cm3 for high and low hillock density devices respectively. Atom probe tomography was performed to study incorporation and distribution of Mg within and at sidewalls of hillock structures.
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