P -type ionization level lowering in ultrawide bandgap Al 1 − x Ga x N [ 11 2 ¯ 0 ] digital alloys

Efficient $p$-type doping is essential and challenging for applications of ultra-wide-bandgap AlGaN with a high aluminum content. Although the band-edge-assisted ionization enabled by the superlattice of [0001] origination can reduce the ionization level due to the raised valence band maximum (VBM), the indispensable electric field induced by the spontaneous and piezoelectric polarization is in the entire range of alloy and quite negative to the photoelectric applications. In this work, we propose that band-edge-assisted ionization can be activated even without the universal electric field, which forms an ultralow ionization level of $\ensuremath{-}0.1$ eV for the ${\mathrm{Al}}_0.8{\mathrm{Ga}}_0.2\mathrm{N}\phantom{\rule{4pt}{0ex}}[11\overline{2}0]$ digital alloy. The energy reduction is triggered by two factors: the intensive VBM states dominated by the coupling between GaN layers and the intriguing decrease in the unoccupied ${p}_z$ level, which is induced by a symmetry reduction from ${C}_3v$-like to ${C}_s$ at the dopant site. The transition of symmetry is triggered by a perturbation from the GaN layers, which is caused by a chainlike coupling of ${p}_x$ states between nitrogen at the VBM and dopant sites and can be partially canceled depending on the dopant sites. In addition to the conventional band bending in band-edge-assisted ionization, the reduction of ionization level activated by long-ranged orbital coupling offers a new view for efficient $p$-type doping in AlGaN systems.