Extremely high-density ($>10^{14}$cm$^{-2}$) with very low sheet resistance 2D electron gases in N-polar AlGaN/GaN heterostructures with GaN/AlN superlattice back barriers grown on sapphire substrates

We report on the realization of extremely high-density ($>10^{14}$cm$^{-2}$) 2D electron gases in N-polar AlGaN/GaN heterostructures grown on sapphire substrates. By introduction the GaN/AlN superlattice (SL) back barrier between GaN buffer layer and AlGaN barrier layer we observed a giant enhancement of the 2D electron gas density at the GaN/Al$_{0.3}$Ga$_{0.7}$N interface from $3\times10^{13}$cm$^{-2}$ (without SL) to $1.4\times10^{14}$cm$^{-2}$ (with SL back barrier) that is only an order of magnitude below the intrinsic crystal limit of $\approx10^{15}$cm$^{-2}$. We found that the changes of 2D electron gas density with SL correlated well with changes of the wafer warp parameter which suggested the strain-induced 2D electron gas density. Simultaneously, the room temperature electron mobility was as high as 169 cm$^2$/Vs, which with the electron density of $1.4\times10^{14}$cm$^{-2}$ gives a very low sheet resistance of 264 $\Omega$/sq (one of the lowest reported so far for the N-polar 2D electron gas channel). The finding of the very high-density 2D electron gas with very low sheet resistance in N-polar III-nitride heterostructures can push the power performance N-polar GaN HEMTs into a new level which is unreachable by the conventional N-polar GaN HEMTs.