High-density polarization-induced 2D electron gases in N-polar pseudomorphic undoped GaN/Al0.85Ga0.15N heterostructures on single-crystal AlN substrates

The polarization difference and band offset between Al(Ga)N and GaN induce two-dimensional (2D) free carriers in Al(Ga)N/GaN heterojunctions without any chemical doping. A high-density 2D electron gas (2DEG), analogous to the recently discovered 2Dhole gas in a metal-polar structure, is predicted in a N-polar pseudomorphic GaN/Al(Ga)N heterostructure on unstrained AlN. We report the observation of such 2DEGs in N-polar undoped pseudomorphic GaN/AlGaN heterostructures on single-crystal AlN substrates by molecular beam epitaxy. With a high electron density of similar to 4.3 x 10(13)/cm(2) that maintains down to cryogenic temperatures and a room temperature electron mobility of similar to 450 cm(2)/V s, a sheet resistance as low as similar to 320 Omega/square is achieved in a structure with an 8 nm GaN layer. These results indicate significant potential of AlN platform for future high-power RF electronics based on N-polar III-nitride high electron mobility transistors. Published under an exclusive license by AIP Publishing.