High-mobility window for two-dimensional electron gases at ultrathin AlN∕GaN heterojunctions

High-conductivity two-dimensional electron gases at AlN∕GaN heterojunctions are reported. The sheet densities can be tuned from ∼5×1012∕cm2to∼5×1013∕cm2 by varying the AlN thickness from 2to7nm. A critical thickness is observed beyond which biaxial strain relaxation and cracking of AlN occur, and a degradation of carrier mobility is seen to occur at extremely high sheet densities. A high-mobility window is identified, within which room-temperature mobility exceeding 1000cm2∕Vs. and sheet densities in the (1–3)×1013∕cm2 are obtained, yielding record low sheet resistances in the range of ∼170Ω∕◻. Interface roughness scattering and strain relaxation are identified as the factors preventing lower sheet resistances at present.