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.