Transport Properties of Polarization-Induced 2D Electron Gases in Epitaxial AlScN/GaN Heterojunctions*

AlScN is an attractive epitaxial heterostructure barrier for GaN high electron mobility transistors (HEMTs) due to its strong spontaneous and piezoelectric polarizations, larger dielectric constant than AlN [Appl. Phys. Lett. 120, 152901 (2022)], and the ability to be lattice-matched with GaN [Appl. Phys. Lett. 110, 162104 (2017)]. We report the epitaxial growth of AlScN/GaN heterostructures by plasma-assisted molecular beam epitaxy (MBE) and the study of transport characteristics of the polarization-induced 2D electron gases (2DEGs) using temperature-dependent magnetotransport measurement. By varying the thickness of lattice-matched AlScN on GaN, 2DEGs with high (~ 2 x 10¹³/cm²) and tunable carrier densities were achieved. The 2DEG formed at AlScN-GaN direct heterojunction exhibits mobilities ~ 300 cm²/V.s between 10 K and 300 K, comparable to those in Si MOSFETs. The insertion of a 2 nm AlN interlayer boosts the 2DEG mobility by more than five times to 1573 cm²/V.s at 300 K and by more than twenty times to 6980 cm²/V.s at 10 K, both are among the highest values reported to date. Theoretical modeling of the temperature-dependent Hall-effect mobilities shows that electron transport is limited by polar optical phonon scattering at temperatures above 200 K and interface roughness scattering at lower temperatures. The ability to epitaxially grow AlScN/GaN HEMTs with high electron mobility and tunable carrier density enables the merging of promising physical properties of epitaxial AlScN with established nitride semiconductors.