Thickness-dependent polarization modulation at AlN interlayers in GaN heterostructures revealed by atomic-scale 4D-STEM

The performance of GaN-based high-electron-mobility transistors (HEMTs) hinges on the two-dimensional electron gas (2DEG) concentration induced by polarization fields at heterojunction interfaces. The AlN interlayer, critical for optimizing interfaces and 2DEG transport, requires atomic-scale understanding of its thickness-dependent polarization modulation, especially at sub-nanoscales (<1 nm). Using four-dimensional scanning transmission electron microscopy, polarization fields at AlGaN/AlN/GaN interfaces with 0.5 nm and 1 nm AlN interlayers are characterized. The sample with a 1 nm interlayer exhibits two opposite electric fields, while the sample with a 0.5 nm interlayer exhibits only one unidirectional field. Geometric phase analysis reveals strain transfer in the sample with a 0.5 nm interlayer, with almost no strain (rather than obvious compressive strain) at the AlGaN lower interface. Quantitative analyses further demonstrate stronger polarization fields and higher negative polarization charge density on the upper interface of GaN in the sample with a 1 nm interlayer, corresponding to the lower on-resistance (higher 2DEG concentration) in HEMTs. This work establishes atomic-scale correlations among AlN thickness, strain, and polarization fields, uncovers sub-nanoscale critical size effects, and guides high-performance HEMT design.