Revisiting the epitaxial Si3N4 crystalline cap on AlGaN/GaN via evolutionary structure search

In our recent experimental work [Luo et al., Appl. Phys. Lett. 125, 122109 (2024)], we observed that crystalline Si3N4 cap layers, a few nanometers thick, can form in situ on GaN surfaces. Compared with amorphous SiO2 and Al2O3 caps, these crystalline caps yield cleaner GaN/Si3N4 interfaces with fewer defects and improved device metrics. These observations motivate two questions: why does Si3N4 farther from the interface become amorphous as the cap thickens, and what is the actual crystal structure of the interfacial Si3N4? Prior work proposed a defect-wurtzite (DW) model constructed heuristically from β-Si3N4 and the AlGaN lattice constants, but it is significantly higher in energy than β-Si3N4 and disagrees with experiment in both interlayer spacings and electronic bandgap. Using a systematic structure search approach under in-plane lattice constraints commensurate with AlGaN, we identify a lower-energy configuration, denoted Lam-Si3N4, with quasi-two-dimensional (laminar) stacking normal to the interface. Under AlGaN-matched metrics, Lam-Si3N4 is about 60 meV/atom more stable than DW-Si3N4 and reproduces the experimentally observed interlayer spacings more closely. The substantial lattice mismatch explains amorphization when the crystalline cap grows far from the interface. Upon full relaxation, both DW- and Lam-Si3N4 exhibit wide ∼4 eV bandgaps. Under AlGaN constraints, the DW gap collapses to ∼1.88 eV, whereas Lam-Si3N4 maintains a larger ∼2.70 eV gap (for reference, PBE gaps: GaN 1.73 eV, AlN 4.05 eV). The wider gap and improved structural match of Lam-Si3N4 rationalize the superior capping performance and provide guidance for optimizing AlGaN/GaN device encapsulation.