Periodic Inversion Domains on Step-Flow Surfaces of N-Polar GaN Grown on <i>m</i> -Plane Offcut 4H-SiC(0001)

Understanding the atomic-scale interfaces in advanced semiconductor heterostructures is essential for controlling defects, optimizing material properties, and ensuring device reliability. In this work, we present a comprehensive study of the atomic structure at Al(Ga)N interfaces in N-polar GaN high-electron-mobility transistor structures (HEMTs) via aberration-corrected annular dark field scanning transmission electron microscopy (ADF STEM). We investigate heterostructures grown on 4H-SiC (0001̅) with different offcut angles toward the m-plane, a crucial platform for high-quality N-polar layers that have not been thoroughly characterized. Here, we demonstrate periodic vertical inversion domain boundaries (IDBs) on step-flow-grown N-polar GaN via a multistep hot-wall metal–organic chemical vapor deposition (MOCVD) process on 4° m-plane offcut 4H-SiC (0001̅). We directly confirm the polarity by ADF STEM and conclude that two terraces with opposing polarity coexist on 4° m-plane offcut surfaces. In contrast, an offcut angle of 1° does not lead to the formation of periodic vertical IDBs but results in hexagonal hillocks and surface pits of nanometer size. The vertical IDBs originate from the interface between the reconstructed 4° m-plane–plane offcut of 4H-SiC and the AlN nucleation layer. The ratio between the N-polar and metal-polar surface area is proportional to the step density at the 4H-SiC surface, which can be controlled by the offcut angle. The presence of these periodic vertical IDBs were undetected by conventional X-ray diffraction measurements and may affect the long-term stability of N-polar GaN-based high-power devices.