Inversion domains (IDs) are common defects in N-polar III-nitride thin films grown on sapphire substrates. In this work, the atomic structure and lateral migration of the randomly distributed Al-polar nanoscale IDs in N-polar AlN films subjected to high-temperature thermal annealing are investigated. With the increasing annealing temperature and time, Al-polar AlN IDs gradually shrunk in sizes. The vertical stripes transformed into cone-shaped caps on top of the AlN columns and were completely removed at last. The annihilation of the IDs was explained in terms of the lateral migration of the inversion domain boundary (IDB) induced by the imbalance of the strain state on two sides of the IDB. This work clarifies the evolution mechanism of AlN IDs during high-temperature annealing, providing a promising approach in the realization of uniform-polar AlN template for the development of high-efficiency optoelectronic and electronic devices.

GaN electronics have hinged on invasive isolation such as mesa etching and ion implantation to define device geometry, which, however, suffer from damages, hence potential leakage paths. In this study, we propose a new paradigm of polarization isolation utilizing intrinsic electronic properties, realizing in situ isolation during device epitaxy without the need of post-growth processing. Specifically, adjacent III- and N-polar AlGaN/GaN heterojunctions were grown simultaneously on the patterned AlN nucleation layer on c-plane sapphire substrates. The two-dimensional electron gas (2DEG) was formed at III-polar regions but completely depleted in N-polar regions, thereby isolating the 2DEG channels with a large 3.5 eV barrier. Structures of polarization-isolated high electron mobility transistors (PI-HEMTs) exhibit significantly reduced isolation leakage currents by up to nearly two orders of magnitude at 50 V voltage bias compared to the state-of-the-art results. Aside from that, a high isolation breakdown voltage of 2628 V is demonstrated for the PI-HEMT structure with 3  μm isolation spacing, which is two-times higher than a conventional mesa-isolation HEMT. Moreover, the PI-HEMT device shows a low off-state leakage current of 2 × 10−8 mA/mm with a high Ion/Ioff ratio of 109 and a nearly ideal subthreshold slope of 61 mV/dec. This work demonstrates that polarization isolation is a promising alternative toward the plasma-damage-free isolation for GaN electronics.