Compatibilize surface crack and crystallinity in 10- μ m-thick AlN epilayer on 4-inch sapphire substrates

Abstract Achieving aluminum nitride (AlN) epilayers with dislocation densities below 10 7 cm −2 on sapphire remains critical for ultraviolet (UV) optoelectronics applications. However, the lattice and thermal mismatches inherent to heteroepitaxial growth hinder the simultaneous suppression of threading dislocations and surface cracking. In this work, a 10.2- μ m-thick, 4-inch AlN film was fabricated on an AlN/sapphire substrate. A strain-modulated buffer was embedded beneath the AlN epilayer to pre-introduce a well-balanced compressive strain, which counteracted tensile strain accumulation during thick-layer growth while maintaining continuous two-dimensional epitaxy for effective defect suppression. This strain management strategy, combined with progressive dislocation annihilation as the layer thickness increased, yielded a surface dislocation density of 7.6 × 10 6 cm −2 and limited cracking to within approximately 2 mm from the wafer edge. This scalable and cost-effective approach enables the growth of crack-suppressed, high-quality AlN epilayers on sapphire, offering a practical pathway for UV optoelectronic devices in light of the current limitations of bulk AlN substrates.