Interfacial engineering for semi-insulating GaN/sapphire template with low dislocation density

•Interfacial engineering for semi-insulating and low-TDD GaN epilayer on sapphire.•Ultrathin sputtered AlN interfacial layer is nano-pixelated into discrete islands.•Adequate 3D growth of GaN on AlN buffer results in a low TDD of 2.7 × 108 cm−2.•EELS mapping reveals O concentration plummets at GaN/AlN interface.•GaN epilayer grown on the ultrathin AlN buffer has a high Rs of 2.43 × 1011 Ω/sq.