Effect of a Lateral Overgrowth Process on the Strain Evolution of AlN Films Grown on a Nanopatterned Sapphire Substrate for Ultraviolet-C Light-Emitting Diode Applications

Strain evolution of AlN grown on hole-type (HT) and pillar-type (PT) nanopatterned sapphire substrates (NPSSs) during the epitaxial lateral overgrowth process is comparably studied. It is found that, after complete coalescence, the residual strain of AlN on HT-NPSS is tensile, whereas AlN on PT-NPSS is almost strain free. The different strain states are associated with different behaviors of crystal merging during the lateral overgrowth process, depending on the type of pattern. Then, ultraviolet-C (UVC) light-emitting diode (LED) wafers are grown on two types of AlN templates with the distinctive strain states. It is confirmed that AlN on HT-NPSS with tensile strain offers the advantage of producing a high-quality AlGaN layer with a fairly flat surface, which is the key point for fabricating high-performance UVC LEDs.