Development of Sandwich AlN on PSS via Layered V/III Ratio Optimization

This study presents a comprehensive investigation into the effects of layer-by-layer V/III ratio optimization on the structural, optical, and morphological properties of AlN films grown on patterned sapphire substrates (PSS) using the PALE technique. A sandwich structure was designed by sequentially applying low and high ammonia flows. Among the samples, the two-layer Sample B, grown with a low V/III ratio followed by a high one, exhibited the best crystalline quality, as indicated by the narrowest peaks in rocking curve analyses. In contrast, Sample A, grown as a single layer with a low V/III ratio, maintained the substrate’s pattern geometry but showed increased structural irregularities due to limited vertical growth. Sample C, despite having a sandwich configuration, showed inferior surface and boundary morphology because the growth started under low ammonia flow, which negatively impacted crystalline alignment. Reflectance measurements revealed no significant shift in the band edge positions across the samples, indicating similar bandgap energies. However, the rougher surface morphology of Sample C led to slightly enhanced light scattering in the ultraviolet range, suggesting a less uniform optical surface. These findings demonstrate that carefully tuned V/III ratio sequencing—considering layer thickness and interface stability—can significantly enhance both the crystalline and surface quality of AlN/PSS heterostructures.