Determination of Sapphire Off‐Cut and its Influence on the Morphology and Local Defect Distribution in Epitaxially Laterally Overgrown AlN for Optically Pumped UVC Lasers

摘要： In this paper a systematic study of the morphology and local defect distribution in epitaxial laterally overgrown (ELO) AlN on c-plane sapphire substrates with different off-cut angles is presented. Precise measurements of the off-cut angle, using a combination of optical alignment and X-ray diffraction with an accuracy of ±5° for the off-cut direction and ±0.015° for the off-cut angle were performed. For ELO AlN growth a transition from step flow growth at a < 0.14° with height undulations on the surface to step bunching with step heights up to 20 nm for a > 0.14° was observed. Furthermore, the terraces of the step bunched surface exhibit curved steps. An analysis of the local defect distribution by scanning transmission electron microscopy and a comparison with atomic force microscopy reveals a bunching of defects in line with the ELO pattern and a roughening of step edges in highly defective regions. In addition, a reduction of the threshold excitation power density for optically pumped UVC lasers with smooth surface morphologies was observed.