Atomic insights into strain-induced nanoscopic compositional fluctuation in AlGaN quantum well epitaxy

Identical UVC light-emitting diode AlGaN growth process was applied on two high-quality AlN buffer layers with different initial dislocation densities. We characterized the AlGaN epi by high-resolution scan transmission electron microscopy (HR-STEM) and atom probe tomography (APT). Geometry phase analysis with HR-STEM revealed the sample grown on the more compressively strained template possesses a more pronounced local lattice constant distortion in the multiple-quantum-well (MQW) region. APT further verified the local lattice distortion originated from the strain-induced Ga segregation in the MQW. The comparative compositional analysis revealed the nanoscopic compositional fluctuation favors further incorporation of Ga during epitaxial growth, which triggered the AlGaN growth front destabilization and new defect nucleation. The 1-D composition profiling from both electron dispersive microscopy and APT suggested nitrogen desorption during the AlGaN stress relaxation.