Stress Relaxation in Low-Strain AlInN/GaN Bragg Mirrors

The structural behaviour of lattice mismatched AlInN/GaN-based multilayer structures grown on sapphire substrates by metal organic vapour phase epitaxy has been studied in-situ by curvature and real-surface temperature measurement, and ex-situ by atomic force microscope, field-emission secondary electron microscope, Nomarski microscope and various X-ray diffraction techniques. Samples grown with AlInN layers obtaining a slightly lower and a higher indium concentration compared to the lattice matched samples show a strong degradation of the AlInN/GaN double layers. The degradation of the multilayer structures both on a microscopic and on a macroscopic scale is investigated, revealing a much stronger roughening of the AlInN surfaces with higher indium concentration. The roughening is attributed to a Stranski--Krastanov transition from two-dimensional to three-dimensional growth mode.