Investigation of coherency stress-induced phase separation in AlN/AlxGa1-xN superlattices grown on sapphire substrates

AlN/AlGaN superlattices (SLs) have been proven promising for various applications including deep ultraviolet light-emitting diodes. However, phase separation in AlGaN of SLs leads to undesirable changes in the optical and electrical properties of SL-based devices. Herein, we investigate the influence of the pulse duration on the compositional structure and surface morphology of the upper AlGaN layer of AlN/AlGaN SLs, grown on a CVD-deposited single crystal AlN template/sapphire substrate by pulsed metal-organic chemical vapor deposition (MOCVD). Spectral transmittance measurements reveal that phase separation happened in the AlxGa1-xN layer of SLs with inhomogeneous distribution of the aluminum composition. The coherency stress and strain profiles of AlN/AlGaN SLs were evaluated by finite element simulations. High compressive stress in the upper AlxGa1-xN layer and tensile stress in the underlying AlN layer were observed for the SLs with a long pulse duration. This increased stress in SLs facilitates the exclusion of aluminum atoms, thus leading to the apparent phase separation in the upper AlxGa1-xN layer of SLs. Additionally, the effect of the shear strain component at the interfaces on the piezoelectric polarization of epitaxial layers was also discussed. This study paves the way for preventing the phase separation during the AlN/AlGaN SL growth by controlling the pulse mode configuration, thereby offering new perspectives for the growth of high-quality AlGaN epitaxial layers targeted for practical applications.