Strain relaxation in AlGaN/GaN superlattices grown on GaN

Abstract

Lattice relaxation of strained AlxGa1−xN/GaN superlattices grown on thick GaN buffer layers is investigated using optical microscopy, x-ray diffraction, and photoluminescence spectroscopy. The results are compared to strained bulk AlxGa1−xN layers particularly with regard to the impact of the superlattice period and the Al content. A relaxation process which keeps the coherency between AlxGa1−xN barriers and GaN wells in the superlattice is found and it is attributed to misfit dislocations at the buffer/superlattice interface. Additionally, the AlxGa1−xN barriers relax via crack channels which form beyond a critical Al content and limit the additional strain energy compared to a free-standing superlattice to a maximum value. Cracks relieve tensile plane stress to an extent similar as in bulk layers, i.e., they do not put the GaN wells of the superlattice under additional plane compression. This is explained by misfit dislocations which nucleate at crack faces and glide into the superlattice at the well/barrier interfaces. The onset of cracking is found to shift to higher tensile stresses in the AlxGa1−xN barriers when increasing the superlattice period which is discussed in view of edge cracks being the starting point of crack channels.