Strain relaxation in AlGaN/GaN superlattices grown on GaN

Lattice relaxation of strained AlxGa1-xN/GaN superlattices grown on thick G aN buffer layers is investigated using optical microscopy, x-ray diffractio n, and photoluminescence spectroscopy. The results are compared to strained bulk AlxGa1-xN layers particularly with regard to the impact of the superl attice period and the Al content. A relaxation process which keeps the cohe rency between AlxGa1-xN barriers and GaN wells in the superlattice is found and it is attributed to misfit dislocations at the buffer/superlattice int erface. Additionally, the AlxGa1-xN barriers relax via crack channels which form beyond a critical Al content and limit the additional strain energy c ompared 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 compressio n. This is explained by misfit dislocations which nucleate at crack faces a nd 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 bar riers when increasing the superlattice period which is discussed in view of edge cracks being the starting point of crack channels. (C) 2001 American Institute of Physics.