Influence of substrate nitridation conditions and buffer layer structures on the crack-free GaN layers on silicon substrate grown by ammonia-assisted molecular beam epitaxy

It is shown that the nitridation conditions of the silicon substrate are not less important than the structure of the buffer layer in influencing the crack formation in GaN-on-Si layers grown by ammonia-assisted molecular beam epitaxy (NH3-MBE). Furthermore, the morphology of the GaN layers is independent of the buffer layer structure, but is determined by the GaN growth conditions. Nevertheless, the use of step-graded AlGaN buffer layers instead of a buffer layer with an inserted AlN layer leads to a higher crystalline quality of the crack -free GaN layers and to lower residual tensile stresses in them.