A special HT-/LT-/HT-AlN sandwich buffer layer was used to diminish the strain between its upper or overlying GaN and underlying Si substrate. It is found that, by simply inserting a LT-AlN layer, one can get an almost crack free GaN layer on Si substrate. SEM images gave a clear observation of the effect of LT-AlN insertion layer on surface morphology. Further analysis of the residual stress in overlaying GaN layer also confirms the function of LT-AlN layer. After comparing the pit densities from AFM images and calculating the dislocations from XRD measurement for all samples, a proper growth temperature for the intermediate layer was found to improve the crystalline quality. On inserting 720 degrees C LT-AlN, the tensile stress could be reduced to only 0.36 GPa and an almost crack-free surface could be achieved. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.