The insertion of an AlN interlayer for tensile strain relief in GaN thin films grown on Si(1 1 1) on-axis and vicinal substrates by nitrogen rf plasma source molecular beam epitaxy has been investigated. The 15 nm AlN interlayer was inserted between a bottom 0.5 mu m GaN layer and the top 1.0 mu m GaN layer. The interlayer was effective in reducing the tensile stress to the level required for complete avoidance of microcracks, which were present in high densities in the case of GaN-on-Si heterostructures grown without an AlN interlayer. The strain in all the layers of the heterostructure was analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Reciprocal space mapping in XRD indicated that the 15 nm AlN interlayer was on average coherently strained with the GaN. However, TEM observations showed that the interlayer was partially relaxed in local regions. The AlN interlayer was also observed to interfere with the GaN growth process. In particular, above morphological features such as v-shaped surface depressions, GaN was overgrown with a high density of threading dislocations and inversion domain boundaries. A synergistic relaxation mechanism is proposed for the AlN interlayer leading to an elastically strained interlayer interconnected by plastically relaxed patches. (C) 2008 Elsevier B.V. All rights reserved.