Significant wafer curvature has been observed for AlGaN/GaN high electron mobility transistor (HEMT) structures grown on SiC substrates by rf plasma molecular-beam epitaxy. The curvature is caused by residual compressive strain in the films, due primarily to the lattice mismatch between substrate and epilayer. The wafers exhibit more bow when an AIN nucleation layer is used, than when GaN/AlGaN is grown directly on SiC. However, in test structures, AIN nucleation layers are found to impart tensile strain in the wafer that is small due to the AIN thickness. Using high resolution x-ray diffraction with reciprocal space maps, thin GaN films are found to relax more readily when grown directly on SiC substrates than on AIN buffer layers. The compressive strain in the thick GaN buffer layer grown on AIN bows the wafer and increases the substrate x-ray diffraction (XRD) linewidth. The GaN buffer, despite its thickness, does not relax fully but retains some residual strain. (c) 2005 American Vacuum Society.