The morphological and structural evolution is presented for GaN grown by metalorganic chemical vapor deposition on 25 nm thick or 150 nm thick AIN initial layers on (0001) 4 H and 6 H SiC substrates. The 25 nm thick AIN on SiC was a rough, partially coalesced film, whereas the 150 nm thick AIN on SiC was smooth and was characterized by a step-terrace structure. Both the 25 and 150 nm thick AIN layers on SiC were nearly free of elastic strains. For both AIN initial layers, the GaN films grew by a coarse islanding mechanism. Measurement of the GaN (0002) interplanar spacing shows that these islands were largely strain relaxed throughout the growth process. Plan view transmission electron microscopy (TEM) showed a well developed misfit dislocation network at the GaN island/AIN interface. Cross sectional TEM revealed that these islands are free of threading dislocations (TDs) prior to coalescence. A simple, low-angle grain boundary model based on island misorientation at coalescence provides reasonable agreement with TD densities measured using plan view TEM. An electron mobility value as high as 818 cm(2)/V s has been measured at room temperature confirming a high degree of material quality. (C) 2004 Elsevier B.V. All rights reserved.