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GaN electronic properties are shown to depend on the AlN nucleation layer (NL) growth temperature for GaN films grown on 6H– and 4H–SiC. Using identical GaN growth conditions except AlN NL growth temperature, 300 K electron mobilities of 876, 884, and 932 cm2/Vs were obtained on 6H–SiC, 4H–SiC, and 3.5° off-axis 6H–SiC. An AlN NL temperature of 1080 °C was used for the planar and 3.5° off-axis 6H–SiC, while an AlN NL temperature of 980 °C was used for 4H–SiC. Atomic force microscope images of the AlN NL grown at 1080 °C reveal smaller AlN grains on the 6H–SiC than those on 4H–SiC, suggesting that the AlN morphology influences GaN film formation and subsequent electron mobility. Transmission electron microscope cross section measurements reveal the absence of screw dislocations in the AlN and a low screw dislocation density near the AlN/GaN interface, consistent with the high electron mobilities achieved in these films. © 2002 American Institute of Physics.