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Interfacial microstructure in GaN nucleation layers was investigated using synchrotron x-ray scattering and transmission electron microscopy. We find that tensile-strained, aligned, interfacial domains coexist with misaligned domains in an as-grown nucleation layer of mostly cubic stacking. The tensile strain originates in a 6/7 matched interfacial structure, wherein 6-Ga atomic distances in GaN match to 7-Al atomic distances in sapphire. The tensile state of the aligned, interfacial domains is preserved during annealing to 1100 °C, while the stacking sequence changes from cubic to hexagonal order. The correlation length of the stacking order is rather short, ∼9 Å in the hexagonal phase, compared to that of the cubic phase in the as-grown nucleation layer, ∼25 Å, due to stacking faults generated during the kinetically limited transformation. © 2001 American Institute of Physics.