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We study the growth mode and strain state of GaN layers grown either directly on 6H–SiC(0001) or on thin (5 nm), coherently strained AlN nucleation layers. Using a combination of structural, optical, and vibrational characterization methods, we show that the 3.4% compressive lattice mismatch strain is fully relieved in the former case, whereas in the latter case a significant amount (0.3%) remains even after 1 μm of growth. This finding is clarified by in situ reflection high-energy electron diffraction and transmission electron microscopy. We demonstrate that the strain state of the GaN layer is determined by its growth mode, which in turn is governed by the degree of wetting of the underlayer rather than by lattice mismatch. © 1999 American Institute of Physics.