When Si was doped into an AlN layer during metalorganic vapor-phase epitaxial growth, the Al density (N Al ) in the AlN layer decreased but the N density (N N ) did not change. The decrease in N Al was almost the same as the Si density (N si ). As N si increased in Si-doped AlN, the lattice constant decreased. These results can be explained by Si atoms replacing Al atoms in Si-doped AlN and subsequent Si–N bond formation. Thus, Si-doped AlN becomes a substitutional solid solution of Al 1- x Si x N ternary alloy. The highest Si density at which the x-ray diffraction peak still appears was 5.8×10 21 cm -3 ( x =12%).