Transmission electron microscope (TEM) analysis has been carried out for the growth process in thick InGaN layers prepared by metalorganic vapor phase epitaxy. In the case of growth on GaN/LT-AlN/α-Al_2O_3 (0001) , a thin InGaN Layer of a good quality can grow with a smooth interface of InGaN/GaN and a smooth (0001) exterior surface. Threading dislocations in GaN penetrate into the InGaN layer and have a pit on each end on the exterior surface of InGaN. By development of the pits, the InGaN layer becomes to have a shape of pyramids with {11^^-01} facet planes. Thereafter, InGaN with a columnar structure is deposited on the pyramids. A two-layer-structure is thus made in the thick layer. Energy-dispersive x-ray spectroscopy (EDX) analysis has confirmed that the lower layer has a lower In content than in the upper one which has the equilibrium composition In_<0.2>Ga_<0.8>N. The lower In content is attributed to the lattice coherence with GaN, or "composition pulling effect". Small grains of InN have been recognized on the surface of the upper layer of InGaN. The grains keep a good lattice coherency with the InGaN layer. On LT-AlN/α-Al_2O_3 (0001) , InGaN with the equlibrium composition grows in a columnar structure. Inside the InGaN layer, domains of a lower In content have been recognized. The domains have a crystal orientation relationship with the matrix of InGaN. These results suggest that the lattice coherency has a strong effect on the formation of inhomogenous regions in InGaN alloys.
