Formation of V-shaped pits in nitride films grown by metalorganic chemical vapor deposition

We have investigated the formation of V-shaped pits in nitride films such as InGaN/GaN and AlGaN/GaN grown on sapphire substrate using transmission electron microscopy. The mechanism of pit formation strongly depends on the indium (In) and aluminum (Al) compositions in In_xGa_1-xN and Al_xGa_1-xN layers, respectively. With increasing the indium composition, V-shaped pits originated from the vertex of threading dislocations to the stacking mismatch boundaries induced by stacking faults and the three-dimensional island growth at the initial stage due to the large lattice mismatch. With increasing the aluminum composition, the origin of the pits also varied from the surface undulation due to the elastic misfit strain to the vertex of threading dislocations.