Etch Pits and Threading Dislocations in GaN Films Grown by Metal-Organic Chemical Vapor Deposition

High quality epitaxial GaN films on (0001) sapphire substrates were grown by a commercial metal-organic chemical vapor deposition system. The etch-pits and threading dislocations in GaN films is studied by a scaning electron microscope (SEM) and a transmission electron microscope (TEM). The SEM images of GaN films etched in mixed acid solution (H₃PO₄:H₂SO₄ = 1:3) and molten KOH exhibit notably different etching pit densities of 5 x 10⁸/cm² and 4 x 10⁷/cm², respectively, which probably indicate that more kinds of etching pits were revealed when etched in mixed acid solution (H₃PO₄:H₂SO₄ = 1:3). Cross section TEM of GaN films with different g vectors showed the portions of different threading dislocations. Theoretical calculation indicates that the lattice and thermal expansion coefficient mismatch may be the main origins of pure edge threading dislocations.