TRANSMISSION ELECTRON-MICROSCOPY STRUCTURAL CHARACTERIZATION OF GAN LAYERS GROWN ON (0001)SAPPHIRE

GaN layers grown by MOCVD or by MBE on (0001) sapphire have been chara cterised by transmission electron microscopy (TEM). We make a review o f the different crystallographic structures found in theses GaN layers . We comment shortly on the nitridation of the sapphire and the struct ure of the buffer layer (BL). We point out that the roughness of the B L can be an important parameter for releasing the residual strain of t he GaN layer. We compute the Keating energies of the main inversion do main boundaries (IDBs) and translation domains boundaries (TDBs) obser ved in some GaN layers. The observed structures correspond to the lowe st energy models. Perfect dislocations have Burgers vectors equal to a , a + c and c. The dislocation lines are generally parallel to the c-a xis. a-Edge dislocations are generally not dissociated and we propose an atomic model for them. Screw dislocations with a Burgers vector equ al to c, can 'open and close' during growth leaving holes (the so-call ed nanopipes) in the structure. (C) 1997 Elsevier Science S.A.