ON THE STRUCTURE OF FAULTED INTERFACES IN ALUMINUM NITRIDE CERAMICS

An unusual two-dimensional structure in hot-pressed wurtzite AlN, composed of a mixture of both curved and flat elements, is analysed by weak-beam and high-resolution electron microscopy. It is demonstrated that the structure, in its elementary form, is a 'dome'-like faulted defect. This defect is characterized by a Frank-type dislocation loop containing a metal-vacancy-induced stacking fault in the basal plane with displacement R(p) = (1/3) [11BAR00] + 0.15[0001] and a curved faulted interface with displacement vector R(c) = (1/3)[11BAR00] + (1/2)[0001]. The elementary dome is found always pointing in the same direction, reflecting the lack of symmetry in the tetrahedral unit structure of AlN with respect to the basal plane. Condensation of supersaturated metal vacancies on both interfaces, combined with the segregation of impurities, mainly oxygen, only on the planar interface, is considered to be the driving force for the nucleation and growth of the structure.