INVERSION DOMAINS IN ALUMINUM NITRIDE

Flat and curved, basal and nonbasal planar defects are frequently observed in sintered AIN ceramics. To prove the existence of inversion domains, TEM techniques sensitive to crystal polarity were used. Dark-field imaging in certain multiple-beam situations with g = (0002) (where g is operating reflection) yielded strong domain contrast giving clear evidence of an inverted crystal structure within the domains. This was confirmed by the dynamic contrast in the disks of convergent-beam electron diffraction. All planar defects observed were found to be associated with inversion domains. The formation of inversion twins may be nucleated by accidental growth or by nucleation and growth within the bulk of a preexisting AIN grain. Both mechanisms are supposed to be associated with the accommodation of impurities.