Dislocation reduction in AlN and GaN bulk crystals grown by HVPE

We analyse the dislocation distribution in GaN and AlN bulk crystals by tra nsmission electron microscopy and X-ray diffraction. The crystals are grown by hydride vapour phase epitaxy onto 6H-SiC[0001] and Si(lll) substrates. Two essentially different dislocation populations are observed: (i) a-type dislocations that show efficient dislocation density reduction (down to 4 x 10(5) cm(-2)) and (ii) a, (a + c) and c-type dislocations each type in a c onsiderable density with less efficient dislocation reduction. We evaluate the dislocation processes that result in this different behaviour as depend ent on the dislocation population.