To study the influence of the growth parameters on structural properties and crystal quality epitaxial layers of aluminum nitride were grown on Si-terminated SiC- and sapphire substrates (0001) by varying substrate temperature, growth rate, and III/V ratio in a RF-plasma enhanced MBE-system. A detailed analysis of dislocations was performed by RES channeling and X-ray diffraction measurements and was compared with TEM results obtained on cross sectional and plan view samples. The annihilation of threading dislocations during the growth process was observed by RBS-channeling depending on the type of substrate and the growth mechanism. Lowest dislocation densities are obtained for 2D-growth on SiC substrate in the range of 2 10(8) cm(-2) The reduction of the growth temperature from 1000 degreesC to 900 degreesC lead to an increase of the dislocation density by about an order of magnitude and in the case of 2D growth we observed for the first time in MBE grown AlN-layers the formation of regularly shaped nanopipes located within a screw dislocation. The spirals are composed by monoatomic steps surrounding nanopipes with diameters of 10 -60 nm. Their formation is discussed according to the Frank theory resulting in a ratio of surface free energy to shear modulus of about 3 10(-3) nm.