Edge-type threading dislocations (TDs) can be reduced by increasing the epilayer thickness of undoped AlN grown on c-plane sapphire Substrate by low pressure MOVPE. Indium as a surfactant helps to reduce cracks in both, undoped and Si-doped AlN epilayers. For high Si concentrations, edge-type TDs bend and bunch together and finally emerge as V-pits on the epilayer Surface. We observed an increasing intensity ratio between the deep level transitions at around 3 eV and the near-band-edge luminescence of the low temperature (10 K) CL spectra for increasing Si concentration, whereas the electrical conductivity decreased in the range of higher doping concentration. A fair electrical conductivity is obtained for a sample having moderate Si concentration of 2 x 10(18) cm(-3). (C) 2008 Elsevier B.V. All rights reserved.