By accurately optimizing the growth conditions for an oxygen doped A1N nucleation layer and for the subsequent epitaxial process the crystal quality of our GaN layers could be improved drastically. In X-ray diffraction analyses we observed FWHM values of 39 and 114 arcsec for the symmetric (0 0 4)- and asymmetric (1 1 4)-reflection, respectively. Consequently, the nominally undoped samples showed semi-insulating behavior in Hall measurements. By in situ deposition of a SiN interlayer, the dislocation density could be reduced by more than a factor of 2, reaching a value of 4 x 10(8) cm(-2) as confirmed by transmission electron microscopy and etch pit density counting. Samples with this low dislocation density showed an extremely narrow X-ray FWHM of 71 arcsec for the asymmetric (1 1 4) reflection along with a narrow linewidth of 870 mu eV in photoluminescence (PL) for the donor bound exciton ((DX)-X-0) at a temperature of 10 K. Atomic force microscopy yielded a very low rms roughness value of about 0.14 nm across a 4 mu m(2) scan area. Finally the excellent crystal quality could be confirmed by growing A1GaN/AIN/GaN high electron mobility transistor structures with reverse breakdown voltages >= 1000 V and a very low sheet resistance of 3 30 Omega/square. (C) 2007 Elsevier B.V. All rights reserved.