We have investigated the microstructure of homoepitaxial N-polar GaN layers grown by rf-plasma-assisted molecular beam epitaxy on freestanding GaN substrates. The structural quality of the epitaxial layers improves when the sheet density of oxygen present on the substrate surface diminishes. An initial sheet density of oxygen of similar to 0.5 monolayer (ML) results in a highly defective epitaxial layer, while an epitaxial layer with no visible threading dislocations was grown on a substrate with an initial oxygen sheet density of similar to 0.08 ML. The significant reduction in oxygen was achieved by using several cycles of Ga deposition and thermal desorption prior to the start of epitaxial growth combined with an initial ultrathin 15-angstrom AlN nucleation layer. These results indicate that reducing the density of oxygen on the surfaces of freestanding N-polar GaN substrates is vital for obtaining high quality homoepitaxial N-polar GaN layers. Published by Elsevier B.V.