Transmission electron microscopy was used to study threading dislocations  in epitaxial ZnO films on (00•1) sapphire substrates produced by a 2-step method. First, ZnO was deposited by pulsed laser deposition. It was found that the sample consisted of a continuous buffer layer with a high density, 7 x 1010cm2, of threading dislocations, with c-aligned nanorods on its top. The nanorods revealed few, if any, threading dislocations. A further layer of ZnO was then grown under conditions favouring nanorod growth, using either chemical vapor deposition or a hydrothermal method. In both cases the nanorods grew laterally and eventually coalesced to form a continuous overgrowth. The nanorods remained mostly free of dislocations until coalescence. New grain boundary dislocations were generated where nanorods coalesced, but many of these dislocations migrated laterally and interacted with other dislocations to form closed loops. The threading dislocation density at the top of the continuous film was thereby reduced to 1 x 109/cm2 and 7 x 109/cm2 in the cases of hydrothermal and chemical vapor deposition treatments, respectively. The mechanism of growth and the means by which threading dislocations were reduced were explained.

Reduction of Threading Dislocations in ZnO/(0001) Sapphire Film Heterostructure by Epitaxial Lateral Overgrowth of Nanorods. Sun, Y., Cherns, D., Doherty, R.P., Warren, J.L., Heard, P.J.: Journal of Applied Physics, 2008, 104[2], 023533