Coalescence overgrowth of pattern-grown GaN nanocolumns on c-plane sapphire substrate with metal organic chemical vapor deposition is demonstrated The subsequent coalescence overgrowth opens a possibility for dislocation reduction due to the lateral strain relaxation in columnar geometry We present further growth optimization and innovative characterization of metal organic chemical vapor deposition layers overgrown on the columnar structure with varying diameters of columns Nano-imprint lithography was applied to open circular holes of 250 300 450 and 600 nm diameter on the SiO(2) layer deposited on the GaN layer on the c-plane sapphire template After the growth of similar to 1 mu m high GaN nanocolumns the further coalescence conditions led to an overgrown layer similar to 2 mu m thickness Photoelectrical and optical properties of the overgrown layers and a reference sample were investigated by time-resolved picosecond transient grating and time-integrated photoluminescence We note a 3-4 fold increase in carrier lifetime in the overgrown epilayers when the diameter of columns increased from 250 to 450 nm This feature is a clear indication of an similar to 4-fold reduced defect density Crown Copyright (C) 2010 Published by Elsevier B V All rights reserved