The growth behavior of GaN layers overgrown on nano-columns on sapphire substrates by RF-plasma molecular beam epitaxy was investigated. Free-standing GaN films were obtained through the coalescence of nano-columns, showing the layered structure supported by underlying columnar GaN. Overgrown GaN layers were characterized with respect to the residual strain. As a result, it was found that overgrown GaN films with layer thickness of 2.7 mum possessed a stress-free property such as Raman frequency of high-frequency E-2 mode of 568.1 cm(-1). (C) 2002 Elsevier Science B.V. All rights reserved.