We report on the maskless epitaxial lateral overgrowth of GaN on structured Si (111) substrates and on its structural properties using transmission electron microscopy and photo-electrochemical (PEC) etching techniques. The Structured silicon substrates are achieved using photolithography and dry etching: 4-mum-deep holes of 1.5 mum in diameter, each separated by 2.5 mum, are etched in the (111) Si surface, The growth process is started by depositing a 10-nm-thick AIN buffer layer at 850 degreesC and then followed by the growth at high temperature (1170degreesC) of the GaN epilayer. The deposition of GaN takes place first on the Si (111) surface covered with AIN in between the holes. i.e. no deposits are formed in the holes. During the growth the GaN layer extends vertically and laterally over the holes until complete coalescence. Transmission electron microscopy shows that regions over the holes only contain dislocations in the basal plane resulting from the bending of dislocations nucleated at the Si/AIN interface and at the coalescence boundary between the two laterally overgrown layers. This results in a drastic decrease of dislocation density ill these areas of the films. The revelation of dislocations is also achieved by PEC etching technique. (C) 2002 Elsevier Science B.V. All rights reserved.