Characterization of overgrown GaN layers on nano-columns grown by RF-molecular beam epitaxy

We report on the successful growth of free-standing GaN films on (0001) sapphire substrates by RF-molecular beam epitaxy. The kev to obtain unstrained GaN layers is employing self-organized GaN nano-columns which involve an air gap structure as a footing layer of overgrown GaN. The residual strain in overgrown GaN films is evaluated by measuring the lattice constant by X-ray diffraction. It is found that the c-axis length of overgrown GaN is estimated to be 5.1848 Angstrom. which is close to the value of strain-free GaN even with a layer thickness of 2.7 mum. Overgrown GaN peeled arbitrarily from GaN nano-columns is observed due to the cleaving process.