Anisotropic X-ray rocking curve due to a damaged surface layer in a freestanding GaN thick film

The structural deformation of freestanding GaN film has been investigated. The specimen was grown on an Al2O3 substrate by using hydride vapor phase epitaxy (HVPE) and was self-detached during the growth. the atomic force microscopy (AFM) image showed much mechanical damages on the surface, which was presumably generated by the surface polishing process. After chemical etching, the near-band-edge to yellow-band (NBE/YB) luminescence intensity ratio was greatly increased. Also, using high resolution X-ray diffraction (HRXRD), we found that the 2 theta-omega scan of (0002) reflection asymmetrically broadened at low diffraction angles, which indicated that a damaged surface layer existed in the film with residual compressive strain. Moreover, a more detailed X-ray analysis showed that the surface had a concave curvature. We concluded that structural distortion of GaN could induce a serious deformation, which could affect not only the structural but also the optical and the electrical properties of the film.