Residual strain evaluation by cross-sectional micro-reflectance spectroscopy of freestanding GaN grown by hydride vapor phase epitaxy

Distributions of residual stresses in the cross-section of as-grown c-plane freestanding GaN substrates grown by hydride vapor phase epitaxy (HVPE) were precisely measured by low temperature cross-sectional micro-reflectance spectroscopy. Based on the measured residual stresses in the cross-section area, the depth profile of in-plane isotropic biaxial intrinsic strain in the as-grown GaN wafer was also established according to the elastic theory for thin films. It is found that the intrinsic strain increases nearly linearly with the increasing distance to the Ga-face of the freestanding GaN substrate, and is greatly reduced by the introduction of three-dimensional (3D) GaN islands growth in the starting stage of the HVPE growth. According to the relationship between the intrinsic strain and threading dislocation density, the current research suggests that the bending of freestanding GaN is caused by the reduction of threading dislocation density along the film growth direction.