Stress and wafer bending of a-plane GaN layers on r-plane sapphire substrates

Abstract

The stress and wafer bending of (112¯0) a-plane GaN layers of different thicknesses grown on (11¯02) r-plane sapphire substrates by hydride vapor phase epitaxy were studied by high-resolution x-ray diffraction and photoluminescence and photoreflectance spectroscopies. The layers are found to be under compression in the growth plane and under tension in the growth direction. The elastic and thermal anisotropies of the GaN and the sapphire crystal result in an in-plane stress and a wafer curvature, both of which are different in the two in-plane directions parallel and perpendicular to the GaN c axis. The GaN unit cell is no longer hexagonal but orthorhombic. The stress relaxes with increasing GaN layer thickness while the curvature of the wafer increases. Different stress relief mechanisms are considered, and the stresses in the layer and the curvature of the wafer are calculated using standard models on wafer bending. The results suggest that the wafer bending is the dominant stress relief mechanism. In addition, the redshift of the near-band-edge photoluminescence and the free exciton photoreflectance peaks with increasing layer thickness is correlated with the strain data determined by x-ray diffraction.