Formation and relief of epitaxial strain in ultra-thin α -(Al,Ga)2O3 films on a-plane sapphire under biaxial stress

We have studied strain formation and relief in thin α-(AlxGa1−x)2O3 layers iso-structurally grown on a-plane sapphire. In a complementary approach of in situ synchrotron-based surface x-ray diffraction and high-angle annular dark-field scanning transmission electron microscopy, we traced macroscopic observations (strain relief) to phenomena on microscopic length scales (formation of partial dislocations and stacking faults). This provided a comprehensive view of the fundamental mechanisms behind. We found two prototypical scenarios for large and moderate biaxial stress, x=0 and 0.54, respectively. In the first case, the formation of misfit dislocations at the interface and stacking faults triggers an instantaneous relief along the in-plane c-direction. Reducing the applied stress halfway to the homoepitaxial limit also reduces the driving forces by half for the relaxation and activates dislocation gliding via half-loop nucleation on neighboring r-planes rather than the direct formation of misfit dislocations at the interface.