Plastic strain relaxation of nitride heterostructures

Thick layers of GaN on AlN, AlN on GaN, and InN on GaN were grown by plasma-assisted molecular beam epitaxy. Their plastic strain relaxation was studied by reflection high-energy electron diffraction (RHEED) and high resolution x-ray diffraction (HRXRD). The results are consistent with a mechanism of progressive introduction of misfit dislocations based on the coalescence of dynamically formed platelets. Due to the lack of proper gliding planes in the wurtzite structure, such dislocations are not mobile, leading to inhomogeneity of the strain state along the growth axis. The agreement between in situ RHEED and ex situ HRXRD measurements provides evidence that the strain state is frozen in during growth.