Comprehensive overview on elastic strain relaxation mechanisms in nitride heterostructures: Stranski–Krastanow versus Frank–Van der Merwe growth mode

Both plastic and elastic strain relaxation of GaN on AlN grown by plasma assisted molecular beam epitaxy have been studied by reflection high energy electron diffraction (RHEED) and high resolution X-ray diffraction (HRXRD). Special attention has been paid to plastic strain relaxation of thick layers which has been found to occur from the beginning of the growth. A mechanism of misfit dislocation introduction based on the coalescence of dynamically formed platelets is proposed. Due to the lack of proper gliding planes in the wurtzite structure, such dislocations are not mobile, leading to an inhomogeneity of the strain state along the growth axis which has been put in evidence by X-ray diffraction measurements. The growth conditions suitable to achieve elastic relaxation through 3D islanding are precised.