X-ray diffraction study of the strain-stress thickness profiling in GaN⁄(〖Al〗_2 O_3 ) heterostructure

Gallium nitride (GaN) films were grown on c-plane sapphire substrates treated with silicon nitride (SiN) by metalorganic chemical vapor deposition. Growth started with nitridation of the substrate of the sapphire, followed by treatment with SiN and then the growth was interrupted at the different stages of the film coalescence. For the different thicknesses, the GaN microstructure state was studied by High-resolution X-ray Diffraction measurements. The a and c lattice constants were measured using symmetric and asymmetric ( /2θ) diffraction spectra. The in-plane and out-of-plane strains were determined and compared to fully relaxed GaN film. As the thickness of the GaN layer increases, the compressive room temperature strain increases and remain constant when the 2D growth mode is achieved.  Then, thermal, hydrostatic, and intrinsic stress levels have been determined for the different film coalescence levels. The growth of GaN layers is accompanied by intrinsic tensile stress generation which is reduced significantly with thickness and remains constant when the film reaches the 2D growth mode. The results demonstrate that all samples exhibit a lower isotopic hydrostatic compression deformation.  It is supposed that the hydrostatic strain in the epilayers is caused by native point defects. The type and concentration of the native defects suspected in the grown GaN thin films have been identified and estimated.