Microstructural dependence of residual stress in reactively sputtered epitaxial GaN films

Abstract Epitaxial GaN films were grown on c -sapphire by rf magnetron reactive sputtering of GaAs at different partial pressures of nitrogen in Ar–N 2 sputtering atmosphere. High-resolution x-ray diffraction and φ -scans reveal the mosaic growth of c -axis oriented, wurtzite GaN films. The c and a parameters were independently determined to obtain the corresponding in-plane and out-of-plane strain components. Raman measurements confirmed the in-plane strain behavior. The surface morphology and elemental composition of films were studied by atomic force microscopy and secondary ion mass spectroscopy, respectively. High-resolution ω- 2 θ, ω , and in-plane φ -rocking curve scans were used to obtain micro-strain, screw and edge dislocation densities, respectively. The films grown at 30%–100% N 2 reveal dominance of edge (∼10 12 cm −2 ) over screw (∼10 10 cm −2 ) dislocations, with both approaching similar densities at lower N 2 percentages. The strain data has been analyzed to separate the hydrostatic and biaxial contributions and their dependences on N 2 percentage. The film grown at 100% N 2 displays large hydrostatic strain and micro-strain due to the presence of excess/interstitial nitrogen. The hydrostatic strain and micro-strain decrease substantially with initial decrease of N 2 percentage, but increase slightly in the films grown below 30% N 2 , primarily due to the incorporation of Ar. The films grown below 75% N 2 display growth-related intrinsic tensile stress, originating from crystallite coalescence. The stress reversal from tensile to compressive, seen in the films grown at higher N 2 percentages is primarily attributed to the incorporation of excess/interstitial nitrogen into grain boundaries and the tensile side of edge dislocations. The decrease of intrinsic tensile stress in the films grown below 30% N 2 is attributed to the incorporation of Ar and their voided structure.