Microstructural Dependence Of Residual Stress In Reactively Sputtered Epitaxial Gan Films

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 -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 omega-2, omega, and in-plane -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 (similar to 10(12) cm(-2)) over screw (similar to 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.