Effects of stress on phase separation in In_xGa_{1- x}N/GaN multiple quantum-wells

In_0.14Ga_0.86N/GaN multiple quantum-wells (MQWs) with well widths of 7, 5 and 2.5 nm were grown using metal-organic chemical vapor deposition on (0 0 0 1) GaN/sapphire composites. The as-grown MQWs were investigated by high-resolution X-ray diffraction (HRXRD), scanning transmission electron microscopy and photoluminescence (PL). The HRXRD ω-2θ scans and high-angle annular dark-field (HAADF) images show that the In _0.14Ga_0.86N/GaN interfaces are flat. Furthermore, the atomic-resolution HAADF images and HRXRD reciprocal space mappings indicate that interfaces between different layers in MQWs are pseudomorphic. The calculations of stresses in MQWs show that the compressive stress in the In _0.14Ga_0.86N wells decreases as the well width increases. Electron energy loss spectroscopy line scans reveal the occurrence of phase separation in the lateral direction only in 7 nm thick wells and inhomogeneous In distribution in the vertical direction as well. PL shows a red-shift of the In_0.14Ga_0.86N peak and decreased peak emission with increased well widths. Arguments will be developed to rationalize these observations.