Phase separation is believed to be responsible for the strong luminescence in In_xGa_1-xN-based light emitting devices Phase separation was observed in In_xGa_1-xN "bulk-like" layers, but not in In_xGa_1-xN quantum-wells. Also, phase separation does not occur during the early growth of "bulk-like" layers. Therefore, a critical question is: can we attribute the differences in the occurrence of phase separation in quantum-wells and "bulk-like" layers to the levels of stresses? To address this question, In_xGa_1-xN/GaN multiple quantum-wells (MQWs) were grown on a (0001) GaN/sapphire composite using metalorganic chemical vapor deposition (MOCVD). All the In_xGa_1-xN wells are pseudomorphic and do not have stacking faults. For In_0.14Ga _0.86N wells with different well widths of 715 and 2.5nm, the calculation shows increased compressive stress in thinner In_0.14Ga_0.86 N wells. Electron energy loss spectroscopy (EELS) line scans reveal the occurrence of phase separation in the lateral direction only in 7nm thick wells and inhomogeneous In (In) distribution in the vertical direction. The suppression of phase separation in 5 and 2.5nm thick wells is ascribed to the increased external compressive stress in thinner wells. For a 2.5nm thick In_xGa_1-xN well, the compressive stress computed increases dramatically as x increases from 14% to 20%. EELS line scans show that phase separation is suppressed in the lateral direction and the In is distributed homogeneously in the vertical direction for a 2.5nm thick In_0.20Ga _0.80N well. This is caused by dramatically increased compressive stress in a In_0.20Ga_0.80N well.

Optical pumping was performed to investigate how laser radiation would interact with the microstructure of In_xGa_1-xN epilayers to simulate what could happen during the operation of LEDs. Dense inversion domains (IDs) were formed through the whole In_0.07Ga_0.93N and GaN layers after 200hrs optical pumping. The atomic resolution HAADF image reveals the atomic structure of those {10-10} IDBs. They are mostly terminated at the GaN/sapphire interface, but sometimes at a ∼1.4nm thick "band" containing oxygen. The "band" has a stacking sequence of ABCBC... from the "band"/matrix interface, which is expected to be (0001) IDB. IDs may be produced by the formation and diffusion of "band" containing oxygen. Improved luminescence after 200hrs optical pumping is likely to be associated with the presence of dense IDBs.