Phase separation and atomic ordering in In_xAl_yGa_1-x-yN layers

We studied the occurrence of phase separation and atomic ordering in quaternary In_xAl_yGa_1-x-yN layers by transmission electron microscopy. Three layers of different compositions were examined: one lattice-mismatched In_0.10Al_0.02Ga_0.88N layer together with two lattice-matched In_0.12Al_0.29Ga_0.59N and In_0.06Al_0.18Ga_0.76N layers. The composition modulations were seen in all the layers. The wavelengths (λ) of composition modulations resulting from phase separation were calculated using selected-area electron diffraction patterns. The smaller λ (9 nm) in In_0.10Al_0.02Ga_0.88N layer in comparison to that in In_0.12Ga_0.88N layer (λ = 20 nm) suggests that the driving force for phase separation in In_xAl_yGa_1-x-yN layers is greater than that in In_xGa_1-xN layers with similar In contents. Energy dispersive spectroscopy line profiles across In_xAl_yGa_1-x-yN/GaN interfaces revealed a gradual increase in Al and In incorporation. Additional (0 0 0 1) diffraction spots in an SADP taken on an 〈 1 over(1, ̄) 0 0 〉 zone from the In_0.06Al_0.18Ga_0.76N layer suggests that atomic ordering can occur in quaternary layers. Arguments are developed to rationalize these experimental observations.