Compositional modulation in InxGa1−xN: TEM and X-ray studies

Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD) have been used to study compositional modulation in InxGa1−x N layers grown with compositions close to miscibility gap. The samples (0.34 < x < 0.8) were deposited by molecular beam epitaxy using either a 200 nm thick AlN or GaN buffer layer grown on a sapphire substrate. Periodic compositional modulation leads to extra electron diffraction spots and satellite reflections in XRD in the θ–2θ coupled geometry. The ordering period Δ measured along c-axis was about Δ = 45 Å for x = 0.5 and Δ = 66 Å for x = 0.78 for samples grown on AlN buffer layer. TEM and XRD determinations of Δ were in good agreement. Compositional modulation was not observed for the sample with x = 0.34 grown on a GaN buffer layer. Larger values of Δ were observed for layers with higher In content and for those having larger mismatch with the underlying AlN buffer layer. The possibility that the roughness of the AlN growth surface promotes strong In segregation on particular crystallographic planes leading to compositional modulation is considered.