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 q-2q coupled geometry. The ordering period D measured along c-axis was about D = 45 Å for x=0.5 and D = 66Å for x=0.78 for samples grown on AlN buffer layer. TEM and XRD determinations of D 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 D were observed for layers with higher In content and for those having larger mismatch with the underlying AlN buffer layer. The ordering starts on polar 10-11 planes where we had previously observed formation of V-defects (pinholes) in InGaN. Also, there was a theoretical prediction that In could preferentially accumulate on these polar planes. The possibility that the roughness of the AlN growth surface promotes strong In segregation on particular crystallographic planes leading to compositional modulation is considered.

This study show an extraordinary possibility to grow InGaN layers uniform in composition for high In content (x = 0.34) by MBE, and also the possibility to grow naturally formed thin layers on nanometer scale. The relationship between structural properties, compositional ordering, and optical properties of the In1-xGaxN films, in particular the presence or absence of a Stokes shift between absorption and photoluminescence, will be discussed.

The work in LBNL is supported by the U.S. Department of Energy under contract No. DE-AC03-76SF00098,

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