Nanoscale x-ray investigation of composition fluctuations in AlGaN nanowires.

In the present study we combined, in the same synchrotron x-ray experiment, reciprocal space mapping, multiwavelength anomalous diffraction and diffraction anomalous fine structure, to determine the strain, crystallographic polarity, alloy composition and ordering at the atomic scale in [0001]-oriented AlGaN nanowires grown by molecular beam epitaxy on GaN nanowire bases. The information that we obtained was averaged over a macroscopic ensemble of NWs. We found from the diffraction anomalous fine structure that there were an isotropic increased number of Ga-Ga pairs in the Ga next nearest coordination shell (cation sublattice) with respect to what is expected for the AlGaN alloy composition determined by anomalous diffraction. This significant deviation from random alloy atomic distribution is present whatever the AlN molar fraction and growth conditions. Our results are consistent with nanoscale composition fluctuations expected from both alloy disorder or kinematically driven spontaneous ordering, both effects being suspected to account for the physical properties of AlGaN ternary alloys.