Spontaneous alloy composition ordering in GaAs-AlGaAs core-shell nanowires.

Nano letters

PubMedID: 23517063

Rudolph D, Funk S, Döblinger M, Morkötter S, Hertenberger S, Schweickert L, Becker J, Matich S, Bichler M, Spirkoska D, Zardo I, Finley JJ, Abstreiter G, Koblmüller G. Spontaneous alloy composition ordering in GaAs-AlGaAs core-shell nanowires. Nano Lett. 2013;13(4):1522-7.
By employing various high-resolution metrology techniques we directly probe the material composition profile within GaAs-Al0.3Ga0.7As core-shell nanowires grown by molecular beam epitaxy on silicon. Micro Raman measurements performed along the entire (>10 µm) length of the [111]-oriented nanowires reveal excellent average compositional homogeneity of the nominally Al0.3Ga0.7As shell. In strong contrast, along the radial direction cross-sectional scanning transmission electron microscopy and associated chemical analysis reveal rich structure in the AlGaAs alloy composition due to interface segregation, nanofaceting, and local alloy fluctuations. Most strikingly, we observe a 6-fold Al-rich substructure along the corners of the hexagonal AlGaAs shell where the Al-content is up to x ~ 0.6, a factor of 2 larger than the body of the AlGaAs shell. This is associated with facet-dependent capillarity diffusion due to the nonplanarity of shell growth. A modulation of the Al-content is also found along the radial [110] growth directions of the AlGaAs shell. Besides the ~10(3)-fold enhancement of the photoluminescence yield due to inhibition of nonradiative surface recombination, the AlGaAs shell gives rise to a broadened band of sharp-line luminescence features extending ~150-30 meV below the band gap of Al0.3Ga0.7As. These features are attributed to deep level defects under influence of the observed local alloy fluctuations in the shell.