Atomic‐scale configuration of catalyst particles on GaN nanowires

Well‐aligned single crystalline GaN nanowires were grown on Al2O3 (0001) by molecular beam epitaxy. An initial 0.3 nm thick Ni layer was employed as catalyst. Atomic‐scale electron microscopy observations revealed the epitaxial growth and the crystal quality of the GaN nanowires with basal stacking faults being the only structural defect. Single crystalline droplets on the tip of the nanowires were identified as a Ni‐based cubic oxide with a 0.208 nm {100} lattice spacing and were oriented along [001]/[0001], (010)/(11 00) with respect to GaN. Crystal fitting of the two mismatched lattices is realized by a dense network of misfit dislocations at the interface, so that the oxide lattice is relaxed close to its strain‐free value. Analytical methods revealed the positions of the misfit dislocation cores, showing a curved and slightly rough oxide/GaN interface. The observations are consistent with the vapour‐liquid‐solid growth mechanism. Controlled radial growth and thickening of the nanowires was achieved by varying the growth regime. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)