Defect characterization and analysis of III-V nanowires grown by Ni-promoted MBE

We report on transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) analysis of GaN and Al0.2Ga0.8N nanowires (NWs) grown by radio frequency plasma assisted Molecular Beam Epitaxy (PAMBE) on c-sapphire substrates. Conventional TEM images of single nanowires removed from the substrate exhibit a high density of basal stacking faults. Further analyses were performed using the aberration-corrected SuperSTEM facility. Geometric phase analysis (GPA) was applied to this data revealing the characteristic lattice displacement introduced by intrinsic basal stacking faults. The nickel-based "seeds", which were employed to promote the NW-type growth, are found to be situated at the tip of each nanowire. Different types of analyses were applied to lattice images of these seeds including GPA, projection method analysis and Fast Fourier transform of the HREM images. All three approaches yielded lattice constants and displacements with good agreement. The measured lattice values in the GaN nanowire body were in good agreement with the lattice constants of relaxed bulk GaN. For the seed particles the resulting lattice spacing was attributable, within the experimental error, to either nickel oxide, Ni3Ga, or an intermediate alloy phase. The defect density was measured in the AlGaN nanowires and was found to be in the order of 10(6) cm(-l). The results are critically discussed ill relation to the possible growth mechanisms of the NWs. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim