Nucleation mechanisms of self-induced GaN nanowires grown on an amorphous interlayer

Abstract

International audienceThe formation mechanisms of GaN nanowires grown on a SixNy amorphous interlayer within a self-induced approach by molecular beam epitaxy have been investigated by combining in situ reflection high-energy electron-diffraction measurements with ex situ high-resolution transmission electron microscopy imaging. It is found that GaN initially nucleates as spherical cap-shaped islands with a wetting angle of 42 +/- 7 degrees. Subsequently, these islands coarsen and undergo a shape transition toward the nanowire morphology at an experimental critical radius of 5 nm. As the epitaxial constraint is very weak on an amorphous interlayer, the equivalent Laplace pressure due to the effects of surface stress has been taken into account. Analytical and finite-element method calculations show that the Laplace pressure results at the nanoscale dimensions in significant volume elastic strain in both spherical caps and nanowires. From thermodynamic considerations, it is revealed that the related strain energy density is slightly in favor of the shape transition toward the nanowire geometry owing to its higher ability to relieve the strain. Nevertheless, the anisotropy of surface energy is an even stronger driving force, since the nanowires are composed of c- and m-planes with very low surface energies. It is deduced that an energy barrier does exit for the shape transition and may be related to edge effects, resulting in a growth condition-dependent critical radius

Statistical Analysis of the Shape of One-Dimensional Nanostructures: Determining the Coalescence Degree of Spontaneously Formed GaN Nanowires

“…In order to answer this question, we next examine selected nanowires of sample D. As explained in Section , this sample exhibits a sufficiently low nanowire density to facilitate the investigation of single nanowires right after their formation. The diameter of both of these nanowires amounts to about 15 nm, just slightly above the critical diameter for nanowire formation of 10 nm experimentally established by Consonni et al 35 Nanowire coalescence at this stage of growth is certainly a very unlikely event, and can even be ruled out for the two exemplary nanowires shown in Fig. 2.…”

“…9 Following the shape transformation of the initial nucleus to the nanowire morphology, 35 the nanowire collects a significant amount of Ga due to the diffusion of Ga adatoms on the substrate in addition to the directly imping- ing Ga flux. The Ga adatom concentration on the nanowire top facet will then, in general, exceed the available concentration of N adatoms.…”

Elastic versus Plastic Strain Relaxation in Coalesced GaN Nanowires: An X-Ray Diffraction Study

“…3(a) and 3(c). Close to the substrate, the nanowires have diameters of 20-30 nm that they attain shortly after nucleation [30]. The nanowires do not grow radially in their bottom parts in the process of axial growth.…”

Recent Advances on p-Type III-Nitride Nanowires by Molecular Beam Epitaxy

“…In fact, a number of recent studies indicate that the MBE-grown III-nitride nanowires could be substrate independent [95][96][97][98][99], i.e., the MBE-grown III-nitride nanowires can be essentially formed on any substrate. For example, GaN, InGaN/GaN dot-in-a-wire, and InN nanowire structures have been demonstrated on silicon oxide [95,[99][100][101]. Moreover, their optical qualities seem to be not compromised, e.g., InN and GaN nanowires grown on silicon oxide show similar photoluminescence characteristics compared to the nanowires grown on Si [99].…”