The characteristic surface morphologies of GaN grown by plasma-assisted molecular beam epitaxy under various growth conditions have been investigated. Three growth regimes (one N stable and two Ga stable) are identified on a surface structure diagram (Ga/N ratio versus substrate temperature). The boundary between the N-stable regime (low Ga/N ratios) and the two Ga-stable regimes (high Ga/N ratios) is determined by the growth rate of the films and is constant over the range of substrate temperatures investigated. The boundary between the two Ga-stable regimes (the Ga-droplet regime and the intermediate regime) is determined by the formation of Ga droplets and has an Arrhenius dependence with substrate temperature. The characteristic morphologies of films grown within each of these regimes are investigated using atomic force microscopy and transmission electron microscopy. N-stable films have rough, heavily pitted morphologies. Films grown within the intermediate phase have areas of flat surface between large, irregularly shaped pits. The pits observed for films grown within both regimes are found to initiate from threading dislocations and to decrease in density with increasing Ga/N ratio at constant temperature. Ga-stable films, grown within the Ga-droplet regime, exhibit atomically flat surfaces with no pit features. The morphology transitions are associated with changes in the growth kinetics caused by variations in the coverage of the GaN surface by excess Ga.

Schottky barrier field effect transistors based on individual catalytically-grown and undoped Si-nanowires (NW) have been fabricated and characterized with respect to their gate lengths. The gate length was shortened by the axial, self-aligned formation of nickel-silicide source and drain segments along the NW. The transistors with 10-30 nm NW diameters displayed p-type behaviour, sustained current densities of up to 0.5 MA/cm2, and exhibited on/off current ratios of up to 10(7). The on-currents were limited and kept constant by the Schottky contacts for gate lengths below 1 microm, and decreased exponentially for gate lengths exceeding 1 microm.

The incorporation of Au during vapor-liquid-solid nanowire growth might inherently limit the performance of nanowire-based devices. Here, we assess the material quality of Au-assisted and Au-free grown GaAs/(Al,Ga)As core-shell nanowires using photoluminescence spectroscopy. We show that at room temperature, the internal quantum efficiency is systematically much lower for the Au-assisted nanowires than for the Au-free ones. In contrast, the optoelectronic material quality of the latter is comparable to that of state-of-the-art planar double heterostructures.

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

International audienceThe formation mechanisms of epitaxial GaN nanowires grown within a self-induced approach by molecular-beam epitaxy have been investigated at the onset of the nucleation process by combining in situ reflection high-energy electron-diffraction measurements and ex situ high-resolution transmission electron microscopy imaging. It is shown that the self-induced growth of GaN nanowires on the AlN buffer layer is initially governed by the nucleation of dislocation-free coherent islands. These coherent islands develop through a series of shape transitions from spherical caps through truncated to full pyramids in order to elastically relieve the lattice-mismatch-induced strain. A strong correlation between the subsequent process of plastic relaxation and the final shape transition from full pyramids toward the very first nanowires is found. The experimental critical radius at which the misfit dislocation nucleates is in very good agreement with the theoretical critical radius for the formation of the misfit dislocation in full pyramids, showing that the plastic relaxation process does take place within full pyramids: this critical size corresponds to the initial radius of the very first nanowires. We associate the plastic relaxation of the lattice-mismatch-induced strain occurring within full pyramids with a drastic change in their total free energy: this gives rise to a driving force for the shape transition toward the very first nanowires, which is mainly due to the anisotropy of surface energy