Shape modification of III-V nanowires: The role of nucleation on sidewalls

The effect of sidewall nucleation on nanowire morphology is studied theoretically. The model provides a semiquantitative description of nanowire radius as a function of its length and the distance from the surface. It is demonstrated that the wire shape critically depends on the diffusion flux of adatoms from the substrate and on the rate of direct impingement to the sidewalls. At high diffusion flux the wire shape is cylindrical. A decrease of diffusion from the surface leads to the onset of nucleation on the sidewalls resulting in the lateral extension and in the reduction of wire length. The wire shape changes from cylindrical to conical, because the supersaturation of adatoms driving the nucleation is higher at the wire foot than at the top. It is shown that the shape modification becomes pronounced at low growth temperatures. Theoretical results are used to model the experimentally observed shapes of GaAs and GaP wires, grown by Au-assisted molecular beam epitaxy at different temperatures.

Figure 1

(Color online) Schematics of the growth model including the wire, the surface layer, and the drop, at three different times. $J$ is the impingement flux, ${\theta }_0$ is the incident angle of the beam to the flat surface, and $\theta$ is the local incident angle. The inset at the top left illustrates the polynuclear lateral growth of the side facet.

Figure 2

(Color online) Forms of potential $G\left(U\right)$ at different $\beta$.

Figure 3

(Color online) Kinetic phase diagram illustrating the domains for the island and the wire formation.

Figure 4

(Color online) Wire shape $\rho (x,l)$ as given by Eq. (21) for fixed $l=10$ and different $\sqrt{A}=13$ (bold cylinder), 7.8, 4.5, 3.3, 2.4, 1.9, 1.5, and $\sqrt{2}$. The lateral growth rate increases as $\sqrt{A}$ decreases. The dashed-dotted curve is the critical curve.

Figure 5

(Color online) SEM images of MBE-grown GaAs wires. The growth was performed at (a) $420°\mathrm{C}$ (45° tilted view) and (b) $580°\mathrm{C}$ (cross view).

Figure 6

SEM images of GaP wires grown by MBE at (a) $420°\mathrm{C}$ (45° tilted view) and (b) $580°\mathrm{C}$ (cross view).

Figure 7

(Color online) Experimental (black squares) and theoretical (solid line) wire shape $2R\left(z\right)$ for the GaAs wire shown in Fig. 5a.