Self-assembled in-plane Ge nanowires on rib-patterned Si (1 1 10) templates

Ge heteroepitaxy on Si (1 1 10) substrates induces the formation of prism-shaped in-plane nanowires bounded with $\left\{105\right\}$ facets. In this work, in-plane nanowires were fabricated via the growth of Ge onto rib-patterned Si (1 1 10) templates oriented in the $\left[55\overline{1}\right]$ direction. Atomic force microscopy (AFM) reveals that a self-elongation of the nanowires occurs, resembling the phenomena observed on rib-patterned Si (0 0 1) templates, which indicates that this is a universal effect for nanowires grown on rib patterns. Finite-element simulations, performed with input from the latest ab initio calculations, reveal that the mechanism behind these phenomena is the minimization of the total energy density of the epilayer under rib-dominated geometry. Ge surface diffusion leads to a broadening of the Ge nanowires at the rib shoulder sites, which is proved to be an effective route to reduce the total energy density. Our results provide a straightforward solution for the realization of a single or a few Ge nanowires for potential device applications.

Figure 1

(a) 3D AFM image (2 × 2 μ${\mathrm{m}}^2$) of the initial rib-patterned Si (1 1 10) template with the ribs aligned along $\left[55\overline{1}\right]$; (b) derivative AFM image (1 × 1 μ${\mathrm{m}}^2$) of the unpatterned Si (1 1 10) substrate after the growth of 4.5 ML Ge at 600 °C.

Figure 2

(a) Derivative and (b) 3D AFM image of Ge nanowires sitting on ribs after the growth of 3.5 ML Ge at 650 °C. Image size: 2 × 2 μ${\mathrm{m}}^2$.

Figure 3

(a) 3D AFM image showing the perfectly self-aligned and self-elongated Ge nanowires on a 1.4-μm-long section of a rib; (b) height profile measured across the white line marked in (a).

Figure 4

(a) 3D rendering of a typical model used for the FEM calculations, in which seven Ge nanowires are sitting on a $\left\{105\right\}$ faceted rib. The rib is tilted by 8.05° to the (0 0 1) surface to simulate the (1 1 10) oriented substrate; (b) schematic profile of the model; (c) and (d) 3D and cross-sectional view of the calculated strain distribution ${\varepsilon }_{yy}$ over the Ge nanowires and the Si substrate underneath.

Figure 5

(a)–(c) Strain, surface, and total energy densities of a single Ge nanowire sitting at either middle (black squares) or shoulder site (red circles), as a function of the number of nanowires on the rib. The yellow diamond represents the corresponding values with a vacancy introduced in the nanowire array with the geometry shown in (d), representing the top, side, and 3D view of the scheme for a faceted vacancy introduced to simulate interrupted nanowires.

Figure 6

(a) Schematic model for the intermixing-induced broadening of the nanowires at the edge of the rib; (b)–(d) comparison of the strain, surface, and total energy densities of nine Ge nanowires sitting at different sites of the rib, with and without wire broadening at the shoulder sites through Si intermixing (triangles) or Ge accumulation (circles).