Optimized Ge nanowire arrays on Si by modified surfactant mediated epitaxy

We demonstrate the formation of Ge nanowire arrays on highly ordered kink-free Si stepped surfaces. The nanowires are grown using Bi surfactant mediated epitaxy. The nanowires are single crystalline and feature minimal kink densities, allowing them to span lengths larger than $1\mu \mathrm{m}$ at a width of $\approx 4\mathrm{nm}$. To achieve desired growth conditions for the formation of such nanowire arrays, we explore a full parameter space of surfactant mediated epitaxy. We show that controlling the surfactant coverage in the surface and/or at step edges modifies the growth properties of surface steps in a decisive way.

Figure 1

(a) Highly ordered $\mathrm{Si}\left(111\right)\text{−}7\times 7$ template. (b) Corresponding height profile. (c),(d) Ge nanowires grown on this template by modified surfactant mediated epitaxy. The long-range order of the template remains conserved. The Bi surfactant changes the surface reconstruction to $\sqrt{3}\times \sqrt{3}$ (d). (e) Apparent height difference measured by STM between Si and Ge areas. The nanowires are Ge bilayers attached to the step edges under the Bi capping (f). They measure $\sim 4\mathrm{nm}$ ($\sim 20$ atoms) in width and $0.3\mathrm{nm}$ in height.

Figure 2

Bi termination of the highly ordered $\mathrm{Si}\left(111\right)\text{−}7\times 7$ template [Fig. 1a]. (a) Standard surfactant mediated epitaxy. The step structure of the template has been destroyed. (b)–(e) Modified surfactant mediated epitaxy. (b) Surface after slow increase of Bi coverage from 0 to $1∕3$ ML. The $1∕3$ ML Bi coverage is indicated by an inhomogeneous $\sqrt{3}\times \sqrt{3}$ structure (c). The modified surfactant mediated epitaxy preserves the original straight step arrangement. (d) Surface after completing the Bi coverage to 1 ML. The 1 ML Bi coverage is indicated by a homogeneous $\sqrt{3}\times \sqrt{3}$ structure (e).

Figure 3

Deposition of Ge on the ordered Bi-terminated surface [Fig. 2d]. (a) Standard surfactant mediated epitaxy. Ge and Bi are deposited simultaneously. The Ge step edges develop facets in the $⟨1\overline{1}0⟩$ directions that are 30° off the original $⟨\overline{1}\overline{1}2⟩$ step direction. (b) Modified surfactant mediated epitaxy. Ge is deposited without Bi flux. The Ge step edges retain the original $⟨\overline{1}\overline{1}2⟩$ step direction.

Figure 4

Changing the Bi content in the surface changes the structure of the step edges and their growth and equilibrium properties. The equilibrium step directions are (a) $⟨\overline{1}\overline{1}2⟩$ on the clean $\mathrm{Si}\left(111\right)\text{−}7\times 7$ surface, (b) $⟨1\overline{1}0⟩$ on the $\mathrm{Si}\left(111\right)\text{−}\sqrt{3}\times \sqrt{3}\text{−}\mathrm{Bi}$ with 1 ML Bi, (c) $⟨\overline{1}\overline{1}2⟩$ on the $\mathrm{Si}\left(111\right)\text{−}\sqrt{3}\times \sqrt{3}\text{−}\mathrm{Bi}$ surface with $1∕3$ ML Bi, and (d) $⟨\overline{1}\overline{1}2⟩$ on the Ge nanowires grown by modified surfactant mediated epitaxy. In (d), the crosses and dots indicate the positions of the adatoms and corner holes of a $3\times 3$ surface reconstruction.