Faceted sidewalls of silicon nanowires: Au-induced structural reconstructions and electronic properties

Si nanowires with a $⟨111⟩$ orientation, synthesized by vapor-liquid-solid process with low silane partial pressure reactant and gold as the catalyst, are known to exhibit sawtooth facets containing gold adsorbates. We report herein the study of the nanowire morphology by means of transmission electron microscopy and scanning tunneling microscopy. The nanowires consist of faceted sidewalls. The number of the sidewalls changes from 12 to 6 along the growth axis, giving rise to nanowires with an irregular hexagonal cross section at their base. The sidewalls are covered with Au-rich clusters. Their facets also exhibit atomic structures that reveal the presence of gold, resulting from the diffusion of gold during the growth. Based on these observations, the tapering of the nanowire is found to be related to two contributions: the reduction in the catalyst particle size during the growth and lateral overgrowth from the direct incorporation of Si species onto the nanowire sidewalls. Because the rearrangement of atoms at surfaces and interfaces might affect the growth kinetics, the trigonal symmetry as well as the higher lateral growth rate on the widest sidewalls are explained from the existence of an interfacial atomic structure with two inequivalent parts in the unit cell. Finally, spectroscopic measurements were performed on the major facets and revealed a metallic behavior at 77 K.

Figure 1

(a) SEM image of a Si NW grown in the [111] direction from a gold island that was synthesized on a Si(111) surface. The orientations of two sidewalls are indicated. (b) SEM image of an annealed NW that has been cleaved and transferred onto a Si(111) surface. Inset: Cross section showing the NW base with an hexagonal shape. (c) TEM image of a 68-nm-diameter Si NW with irregular sawtooth-faceted sidewalls. (d) TEM image of a 300-nm-diameter Si NW with periodic sawtooth-faceted sidewalls. The facet orientations are indicated. The thin light gray layer in the TEM images corresponds to the oxide layer. (e) TEM image of a 71-nm-diameter Si NW showing a large and small corrugation for the top and bottom sidewalls, respectively. (f) HAADF STEM image of a 65-nm-diameter Si NW, where gold-rich clusters appear as bright protrusions decorating the NW sidewalls. (g) Lattice-resolved TEM image of a 75-nm-diameter Si NW. In the different images of the cleaved NWs, the horizontal arrow points toward the [111] growth direction. S and L indicate facets with small and large widths, respectively. The scale bars correspond to (a)–(d) 50 nm and (e) and (f) 20 nm.

Figure 2

(a) Large-scale three-dimensional STM image of an annealed Si NW grown along the [111] direction. Facets on the top sidewall are resolved as well as small gold-rich clusters. The average diameter of the NW is 170 nm. The nanowire is deposited on a $\text{Si}\left(111\right)\text{−}\left(7\times 7\right)$ surface. Scale bar: 100 nm. Inset: Topographic height profile above a schematic transverse section of the NW. (b) STM image of a NW sidewall, where the facet orientation is indicated. The average diameter of the NW is 120 nm. (c) Height profile measured along the dash line of the topographic image (b). Upper inset: Table giving the tilt angles $\theta$ between the $\left[\overline{1}\overline{1}1\right]$ direction and the direction of the other facets. Lower inset: Topographic height profile above a schematic transverse section of the NW shown in (b). The STM images were obtained at a sample voltage, tunneling current, and temperature of $+2.0\text{V}$, 100 pA, and 77 K.

Figure 3

Filled and empty state STM images for the (a), (b) Si{111}, (c), (d) Si{115}, and (e), (f) Si{119} facets of an annealed NW. STM images for the (g) Si{117} and (h) Si{1111} facets. The images (a) and (b) and (e) and (f) were acquired simultaneously. The feedback parameters (sample voltage, tunneling current) were (a) $-1.3\text{V}$, 100pA, (b) 1.3 V, 100pA, (c) $-2.0\text{V}$, 100pA, (d) 2.0 V, 500pA, (e) $-2.0\text{V}$, 500pA, (f) 2.0 V, 500pA, (g) $-1.3\text{V}$, 100pA, and (h) 2.0 V, 100pA. The sample temperature was 77 K. The vertical arrow indicates a $⟨110⟩$ direction. The unit cells are indicated for the (a) and (b) $6\times 6$ and (c) and (d) $4\times 2$ reconstructions. The unit cell of the $6\times 6$ reconstruction is divided in two inequivalent parts. All image sizes are $10\text{nm}\times 10\text{nm}$.

Figure 4

Three-dimensional view of an annealed Si NW sidewall, where gold clusters are seen on the high-index planes only. The STM image was obtained at a sample voltage, tunneling current, and temperature of $-1.6\text{V}$, 100 pA, and 77 K. Scale bar 20 nm.

Figure 5

Differential conductivity spectra measured on (a) Si{111} and (b) Si{115} facets of an annealed NW. The insets show the tunneling current spectra on a smaller voltage range. The feedback parameters (sample voltage, tunneling current) were (a) $-2.5\text{V}$, 100pA and (b) $-2.0\text{V}$, 100pA at a temperature of 77 K. Both spectra were acquired with a tip-sample variable distance method where the tip was approached linearly toward the surface at a rate of 0.80 and $0.67\text{Å}/\text{V}$, respectively.

Figure 6

(a) TEM image of a Si NW grown with a silane partial pressure of 1 mbar at a temperature of $430°\text{C}$. Dark protrusions visible on the shaft below the seed particle correspond to Au-rich clusters. Inset: zoom on the NW edge to show the straight sidewall. (b) TEM image of a Si NW grown with a silane partial pressure of 0.08 mbar at a temperature of $550°\text{C}$. The NW sidewalls are faceted. Scale bar: 20 nm.

Figure 7

(a) SEM images of a $⟨111⟩$-oriented Si NW viewed from different directions with the sample stage either rotated at (a1) $26°$ or (a2) and (a3) $10°$. The orientations of the most visible sidewalls are indicated. The diameter of the seed particle, the width of the L and S sidewalls at the end of the transitional region and at the NW base, and the height between the transitional region and the NW base are labeled D, ${\text{W}}_{\text{L}1}/{\text{W}}_{\text{S}1}$, ${\text{W}}_{\text{L}2}/{\text{W}}_{\text{S}2}$, and H, respectively. (b) SEM images of the (b1) top and (b2) base of a $⟨111⟩$-oriented Si NW transferred onto a Si(111) surface. Scale bar: 50 nm.

Figure 8

Variation in the sidewall width as a function of (a) the seed particle diameter $D$ and (b) the height $H$. $D$ and $H$ are defined in Fig. 7. The ratios between the width at the end of the transitional region and the width at the base are plotted in filled and open squares for the L and S sidewalls, respectively.

Figure 9

(a) Measured $D_{bm}$ and estimated diameter $D_{b\text{Au}}$ at the base of the NWs, knowing their diameter $D$ and total height $h$ at the end of the growth. $D_{b\text{Au}}$ corresponds to the diameter of the catalyst particle before the growth in case where there is no lateral growth and the reduction in the particle size is only caused by gold diffusion. (b) Simplified geometry of the NW to determine the variation in the diameter due to gold diffusion.

Figure 10

(a) SEM image of seed particles located at the top of $⟨111⟩$-oriented Si NWs. Scale bar: 80 nm. (b) Variation in the contact angle as a function of the catalyst particle for the L and S sidewalls. Inset: Enlarged view of the interfacial region between the top of the Si NW and the catalyst particle. (c) Tilted SEM image of a $⟨111⟩$-oriented Si NW, where the deformation of the particle to wet the different types of sidewalls is visible. The long, short, and truncated sidewalls are labeled L, S, and T, respectively.

Figure 11

(a) LEED pattern of the $\text{Au}/\text{Si}\left(111\right)\text{−}\left(6\times 6\right)$ surface observed at $350°\text{C}$ with a primary energy of 100 eV. (b) LEED pattern of the $\text{Au}/\text{Si}\left(111\right)\text{−}\beta \left(\surd 3\times \surd 3\right)$ surface observed at $550°\text{C}$ with a primary energy of 100 eV.