**5. Application of 3D architected Si**

The creation and observation methods for well-defined surfaces enable the epitaxial growth of an arbitrary geometry, a key technique for nanoconstruction in 3D space [26–31]. Therefore, our established methodology contributes to the realization of well-ordered 3D nanofabrication, where the material stacking direction can be perfectly switched between the out-of-plane and in-plane directions. Novel 3D nanostructures are also expected to help unveil the underlying 3D surface science phenomena. Finally, two demonstrations utilizing a 3D architected Si platform are shown.

#### **5.1 Platform for material growth on 3D surfaces**

Atomically well-defined side-surfaces on a substrate can make an enormous contribution to nanofabrication [26–31]. To demonstrate the applicability of material growth on such side-surfaces, we produced Si 111 f g-Fe and Si 111 f g-Ag reconstructed side-surface structures on 3D Si with vertical 111 f g7�7 side-surfaces. Ag and Fe layers with thickness of 1.0 and 0.4 nm were deposited on the 111 � � leftside and 111 � � right-side surfaces, respectively, and the sample was subsequently annealed at 500°C in UHV.

**Figure 14(a)** and **(b)** show typical RHEED patterns obtained from the left-side and right-side surfaces, respectively. We can confirm ffiffiffi 3 <sup>p</sup> � ffiffiffi <sup>3</sup> <sup>p</sup> (streaky) spots in L0 (orange arrows) in **Figure 14(a)**. In **Figure 14(b)**, we can confirm 2�2 (streaky) spots (orange arrows), showing the formation of c-FeSi [18]. Simultaneously, Si(111) ffiffiffi <sup>3</sup> <sup>p</sup> � ffiffiffi <sup>3</sup> <sup>p</sup> -Ag [4, 24, 31, 32] was obtained on the left-side surface. These results show that highly developed thin-film formation techniques are applicable for the vertical side-surface of 3D patterned substrates and the material stacking direction can be perfectly switched between the out-of-plane and in-plane directions.

**Figure 14(c)** and **(d)** show cross-sectional TEM images of the 0.4-nm-thick Fe layer deposited on the 111 � � right-side surface and the 5.0-nm-thick Ag deposited on the 111 � � left-side surface. We can see four and five MLs of *α*-Fe [18] on the 111 � � right-side surface (**Figure 14(d)**), where a smooth in-plane heteroepitaxial interface with a length of 50 nm or more was formed between *α*-Fe and Si. The orientation relations between the *α*-Fe and the Si right-side surface are 111 � �Fe∥ 111 � �Si and <sup>112</sup> � �Fe<sup>∥</sup> <sup>112</sup> � �Si, similar to those indicated in previous reports on Fe on a 2D Si 111 ð Þ surface [18, 33, 34]. A cross-sectional TEM image for Ag deposited on the 111 � �

**Figure 12.**

**Figure 13.**

**102**

*(a) RHEED pattern from the* f g 110 *vertical sample observed at* θ *= +0.4*° *and* ϕ *= +1.1*° *with eye guides of Laue zone. Schematics of 2D reciprocal lattices on (b) Si*ð Þ 100 *2*�*1 and (c) Si*ð Þ 011 *16*�*2, corresponding to the*

*(a) RHEED pattern for the* f g 111 *facet sample at* θ *= +0.7*° *and* ϕ *= +2.7*°*. The pattern consists of a* ð Þ 111 *7*�*7 pattern tilted* �*36*° *in the counterclockwise direction (yellow circles) and a faint 16*�*2 pattern (cyan circles). (b) Simulated RHEED pattern (left) reflecting geometric relationship (bottom right). The upper right figure*

*represents the corresponding 2D reciprocal lattice normal to the facet direction.*

*top/bottom and right-side surfaces, respectively, as shown in (d).*

*21st Century Surface Science - a Handbook*

#### **Figure 14.**

*RHEED patterns obtained from (a) Si*ð Þ 111 *-Ag left-side surface and (b) Si*ð Þ 111 *-Fe right-side surface annealed at 773 K in UHV. Spots in the L0 Laue zone indicated by orange arrows correspond to the* ffiffiffi <sup>3</sup> <sup>p</sup> � ffiffiffi 3 p *reciprocal lattice rods in (a), and spots indicated by orange arrows correspond to the 2* � *2 superstructure in (b). Cross-sectional TEM images of (c) Ag-deposited and (d) Fe-deposited side-surfaces at RT.*

left-side surface (**Figure 14(c)**) showed that fcc-Ag epitaxially grew with an atomically matched interface, Ag 111 � �∥ Si 111 � �, without any visible defects or dislocations.

Our results clearly show that a coherently grown (ultra) thin film was realized on the vertical side-surface with the growth alternating between the out-of-plane and inplane directions. The siliciding reaction can be controlled on the side-surfaces. Therefore, highly developed thin-film formation techniques are applicable for the vertical side-surface of 3D patterned substrates, and the material stacking direction can be perfectly switched between the out-of-plane and in-plane directions.
