**3. Experimental procedure**

The authors demonstrated the creation of atomically ordered side- and facetsurfaces owing to the arbitrary selection of planes (**Figure 6**). Atomically reconstructed Si 100 f g, 110 f g, and 111 f g vertical side-surfaces and 111 f g tilted facet-surfaces were first realized on 3D patterned Si substrates, and the perfection of the 3D surface structures was examined. The 100 f g2�1, 110 f g16�2, and 111 f g 7 �7 diffraction spots from the 3D surfaces were confirmed by RHEED and LEED. flow rate of 51 sccm, a pressure of 4.0 Pa, and a bias power of 60 W using an RIE

*(a) Photograph of a 3D patterned Si*ð Þ 110 *substrate consisting of* f g 111 *vertical side-surfaces. (b) Top and (c) cross-sectional SEM images of a patterned area. (d) Cross-sectional and (e) top SEM images of a sample with*

*Creation and Evaluation of Atomically Ordered Side- and Facet-Surface Structures of Three…*

After dry etching, wet etching was performed to reduce the side- and facetsurface roughnesses. The base solution used in the wet process removes the damage and contaminants [19–21], and flash annealing promotes reconstruction through the sublimation of the contaminants [22]. Unoptimized conditions never led to reconstructed patterns on side- or facet-surfaces. In this process, the etching recipe was optimized by considering the plane-dependent etching properties [23]. A 100 f g sample was dipped in 25 wt% tetramethylammonium hydroxide (TMAH) at 70°C for 3 min. A 110 f g sample was etched in 25 wt% TMAH with 0.1 vol% surfactant (iso-octylphenoxy polyethoxyethanol) at 70°C for 1 min [7]. Then, the sample was rinsed with pure water, dried by blowing with N2, and introduced into an ultra-high vacuum (UHV) chamber. The 3D Si sample was degassed and flashed by direct-

RHEED patterns were obtained at RT using an electron beam with an *Ep* of 15 keV and a diameter of �0.5 mm. Au advantage of the RHEED technique is the accessibility of various 3D surfaces; that is, the diffraction pattern consists of all diffractions from the substantial crystal surfaces in 3D space. Actually, the selection of the incident e-beam direction enables the identification of 3D surface structures. In this study, the direction of the incident electrons was defined by the glancing angle *θ* and azimuth angle *ϕ* for the top and bottom Si surfaces (**Figure 8**). The RHEED patterns were also obtained from the 3D samples with vertical side-surface or tilted facet-surfaces by controlling *θ* and *ϕ*. For instance, for 3D Si with a vertical f g 111 side-surface, *θ* is the angle between the incident electron beam and the top ð Þ 110 surface, which was changed from �0.4° to +1.2°. *ϕ*, which is defined as the angle from 112 in the in-plane direction, was changed from �4.5° to +4.7°. The RHEED patterns were filtered by a computer to emphasize the spot features in the background. LEED can also be used to observe 3D surface ordering, while 3D accessibility is less than that of RHEED because of the limited range of diffraction conditions. LEED patterns were also observed using a typical *Ep* of 40–200 eV and a

current heating at �1150–1200°C at a pressure below 2 � <sup>10</sup>�<sup>8</sup> Pa.

**3.2 RHEED observation of 3D structured samples**

beam size of �1 mm.

**97**

system (RIE-10 NR, Samco) with SF6 gas [9].

*DOI: http://dx.doi.org/10.5772/intechopen.92860*

**Figure 7.**

*tilted* f g 111 *facet-surfaces.*

#### **3.1 Sample fabrication with combination of dry and wet etching processes**

The 3D architected Si samples with arbitrary faces discussed here were produced on commercial mirror-polished Si substrates by the following dry and wet etching procedures. All processes were performed at room temperature (RT). First, a line mask pattern was drawn using a photoresist. To produce the vertical side-surfaces (**Figure 7(a)**–**(c)**), Si was etched in an inductive coupled plasma (ICP)-reactive ion etching system (RIE-400iPB, Samco). The process parameters were an ICP source power of 300 W, a bias power of 10 W, and a working pressure of 4 Pa. Mixture gases of 10 sccm SF6, 5 sccm O2, and 200 sccm Ar were used in the etching cycle, and 40 sccm C4F8, 5 sccm O2, and 200 sccm Ar were used in the passivation process [6–9]. Depending on dry RIE conditions, various curved structures from trapezoid to triangle shapes were obtained. When we produced tilted facet-surfaces (**Figure 7(d)** and **(e)**), the optimized conditions for a triangle-like shape were a

*Creation and Evaluation of Atomically Ordered Side- and Facet-Surface Structures of Three… DOI: http://dx.doi.org/10.5772/intechopen.92860*

#### **Figure 7.**

3D fabricated materials, the authors have demonstrated that not only substrate surfaces but also surfaces inclined from or perpendicular to a substrate plane exhibit atomically ordered structures where surface spots elongated along directions inclined from or perpendicular to the substrate normal direction appear in RHEED patterns (**Figure 5(b)**) [6, 7, 9]. We emphasize that a simple surface property of 3D fabricated materials, that is, a surface direction, can be confirmed by the elongation of surface spots. Low-energy electron diffraction (LEED) with a typical *Ep* of 50–100 eV focuses the interference caused by the backward scattering of an electron incident to an atom, while RHEED focuses the interference caused by forward scattering. Both LEED and RHEED are sensitive to surface structures. A LEED pattern at the normal incidence corresponds to a top view of reciprocal lattice rods intersecting with an Ewald sphere from the surface normal direction. The diffraction spots move to the 00 ð Þ spot arising from the 00 ð Þ rod with increasing *Ep* [8, 18]. For an inclined surface, the arrangement of diffraction spots in alignment changes that in an arc, the center of which is the 00 ð Þ spot. The diffraction spots also move to the 00 ð Þspot with increasing *Ep*. Thus, we can confirm the surface direction of 3D fabricated materials by the arrangement and *Ep*-

The authors demonstrated the creation of atomically ordered side- and facet-

surfaces owing to the arbitrary selection of planes (**Figure 6**). Atomically reconstructed Si 100 f g, 110 f g, and 111 f g vertical side-surfaces and 111 f g tilted facet-surfaces were first realized on 3D patterned Si substrates, and the perfection of the 3D surface structures was examined. The 100 f g2�1, 110 f g16�2, and 111 f g 7 �7 diffraction spots from the 3D surfaces were confirmed by RHEED and LEED.

**3.1 Sample fabrication with combination of dry and wet etching processes**

*(a)–(c) Schematic relationship among flat surface, vertical side-surfaces, and tilted facet-surfaces.*

The 3D architected Si samples with arbitrary faces discussed here were produced on commercial mirror-polished Si substrates by the following dry and wet etching procedures. All processes were performed at room temperature (RT). First, a line mask pattern was drawn using a photoresist. To produce the vertical side-surfaces (**Figure 7(a)**–**(c)**), Si was etched in an inductive coupled plasma (ICP)-reactive ion etching system (RIE-400iPB, Samco). The process parameters were an ICP source power of 300 W, a bias power of 10 W, and a working pressure of 4 Pa. Mixture gases of 10 sccm SF6, 5 sccm O2, and 200 sccm Ar were used in the etching cycle, and 40 sccm C4F8, 5 sccm O2, and 200 sccm Ar were used in the passivation process [6–9]. Depending on dry RIE conditions, various curved structures from trapezoid to triangle shapes were obtained. When we produced tilted facet-surfaces (**Figure 7(d)** and **(e)**), the optimized conditions for a triangle-like shape were a

dependent motion of diffraction spots.

*21st Century Surface Science - a Handbook*

**3. Experimental procedure**

**Figure 6.**

**96**

*(a) Photograph of a 3D patterned Si*ð Þ 110 *substrate consisting of* f g 111 *vertical side-surfaces. (b) Top and (c) cross-sectional SEM images of a patterned area. (d) Cross-sectional and (e) top SEM images of a sample with tilted* f g 111 *facet-surfaces.*

flow rate of 51 sccm, a pressure of 4.0 Pa, and a bias power of 60 W using an RIE system (RIE-10 NR, Samco) with SF6 gas [9].

After dry etching, wet etching was performed to reduce the side- and facetsurface roughnesses. The base solution used in the wet process removes the damage and contaminants [19–21], and flash annealing promotes reconstruction through the sublimation of the contaminants [22]. Unoptimized conditions never led to reconstructed patterns on side- or facet-surfaces. In this process, the etching recipe was optimized by considering the plane-dependent etching properties [23]. A 100 f g sample was dipped in 25 wt% tetramethylammonium hydroxide (TMAH) at 70°C for 3 min. A 110 f g sample was etched in 25 wt% TMAH with 0.1 vol% surfactant (iso-octylphenoxy polyethoxyethanol) at 70°C for 1 min [7]. Then, the sample was rinsed with pure water, dried by blowing with N2, and introduced into an ultra-high vacuum (UHV) chamber. The 3D Si sample was degassed and flashed by directcurrent heating at �1150–1200°C at a pressure below 2 � <sup>10</sup>�<sup>8</sup> Pa.

#### **3.2 RHEED observation of 3D structured samples**

RHEED patterns were obtained at RT using an electron beam with an *Ep* of 15 keV and a diameter of �0.5 mm. Au advantage of the RHEED technique is the accessibility of various 3D surfaces; that is, the diffraction pattern consists of all diffractions from the substantial crystal surfaces in 3D space. Actually, the selection of the incident e-beam direction enables the identification of 3D surface structures. In this study, the direction of the incident electrons was defined by the glancing angle *θ* and azimuth angle *ϕ* for the top and bottom Si surfaces (**Figure 8**). The RHEED patterns were also obtained from the 3D samples with vertical side-surface or tilted facet-surfaces by controlling *θ* and *ϕ*. For instance, for 3D Si with a vertical f g 111 side-surface, *θ* is the angle between the incident electron beam and the top ð Þ 110 surface, which was changed from �0.4° to +1.2°. *ϕ*, which is defined as the angle from 112 in the in-plane direction, was changed from �4.5° to +4.7°. The RHEED patterns were filtered by a computer to emphasize the spot features in the background. LEED can also be used to observe 3D surface ordering, while 3D accessibility is less than that of RHEED because of the limited range of diffraction conditions. LEED patterns were also observed using a typical *Ep* of 40–200 eV and a beam size of �1 mm.

and right-side Si 111 f g7�7 patterns depends on *ϕ*, as shown in **Figure 9(c)** and **(d)**. *ϕ* is the azimuth angle for the Si 110 ð Þ top-/bottom-surfaces and simultaneously corresponds to the glancing angle for the Si 111 f g side-surfaces. On the other hand, *θ* is the glancing angle for Si 110 ð Þ and is also the azimuth angle for Si 111 f g. **Figure 9(a)** summarizes the dependence of the observable and non-observable conditions in the RHEED patterns on the polarity of *θ* and *ϕ*. When *θ* (the glancing angle for Si 110 ð Þ) decreases, the RHEED pattern from Si 110 ð Þ disappears, while a pattern from Si 111 f g is present. Indeed, in **Figure 9(b)** (*θ* = 0.0° and *ϕ* = +1.9°), a quarter circle 7�7 pattern with faint 2�16 spot can be seen. The diffraction spots on the left and the right sides (**Figure 7(c)**) are slightly elongated in the horizontal direction. In general, the elongation (streaky) direction corresponds to the surface normal direction [4], as illustrated in **Figure 8**. Thus, these slightly streaky spots indicate the existence of

*Creation and Evaluation of Atomically Ordered Side- and Facet-Surface Structures of Three…*

Let us analyze the curious RHEED patterns in more detail. In **Figure 9(c)** (*θ* = +0.3° and *ϕ* = �1.6°) and **Figure 9(d)** (*θ* = +0.3° and *ϕ* = +1.1°), diffraction spots from the direct beam (DB) can be observed in the 1/7th-order Laue zones (L1/7-L6/7) on the left and right quarter sides, respectively, as well as seven spots within the Kikuchi band width (e.g., indicated by a pink arrow in **Figure 9(c)**). These patterns clearly correspond to Si 111 f g7�7 reconstruction [4], having shadow edges in the horizontal and vertical directions. **Figure 9(c)** (**Figure 9(d)**) corresponds to Si <sup>111</sup> (Si 111 ) 7�7 diffraction on the surface of the left-side (right-side) wall of the 3D patterned structure. Note that a specular spot 00 ð Þð Þ <sup>111</sup> ( 00 ð Þð Þ <sup>111</sup> ) from the DB appears on the left (right) side. In addition, strong Kikuchi lines and bands were observed in the side RHEED pattern. These results indicate that atomically flat sidesurfaces were achieved on 3D patterned Si 110 ð Þ by etching and UHV annealing. One can see the characteristic *ϕ* and *θ* dependences of the RHEED patterns (movie) in Supporting Information of Ref [5]. These RHEED patterns clearly show that all the surfaces on 3D Si, that is, the 110 ð Þ top-/bottom-surfaces, the 111 rightside surface, and the 111 left-side surface, have atomically ordered structures.

**Figure 10** shows the LEED patterns observed from the figured 111 side-surface. Because in LEED we observe backscattering diffraction, while RHEED reflects forward scattering diffraction, a unique 3D Si sample with a wider side-surface was prepared [9]. A clear 7�7 pattern can be seen in **Figure 10(a)**, where the incident electron beam is along a direction almost normal to the 111 side-surface. Characteristic LEED patterns were observed when the sample was rotated; the electron beam probed both a <sup>111</sup> side-surface and a 110 ð Þ top-surface. An example with an incident angle of �37° is shown in **Figure 10(b)**, which reflects surface reconstructions for both the figured

The LEED patterns are in good agreement with those simulated by considering the crystal orientation and electron beam (i.e., the cross sections of the reciprocal lattice rods and an Ewald sphere [22]) shown in the lower-right panels of **Figure 10**. It is possible to identify diffraction spots on Laue zones of both <sup>111</sup> and 110 ð Þ surfaces in the patterns, demonstrating the observation of atomic crystalline order-

**Figure 11(a)** shows a typical filtered RHEED pattern obtained from 3D Si with f g 100 side-surfaces. The 100 f g side-surfaces were created on a commercial mirrorpolished Si 100 ð Þ substrate (**Figure 6(a)**). In **Figure 11(a)** (*θ* = +0.4° and *ϕ* = �1.2°), the two overlapping Si 100 f g2�1 reconstructed diffraction patterns [4, 24] originated

<sup>111</sup> <sup>7</sup>�7 side-surface and the pristine 110 ð Þ <sup>16</sup>�2 top-surface [6].

ing for the 3D surfaces using LEED.

**4.2 100** f g **vertical side-surfaces**

**99**

vertical side-surfaces.

*DOI: http://dx.doi.org/10.5772/intechopen.92860*

#### **Figure 8.**

*Schematic of the diffraction from the top- and side-surfaces of a 3D patterned Si*ð Þ 110 *substrate. The control of the incident electron beam direction defined by* θ *and* ϕ *enables the observable faces in the 3D space to be selected.*

### **4. Surface structures on 3D architected Si sample**
