Porosification-Induced Back-Bond Weakening in Chemical Etching of

Jan 10, 2013 - Department of Physics, Institute of Advanced Materials, Hong Kong Baptist ... metal-assisted chemical etching (MACE) to engineer the su...
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Porosification-Induced Back-Bond Weakening in Chemical Etching of n‑Si(111) Fan Bai, Wai-Keung To, and Zhifeng Huang* Department of Physics, Institute of Advanced Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR ABSTRACT: It is imperative to control the etching direction in metal-assisted chemical etching (MACE) to engineer the surface crystalline orientation of as-generated silicon nanowires (SiNWs). At room temperature, MACE of n-Si(111) carries out along the intrinsic back-bond etching direction of [111] under [HF]/[AgNO3] ≥ 50. When n-Si(111) is heavily doped, MACE generates mesoporous SiNWs (mp-SiNWs) standing on a mesoporous silicon (mpSi) layer. The porosification substantially weakens the back bonds underneath the sinking metal particles, leading to a deviation of etching from [111]. The selection of etching direction is governed by an angle of the selected direction to [111], and small angle is preferential. Due to the thermodynamic preference of the intrinsic back-bond etching, the etching along [111] is retained at ≥30 °C. Zigzag mp-SiNWs are first created under high [HF] and solution stirring, attributed to the HF-induced heating and stirring-stimulated H2 evaporation. This work would potentially pave a way to employ mp-SiNWs with controllable surface orientations in catalysis, electronics, optoelectronics, sensors, and surface functionalization. HF + 0.01/0.04 mol/L AgNO3)21 and (4.8 mol/L HF + 0.4 mol/L H2O2)22 below 25 °C, instead of [100]. However, Huang et al. reported etching was along [111] in 4.6 mol/L HF and 0.1 mol/L H2O2, but switched to [311] when the concentration of H2O2 (i.e., [H2O2]) was reduced to 0.02 mol/ L, indicating the preferential direction of [311].23 In terms of lightly doped p-Si(111), contradictory results were also reported. In 4.6 mol/L HF and 0.04 mol/L AgNO3, the preferential etching is either along [100]24 or [111].21 When Fe(NO3)3 or H2O2 was used as the oxidant, etching is along [111].15,25 By tuning [H2O2] and etching temperature, it was deduced that [100] is preferential over [111].22,23,26 Note that a remarkable number of studies have identified that [111] is preferential over [100] on Si(111), in contrast to the back-bond theory. MACE of heavily doped wafers produces mp-SiNWs,27 involving two subetching processes. Some metal nuclei grow with continuous reduction of metal ions at the solution/silicon interfaces and electrochemically scratch bulk wafers, leaving the residues as s-SiNWs. Simultaneously, the rest, nongrown metal nuclei randomly etch s-SiNWs in a short period of time, and porosify s-SiNWs. The heavily doped wafers consume metal ions sufficiently, and reduce the gradient of metal ion concentration from the bulk solution to the interfaces. As a result, the diffusion of metal ions is deteriorated; hence, only a portion of nucleated catalysts can continuously grow, and the rest stop growing due to a lack of source materials. When the

1. INTRODUCTION Since the beginning of the 2000s,1,2 MACE has been developed as a simple, low cost approach to generate silicon nanowires (SiNWs), one type of significant nanomaterials potentially applied in batteries,3 photovoltaic,4 field-effect transistors,5 thermoelectronics,6 and cellular culture monitoring.7,8 Before 2009, MACE was mainly carried out to create solid SiNWs (sSiNWs) without pores;9,10 then it was developed to fabricate mp-SiNWs, which have pores with distributed sizes smaller than 50 nm and adjustable porosity in s-SiNWs.11,12 As is wellknown, surface crystalline structures of nanomaterials play a vital role in catalysis,13 electronics,14 and surface chemistry. It is generally adopted in MACE that metal particles functioning as the etching catalysts electrochemically etch/scratch silicon wafers, leaving the residues as an array of s-SiNWs standing on the intact wafers.15 As a result, surface crystalline orientations of as-generated SiNWs are substantially dependent on the etching direction of metal particles.16 The back-bond theory addresses that two Si−Si back bonds attached to surface Si atoms need to break along [100], and three along [111]; and thus the direction [100] is preferential over [111], which is inert to MACE.17 In general, MACE of Si(100) produces vertical s-SiNWs under ambient conditions, manifesting the preference along [100].18−20 The inertness of [111] against MACE predicted in the back-bond theory evokes research in the scratching track of catalyst particles in Si(111), finding that the etching direction is comprehensively determined by the concentration of etchants (HF) and oxidants (AgNO3, Fe(NO3)3, or H2O2), etching temperature, and doping level of mother wafers. For lightly doped n-Si(111), particles migrate along [111] in (4.6 mol/L © 2013 American Chemical Society

Received: December 6, 2012 Revised: January 8, 2013 Published: January 10, 2013 2203

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Figure 1. MACE (2.4 mol/L HF and 0.02 mol/L AgNO3) of n+2-Si(111) at room temperature, lasting 10 min. (a) An oblique-viewing SEM image. Inset: a cross-sectional SEM image showing the etching is along [311]. (b) TEM image of a mp-SiNW with SAED pattern as inset.

Table 1. α of Diverse Etching Directions with Respect to [111]a

a

Refer to http://www.cleanroom.byu.edu/EW_orientation.phtml.

2. EXPERIMENTAL SECTION The wafers for etching are n-Si(111), lightly doped (with ρ of 1−10 Ω·cm, i.e., n−-Si(111), Semiconductor Wafer, Inc.) and heavily doped (1−5 mΩ·cm, i.e., n+2-Si(111), Semiconductor Wafer, Inc.). The MACE method was elaborated previously.19,30 The wafers were cleaned via sufficient ultrasonication in acetone and ethanol, degreased in Piranha (98% H2SO4: 30% H2O2 = 3:1, v/v) at room temperature, and dipped in 5% HF to remove native oxides. Then the treated wafers were immersed in a Teflon beaker, containing an aqueous solution containing HF and AgNO3 (analytical reagent grade, Fisher Chemical). An aluminum foil shielded the beaker from solar illumination, leading to uniformly generating nanostructures on a wafer with an area of 1.5 × 1.5 cm2. A set of etching conditions were tuned, including the ratio of [HF]/[AgNO3] in a range of 2.5− 480, etching temperature of 10−55 °C, and etching duration of 3−20 min. Without specific description, the etching was carried out without stirring etchant solutions. When the etching was over, the samples were immediately dipped into 68% HNO3 to dissolve the as-generated Ag dendrites on the wafer surfaces. After each acid treatment, the wafers were thoroughly rinsed with the DI water (18.2 MΩ, Milli-Q reference water purification system fed with campus distilled water) and blown by N2. The as-etched wafers were mechanically split, leaving the freshly split surfaces for SEM characterization (field emission SEM, Oxford, LEO 1530) without coating metals. SiNWs were scratched off from the as-etched wafers and transferred to isopropanol. After 5 min ultrasonication, SiNWs well dispersed in isopropanol. Several drops of the mixture were

former subetching is faster, MACE creates mp-SiNWs with nonuniform porosity along the NW long axis, which directly stand on bulk wafers.28 In contrast, the porosification surpasses the scratching of growing metal particles, creating a mp-SiNW/ mpSi/wafer triple-layer structure.29,30 In this case, since the scratching of catalyst particles lags behind the porosification, the porosification may disturb the etching direction. Therefore, it is intuitive to ask how the porosification perturbs the sinking of catalyst particles in MACE of heavily doped Si(111). It was reported that MACE of heavily doped (electric resistivity ρ: 4− 8 mΩ·cm) p-Si(111) produced s-SiNWs along [100] in (4.8 mol/L HF + 0.15 mol/L H2O2),22 and contradictorily along [111] in (10% HF + 0.6% H2O2).31 Although the reason was ambiguous, both works produced s-SiNWs instead of mpSiNWs on the heavily doped wafers, and thus the perturbation of porosification to the etching was not discussed. This work demonstrates the fabrication of the two-layer porous nanostructures (mp-SiNWs on mpSi) via MACE (HF + AgNO3) of the heavily doped n-Si(111), and for the first time investigates the porosification-caused disturbance to the etching directions with etching temperature and etchant concentration. Then we first create zigzag mp-SiNWs, and explore the formation mechanism. The engineering of surface crystalline orientation and shape may impose novel functions on mpSiNWs to develop the applications of energy storage,32 batteries,33 photocatalysis,34 drug delivery35 and gas sensing.36 2204

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applied to a lacey carbon film on a grid structure (Electron Microscopy Sciences). The grid was ambiently dried and inspected by TEM (Tecnai G2 20 S-TWIN) and SAED (selected area electron diffraction, integrated in TEM).

Equation 3 indicates that silicon is directly dissolved in HF via the back-bond etching without silicon oxidation, and the intrinsic back-bond etching should be investigated under high [HF]/[AgNO3]. When [HF]/[AgNO3] is 2.5, the etching of n−-Si(111) at room temperature creates oblique s-SiNWs with α of 34.9 ± 1.7, indicating the ⟨110⟩ etching (Figure 2a). At

3. RESULTS AND DISCUSSION At room temperature, MACE (2.4 mol/L HF and 0.02 mol/L AgNO3) of n+2-Si(111) produces oblique SiNWs on a mpSi layer (Figure 1a), and SiNWs appear to be mesoporous (Figure 1b). Since MACE generally creates single crystalline s-SiNWs, SAED was used to study the etching direction of s-SiNWs.21,31 However, the random porosification creates mesopores with polycrystalline surfaces, resulting in a polycrystalline SAED pattern of mp-SiNWs (as inset in Figure 1b) to prevent identifying the etching direction. Alternatively, the angle α of the NW growth orientation with respect to the surface normal (as shown in the scheme of Table 1) was measured in crosssectional SEM images of the as-etched samples (e.g., inset in Figure 1a), to evaluate the etching direction according to Table 1.16,21 The angle α was statistically measured from multiple cross-sectional SEM images taken along the freshly split sample surfaces, evaluated as an average angle with standard deviation. Note that the length of mp-SiNWs increases with etching duration, and long mp-SiNWs tend to aggregate due to the porosification-caused weakening of NW mechanical strength.19 The aggregation makes mp-SiNWs deform away from the original growing orientation. Hence, the etching processed less than 20 min in this work to guarantee that NWs have sufficient length without aggregation for reliably evaluating the etching direction. For instance, the inset SEM image of Figure 1a clearly shows that 10 min etching generates mp-SiNWs with a length of ∼650 nm and α of 29°, indicating the etching along [311]. The as-generated two-layer mesoporous structures (Figure 1a) indicate the disturbance of porosification to the etching. To study the perturbation effect of porosification, first of all it is intuitive to eliminate the porosification disturbance and solely investigate the intrinsic etching in Si(111), via MACE of n−Si(111) to fabricate s-SiNWs. MACE involves a set of electrochemical reactions at the solution/Si interfaces.30 Silicon oxidizes (eq 1a) and donates electrons to reduce Ag+ ions (eq 2), which nucleate on the wafer surfaces. HF dissolves the oxidized SiO2 underneath Ag nuclei (eq 1b), and creates the pits to trap the nuclei on the surfaces. Si + 2H 2O − 4e− → SiO2 + 4H+

(1a)

SiO2 + 6HF → H 2SiF6 + 2H 2O

(1b)

Ag + + e− → Ag

Figure 2. MACE of n−-Si(111) at room temperature, under different etching conditions as denoted in Table 2. (a−e) SEM cross-sectional images.

[HF]/[AgNO3] over 50, s-SiNWs tend to grow vertically along [111] (Figure 2b−e), in coincidence with the previous report.21 As a result, the intrinsic back-bond etching of n-Si(111) which occurs at [HF]/[AgNO3] ≥ 50 is preferential along [111] over [100], even though three back bonds should be cut off along [111] rather than two along [100]. After clarifying the intrinsic back-bond etching of n-Si(111), the porosification is additionally taken into account via MACE of n+2-Si(111). Given [HF]/[AgNO3] ≥ 50 for the intrinsic back-bond etching, for example, at the concentration ratio of 120, MACE was carried out under controlled temperature of 10−55 °C (Figure 3). The two-layer mesoporous structures are formed under all temperatures (Figure 3), indicating the general existence of the porosification disturbance to the etching. At 20 °C, NWs tend to obliquely grow along ⟨311⟩ (with α of 29.3 ± 8.3°, Figures 1 and 3b), instead of [111] without the porosification (Figure 2c). When temperature is reduced to 10 °C, the etching occurs along ⟨211⟩ (with α of 19.2 ± 6.7°, Figure 3a). α decreases from 6.0 ± 2.5° at 30 °C to 2.1 ± 1.9° at 55 °C (Figure 3c−f), indicating the retention of the intrinsic back-bond etching along [111] over 30 °C. When the porosification surpasses the sinking of metal particles, the random porosification may effectively weaken back bonds underneath the scratching particles before the intrinsic etching, leading to the deterioration or elimination of the preference along [111]. It was proposed on the Si(111) surfaces that metal particles are apt to vertically scratch along [111] at the initial etching stage, and the vertical etching may switch to a direction by overcoming an energy barrier which increases with α.21 Table 1 clearly illustrates that the etching tends more readily to switch from [111] to [211] (with α of 19.47°) or [311] (with α of 29.50°) than other directions, and [100] is highly prohibited due to a relatively large α of 54.74°. Consequently, the thermal

(2)

When the three reactions occur subsequently and repeatedly, Ag nuclei continuously grow and sink into the bulk wafer, leaving the residues as an array of s-SiNWs. Two models have been proposed to further discuss the etching process,23 in terms of the ratio of [HF]/[AgNO3]. When the concentration ratio is small, the silicon oxidation (eq 1a) is faster than the oxide dissolution (eq 1b). Since eq 1b is the rate-determining step (RDS), the etching tends to be isotropic.37 In contrast, high [HF]/[AgNO3] makes the silicon oxidation (eq 1a) as the RDS. Combine eq 1a and 1b to produce eq 3, Si + 6HF − 4e− → H 2SiF6 + 4H+

(3) 2205

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Table 2. MACE Conditions, α, and Etching Direction of n−-Si(111)

Figure 3. MACE of n+2-Si(111) at (a) 10 °C, (b) 20 °C, (c) 30 °C, (d) 35 °C, (e) 45 °C, and (f) 55 °C. The areas marked between two red dashed lines represent the as-generated mpSi layers. Table 3 represents the detailed MACE conditions.

Table 3. MACE Conditions, α, and Etching Direction of n+2-Si(111)

proposed.30 One factor is the heat produced by the dissolution of SiO2 in HF, having a reaction enthalpy of −(138.22 ± 1.36) KJ/mol (eq 1b).42 Another originates from the transfer of kinetic energy from the evaporating H2 to the scratching particles. The reduction of H+ ions produced via eq 1a or eq 3 generates H2, given by

activation accounts for the fact that the etching switches to ⟨211⟩ at 10 °C, and further to ⟨311⟩ at 20 °C. The retention of the vertical etching above 30 °C illustrates that the intrinsic back-bond etching along [111] is thermodynamically preferential. It is imperative to engineer the shape of NWs for exploring the shape-dependent properties and functions.38−40 For the MACE technique, engineering of NW shapes lies in control over etching directions. For instance, s-SiNWs have been sculptured in zigzag,16,21,23,41 which is in void for mp-SiNWs. The zigzag structures are formed by alternately switching the etching direction under external stimulation to overcome the αdependent etching energy barrier, ascribed to two factors as

2H+ + 2e → H 2↑

(4)

Equation 4 competes with the Ag reduction (eq 2) through competitively consuming the electrons donated from the silicon oxidation (eq 1a). It is thermodynamically indicated in eq 1b and eq 3 that high [HF] facilitates the generation of both heat and H+ ions, and consequently the increase of [H+] promotes 2206

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Figure 4. MACE (5 mol/L HF and 0.02 mol/L AgNO3) of n+2-Si(111) at room temperature, without (a) and with (b−f) solution stirring, as denoted in Table 4. (a−f) SEM cross-sectional images. The green arrows point along [111], and blue arrows point along non-[111] directions as marked in (d)−(f).

Figure 5. TEM images of zigzag mp-SiNWs, fabricated by MACE (5 mol/L HF and 0.02 mol/L AgNO3, stirring for 15 min at room temperature) of n+2-Si(111).

Table 4. MACE Conditions, α, and Etching Direction of n+2-Si(111)

generates zigzag NWs (Figure 4b,c) with mesoporous structures (Figure 5a−c), probably attributed to the stirringpromoted H2 evaporation. Further structural analysis shows that the etching moves along diverse directions, as denoted in Figure 4d−f. Note that switching the etching from [111] to a direction with large α stems from overcoming a high energy barrier. Among the various directions of the zigzag branches, ⟨410⟩ (as shown in Figure 4e) has the biggest α of 45.56° (Table 1). It is indicated that the combination of HF-induced heating and stirring-promoted H2 evaporation is sufficient to drive the etching switch among a wide diversity of directions

the evaporation of H2 according to eq 4. Thus, high [HF] facilitates the production of zigzag SiNWs. Without stirring, MACE (2.4 mol/L HF and 0.02 mol/L AgNO3) of n+2-Si(111) produces titled cylindrical mp-SiNWs at 20 °C (Figure 1 and Figure 3b). When [HF] is increased to 5 mol/L, mp-SiNWs tend to grow vertically along [111], with α of 3.6 ± 1.8° (Figure 4a). Since [111] is thermodynamically preferential above 30 °C (Figure 3c−f), the vertical growth at room temperature (Figure 4a) strongly supports the heating effect induced by high [HF]. Apparently, however, the heating effect is not sufficient to create zigzag structures. The application of stirring etchant solution throughout MACE effectively 2207

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Figure 6. MACE (1 mol/L HF and 0.02 mol/L AgNO3) of n+2-Si(111) at room temperature. (a−c) SEM cross-sectional images. The green arrows point along [111], and blue arrows point along non-[111] directions as marked in (a−c).

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with α < 45.56°; hence, the etching switch to [100] with α of 54.74° is energetically prohibited. As shown in Table 4, the ambient etching without stirring leads to the monodirection etching of n+2-Si(111) under [HF]/ [AgNO3] < 250. When [HF]/[AgNO3] is reduced to 50, however, MACE produces zigzag mp-SiNWs, which is preferentially along ⟨521⟩ eventually (Figure 6a−c). For the comparison, MACE of n−-Si(111) under the same conditions produces vertical s-SiNWs (Figure 2b). The origin of the unexpected zigzag structures is not clarified yet, and the exploration is being carried out currently.

ACKNOWLEDGMENTS We gratefully acknowledge Mr. Benson Siu-Cheong Leung (Physics, HKBU) for his technical assistance in TEM and SAED (funded by SEG_HKBU06). This work is supported by HKBU8/CRF/11E (F.B.), TDG/1011/11 (F.B., W.K.T.), and HKBU/FRG2/10-11/006 (W.K.T.).

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4. CONCLUSIONS In summary, MACE of n-Si(111) in HF and AgNO3 processes along the intrinsic back-bond etching direction of [111] at room temperature, under [HF]/[AgNO3] ≥ 50. When the wafers are heavily doped, the evolution of the porosification surpasses that of the scratching of metal particles, leading to the generation of two-layer mesoporous structures (mp-SiNWs above mpSi). The porosification substantially weakens the back bonds underneath the sinking particles, leading to a deviation of etching from [111]. The etching preferentially switches to a direction with small α, due to low direction-switching barrier which increases with α. The etching is preferential along ⟨211⟩ (α of 19.47°) at 10 °C, and ⟨311⟩ (α of 29.50°) at 20 °C. The etching along [111] is reserved above 30 °C, indicating the thermodynamic preference of the intrinsic back-bond etching. The etching does not occur along [100], which is predicted to be preferential by the back-bond theory, mainly owing to a relatively large α of 54.74°. Therefore, it is indicated the etching is governed by not only the back-bond etching but also crystalline structures of the as-etched mother wafers. At room temperature, high [HF] together with solution stirring create zigzag mp-SiNWs, attributed to the HF-induced heating effect and stirring-stimulated H2 evaporation. The growth directions in the zigzag structures are randomly diverse, and the engineering of sculptured directions is under exploration currently. This work originally studies the control over surface orientations of mp-SiNWs, and would potentially pave a way to employ mp-SiNWs with controllable surface orientations in catalysis, electronics, optoelectronics, sensors, and surface functionalization.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2208

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