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Controlling Kink Geometry in Nanowires Fabricated by Alternating Metal-Assisted Chemical Etching Yun Chen, Liyi Li, Cheng Zhang, Chia-Chi Tuan, Xin Chen, Jian Gao, and Ching-Ping Wong Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04410 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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Controlling Kink Geometry in Nanowires Fabricated by Alternating Metal-Assisted Chemical Etching Yun Chenab, Liyi Lib, Cheng Zhangbc, Chia-Chi Tuanb, Xin Chena*, Jian Gaoa, Ching-Ping Wongbd* a

School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, 510006, China

and Key Laboratory of Mechanical Equipment Manufacturing & Control Technology b

School of Materials Science and Engineering, Georgia Institute of Technology, 711 Ferst Drive, Atlanta, GA 30332, USA

c

School of Materials Science and Engineering, Southeast University, Nanjing, 211189, China d

School of Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong

KEYWORDS: kinked nanowires, controlling kink geometry, alternating MACE

ABSTRACT: Kinked silicon (Si) nanowires (NWs) have many special properties that make them attractive for a number of applications, such as microfluidics devices, microelectronic devices, and biosensors. However, fabricating NWs with controlled 3D geometry has been

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challenging. In this work, a novel method called alternating metal-assisted chemical etching, or alternating MACE, is reported for the fabrication of kinked Si NWs with controlled 3D geometry. By the use of multiple etchants with carefully selected composition, one can control the number of kinks, their locations, and their angles by controlling the number of etchant alternations and the time in each etchant. The resulting number of kinks equals the number times the etchant is alternated, the length of each segment separated by kinks has a linear relationship with the etching time, and the kinking angle is related to the surface tension and viscosity of the etchants. This facile method may provide a feasible and economical way to fabricate novel silicon nanowires, nanostructures, and devices for broad applications.

Silicon nanowires have many potential applications, including in devices for photovoltaics, 1-3

microfluidics, 4 and microelectronic, 5 as well as in sensors for life science. 6 Several novel

fabrication methods have been developed for Si NWs, such as the vapor-liquid-solid (VLS) method, 7, 8 dry etching, 9 and wet etching. 10, 11 However, with these methods it is difficult to fabricate non-straight nanowires.

12, 13

Both the VLS and dry-etching methods require

expensive equipment and high temperature. 14 Because of the isotropic etching mechanism, traditional wet-etching methods can only develop small aspect features; therefore, fabricating non-straight nanowires is nearly unmanageable for traditional wet-etching methods. 15 Non-straight nanowires, especially kinked nanowires, have many special properties, such as a higher photon absorption coefficient, 16 good thermal isolation capability, 17, 18 heightened strain effect, 20, 21 and excellent suitability for biosensors. 22-24 Developing facile approaches to fabricate non-straight nanowires is essential in order to exploit these properties in a practical way. Metal-assisted chemical etching (MACE), a recently developed method for wet etching of Si, may have the potential to fabricate non-straight nanowires. 25-28 Many researchers have


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demonstrated such capability and successfully fabricated non-straight nanowires using MACE. For example, slanted nanowires can be fabricated on silicon samples with orientations other than (100), such as Si (110) and Si (111), as is the favored etching direction for MACE. 29 In addition, curved or tilted nanowires can be fabricated by adding methanol, ethanol, 2-propanol, or acetonitrile. These organic additives can lower the surface tension of etchants, decrease the etching rate along the vertical etching direction, and allow etching along other major crystallographic directions or some vector combinations of them. 30 However, no kinks are formed in nanowires fabricated with this simple one-step MACE method. Zigzag nanowires with periodical kinks have been fabricated by a two-step method. 31 In this method, an etchant with a high ratio of concentration of HF (denoted as [HF], with the same notation used for H2O2 concentration in the following) over [H2O2] was used to remove the thin layer of porous silicon oxide formed on the surface of straight nanowires fabricated in the first etching step with an etchant with a low ratio of [HF]/[H2O2]. Similarly, by using solutions with different concentration of oxidant, zigzag nanowires can be formed on both Si (111) and Si (110) samples with a constant angle of 115° or 90° between consecutive Si segments, as etching preferentially maintains the oriented direction on both Si (110) and (111)-oriented samples. 32 Chen et al. also found that by controlling etching temperature, zigzag nanowires with a constant angle of 90°, 125°, or 150° can be fabricated on (111)oriented silicon samples.

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However, kinks on these nanowires originated from random

fluctuations. Therefore, neither the location, nor angle, nor number of kinks could be controlled. In nanowire-based three-dimensional devices (e.g., gate-all-around nanowire field effect transistors, biosensors), it is essential to form certain kinks at well-controlled locations, orientations, and sidewall geometry. 12, 34-37


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In this work, an alternating MACE method is reported to fabricate kinked nanowires with full control over the kink geometry, as shown in Figure 1. First, patterns of polystyrene (PS) microspheres were formed on Si through a self-assembly process.

38, 39

Then, 3-nm-thick

titanium (Ti) and 30-nm-thick gold (Au) are deposited as the catalysts on Si. After that, to form kinks at desired positions, Si is etched for specific times in different etchant solutions containing various additives. Firstly, the silicon sample is immersed into etchant A for time X1, when slanted NWs of length L1 are formed. Then, the Si is removed from etchant A and immersed into another etchant solution, designated as etchant B, for time Y1. Because the etching directions are different in the two etchants, kinks with angle α are formed. After that, the silicon sample is re-immersed into etchant A to etch another slanted segment for each nanowire. However, in order to change the length of the second and third slanted segments to be L2 and L3, respectively, the etching time is changed to X2 and X3, correspondingly. Similarly, in order to control the angle β of the second kink, the third kind of etchant named as etchant C is used. From here on, we designate the sequence of etchants as a series of appropriate letters, such as A-B-A-C-A and the corresponding etching times as sequence of times in minutes written as X1-Y1-X2-Y2-X3. By repeating similar procedures, more kinks can be obtained. In addition, by changing the etching time and etchant composition for each segment, the location, number, and angle of the kinks can also be precisely controlled.


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Figure 1. Schematic diagram of alternating MACE. (a) bare Si; (b) self-assembly of PS microspheres; (c) metal deposition; (d) etching in etchant A for time X1; (e) etching in etchant B for time Y1; (f) etching in etchant A for time X2; (g) etching in etchant C for time Y2; (h) etching in etchant A for time X3; (i) 3D view of kinked NWs.

Figure 2 shows highly ordered, uniform kinked nanowires fabricated by alternating MACE. It also demonstrates that the method proposed in this work has the ability to form various kinked nanowires on very large scale. In addition, the developed alternating MACE method has the advantage of being solution based, which may reduce cost.


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Figure 2. Typical kinked nanowire fabricated by alternating MACE. (a) nanowire with only one kink after changing etchant 1 time; (b) nanowire with two kinks after changing etchant 2 times; (c) nanowire with four kinks after changing etchant 4 times; (d) Periodically multiply kinked nanowires with four kinks after changing etchant 4 times.

Controlling the number of kinks. To form kinks in NWs, usually more than two kinds of etchant should be used. In this serial experiments, three kinds of etchant were used. Etchant A consists of 20 ml deionized (DI) water, 2 ml hydrogen peroxide (H2O2) and 10 ml hydrofluoric acid (HF), etchant B consists of 15 ml DI water, 5 ml glycerol, 2 ml H2O2, and 10 ml HF, and etchant C consists of 10 ml DI water, 20 ml ethanol, 2 ml H2O2, and 10 ml HF. The viscosity of glycerol (945.0 mPa*S) is about 1000 times larger than that of the other components (~1 mPa*S), while the surface tension of ethanol (22.39 mN/m) is about 1/3 that of DI water (72 mN/m). Details are in the supporting information. These different physical properties greatly affect the diffusion of HF and H2O2 from bulk solution to the reaction interface, and, thus, result in a change of the etching direction. When the sequence of etchants was A-B and the sequence of etching times is 2-2, there is only one kink formed on each nanowire (Figure 2(a)) corresponding to the single change in etchant. In this study, only the inflection point and the undulation point are considered as


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kinks. Mathematically, the inflection point is the point of the curve where the curvature changes its sign while a tangent exists; the undulation point is a point where the curvature vanishes but does not change sign. As the sign of curvature of smoothly curved sections is not changed, no kink is considered to exist as long as the curvature does not vanish. Following the same procedure, when the etchant sequence was A-B-A and the sequence of etching times was 1-3-1, i.e., two etchant changes, there are two kinks formed on each nanowire, as shown in Figure 2(b). To form more kinks, we used the etchant sequence A-B-A-C-A with corresponding etching times 2-3-2-3-2. With four changes of etchant, there are four kinks formed on each nanowire, as shown Figure 2(c). Four kinks can also be formed by alternating between only two etchants, as for example in Figure 2(d), where the etchant was changed four times in the sequence A-B-A-B-A. The sequence of etching times used in Figure 2(d) was 2-4-2-4-2. From the above results, we conclude that the number of kinks is equal to the number of times the etchant is changed. Therefore, to create kinks in the NWs requires only finding at least two types of additives whose surface tension or viscosity are sufficiently different so as to significantly affect the etching direction. Length control. To form kinked nanowires with different segment lengths, we used the etchant sequence A-B-A and the varied etching time for the first segment in each of four cases, as listed in Table 1. Cross-sectional SEM images of the Si wafers after etching are shown in Figure 3. In cases (b) and (c), the first kinks are not well defined. Therefore, the total penetration depth of the first two segments was measured. The total etching times for these two segments for cases (a) through (d) were 4, 5, 6, and 7 min, respectively. Multiple nanowires were measured for each case (Figures S1–S4 in the supporting information), and the average value was used to represent the length. The average measured lengths were 1.97, 2.64, 3.53, and 4.42 µm for


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cases (a)–(d), respectively, as plotted in Figure 3(d). The length increases linearly with the etching time. Thus, the length of each segment can be controlled by adjusting the etching time. These results demonstrate that, by changing the etching duration, it is feasible to form a kink in any desired location. Table 1. Etching time (min.) Case

Etchant A

Etchant B

Etchant A

a

1

3

1

b

2

3

1

c

3

3

1

d

4

3

1

Figure 3. Length control for kinked nanowire using etchants A and B. Etchant sequence is A-B-A for all four cases. Etching times for cases (a), (b), (c), and (d) are given in Table 1. (e) Linear relationship between etching time and segment length.

Angle control. To form kinked nanowires with different kinking angles, etchants with different concentration of glycerol are used. The etchant sequence was A-Bx-A, where x in Bx represents the cases designated (a), (b), and (c). The sequence of etching times was 1-4-1. The compositions of each etchant are listed in Table 2. Cross-sectional SEM images of the Si wafers after etching are shown in Figure 4. Several nanowires were also selected to be measured for each case (Figures S5–S7 in the supporting information), and the average value was used to represent the kinking angle. The average kinking angles were 177°, 163°, and


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132° for cases (a)–(c), respectively, as plotted in Figure 4(d). The average angle decreases linearly with the volume of glycerol. This means the kinking angle can be controlled by adjusting the concentration of glycerol in the etchant solution. Table 2. Etchant compositions Etchant

HF(mL)

H2O2(mL)

DI water(mL)

Glycerol(mL)

A

10

2

20

0

Ba

10

2

17.5

2.5

Bb

10

2

15

5

Bc

10

2

20

10

Figure 4. Angle control for kinked nanowire. The sequence of etching times is 1-3-1 for all the three cases (a) etchant sequence A-Ba-A; (b) etchant sequence A-Bb-A; (c) etchant sequence A-Bc-A. (d) Linear relationship between kinking angle and glycerol volume.

Periodically kinked nanowires. To further demonstrate the control of kinking angles, experiments were designed to form periodically multiply kinked nanowires. The etching sequence was A-B-A-B-A, and etching times were 2-4-2-4-2, respectively. Cross-sectional SEM images of the Si wafer after etching are shown in Figure 2(d). The lengths and angles indicated in the figure are average values (Figures S8–S9 in supporting information). The first and third segments were etched in the same etchant for the same time, and the lengths are 0.98 and 1.06 µm, respectively; the deviation is only about 5.6%. Similarly second and


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fourth segments were fabricated in the same way. Their lengths are 2.99 and 2.95 µm, respectively, which means the deviation is only about 1%. The kinking angles are 135.2°, 134.6°, and 137.7°, the deviation of kinking angles is only 1.2%. These NWs were characterized by transmission electron microscopy (TEM, Figure 5). Figures 5(b) indicates that a given NW is single crystalline, which is intrinsic since NWs are etched from single-crystal Si wafers. From the lattice-resolved TEM images, the etching direction can be identified as firstly along [100] when in etchant A (Figure 5(c)), since the number of Si back-bonds that have to be broken is the lowest for this direction. However, the etching direction changed to [21-1] when in etchant B (Figure 5(c)). As the H2O2 is more likely to be trapped by the high viscosity glycerol than by HF through hydrogen bonding, 40, 41 H2O2 locally diffuses much more slowly than does HF at the reaction sites, resulting in a localized high HF concentration. Thus, sufficient electronic holes provided by HF allow etching towards other crystallographic directions or some vector combinations of them. 25, 27, 30, 31, 42 Afterwards, in etchant A (Figure 5(d)), as the concentration of electronic holes returns to normal, the etching direction returns to [100]. After that, it changed to [21-1] again when in etchant B (Figure 5(e)) and back to [100] when in etchant A (Figure 5(f)).


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Figure 5. Crystallographic structure of kinked silicon NWs after alternating MACE. (a) Bright-field TEM image of multiply kinked silicon NWs. There are four kinks in each NW, labeled as I, II, III, and IV on one of the NWs. (b) Selected area electron diffraction (SAED) patterns recorded from Kink IV. The SAED patterns were recorded along the [011] zone axis. Lattice-resolved TEM images from (c) Kink I, (d) Kink II, (e) Kink III, and (f) Kink IV. Arrows denote etching directions.

In summary, a novel method, called alternating MACE, is proposed to form kinked nanowires and to control the geometry of the kinks. The core concept of this method is alternately etching Si with different types of etchant, each for a specific, predetermined time. Based on this idea, kinked nanowires with two or more kinks, with segments of the same or


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different length, and with small or large kinking angle, were fabricated in our experiments. The number of kinks is simply equal to the number of times the etchant is changed, and the length of each segment has a linear relationship with etching time. The angle of kinks is related to the surface tension and viscosity of etchant. By controlling the etching time, selecting the number of etchant alternations, and carefully tailoring the composition of each etchant, the number of kinks and their locations and angles can be fully controlled. Kinks can be engineered to be at any desired on any location, and various types of kinked nanowires can be fabricated. This method may provide a feasible and economical way to fabricate novel nanowires, nanostructures, and devices. Methods: Silicon wafers used in the experiment are p-type single-crystalline Si wafers ((100)oriented, boron doped, resistivity: 1–10 Ω cm) purchased from University Wafer, MA, USA. The polystyrene (PS) microspheres were purchased from Polysciences Inc. (Warrington, PA). The microspheres (500 nm in diameter) were received in 2.5% (w/v) aqueous solution. All the other chemicals were purchased from VWR International LLC and used without further processing. Si wafers were firstly cleaned with a piranha solution (H2SO4 (96 wt.%) and H2O2 (30 wt.%) with a volumetric ratio of 1:1) at 120 °C for 10 min to remove any oxides. This was followed by rinsing in deionized (DI) water and drying in flowing N2. The as-received PS microsphere solution was dropped directly onto the Si wafers without further processing and dried in air at 25° C. The resulting surface was covered with a close-packed pattern of PS microspheres. The PS-covered wafers were treated in a Vision RIE system (Advanced Vacuum) for 2 min. (O2 and Ar were used as the etch gas with flow rate of 5 sccm and 45 sccm, respectively. The chamber pressure was 100 mTorr, and the power was 200 W.) After that, 3-nm-thick titanium (Ti) and 30-nm-thick gold (Au) were deposited as the catalysts for


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MACE by an electron beam evaporator (Denton Explorer) at a rate of 0.5 Å/s in a vacuum of 3.0×10−6 Torr. During the experiment, the silicon wafers were cut into square samples about 1×1 cm2 in size. Reagents used for etching were aqueous HF and H2O2 with concentrations 49 wt.% and 30 wt.%, respectively. The Si samples were immersed in a chemical etching bath containing HF, H2O2, DI water (18 MΩ cm), and an additional solvent (ethanol, or glycerol). Viscosity and surface tension are shown in the supporting information. The ratio of water, HF, H2O2, and co-solvent are varied for different etching conditions. After etching, scanning electron microscopy data (SEM, Zeiss LEO 1550 and Hitachi SU8010) and transmission electron microscopy data (TEM, FEI Tecnai G2 F30) were recorded to study the etching morphology. Corresponding Author *[email protected]; *[email protected]. Author Contributions Y.C. and L.L. wrote the main manuscript text. Y.C., C.Z and C.C.T performed and analyzed the experiments. X.C., J.G., and C.P.W. contributed to the principal aspects and supervised the progress of the research. All authors reviewed the manuscript. Notes The authors declare no competing financial interest. Supporting Information (a) Detailed measurements of segment length (b) Detailed measurements of kinking angle (c) Detailed measurements of periodically nanowires


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(d) Viscosity and surface tension of etchant

ACKNOWLEDGMENT The work presented in the paper is partially supported by National Natural Science Foundation of China (51605100, U1601202), National Science Foundation (CMMI 1130876), Hong Kong Research Grants Council (RGC 417513, 14243616) and Fund of Guangdong R&D Science and Technology (2016A010102016, 2016B090905001). The authors acknowledge the help of Dr. Ding Yong, Georgia Institute of Technology, USA, for assistance with TEM. REFERENCES 1.

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Nano Letters

Table of Contents Graphic Title: Controlling Kink Geometry in Nanowires Fabricated by Alternating Metal-Assisted Chemical Etching

Alternating MACE is reported for the fabrication of kinked Si NWs with controlled 3D geometry. By the use of multiple etchants with carefully selected composition, one can control the number of kinks, their locations, and their angles by controlling the number of etchant alternations and the time in each etchant. .


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Controlling Kink Geometry in Nanowires ... - ACS Publications

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