Controlling the Geometries of Si Nanowires through Tunable

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Controlling the Geometries of Si nanowires through tunable nanosphere lithography Luping Li, Yin Fang, Cheng Xu, Yang Zhao, Kedi Wu, Connor Limburg, Peng Jiang, and Kirk J Ziegler ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09959 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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ACS Applied Materials & Interfaces

Controlling the Geometries of Si nanowires through tunable nanosphere lithography Luping Li,† Yin Fang,† Cheng Xu,† Yang Zhao,† Kedi Wu,† Connor Limburg,‡ Peng Jiang,‡ and Kirk J. Ziegler†,‡,* †

Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611 USA

‡

Department of Materials Science & Engineering, University of Florida, Gainesville, Florida 32611 USA

*

Corresponding author. Email address: [email protected] (Kirk Ziegler)

Abstract A tunable nanosphere lithography (NSL) technique is combined with metal-assisted etching of silicon (Si) to fabricate ordered, high-aspect-ratio Si nanowires. Non-close-packed structures are directly prepared via shear-induced ordering of the nanospheres. The spacing between the nanospheres is independent of their diameters and tuned by changing the loading of nanospheres. Nanowires with spacings between 110-850 nm are easily achieved with diameters between 100-550 nm. By eliminating plasma or heat treatment of the nanospheres, the diameter of the nanowires fabricated is nearly identical to the nanosphere diameter in the suspension. The elimination of this step helps avoid common drawbacks of traditional NSL approaches, leading to the high-fidelity, large-scale fabrication of highly-crystalline, nonporous Si nanowires in ordered hexagonal patterns. The ability to simultaneously control the diameter and spacing makes the NSL technique more versatile and expands the range of geometries that can be fabricated by top-down approaches.

KEYWORDS: tunable nanosphere lithography, shear-induced ordering, silicon nanowires, metal-assisted chemical etching, spacing control

1 ACS Paragon Plus Environment


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Introduction Si nanowires (SiNWs) have unique physical and chemical properties that make them versatile

building

blocks

in

various

fields,

including

photonics,1

photovoltaics,2

thermoelectrics,3 batteries4 and sensors.5 The synthesis of SiNW arrays falls into either of two paradigms: bottom-up or top-down fabrication.6 Bottom-up approaches, such as the vaporliquid-solid (VLS) growth mechanism,7, 8 rely on the continuous accumulation of Si atoms into a metal catalyst (such as Au nanoparticles) on the surface of the substrate until supersaturation is reached and phase separation subsequently results in nanowire growth. The large-scale synthesis of high-density, vertically-aligned SiNW arrays is generally considered challenging using the VLS method,9, 10 primarily from concerns over catalyst contamination,11 low crystallinity,6 and difficulty in orientation control.12,

13

Top-down approaches, on the

other hand, carve bulk Si substrates into nanowire arrays in a subtractive manner via etching. Using appropriate etching masks has enabled the fabrication of high-density SiNW arrays with high throughput.11 The challenges associated with these approaches have been to develop nanoscale masks that can control both the diameter and spacing (the distance between the centers of two nearest nanostructures) without complex lithographic techniques like e-beam lithography. Metal-assisted chemical etching is a key step for fabricating SiNW arrays in many topdown approaches. Although isolated and randomly distributed Ag nanoparticles can be used, this approach imposes little control over the diameter, location, and spacing of the resultant nanowires.14 This control over the geometry is important since the performance of devices based on SiNW arrays frequently relies on precise control over nanowire geometries (i.e., the diameter, spacing, and aspect-ratio).1,

2

Therefore, patterned metal thin films with ordered

nanopores have been used extensively for the chemical etching step in recent years.6, 14, 15


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Various techniques have been used to prepare the patterned metal thin films needed for chemical etching, including the preparation of lithographic masks using nanospheres (NSL),10, 14, 16, 17

block copolymers (BCL),11,

18

and anodized aluminum oxide (AAO).13,

15, 19, 20

Although AAO masks have exceptional control over the diameters of SiNWs,13, 15, 19 AAO masks are extremely delicate and large-scale mask fabrication is difficult. These approaches also suffer from non-uniform metal deposition19,

20

and pattern deformation after mask

removal.20 In BCL, the phase separation of the copolymer drives the polystyrene (PS) nanospheres into hexagonal patterns.11 Although SiNWs with sub-20-nm diameters have been fabricated, these nanowires showed irregular shapes and lacked vertical alignment.11 The multiple steps used in BCL may also render this technique impractical for large-scale nanowire fabrication. NSL has been the most widely used technique for preparing the patterned metal thin films.14, 16, 17 However, intense plasma or heat treatment is required to reduce the size of the nanospheres.14,

16, 17

These intense processes inevitably make it more difficult to achieve

uniform etching of the nanospheres, often resulting in nanospheres with rough surfaces, irregular shapes, and non-uniform sizes.16, 17, 21 As illustrated by Yeom et al.,17 these issues become especially problematic when size reduction greater than 50% is required. The presence of any defects in the nanosphere morphology caused by size reduction is subsequently transferred to the nanowires. In many cases, large size reduction prevents metal lift-off, resulting in localized regions of SiNW arrays.6 The range of feasible processing parameters available to NSL techniques places practical limits on the range of achievable diameters and spacings in the array. Although cryogenic reactive ion etching (RIE) has been effective at reducing some of these factors,17 there is still a need for simpler approaches that can achieve ordered structures reliably. The NSL technique described here provides another approach to manipulate the dimensional parameter space, helping to expand the applications of SiNWs prepared by metal-assisted chemical etching. 3 ACS Paragon Plus Environment


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Experimental Details Preparing colloidal suspensions: Spin-coating procedures were modified from our previous work22,23 to generate non-close-packed hexagonal patterns on Si substrates. Monodisperse SiO2 nanospheres suspended in ethanol-ammonia solution with diameters between 90-520 nm were purchased from Particle Solutions, USA. These nanospheres were synthesized by Stober’s method,24 and no surface modifications were performed after synthesis. Nanospheres of desired diameters were cleaned three times by consecutive centrifugation and redispersion in 200-proof ethanol. After a final centrifugation, the thick slurry of SiO2 nanospheres were dispersed in a monomer (ethoxylated trimethylolpropane triacrylate, SR454, Sartomer Arkema, Inc.) to give a nanosphere vol% between 15-40%. A photoinitiator (Darocur 1173, Ciba Specialty Chemicals) was added to the suspension (~ 1 wt.%), which was subsequently mixed on a vortex mixer. The viscous suspension appears transparent, indicating no flocculation of nanospheres. It can be stored in the dark indefinitely and used later for spin-coating. Generating non-close-packed patterns: Highly-doped n-type (100) Si wafers with a resistivity of 0.0001-0.005 Ohm·cm (University Wafers, USA) were primed with 3acryloxypropyl trichlorosilane (SIA0199.0, Gelest) and placed on a spin-coater (WS-650MZ23NPPB, Laurell). Suspensions with different loadings of nanospheres were used to tune the spacing of the patterns formed. The suspension was added drop-wise onto primed Si to form a continuous layer before spin-coating started. For 90 nm nanospheres, the typical spin-coating sequence was 200 rpm for 1 min, 300 rpm for 1 min, 1000 rpm for 30 s, 3000 rpm for 10 s, 6000 rpm for 10 s, 8000 rpm for 10 s, and 10 k rpm for 26 min. The slow progression of spinning speed is necessary to achieve good hexagonal packing. For larger nanospheres (>90 nm), the sequence was altered to 200 rpm for 2 min, 300 rpm for 2 min, 1000 rpm for 1 min, 3000 rpm for 20 s, 6000 rpm for 20 s, and 8000 rpm for 5 min. The monomer within the


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resulting films was then polymerized for 8 s using a pulsed UV curing system (RC 742, Xenon). Depositing Au films: The polymer matrix embedding the nanospheres was subsequently removed by RIE (Unaxis Shuttlelock) at a power of 100 W for 2 min with a stream of oxygen flowing at 20 sccm. Our prior work found that the majority of the polymer is removed during RIE, leaving tiny polymer islands underneath the nanospheres.25 The remaining polymer will not affect the pattern formation during subsequent metal deposition because these are line-ofsight processes. A thin layer (1 nm) of Ti was deposited as an adhesion layer on the SiO2/Si surfaces using electron beam deposition (PVD Products, Inc.). The active Au layer was then deposited with thicknesses of 10 nm (for 90-nm nanospheres) or 20 nm (>90 nm). Fabricating SiNW arrays: SiO2 nanospheres were removed by ultrasonicating the Si substrate in ethanol briefly, leaving the porous Au thin film on Si. If the polymer matrix is not removed with the nanospheres, the metal-assisted chemical etching process will not be impacted because etching primarily occurs where the metal is deposited. For chemical etching of Si, hydrofluoric acid (48-51%, Acros Organics), hydrogen peroxide (35%, Acros Organics), and ethanol were mixed at a volume ratio of 10:1:1. The Au/Si samples were first dipped in ethanol before immersing them into the etching solution. This sequence prevented the detachment of the Au film from the Si substrate upon contact with the etching solution. Typical etching times were about 5-10 min and nanowires were thereafter gently rinsed in isopropyl alcohol and naturally dried in air. To obtain long nanowires, Si was etched for up to 2 h and a supercritical point dryer (Tousimis 915B) was used to dry the nanowires to avoid bundling. Characterization and statistical analyses: An FEI SEM (Nova NanoSEM 430) was used to examine the nanostructures at different processing steps. ImageJ (Version 1.48v) was used to calculate the average diameters and spacing (the distance between the centers of two



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nearest nanostructures). Typically, more than 300 nanostructures were counted in each case to obtain adequate statistics.

Results and Discussion In traditional NSL, PS or SiO2 nanospheres are first assembled into close-packed hexagonal patterns, which are then transformed to non-close-packed patterns by reducing the diameter of the nanospheres, resulting in the structures shown in Figure 1a. A thin film of Au

Figure 1. Top panel: Schematic of the major steps for SiNW fabrication. (a) PS/SiO2 nanospheres with a non-close-packed hexagonal pattern formed on Si by either reducing the diameter from the close-packed hexagonal pattern (traditional NSL) or controlling the loading of nanospheres in a colloidal suspension (NSL based on shear-induced ordering). (b) A porous metal film with the same pattern and dimension as the nanospheres formed after Au/Ag deposition and mask removal. (c) SiNW arrays with precisely controlled diameters and spacings formed after chemical etching. Middle panel: Diagrams illustrating how specific dimensions are determined in (d) traditional and (e) shear-induced ordering. (f-h) SEM images corresponding to the processing steps of NSL based on shear-induced ordering demonstrated in (a-c), respectively. The images are based on 200 nm SiO2 nanospheres. Insets are HRSEM images showing no morphological defects, indicating the high fidelity of this technique.


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or Ag (10-50 nm) is then deposited using the nanospheres as a mask. The nanosphere mask is subsequently removed by ultrasonication, leaving a porous metal film on the Si with nanopores whose diameters and spacings are the same as the non-close-packed nanosphere pattern, as shown in Figure 1b. The patterned Si is then immersed in a HF-based chemical etching solution to produce a SiNW array, as shown in Figure 1c. Although the nanowire diameter (d) can be controlled by the amount of size reduction of the nanospheres, the spacing (S) between the nanowires is fixed by the initial diameter of the nanospheres (D),14, 16 as shown in Figure 1d. To expand the range of dimensions available to NSL processes, we introduce a tunable NSL technique where the nanosphere spacing is controlled by the loading of nanospheres in a colloidal suspension so that subsequent size reduction is unnecessary. Consequently, the diameter and spacing of the resultant SiNWs are precisely and independently controlled over a wide range of values. Because nanosphere size reduction is avoided, problems associated with non-uniform particle etching and metal lift-off are eliminated. Therefore, the approach is able to achieve high-aspect-ratio SiNW arrays with highly-ordered hexagonal patterns more reliably. The ability to independently control both the diameter and spacing of SiNWs is made possible by the way the nanosphere mask is generated. Rather than tuning the nanowire diameter by reduction of the nanosphere diameter from their close-packed patterns, the nanowire diameter is selected by preparing colloidal suspensions containing SiO2 nanospheres with nearly identical diameters (D = d). These suspensions are spin-coated onto Si to form a non-close-packed hexagonal pattern. The polymer matrix embedding the SiO2 nanospheres is then removed by RIE, yielding the pattern shown in Figure 1a. Although RIE is used, a significant difference from prior approaches is the intensity of RIE. For example, up to 2500 W plasma power has been used to reduce nanosphere diameters,17 whereas only 100 W is used in this work. The pattern can be generated uniformly on a wafer-scale, as shown in our 7 ACS Paragon Plus Environment


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previous work, with domain sizes greater than 50 µm.22,23 NSL has been used to generate patterns on other substrates as well, including quartz 27 and glass.28 The formation of a non-close-packed hexagonal pattern during spin-coating is attributed to shear-induced ordering.22,23,25 The high spin speed (up to 10k rpm) induces a high shear rate in the nanosphere suspensions, causing a gradual convective flow of nanospheres in the radial direction. The balance of centrifugal and viscous drag forces acting on the nanospheres during spinning determines the spacing between them. When the nanospheres in the suspension are uniform in diameter, the force balance assembles the nanospheres into a hexagonal pattern.22 The successful assembly of the nanospheres depends on several parameters, including suspension viscosity, particle diameter and loading, spin speed and sufficient time to reach the balanced state. Generally, nanosphere loading determines the nanosphere spacing after spin coating and the same spin coating parameters can be used for different loadings. Smaller nanosphere diameters require higher top spin speed to achieve ordered hexagonal packing. The need for higher speeds may be due to the reduced mass of smaller nanospheres, which requires higher rotational speed for radial movement. The force interaction and transient dynamics of the process have been extensively investigated both experimentally26,27 and computationally.28,29 As Figure 1e illustrates, the diameter of SiNWs (d) is predetermined by the diameter of nanospheres (D). The spacing in NSL with shear-induced ordering is no longer fixed, as it is for more traditional NSL approaches, as shown in Figure 1d. This versatility occurs because the spacing can be tuned by the nanosphere loading in the colloidal suspension, which changes the viscosity and force balance. Therefore, this new approach offers independent control over the diameter and spacing of SiNWs. Figure 1f-h shows SEM images of the arrays prepared by shear-induced ordering that correspond to the processing steps described in Figure 1a-c, respectively. The 200 nm SiO2 nanospheres used here form a highly-ordered non-close-packed hexagonal pattern after spin8 ACS Paragon Plus Environment

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coating (see Fig. 1f). A nearly identical pattern with a slightly broader diameter distribution forms within the Au film after nanosphere removal (see Fig. 1g). This Au film pattern then assists the etching of Si, leading to SiNWs (see Fig. 1h) whose diameter and spacing are simply controlled by the spin-coating step rather than the amount of etching to the nanospheres used in typical NSL approaches. To demonstrate control over the geometry, SiNWs with diameters between 100 to 550 nm and spacings between 110-850 nm were fabricated. Figure 2a-c shows SEM images of the nanosphere patterns generated using 270-nm SiO2 nanospheres at 30, 20, and 10 vol% loading, respectively. The resulting SiNW arrays are shown in Figure 2d-f, respectively. In addition to the 200 and 270 nm nanospheres shown in Figures 1 and 2, respectively, SiNW arrays were also fabricated with 90 and 520 nm diameters, as shown in the Supporting Information. Although non-close-packed hexagonal patterns were observed for all nanospheres, nanospheres with diameters of 100 nm or less yielded less-ordered patterns due to more polydispersity in nanosphere diameter and the increased importance of Brownian motion. Similarly, the arrays are not as well ordered at large spacings, such as the array shown in Figure 2c. Under these conditions, the forces that help arrange the nanospheres compete with

9 Figure 2. SEM images of SiO2 nanospheres on Si after spin-coating suspensions containing 270-nm ACS Environment nanospheres at (a) 30%, (b) 20%, andParagon (c) 10% Plus volume loadings. The corresponding SEM images of the resultant SiNW arrays are shown in (d-f).


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brownian motion. Nonetheless, these results clearly demonstrate the ability to simultaneously control both the nanowire diameter and spacing over a broad range using shear-induced ordering of nanospheres. Controlling the spacing via nanosphere loading in the suspension rather than size reduction of the nanospheres eliminated the use of intense plasma or heat treatment. The advantage of this approach is that the roughness factor and aspect ratio of the nanospheres is not altered. For example, Yeom et al.17 showed that typical RIE processes lead to a 30-40% increase in both roughness factor and aspect ratio when reducing nanospheres by more than 50%. Although they demonstrate better control over both (< 10% increase), cryogenic cooling was needed to prevent nanosphere disintegration or movement. In contrast, both of these parameters are constant in the shear-induced ordering of nanospheres. Therefore, the original size and shape of the nanospheres were subsequently transferred to the nanowires, yielding high-aspect-ratio SiNW arrays with uniform diameter and shape. Figure 3a-c show SEM images of 100-, 290-, and 550-nm SiNW arrays after etching for 40 min (a), and 2 h (b and c), respectively. The corresponding HRSEM images in Figure 3d-f show the respective nanowire diameters. The length for the nanowires is 7.5 µm (for 100-nm nanowires), 35 µm (290 nm), and 31 µm (550 nm), representing a high aspect ratio of 75, 121, and 56, respectively. The high-aspect-ratio nanowires have near perfect hexagonal patterns without morphological defects, such as porosification and tapering. It has been shown that the etching conditions are crucial factors in eliminating these types of defects.15,17 Higher HF:H2O2:H2O etchant ratios, such as the one used in this work, help eliminate the defects. Interestingly, highly-doped Si (< 0.05 Ohm·cm) and long etch times typically lead to more porous nanowires.30 However, the SiNWs fabricated in this work were highly doped (120 are obtained. The NSL technique based on shear-induced ordering expands the range of SiNW geometries fabricated by top-down approaches. The fabrication of SiNW arrays with expanded—yet precisely controlled— geometries are desirable in many applications. Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org/. The Supporting Information is available free of charge on the ACS Publications website at DOI: 15 ACS Paragon Plus Environment


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Cross-sectional SEM images of SiNW arrays using 270-nm nanospheres, SEM of nanostructures fabricated using 90 and 520 nm nanospheres and their statistical distributions, plot of nanosphere loading vs the resultant nanosphere spacing. Acknowledgements The authors acknowledge partial support from the Donors of the American Chemical Society Petroleum Research Fund, the University of Florida Opportunity Fund, the National Science

Foundation

(CBET-1033736),

and

US National

Aeronautics

and

Space

Administration (NASA) under Grant Award No. NNX14AB07G.

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Controlling the Geometries of Si Nanowires through Tunable

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