High-Density Silicon Nanowires Prepared via a Two-Step Template

Feb 11, 2014 - The two-step template method for the preparation of high-density Si ... In the MACE method, the Si substrate under noble metal coverage...
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High-Density Silicon Nanowires Prepared via a Two-Step Template Method Dayong Teng, Luo Wu, Weiwei He, and Changhui Ye* Anhui Key Laboratory of Nanomaterials and Technology and Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China S Supporting Information *

ABSTRACT: High density ordered Si nanowire arrays can be fabricated from a Fe2O3 template annealed from polystyrene (PS) microsphere layers via a metalassisted chemical etching method. The metal mesh films, containing positionand density-defined pores that determine the position and density of the remaining structures after etching, are extremely important for achieving high quality Si nanowires. By adding a structural inversion process, a Au metal mesh with arrays of high density nanopores is devised as a catalyst for metal-assisted chemical etching of silicon. The density of Si nanowires can be increased to two times that of the single-layer PS microspheres and further to three times when a double layer of PS microspheres is introduced. The two-step template method for the preparation of high-density Si nanowires shows great potential in the fields of nanofabrication and nanoelectronics.

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INTRODUCTION Nanostructures of silicon, the most important material for the current semiconductor industry, are well-documented as promising building blocks for devices in the fields of nanoelectronics,1,2 opto-electronics,3 energy conversion,4−9 energy storage,10,11 and bio- and chemical sensors.12,13 Characteristic parameters, such as crystalline orientation,14,15 crystalline quality,16 strain,15,17 orientation relative to the substrate,5 and size, affect the properties of Si nanostructures18 and are thus important for their application in devices. Numerous methods have been developed to prepare nanowires or nanowire arrays.19−21 For the preparation of Si nanostructures, top-down and bottom-up approaches, such as vapor−liquid−solid growth,22 reactive ion etching (RIE), electrochemical etching, or metal-assisted chemical etching (MACE),23 have also been well documented. Among these methods, the MACE method has attracted increasing attention in recent years for the following reasons: (1) it is a simple and cost-effective method for fabricating various Si nanostructures with the ability to control various parameters; (2) it enables the control of the orientation of Si nanostructures relative to the substrate; and (3) it is more versatile and can be used to make higher surface-to-volume ratio nanostructures. Therefore, the nanostructures fabricated by MACE method have demonstrated their application potentials in fields ranging from solar energy conversion4−9 to thermal power conversion24 to chemical and biological sensing.25,26 In the MACE method, the Si substrate under noble metal coverage is etched much faster than Si without noble metal coverage. MACE combined with polystyrene (PS) microsphere or nanosphere lithography is a popular method presented by Huang et al.27,28 Starting from self-assembly of a monolayer of a © 2014 American Chemical Society

PS microsphere array on the Si substrate, size reduction of the PS microspheres was achieved by an RIE process, transferring the close-packed PS microspheres into non-close-packed ones. Subsequently, a noble metal film was deposited by thermal evaporation onto the Si substrate, with the non-close-packed PS sphere functioning as a mask, which resulted in a continuous layer of noble metal with an ordered array of pores. The Si substrate covered with the continuous metal film with pores (mesh) was etched in an etchant containing HF and H2O2. During the etching process, the noble metal mesh sank vertically into the Si substrate. The unetched Si protruded from the etched surroundings on the mesh, producing a Si nanowire (SiNW) array. The diameters of the pores were determined by the remaining diameter of the RIE-etched PS microspheres,28 and the length/diameter ratio can be controlled by the chemical etching duration. In electronic devices, such as optoelectronics for data processing, the density of SiNWs is one of the determining factors for the speed of processing. Efforts have been made to prepare SiNWs with a high density; for example, by using interference lithography and applying the mask to the metalassisted chemical etching, SiNWs with densities of 3.5 × 107/ cm2 to 4 × 108/cm2 have been grown by Choi et al.29 Huang et al. reported the growth of SiNWs of ultrasmall diameter by using an anodic aluminum oxide (AAO) template with small pore size; however, the density of SiNWs was still limited by the density of pores in the AAO template which is extremely difficult to increase beyond the 1010/cm2 level.30 With Received: December 25, 2013 Revised: February 10, 2014 Published: February 11, 2014 2259

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Preparation of SiNW Arrays. Fe2O3 porous template was coated with gold film by using a sputter coating machine (K550X). The Si substrate covered with the Au/Fe2O3/PS film was etched in an etchant containing HF and H2O2 (4.6 mol/L of HF and 0.4 mol/L of H2O2) at 25 °C for 2 min. Finally the samples were washed in aqua regia solvent for 5 min to remove Fe2O3 template and Au film. Characterization. Morphologies of PS microsphere arrays, the Fe2O3 template, and the different densities of SiNW arrays were characterized by using a Sirion 200 field emission scanning electron microscope (FESEM). The improvement in antireflection performance of Si substrates with different densities of SiNWs was characterized with a SolidSpec 3700DUV UV−vis spectrophotometer.

microsphere or nanosphere lithography, SiNWs with a density of 4.6 × 108/cm2 has been reported by Wang’s group.31 However, the disadvantage of this method is that the density of SiNWs cannot be adjusted in a large range because the density of the PS microsphere layer was limited since currently it is extremely challenging to assemble PS microspheres with diameters smaller than 200 nm into large-area ordered arrays.32 Therefore, the preparation method toward a high-density of SiNWs is still a research field which deserves more research efforts. In this paper, we used the MACE method to prepare SiNW arrays of high density via a new PS microspheres-based twostep template method. Through controlling the plasma etching treatment of PS microspheres and adjusting the content of Fe(NO3)3 solvent during the first template fabrication,33,34 we achieved Si NWs of two times the density of the monolayer PS microspheres. Moreover, by using a bilayer PS microsphere template, we obtained Si NWs of three times the density of the PS microspheres in one layer.

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RESULTS AND DISCUSSION Noble-Metal-Assisted Chemical Etching Reactions. The mechanism of the galvanic process can be summarized by two half-cell reactions:31,37−39 Cathode reaction at the metal

EXPERIMENTAL DETAILS

H 2O2 + 2H+ → 2H 2O + 2h+

(RI 1)

2H+ + 2e− → H 2

(RI 2)

Anode reaction at Si

Materials and Chemicals. Si wafers were of single crystalline ptype (100) (ρ = 1−10 Ω·cm; Jingyifang Electronics Co.). Polystyrene microspheres (5 wt %, Alfa Aesar) of different diameters (2000 and 500 nm) were used without further treatment. HF (GR reagent, 40 wt %), H2O2 (AR reagent, 30 wt %), Fe(NO3)3-9H2O (GR reagent, 98.5 wt %), HCl (AR reagent, 36−38 wt %), and HNO3 (AR reagent, 65− 68 wt %) were used as received (Jinchuang New Materials Co.). Aqua regia (concentrated nitric acid mixed with concentrated hydrochloric acid of 1:3 in volume ratio) was formulated in our lab. Preparation of Clean Silicon Substrates and PS Microsphere Templates. Si substrates were cut into small pieces (1 × 1 cm2) from Si wafers. To create a hydrophilic surface, the wafers were ultrasonically cleaned in ethanol, acetone, and deionized water for 20 min each, and then in the piranha solution (3:1, v/v, 98%H2SO4/ 30%H2O2) at room temperature for 8 h. PS microsphere templates were assembled according to literature method.35,36 A certain volume (5 μL) of PS microsphere solution was then spin-coated on the Si substrates (coated area is around 1 cm in diameter) at a certain speed (500 rpm) to make a monolayer PS microsphere. We increased the volume of the PS microsphere solvent to 10 μL (coated area is still around 1 cm in diameter) and used a slower spin-coating speed (400 rpm) to make bilayer PS microspheres (when the diameter of coated area increased from 1 to 2 cm, the volume of PS microsphere solution used must increase to 35−40 μL, and the spin-coating speed must be reduced to 300−350 rpm). The size of PS microspheres was then reduced by using Ar plasma etching at an input power of 100 W (PDC-32G-2) for a certain duration. Preparation of Fe2O3 Template. PS microspheres-coated Si substrates were immersed in Fe(NO3)3 solution of a certain concentration for 5 s and then dried naturally, followed by annealing in a tube furnace at 400 °C for 1 h under atmosphere. The experiment parameters are listed in Table 1.

Si + 2H 2O + 4h+ → SiO2 + 4H+

(RII 1)

SiO2 + 6HF → H 2SiF6 + 2H 2O

(RII 2)

Overall reaction Si + H 2O2 + 6HF → 2H 2O + H 2SiF6 + H 2

(RIII)

In the process, the metal layer adhering to the Si surfaces has a greater electronegativity than Si, and thus electrons are attracted from Si to metal, making the metal layer negatively charged. Subsequently, O− ions from H2O2 capture electrons preferentially from the negatively charged metal layers and become O2‑ ions in RI. This charge transfer causes the local oxidation of the Si underneath the metal patterns. The produced SiO2 is then continuously etched away by HF, leading to the penetration of metal into Si substrates in RII. As a result, the pattern defined by PS microspheres is transferred to the substrates, and eventually SiNWs are obtained when metal penetration reaches a certain depth in RIII and the process can be visually described in Figure 1. In our work, we used two materials, Fe2O3 and Au, to catalyze the chemical etching reactions as shown in Figure 2. As we explained in the MACE reaction, the normal process can be described as a flowchart (a−c) in Figure 2 a,nd in our method,

Table 1. Experiment Parameters: Layers of PS Microsphere on Si Substrate, Plasma Etching Time, Fe(NO3)3 Concentration, Annealing Condition, and Pore Density within the Fe2O3 Template

layer of PS microsphere single single double

plasma etching duration (min) 9 4 9

Fe(NO3)3 concentration, mmol/L

annealing duration time/ temperature

pore density/ PS microsphere density

75 50 75

1 h/400 °C 1 h/400 °C 1 h/400 °C

one time two times three times

Figure 1. Two-step metal-assisted chemical etching process: reaction (RI) took place at metal/etching solution and reaction (RII) took place at metal/Si interface. 2260

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Figure 2. (I) MACE method with Au/Si system (a−c) and MACE method with Au/Fe2O3/Si system (d−f). (II) Energy band diagram for MACEAu/Si system. (III) Energy band diagram for MACE-Au/Fe2O3/Si system.

Figure 3. Left panels: (a) 500 and (d) 2000 nm diameter PS microsphere arrays on Si substrate; middle panels: (b) 500 and (e) 2000 nm PS microsphere arrays after plasma etching treatment; right panels: (c and f) SiNWs prepared based on normal MACE method.

Au/Fe2O3 contact in the system belongs to Schottky junction, and Fe2O3/Si contact belongs to a typical P−N junction. In the Schottky junction, Au has a higher work junction value than Fe2O3. Because of the electric potential difference, electrons were attracted and flowed from Fe2O3 into Au; the Fermi energy level of Fe2O3 decreased due to the loss of electrons and the energy barrier decreased accordingly. In the Fe2O3/Si system, which is a typical P−N junction, Si acted as a hole receiver and holes flowed from Fe2O3 into Si. During this Au/

Fe2O3/Si sandwich system, Fe2O3 acted as a shuttle and provided the system directional charge transportation. Because of this system, the etching rate of Si in Au/Fe2O3/Si was higher than normal Au/Si, and this is the key point to our following results. It is worth emphasizing that Fe2O3 is not only an electron transport acceleration layer but also a pore positionand density-defined template layer (we will discuss this later). Narrow Space to Improve SiNWs Density with Decreasing the Size of PS microspheres. We tried two 2261

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Figure 4. Flowchart 1: preparation steps for one time density SiNWs from a → f → g → h → i. Flowchart 2: preparation steps for two times the density of SiNWs from a → b → c → d → e. (a) One PS microsphere layer on the Si substrate; size decreasing of PS microspheres after (b) 4 min and (f) 9 min plasma etching; (c) two times and (g) one time the density pores in Fe2O3 template were formed by immersing the samples in Fe(NO3)3 solution and then annealed at high temperature; (d and h) Au film was sputtered on the Fe2O3 template; (e) two times and (i) one time the density SiNW arrays were obtained after standard MACE.

HF and H2O2 for a certain duration, one time the density (namely, the density of PS microspheres) SiNWs in Figure 4i with a SiNW density of 4.6 × 108/cm2 were obtained. The situation was totally different in flowchart 1, where a shorter plasma etching time and lower Fe(NO3)3 concentration were used. Because of the short plasma etching time, PS microsphere sizes were decreased only slightly and the contact between PS microspheres and Si substrate was changed from previous nanoplane contact to point contact as shown in Figure 4b, and this point contact between PS microspheres and Si substrate can be easily broken and covered by Fe2O3 film during a later annealing process because of the weak bonding strength (therefore, the Fe2O3 film was continuous). We observed that the gaps between the neighboring PS microspheres were narrow and the connection of the Fe2O3 template among the gaps can be easily broken during the annealing process because a lower concentration of Fe(NO3)3 was applied. We identified clearly that additional pores were formed among previous PS microspheres because of the high temperature annealing process in Figure 4c. These pores in the Fe2O3 template after a high temperature annealing process will “grow” SiNWs after MACE as we explained. We calculated the density of SiNWs as follows: each pore was shared by three PS microspheres, and each PS microspheres have six pore around as in shown in Figure 4d, equivalent to two SiNWs for one PS microsphere and SiNW density of 9.2 × 108/cm2 was achieved (Figure 4e). Preparation of Three Times the Density of SiNWs. The experiment flowchart and steps to prepare three times the density SiNWs were similar to previous experiments as we discussed. The key point was to form the three times the density pores on Fe2O3 template before MACE process. We

kinds of PS microspheres, 2000 and 500 nm in diameter, to compare the SiNWs density within a fixed Si substrate as shown in Figure 3. We compared the images a−c to d−f in Figure 3 and found that with the decrease of the size of PS microspheres from 2000 to 500 nm, the density of SiNWs increased from 2.8 × 107/cm2 to 4.6 × 108/cm2. The density of SiNWs is the same to that of PS microspheres. It is difficult to assemble PS microsphere with diameter smaller than 200 nm into large-area ordered arrays, therefore, we have to go beyond the simple PS nanolithography to improve the density of SiNWs. Si nanowires prepared with the etching method are highly crystalline (Figure S1). Energy dispersive X-ray spectroscopy (EDS) analysis indicated clearly the distribution of Au, Fe, and Si following each step of the fabrication processes (Figure S2). Preparation of One and Two Times the Density of SiNWs. Experiments were started from Si substrate coated with PS microsphere (500 nm diameter) monolayer achieved by normal spin-coating technique as in Figure 4a, and then we went through two flowcharts as mentioned in Figure 4. In flowchart 1, with longer plasma etching time, PS microspheres became smaller in Figure 4f compared to those in Figure 4b. Due to the longer plasma treatment, the contact area between PS microspheres to the Si substrate was much bigger than the situation in flowchart 2 because of the higher melting level of PS microspheres. Additionally, the higher concentration of Fe(NO3)3 solution applied as mentioned in Table 1 resulted in the connection of Fe2O3 being strong enough to survive the annealing process as shown in Figure 4g. Under this circumstance, only the contact area between seriously melted PS microspheres and the Si substrate became pores in the Fe2O3 template in Figure 4g. After sputtering a layer of 35 nm (see Figure 4h) gold film and being etched in 2262

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Figure 5. Schematic illustration for three times the density of SiNWs preparation: (a) Bilayer PS microspheres on Si substrate; (b) size decreasing of PS microspheres after plasma etching; (c) Fe2O3 template formation after immersing the samples in Fe(NO3)3 solution and annealing at high temperature; (d) Au film was sputtered on the Fe2O3 template; (e) SiNW arrays were obtained after standard MACE; (f) three times the density of SiNWs after aqua regia treatment to remove the Au/Fe2O3 template.

Figure 6. Schematic illustration and SEM images for three times the pore formation on the Fe2O3 template: (a) bilayer PS microspheres on Si substrate, inset is the enlarged view; (b) size decreasing of PS microspheres after plasma etching; (c) Fe2O3 template formed after annealing at high temperature; (d) deposition of Au layer; (e) HF/H2O2 chemical etching; (f) three times the density of SiNWs, inset is the cross-sectional view (with a long HF/H2O2 etching time).

Figure 7. Fe2O3 template structures based on the solvents with different Fe(NO3)3 concentrations: (a) 50, (b) 75, and (c) 100 mmol/L.

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CONCLUSION In this work, we developed a method to fabricate high density SiNWs by using the MACE process based on a two-step template method. We successfully manufactured high-density SiNWs, with a density of 9.2 × 108/cm2 (two times the density of PS microspheres (4.6 × 108/cm2)), by controlling the plasma treatment time to the PS microspheres and the concentration of Fe(NO3)3 solution. We further achieved three times the density (1.4 × 109/cm2) of SiNWs by using bilayer PS microspheres on Si substrate based on the proper time for plasma treatment and the right quantity of Fe(NO3)3 solution. Combinative usage of Fe2O3/Au, which acts as an electron transport acceleration layer and pore position- and density-defined template layer, was another highlight of our research, and it played an important part in our process. Finally, the idea of the SiNWs density increasing causes a dramatic improvement in the antireflection performance. The ease, reliability, and versatility of this two-step template method may show great potential in well-controlled high-density SiNWs production for future electronic device applications.

explain the formation processes in Figure 5 and show SEM images of the structures in Figure 6. As we realized that the key point was to form three times the density of pores based on the Fe2O3 template before the MACE process, we used bilayer PS microspheres as shown in Figures 5a and 6a. We successfully achieved the structure by using larger PS content and lower spin-coating speed as mentioned in Table 1. In the experiments, with enough plasma etching time and proper Fe(NO3)3 concentration, we achieved the structure in Figure 5b, matching well with the image in Figure 6b. Because of this superposed structure, we successfully prepared three times the density of pores on the Fe2O3 template after high temperature annealing treatment which can be easily understood based on Figures 5c and 6c. Once the three pores appeared within one PS microsphere size area, we went ahead with our standard MACE process as described in Figure 5d−f that characterized by the SEM images in Figure 6d−f with a high SiNWs density value of 1.38 × 109/cm2. In this experiment, we realized that the concentration of Fe(NO3)3 in the solution was one of the important parameters to the final result, and we performed experiments with low and high concentrations of Fe(NO3)3 solution and obtained the results in Figure 7. If the concentration of Fe(NO3)3 was too low, the connection of the Fe2O3 template will be broken during the annealing process in Figure 7a, and the result is bad too with higher Fe(NO3)3 concentration in Figure 7c. Based on a series of experiments, we obtained the proper Fe(NO3)3 concentration for our system as 75 mmol/L as shown in Figure 7b. After successfully achieving three kinds of samples with different SiNWs density, we identified the difference among the samples by testing the antireflection properties because many potential applications relied on this performance. We obtained the curves in Figure 8. By introducing a surface nanostructure,

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures (TEM images and EDS analyses) showing the crystallinity and elemental distribution information of Si nanowires. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +86-551-65595629. Fax: +86-551-65591434. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by thte National Basic Research Program of China (973 Program, Grant No. 2011CB302103), National Natural Science Foundation of China (Grant No.11274308), and the Hundred Talent Program of the Chinese Academy of Sciences.

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