Fabrication of Orientation-Tunable Si Nanowires on Silicon Pyramids

Apr 3, 2017 - ABSTRACT: In this work, the different orientation of SiNWs on Si pyramids by a two step MACE method have been fabricated. By tuning the ...
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Fabrication of Orientation-Tunable Si Nanowires on Silicon Pyramids with Omnidirectional Light Absorption Zhibin Pei,†,‡ Haibo Hu,*,† Shuxin Li,† 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 ‡ University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: In this work, the different orientation of SiNWs on Si pyramids by a two step MACE method have been fabricated. By tuning the structure of Ag catalyst film and controlling the concentration of H2O2 or the etching temperature, the tunability of the orientation of SiNWs from to on Si pyramids was realized. Si structures composed of Si pyramids and SiNWs exhibit better omnidirectional light-trapping ability by multiple reflections. Si structures with structural tunability and enhanced light harvesting performance will find a wide variety of significant applications in solar cells, photodetectors, and optoelectronic devices.

1. INTRODUCTION It is known that Si-based optoelectronic devices, such as solar cells and photodetectors, could only harvest a certain portion of sunlight with nearly 40% being wasted of the total incident light due to the reflection.1,2 To improve the light utilization efficiency, a lot of studies have been focused on reducing the light reflection for Si-based photovoltaic devices. Si structures, such as nanowires, nanopillars, nanocones, and pyramids, are considered as ideal light absorbers that can prolong the optical path length of incident light and reduce the escape of light from the surface.3−6 Si nanowire (SiNW) array could suppress light reflection and trap light efficiently. However, it only works the best under direct sunlight illumination, and could not face the sun at the appropriate angle from morning to evening.7 There have also been sun-tracking systems for the purpose of receiving direct light illumination; however, it is not costeffective for practical applications. An ideal Si structure should lead to efficient omnidirectional light absorption over an extended solar spectral range irrespective of the light incident angle.8,9 Pyramids on the surface of Si wafer, also called “black silicon”, have been used in solar cell manufacturing industry for making three-dimensional p−n junction.10 A 30% reduction in reflectance can be obtained by multiple reflection in the micrometer-sized pyramid structure in the whole solar spectrum, compared to polished Si wafer.11 Intensive efforts have also been devoted to the use of SiNWs due to their advantageous light trapping effect.3,7 So far, a complex structure combining the advantages of pyramids and SiNWs has attracted significant research interest in achieving omnidirectional light absorption and overcoming the directional dependence of photovoltaic performance.8−17 Methods to fabricate complex Si © XXXX American Chemical Society

structures include dry-etching method and wet-chemical etching method. Dry-etching method, such as reactive ion etching, was widely used in industry and could make a variety of structures on Si wafer.17 However, this method may result in nonuniform lengths of SiNWs on Si pyramids, and is difficult in producing orientation-tunable SiNWs on Si wafer. By wetchemical etching method, such as metal-assisted chemical etching (MACE), it is much easier to overcome this challenge. By virtue of different anisotropic etching rates of singlecrystalline Si in the etching solution, wet-chemical etching method could not only tune the orientation of SiNWs, but also produce SiNWs with uniform lengths on Si pyramids. However, so far, there have been few studies focusing on fabricating orientation-tunable SiNWs on Si pyramids. Herein, we report a systematic investigation of the growth and optical properties of orientation-tunable SiNWs on Si pyramids. First, Ag film with a suitable aggregation state was formed on Si pyramids by using electroless deposition method. Then we tuned the orientation of SiNWs on Si pyramids through changing the concentration of H2O2 and the reaction temperature. It was found that the dissolution and redeposition of Ag films on Si pyramids was the key factor affecting the orientation of SiNWs.

2. EXPERIMENTAL SECTION Fabrication of Pyramid-Textured Si. Si wafers were of singlecrystal, n-type Si (100) with resistivity of 1−3 Ω·cm, n-type Si (111) Received: November 11, 2016 Revised: December 26, 2016 Published: April 3, 2017 A

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Langmuir with resistivity of 5−10 Ω·cm. The wafers were sequentially cleaned in acetone, ethanol, and deionized (DI) water for 20 min each at room temperature and then dipped in the mixed solution (25−28% NH4OH/30% H2O2/H2O, v/v/v = 1/1/5) at 70 °C for 1 h. Random Si pyramids were fabricated by anisotropic etching of Si in a mixed solution of potassium hydroxide (KOH) (5 wt %) and isopropanol (5 vol %) at 80 °C for 30 min. The textured Si substrates were then immersed in dilute hydrochloric acid for 10 min and hydrofluoric acid (HF) for 5 min to remove any residue KOH and silicon dioxide, respectively. Fabrication of SiNW Arrays on Pyramid-Textured Si. Si pyramids were cleaned in DI water for 20 min at room temperature then cleaned in mixed solution (1:1, v/v, 25−28% NH4OH/30% H2O2) at 70 °C for 1 h. Afterward, the cleaned samples were rinsed with DI water. Ag films were deposited on precleaned Si wafers by immersing the wafers in a solution of 4.6 M HF and 5 mM AgNO3 for 2 min at room temperature and then rinsed by DI water for 10 s. Subsequently, the Si pyramids were immersed in the mixed etching solution of HF (4.6 M) and different concentrations of H2O2 at various temperatures for several minutes (more details are in Table 1,2). Finally, all the samples were immersed in nitric acid for 1 min to remove the Ag films on the surface of the samples.

Figure 1. Cross-sectional view SEM images of nanostructures on Si pyramids by etching in 4.6 M HF at 25 °C via changing H2O2 concentration and the etching time. (a) 20 mM H2O2 for 10 min, (b) 40 mM H2O2 for 10 min, (c) 100 mM H2O2 for 5 min, and (d) 400 mM H2O2 for 3 min.

solution of HF/H2O2 at room temperature. The etching results show two typical behaviors, ⟨111⟩-oriented SiNWs at a low concentration of H2O2 (e.g., 20 mM) (Figure 1a) and ⟨100⟩oriented SiNWs at a high concentration of H2O2 (e.g., 400 mM) (Figure 1d). For a medium concentration of H2O2, for example, 40 mM, Figure 1b exhibits curved SiNWs at the upper part and ⟨111⟩-oriented SiNWs at the lower part of Si pyramid. The curved SiNWs initially had a straight ⟨111⟩ orientation normal to the surface of Si pyramid and subsequently changed into ⟨100⟩ orientation. Finally, the curved SiNWs turned to ⟨111⟩ orientation and became zigzag structures on Si pyramids (inset in Figure 1b). When the concentration of H2O2 was changed from 40 to 100 mM, SiNWs on Si pyramids were similar to those shown in Figure 1b. However, these SiNWs did not possess zigzag structures (Figure 1c). These observations present a simple deduction of a H2O2-concentration-dependent effect on the orientation of SiNWs. If the concentration of H2O2 is sufficiently small (e.g., 20 mM), the etching preferred ⟨111⟩ orientation, whereas if the concentration of H2O2 is sufficiently high (e.g., 400 mM), the etching preferred ⟨100⟩ orientation. It implies that the concentration of H2O2 can affect the orientation and the morphology of SiNWs on Si pyramids. To elucidate the mechanism under the orientation tuning, a series of experiments have been carried out in low and high concentrations of H2O2 at different temperatures. During the etching reaction, the etching rates would accelerate along with the increase of temperature. In order to avoid being affected by the solution-dependent etching,18 the reaction time was controlled to get the same length of SiNWs as far as possible. Figure 2a,c shows cross-sectional view SEM images of samples etched in a low concentration of H2O2 (e.g., 20 mM) at different temperatures ranging from 4 to 50 °C. The images show ⟨111⟩-oriented SiNWs normal to the surface planes of Si pyramids at 4 and 25 °C (Figure 2a,b). At 50 °C, the etching results in curved structures at the upper part and ⟨111⟩oriented SiNWs at the lower part of Si pyramids (Figure 2c). Similar results were observed at a high concentration of H2O2 (e.g., 400 mM), where curved SiNWs occurred at 20 °C (Figure 2e). Moreover, when etched in a high concentration of H2O2, ⟨111⟩-oriented SiNWs were formed at 4 °C (Figure 2d) and ⟨100⟩-oriented SiNWs were formed at 50 °C (Figure 2f), respectively. Furthermore, high-resolution transmission electron microscopy investigations showed the corresponding

Table 1. Preparation Conditions of Orientation-Tunable SiNWs on Si Pyramids with Different Concentrations of H2O2 at Room Temperature sample

H2O2 (mM)

reaction temp (°C)

reaction time (min)

1 2

20 40

25 25

10 10

3

100

25

5

4

400

25

3

preferential orientation ⟨111⟩-oriented SiNWs curved SiNWs (upper) and ⟨111⟩-oriented SiNWs (lower) curved SiNWs (upper) and ⟨111⟩-oriented SiNWs (lower) ⟨100⟩-oriented SiNWs

Table 2. Preparation Conditions of Orientation-Tunable SiNWs on Si Pyramids at Different Temperatures sample

H2O2 (mM)

reaction temp (°C)

reaction time (min)

1 2 3

20 20 20

4 25 50

20 10 5

4 5

400 400

4 20

10 5

6

400

50

1

preferential orientation ⟨111⟩-oriented SiNWs ⟨111⟩-oriented SiNWs curved SiNWs (upper) and ⟨111⟩-oriented SiNWs (lower) ⟨111⟩-oriented SiNWs curved SiNWs (upper) and ⟨111⟩-oriented SiNWs (lower) ⟨100⟩-oriented SiNWs

Sample Characterization. The morphology of Si pyramids and SiNWs were characterized by using a Sirion 200 field emission scanning electron microscope (FESEM). The crystallographic structures of SiNWs were imaged by using a JEM-2010 high resolution transmission electron microscope (TEM). Optical spectra of the samples were measured by using a optical spectrometer (UV−vis−IR spectrometer, PE Lamda 750) at room temperature.

3. RESULTS AND DISCUSSION In the metal-assisted chemical etching (MACE) process, the concentration of H2O2 plays an important role in the direction of etching.18−20 Figure 1 illustrates the effect of different concentrations of H2O2 on the fabrication of orientationtunable SiNWs on Si wafers. Ag film was deposited on Si pyramids via electroless displacement reaction for 2 min at room temperature, and then the sample was etched in a mixed B

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Figure 2. Cross-sectional view of SEM images for Si pyramids coated with Ag films and etched in the solution (4.6 M HF, 20 mM H2O2) for a) 20 min at 4 °C, (b) 10 min at 25 °C, and (c) 5 min at 50 °C and in the solution (4.6 M HF, 400 mM H2O2) for (d) 10 min at 4 °C, (e) 5 min at 20 °C, and (f) 1 min at 50 °C, respectively.

crystallography of SiNWs etched at different temperatures (Figure S1). It seemed that each concentration of H2O2 would lead to ⟨111⟩-oriented SiNWs at a low temperature and ⟨100⟩-oriented SiNWs at a high temperature. Different concentrations of H2O2 induce a different transition temperature for the etching direction. The higher the concentration of H2O2, the lower the transition temperature. More details were shown in Figure 3. For curved SiNWs, the time of crystallographic change at the

Figure 4. SEM images of Ag films deposited on different Si substrate via electroless plating for 2 min: (a) Si pyramids, (b) planar Si (111) wafer, and (c) planar Si (100) wafer.

pyramids immerse in the HF/Ag+ solution, the Ag+ was reduced on the surface of Si pyramids to form Ag nuclei because the electrochemical potential of Ag+/Ag was more positive than Si atom. With the increase of time, the growth of Ag nuclei into large particles needed more Ag+ from the solution. In the case of plane Si substrates, the supplement of Ag+ was quick and uniform from the etching solution. So, the Ag particles grew up and connected to each other to form uniform mesh structure. However, due to the nonplane structure of pyramid, the concentration of Ag+ at the upper part of Si pyramids was higher than the lower part of pyramids because the consumed of Ag+ can rapidly supplied from the HF/Ag+ solution. Therefore, Figure 4a shows that the aggregation status of Ag films changes dramatically from the upper to the lower part of Si pyramids, varying from meshlike structure at the upper part to particulate film at the lower part of Si pyramids. According to the others studies, the variation of Ag structure was thought to be caused by the nonuniform distribution of Ag+ ion surrounding Si pyramids.12,24 The orientation-tunable etching mechanism can be explained by a combination of the aggregation status of Ag films and the nonuniform concentration of H2O2 at different regions of Si pyramids. For a plane substrate etched with isolated Ag particles, the movement of Ag particles proceeded along inclined ⟨100⟩ directions. If the Ag particles interconnected into a continuous film, the internal stress between the particles will increase, and, further, the lateral movement of the particle will be restricted. As a result, the etching direction inclined the vertical direction of the surface.20,21 For a large-area Ag mesh, it could only move vertically to the surface of Si wafer. As shown in Figures S2 and S3, no matter how the etchant component and reaction temperature changed, the nanowires on planar Si substrate were always perpendicular to Si wafer. It could be deduced that Ag film on planar Si (111) and (100) substrate were sole mesh structure, similar to that at the upper and medium part of Si pyramids (Figure 4a). Due to the interaction-dependent etching direction theory, Ag mesh on Si pyramids should move perpendicular to the surface planes of Si pyramids ({111} planes), leading to ⟨111⟩-oriented SiNWs.

Figure 3. Cross-sectional view SEM images of nanostructures on Si pyramids by etching in 4.6 M HF with (a) 20 mM H2O2 at 50 °C for 10 min, (b) 100 mM H2O2 at 25 °C for 5 min, (c) 400 mM H2O2 at 20 °C for 5 min. The turning points were marked with a dashed red line.

upper part of Si pyramids was earlier than that at the lower part of Si pyramids in low (e.g., 20 mM) and medium (e.g., 100 mM) concentration of H2O2 (Figure 3a,b), whereas the lower part of Si pyramids changed earlier than the upper part of Si pyramids in the high (e.g., 400 mM) concentration of H2O2 (Figure 3c). As we know, if the amount of holes (h+) is high enough (i.e., at a high concentration of H2O2) and the rate of injection is fast enough (i.e., at a high temperature), etching will proceed vertical to the surface, otherwise, preferred ⟨100⟩ etching.18,19 In our experiments, when etched in a high concentration of H2O2 or at a high temperature, SiNWs on Si pyramids were solely along the ⟨100⟩ orientation. Therefore, the transition temperature indicates that the injection of positive holes theory could not completely explain the phenomena of orientationtunable formation of SiNWs on Si pyramids. It has been found that the morphology of catalyst metal can influence the etching direction, and isolated Ag nanoparticles and meshlike films have different etching behaviors.21−24 Ag films deposited on various Si substrates using electroless plating method exhibited different morphologies, as shown in Figure 4. Electroless plating method is a simple and low-cost method for the deposition of Ag morphology on Si substrates. When the Si C

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Langmuir In fact, the direction of SiNWs tended to be along the ⟨100⟩ direction at a high concentration of H2O2 or at a high temperature, as shown in Figures 1d and 2f. The morphology of Ag films could vary dynamically during the etching process, because of the injection of positive holes.21,25−28 During the etching process, the reaction of Ag (i.e., Ag+ → Ag(s) + h+) was followed by the decomposition of H2O2 (i.e., H2O2 + 2H+ → 2H2O + 2h+).29 In fact, the dissolution of Ag became fast as the amount of holes increased. Accordingly, in our experment, with a low concentration of H2O2 (e.g., 20 mM) and at a low temperature (e.g., 4 °C), generation of holes would be limited. During MACE process, the dissolution and the redeposition of Ag film would be less pronounced. The interaction between Ag structures would retain the meshlike structure and make Ag film move in the same direction, resulting in ⟨111⟩-oriented SiNWs (Figure 2a). When the temperature was increased to 50 °C, the holes at the reaction interface would be consumed rapidly and the concentration of H2O2 close to the interface would be reduced, resulting in a nonuniform concentration profile of H2O2. Under this condition, the amount of holes was expected to be increased at the upper part of Si pyramids and Ag mesh would be dissolved therein and separate into pieces of isolated nanoparticles at the upper part of Si pyramids. Therefore, Ag nanoparticles would etch along the crystallographically preferred ⟨100⟩ direction. At the lower part of Si pyramids, Ag film might retain the particulate film structures because of the lacking of holes. The curved SiNWs occurred at the upper part of Si pyramids and ⟨111⟩-oriented SiNWs formed at the lower part of Si pyramids as shown in Figures 2c and 3a. Similar conclusions apply to the medium concentration of H2O2 (e.g., 100 mM). With a high concentration of H2O2 (e.g., 400 mM) and at a low temperature (e.g., 4 °C), the generation of holes would also be limited. Similarly, ⟨111⟩-oriented SiNWs occurred. When the temperature was increased to 20 °C, the consumption rate of holes increased, generating a nonuniform concentration of H2O2. However, the consumed of H2O2 was rapidly supplied by diffusion from the high concentration solution. Ag film at the upper part of Si pyramids will separate into pieces of little mesh and retained the perpendicular movement to the surface planes. As time went on, Ag mesh would further separate into nanoparticles, resulting in curved SiNWs (Figure 3c). At the lower part of Si pyramids, Ag film would separate into nanoparticles initially, leading to ⟨100⟩-oriented SiNWs (Figure 2e, 3c). When the temperature was increased to 25 °C or higher, all Ag films on Si pyramids would separate into nanoparticles, resulting in ⟨100⟩-oriented SiNWs (Figures 1d and 2f). Other researchers have also reported that Ag mesh preferentially moved along the direction normal to ⟨111⟩ planes, whereas isolated Ag nanoparticles sank along equivalent [100] directions of Si crystal13,16 in a high concentration of H2O2 at room temperature. If the catalyst metal did not dissolve, the etching behavior would be different. For example, Wei et al.8 deposited a 5 nm Au layer by sputtering method on Si pyramids then etched in the mixed solutions of HF/H2O2 (4.8 M HF, 400 mM H2O2) at room temperature, resulting in ⟨111⟩-oriented SiNWs. To finally verify the dissolution of Ag structure plays an important role in the control of the orientation of SiNWs on Si pyramids, we carried out SEM investigations of Ag nanoparticles in SiNWs after etching in 4.6 M HF at 25 °C via changing the concentration of H2O2. Figure S4 shows the results from low concentration (e.g., 20 mM) to high

concentration (e.g., 400 mM). The SEM images confirm that the size of nanoparticles became thinner in the high concentration of H2O2, we can concluded that the more amount of the generated holes (h+), the more Ag structure would dissolved, which conforms our deduction. Therefore, we could readily tune the orientation of SiNWs on Si pyramids via changing the etchant concentration, reaction temperature, and the morphology of Ag films. As the basis of complex structure, Si pyramids have been reported in many literatures, and light paths of incident light at normal incidence or at wide angles of incidence (AOI) were also explained.30−32 A schematic illustration of light reflection on Si pyramids via wide AOIs is shown in Figure 5. We define

Figure 5. Schematic diagram of light reflectance at specific incidence angles θ on Si pyramids. (a) θ = 0°, (b) θ = 18°, (c) θ = 36°, (d) θ = 54°.

the ‘A’ point as the splitting point of the first and the second reflection. When AOI is 18°, the ‘A’ point occurs at the peak of Si pyramids. The area above the ‘A’ point only has a first reflection and the area below the ‘A’ point has a second reflection which leads to less overall reflectance (Figure 5b). If AOI is 36°, the incident light is parallel to one neighboring plane of the pyramids and the ‘B’ point occurs. We define the ‘B’ point as the dead point, below which it hardly reflects light (Figure 5c). When AOI is 54°, the incident light is perpendicular to one plane of the pyramids. Under such condition, ‘A’ and ‘B’ points coincide and the area above this point only has a first reflection (Figure 5d). As can be concluded, the light reflectance is closely related to AOI. Figure 6 shows the optical properties of Si in the wavelength range of 400−1000 nm. As can be seen, at normal incidence (θ = 0°), the reflectance of Si pyramids is much lower than that of planar Si wafers, however, the reflectance of Si pyramids gradually increases with AOI (Figure 6b), whereas the reflectance of planar Si wafer decreases with AOI. At normal incidence, the reflectance of planar Si is much larger than that of Si pyramids. Although Si pyramids are promising as “black silicon”, they have difficulty to be used as an omnidirectional light absorber. Hierarchical structures that combine pyramids with SiNWs would further suppress the reflection in the visible light range.8−11 The overall reflectance of Si pyramids with SiNWs of different orientation has not been reported over wide AOIs. Figure 6c,d,e shows the optical reflectance of Si pyramids with D

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Figure 6. Overall reflectance spectra and schematic diagram of (a) planar Si, (b) Si pyramids, (c) Si pyramids with ⟨111⟩-oriented SiNWs, (d) Si pyramids with curved SiNWs (upper) and ⟨111⟩-oriented SiNWs (lower), (e) Si pyramids with ⟨100⟩-oriented SiNWs, and (f) comparison of the performance of the rate of reflectance change at 600 nm.

⟨111⟩-oriented SiNWs, with curved SiNWs (upper) and ⟨111⟩oriented SiNWs (lower), and with ⟨100⟩-oriented SiNWs, respectively. The length of SiNWs are approximately 1 μm. Figure 6e further compares the reflectance of these three nanostructures at 600 nm. It was found that Si pyramids with ⟨111⟩-oriented SiNWs exhibit the best antireflection behavior, and the reflectance is smallest when AOI is 15°. According to the schematic illustration of light reflection on Si pyramids, when AOI is smaller than 18°, all the incident light would have second reflection on the corresponding sides that more light will be absorbed. However, the optical absorption of SiNWs varies with AOI due to the multiple scattering in SiNWs.7 Therefore, the reflectance at 15° is smaller than at 0° because the equivalent AOIs at right side increased along with the AOIs (Inset in Figure 6c). We notice that the reflectance of Si pyramids with ⟨100⟩-oriented SiNWs is a little higher than that of with ⟨111⟩-oriented SiNWs, that is because the space between ⟨100⟩-oriented SiNWs is much bigger than that between ⟨111⟩-oriented SiNWs (as shown in Figure S5), namely, the latter is much denser in areal number. It is expected that ⟨111⟩-oriented SiNWs may match the geometrical structure with the surfaces of Si pyramids more perfectly. The reflectance of Si pyramids with curved SiNWs (upper) and

⟨111⟩-oriented SiNWs (lower) are between that of with ⟨100⟩and ⟨111⟩-oriented SiNWs. A similar trend was reported by other researchers that Si pyramids with ⟨111⟩-oriented SiNWs demonstrated lower reflection than that with ⟨100⟩-oriented SiNWs at normal incidence (θ = 0°).16

4. CONCLUSION In summary, we report the fabrication of SiNWs with different orientation on Si pyramids by a two step MACE method. By tuning the structure of Ag catalyst film and controlling the concentration of H2O2 or the etching temperature, we realize the tunability of the orientation of SiNWs from ⟨111⟩ to ⟨100⟩ on Si pyramids. Si structures composed of Si pyramids and SiNWs exhibit better omnidirectional light-trapping ability by multiple reflections. Si structures with structral tunability and enhanced light harvesting performance will find a wide variety of significant applications in solar cells, photodetectors, and optoelectronic devices. E

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b04068. Additional information, figures, and tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Telephone: +86-551-65592033; Fax: +86-551-65591434. *E-mail: [email protected]. ORCID

Haibo Hu: 0000-0001-7494-1469 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos. 11274308, 21401202, and 51502295), and the Hundred Talent Program of the Chinese Academy of Sciences. We thank Prof. J. C. Ho at City University of Hong Kong for the assistance in measuring optical spectra.

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DOI: 10.1021/acs.langmuir.6b04068 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b04068 Langmuir XXXX, XXX, XXX−XXX