Template-free fabrication of silicon micropillar/nanowire composite structure by one-step etching
© Bai et al.; licensee Springer. 2012
Received: 19 August 2012
Accepted: 25 September 2012
Published: 8 October 2012
A template-free fabrication method for silicon nanostructures, such as silicon micropillar (MP)/nanowire (NW) composite structure is presented. Utilizing an improved metal-assisted electroless etching (MAEE) of silicon in KMnO4/AgNO3/HF solution and silicon composite nanostructure of the long MPs erected in the short NWs arrays were generated on the silicon substrate. The morphology evolution of the MP/NW composite nanostructure and the role of self-growing K2SiF6 particles as the templates during the MAEE process were investigated in detail. Meanwhile, a fabrication mechanism based on the etching of silver nanoparticles (catalyzed) and the masking of K2SiF6 particles is proposed, which gives guidance for fabricating different silicon nanostructures, such as NW and MP arrays. This one-step method provides a simple and cost-effective way to fabricate silicon nanostructures.
Silicon nanostructures, including silicon nanohole, silicon nanowire, and silicon nanopillar, have attracted wide attention due to their potential in various fields of application, such as solar cells [1–3], lithium batteries , insulator transistors , and gas and chemical sensors [6, 7]. In particular, silicon micropillar (MP)/nanowire (NW) composite structure becomes more interesting recently due to its excellent light trapping and efficient carrier collection, which is applied to design and construct high performance radial p-n junction solar cells [8, 9]. At present, the effective fabrication method for silicon MP/NW structure is the wet or dry etching of silicon substrate combined with various templates, such as silica dot array  and circular-shaped photoresist dots [9, 10]. However, the use of these templates increases the fabrication cost and also makes the manufacturing process complicated. Therefore, searching a simple and cost-effective fabrication approach for silicon MP/NW structure is necessary.
Recently, K2SiF6 crystallites have been observed during the different wetting etchings of silicon substrate in the presence of HF and K+ ions, such as chemomechanical polishing , stain etching , laser-assisted etching , and metal-assisted electroless etching (MAEE) [14, 15]. These K2SiF6 crystallites are insoluble in dilute HF solution and to some extent can prevent the etchant solution from the contact with silicon surface. Therefore, K2SiF6 crystallites offer a possible mask approach to selectively remove silicon materials. Previous studies focus on the effect of K2SiF6 crystallites on the formation of porous silicon and its photoluminescence property, but utilizing K2SiF6 as a template to fabricate silicon nanostructures has not been studied.
In this work, a template-free fabrication method for silicon MP/NW structure is presented. By utilizing an improved MAEE of silicon in KMnO4/AgNO3/HF solution, silicon MP/NW structure was achieved under the mask of self-growing K2SiF6 particles. This simple one-step fabrication method integrates the masking process and the etching process, avoiding conventional masking procedures, which provide a simple and cost-effective route to fabricate silicon nanostructures.
In these experiments, p-type Si(100) wafers with a resistivity around 7 to 13 Ω·cm were used. The wafer was cut into 1.5 × 1.5 cm2 pieces and used as test samples. Silicon samples were ultrasonically cleaned in acetone, absolute alcohol, and deionized water successively. Then, the cleaned Si samples were dipped into dilute HF solution to remove native oxide. Following the cleaning step, the etching process was performed through immersing the silicon samples into the etchant solution, which contains 5 M HF, 0.02 M AgNO3, and KMnO4 with different concentrations. The reaction time varied from 15 to 90 min. After etching process, Si samples were rinsed with deionized water and then immersed into the concentrated HNO3 to remove the retaining silver and other residue. All treatments were performed at room temperature.
The morphologies of the silicon nanostructures were observed by the scanning electron microscope (SEM) with FEI Quanta 200 F (FEI Company, OR, USA). Crystals covered on the Si samples were analyzed using the energy dispersive X-ray (EDX), and X-ray diffraction (XRD) by Bruker D8 Focus X-ray powder diffractometer (Bruker Corporation, MA, USA) with Cu Kα radiation (λ = 1.5406 Å).
Results and discussion
K2SiF6 crystallite, spontaneously generating when the solubility product of K2SiF6 exceeded to 6.3 × 10−7 mol3dm−9, is a byproduct during the forming process of porous silicon . Hadjersi et al. reported that an insoluble solid-phase film (K2SiF6) covered the top of porous silicon layer by the etching of silicon-coated silver film in HF-oxidizing solution . Also, the existence of K2SiF6 layer causes the decrease of the etching rate of silicon . These results demonstrated that K2SiF6 has an ability to form a masking layer during MACE process. Based on these, an improved MACE approach, integrating three processes (including the deposition of catalyzed silver nanoparticles (NPs), the formation of K2SiF6 mask, and the electroless etching of silicon) was utilized to realize one-step fabrication of silicon nanostructures.
At initial stage of etching, since the standard reduction potential of MnO4− (1.51 eV) is larger than that of Ag (0.78 eV) , injected holes are provided mainly from S2. As the reactions (S2 and S3) continuously proceed, the concentration of SiF62− increases gradually. When the concentration of SiF62− is accumulated sufficiently, K2SiF6 can heterogeneously nucleate at the silicon surface and grow up to K2SiF6 particles, covering dense silver NPs (as described in Figure 4B). So, K2SiF6 particles shelter parts of Ag NPs and further prevent the etchant solution from the contact with Ag NPs. It is difficult for hole injection from Ag NPs to silicon areas covered by K2SiF6 particles. At the same time, at the areas of silicon surface without K2SiF6 particles, silicon is still subjected to the etching assisted by the catalysis of Ag NPs. Therefore, the silicon under K2SiF6 particles was retained, while the silicon not covered with K2SiF6 particles was etched away, leading to micropillar structure on the silicon substrate. As the reaction continuously performs, the concentration of MnO4− in the solution is reducing; injected holes are provided mainly by S1. On the surface of silicon without K2SiF6 particles covered, nanowire array form closely around micropillars and simultaneously accompanied the deposition of silver dendrites (as described in Figure 4C), which is clearly illustrated by the formation model of SiNW array in HF/AgNO3 solution [2, 23]. Finally, the silicon MP/NW composite structure was obtained after the cleaning by HNO3, as described in Figure 4D.
A simple fabrication method integrating the masking process of K2SiF6 particles and the silver-assisted electroless etching process is presented to fabricate silicon nanostructures. Using this method, silicon MP/NW composite structure was successfully fabricated, and their lengths can be controlled by adjusting reaction parameters. By the observation of EDX and XRD, it is demonstrated that the electroless etching under the mask of K2SiF6 particles causes the formation of silicon MP/NW structure. Further, a formation mechanism of the silicon MP/NW composite structure in KMnO4/AgNO3/HF solution was proposed. Based on these, different silicon nanostructures such as nanowire and pillar arrays can also be achieved by adjusting the size and distribution of K2SiF6 particles.
FB, a Ph.D. candidate, is under the supervisory of Prof. ML. He obtained his masters degree at the Wuhan University of Technology in 2010. At present, his research interests are as follows: silicon-based solar cells, fabrication of large-areas graphene, and light trapping silicon surface structure. ML is a professor in Renewable Energy and Clean Energy. He is the director of New Energy Materials and PV Technology Center, State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources. He worked in the University of Cambridge as a research fellow from 2004 to 2006. His expertise is in the fields of design and fabrication of energy materials, functional micro-nanostructures, and energy conversion devices.
We acknowledge the support from the National Natural Science Foundation of China (grant nos.: 51172069, 50972032, 61204064, and 51202067), and the Ph. D. Programs Foundation of Ministry of Education of China (grant no.: 20110036110006), and the Fundamental Research Funds for the Central Universities (key project 11ZG02).
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