Silver-assisted chemical etching on silicon with polyvinylpyrrolidone-mediated formation of silver dendrites.

DOI: 10.1002/cphc.201402634

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Silver-Assisted Chemical Etching on Silicon with Polyvinylpyrrolidone-Mediated Formation of Silver Dendrites Chia-Yun Chen* and Po-Hsuan Hsiao[a] particles,[17] thus enabling diverse applications in solar cells,[18] bio-sensing[19] and Li-ion batteries.[17, 20] The typical MaCE process involves the electrochemical reaction between Si and Ag + ions, where the etching of Si is initialized by the reduction of Ag + ions owing to the higher potential of Ag + /Ag compared to the Femi level of Si. Simultaneously, the galvanic oxidation of Si is accompanied by hole injection from the Ag + to Ag + /Si interface. The oxidized Si beneath the newly born Ag clusters is subsequently removed by acidic etchant, that is, normally HF is utilized as the etchant of oxidized Si. The succeeding etching is maintained by such an electrochemical cycle, accompanied by the generation of more Ag clusters, finally leaving the aligned Si nanowires in the vicinity of the etching pores. Notice that the reduced Ag clusters grow throughout the Si surfaces as the dissolution of Si takes place, and they preferentially form dendrite architectures that are the energetically favored morphology for Ag growth,[21] similarly to Ag displacement on other substrates.[22, 23] Nevertheless, the self-formation of Ag dendrites during Si etching has rarely been studied, and the detailed understanding of the role of these Ag dendrites on affecting the dynamics of Si dissolution via the MaCE process has not been unveiled yet. To apply the MaCE technique for practical purposes, it is a prerequisite to unveil the functionality of these Ag dendrites, especially for their morphologies and crystallographic natures on the dynamics of Si etching. For example, recent papers have reported nanopore formation on the surface of etched nanowires due to the excess hole supply to Si during catalytic etching with AgNO3/HF as the reactive electrolyte,[24, 25] but the effects of hole transport supplied by the Ag dendrites are still unclear. Therefore, in this study, the MaCE process is induced by immersing the Si substrate in AgNO3/HF/PVP electrolyte solutions, where the effects of PVP on the in situ modulation of the Ag dendrites, accompanied by Si etching, are systematically investigated. Figure 1 shows SEM images of Ag dendrites formed on Si surfaces at RPVP values of 5, 1, 0.01 and 0. After 20 min reaction, a branch of Ag dendrites densely grows and covers the Si surface, regardless of the electrolyte reactants used in the reaction. Nevertheless, the morphology of the grown Ag dendrites is significantly altered by the RPVP, as evidenced in the inset of Figures 1 a–d. It can be clearly observed that the formed Ag dendrites favorably possess apparent crystalline facets, while the PVP macromolecules are introduced in the etching electrolytes. Furthermore, the extent of facet formation is more likely to be determined by the amount of PVP addition. Such a spontaneous formation of faceted Ag structures along with Si disso-

Metal-assisted chemical etching (MaCE) on silicon (Si)—mediated by polyvinylpyrrolidone (PVP)—is systematically investigated herein. It is found that the morphologies and crystallographic natures of the grown silver (Ag) dendrites can be significantly modulated, with the presence of PVP in the MaCE process leading to the formation of faceted Ag dendrites preferentially along the (111) crystallographic phase, rather than along the (200) phase. Further explorations of the PVP-mediated effect on Si etching are also revealed. In contrast to the aligned Si nanowires formed by MaCE without PVP addition, only distributed nanopores with sizes of 200 to 400 nm appear on the Si surfaces in the presence of PVP. The origin of surface polishing on Si in the PVP-mediated MaCE process can be attributed to the distinct transport pathway of holes supplied by the Ag + ions, where the holes are injected directly into the primary Ag seeds, rather than through Ag dendrites, thus leading to the isotropic etching of the Si surface.

Low-dimensional silicon (Si) nanomaterials, such as nanoparticles,[1] nanosheets[2] and nanowires,[3, 4] have attracted enormously growing attention owing to their intriguing electric, optical, and mechanical properties. In particular, Si nanowires are considered the most promising candidates the for advanced development of optoelectronic devices[5, 6] and solidstate electronics,[7] not only because of the unique quantum effect in a one-dimensional confinement, but also resulting from many applicable fabrication techniques and reliable processing for practical industrial applications. So far, many methods have been employed to synthesize Si nanowires, such as vapor–liquid–solid (VLS) growth,[8] oxide-assisted growth,[9] solid–liquid–solid (SLS) growth,[10] vapor–solid–solid (VSS) growth[11] and metal-assisted chemical etching (MaCE).[12–14] Among them, the MaCE method emerges as an inexpensive, rapid and facile etching technique to fabricate single-crystalline Si nanowires on various Si substrates including planar wafers,[12–14] micro-trenches,[15] micro-pyramids,[16] and micro-

[a] Prof. C.-Y. Chen, P.-H. Hsiao Department of Applied Materials and Optoelectronic Engineering National Chi Nan University No.1, Daxue Rd., Puli Township Nantou County 545, Taiwan (R.O.C.) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402634.

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Figure 1. SEM images of Ag dendrites formed at various RPVP values: a) RPVP = 5, b) RPVP = 1, c) RPVP = 0.01, and d) RPVP = 0.

lution is believed to be critical for the etching of Si, as will be discussed later. By contrast, without PVP introduction in the etching solutions on Si, only round Ag dendrites can be found after the MaCE process. To further examine the crystallographic phases of the Ag nanostructures, XRD characterizations were performed, as plotted in Figure 2 a. These exhibit several distinct XRD peaks corresponding to the Ag dendrites on the Si substrates, with Bragg angles (2q) and corresponding crystal planes of 388 (111), 448 (200), 648 (220), and 778 (311), indicating polycrystalline fcc patterns of the formed Ag dendrites. Apart from similar XRD signatures of samples formed at various contents of PVP, the relative intensity of Ag(111) to Ag(200) is significantly determined by the value of RPVP. These findings can be further supported by comparisons between the Ag(111) and Ag(200) XRD peaks, where the increased addition of PVP renders the favored formation of Ag(111) planes, in contrast to the Ag(200) planes, as evidenced in Figure 2 b. Furthermore, this trend is explicitly convincing, as the relative XRD intensity at the peak of Ag(111) to Ag(200), extracted from the measured XRD patterns, is positively dependent on the value of RPVP, as shown in the insert of Figure 2 b. These results imply that the presence of PVP in the MaCE process leads to the formation of faceted Ag dendrites, preferentially along the (111) crystallographic phase. In fact, recent studies have shown that PVP macromolecules tend to selectively interact with Ag(100) planes rather than with Ag(111) planes, which strongly inhibits the continued growth of Ag structures with (100) crystal planes, and thereby lead to the formation of shape-controlled Ag nanocrystals.[26, 27] To gain insights into the morphology of Ag dendrites in contact with etched Si, the Ag dendrites prepared with RPVP = 0.1 and RPVP = 0 were transferred to the flexible tape, respectively, following the process flow illustrated in Figure 3 a. The micrographs of etched Si substrates after the transfer process are

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shown in Figure 3 b, showing the distinct transfer capacities of Ag dendrites fabricated in these two cases. Interestingly, the successful transfer process is only achieved in PVP-mediated Ag dendrites (Figure 3 b, sample in the right), indicating the fact that the formed Ag dendrites are loosely in contact with underlying Si substrates, and thus enable a facile transfer process onto the flexible tape, as also evidenced in the SEM observation (Figure 3 c). Furthermore, such characteristics may be beneficial for the realization of the flexible surface-enhanced Raman scattering (SERS) platform.[23, 26] However, in addition to the network formation of Ag dendrites made by the MaCE process, there are abundant Si nanowires

Figure 2. a) XRD characterizations of Ag dendrites formed at various RPVP ratios, indicating that all the formed Ag dendrites during Si dissolution are polycrystalline. b) Comparisons of the two crystalline peaks of Ag for three different samples: Ag(111): 2q = 388 and Ag(200): 2q = 448. The inset shows the intensity ratio of Ag(111) to Ag(200), extracted from the XRD data.

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Figure 3. a) Process of transferring the Ag dendrites to a flexible tape. b) Micrographs of the etched Si substrates after the transfer process. Left: Sample prepared without PVP addition (RPVP = 0). Right: Sample prepared with PVP addition (RPVP = 0.1). Top-view SEM image of etched Si substrates prepared: c) with PVP addition (RPVP = 0.1) and d) without PVP addition (RPVP = 0) after the transfer process,.

as no PVP was added to the etching solutions. It is found that the dense attachment between Ag dendrites and underneath the Si nanowires leads to the unsuccessful transfer process of Ag dendrites, as shown in Figure 3 d. Apart from that, the surface wettability of the transferred Ag dendrites on tape was also examined by contact angle measurements, showing distinct surface wettabilities of the Ag dendrites prepared at RPVP = 0.1 and RPVP = 0, as described in the Supporting Information. Such a hydrophilic feature appearing in the PVP-free MaCE process also favors the dense attachment of Ag dendrites and underlying nanowire surfaces, which therefore hinders the transfer process of Ag dendrites onto the tape (Figure 3 d). Figure 4 shows a cross-sectional view of the Si surface at various etching conditions, including RPVP = 1, 0.1, 0.01, and 0. It has been reported that the etching of Si is essentially initiated by the oxidation of Si and followed by the removal of oxidized Si. Thus, the Ag + /HF electrolytes cause the directional dissolution of Si preferentially along the < 100 > directions, and further result in the formation of aligned Si nanowires.[13, 16] However, in contrast to the vertically aligned Si nanowires appearing in the case of RPVP = 0 (Figure 4 d), the addition of PVP to the etching electrolytes exceptionally causes surface polishing on Si instead of directional etching, as shown in Figures 4 a,b. An intermediate behavior is observed at RPVP = 0.01 (Figure 4 c), where only few vertical pores can be seen due to the extremely low concentration of PVP used. To clearly examine the morphology of the Si surface after etching, concentrated HNO3 (65 %) was used to remove entire Ag dendrites from the samples in both cases of RPVP = 0 and RPVP = 0.1, as shown in Figure 5. One can clearly observe Si nanowires with diameters of 50 to 150 nm, vertically oriented to the Si substrates, similar to previous reports,[12, 13] as presented in Figures 5 a,b. By contrast, only distributed nanopores with sizes of 200 to 400 nm appear on the Si surfaces (Figures 5 c,d), and these pores are ChemPhysChem 0000, 00, 0 – 0



found to be approximately isotropic since their depths are approximately similar to the pore diameters, as shown in the inset of Figure 5 c. In an attempt to reveal the PVP-mediated effect on the MaCE process, it is of particular importance to examine the PVP mediation at the initial stages of the Ag + –Si electrochemical reaction. Explorations of this issue were performed by a two-step process. Firstly, the cleaned Si substrate was dipped in a AgNO3 (0.02 m)/HF (4.5 m) solution for 5 s to form the primary Ag seeds on the Si surface, as illustrated in Figure 6 a. In addition, the morphology of the re-

Figure 4. Cross-sectional SEM images of samples after a 20 min MaCE process: a) with RPVP = 1, b) with RPVP = 0.1, c) with RPVP = 0.01, and d) without PVP addition.

sulting Ag-loaded Si is also shown in the SI. After quickly rinsing with deionized water, the Ag-loaded Si substrate was immersed in a AgNO3 (0.02 m)/HF (4.5 m)/PVP (0.002 m) solution for several minutes, and the resulting etching morphologies of the Si surfaces were investigated by SEM. The summarized kinetic results of reaction time versus nanowire length are plotted in Figure 6 b. In comparison, samples prepared at both etching conditions of AgNO3 (0.02 m)/HF (4.5 m) and AgNO3 (0.02 m)/HF (4.5 m)/PVP (0.002 m), directly on bare Si, were also examined, as represented by the black and green lines in Figure 6 b, respectively. A clearly linear dependence of the nano3


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Communications ions with the Si-exposed surfaces. Specifically, these selfformed Ag dendrites during the MaCE process bridge the Ag + ions (in the electrolytes) toward the primary Ag seeds (at the Si surfaces), and facilitate the kinetically stable hole transport injecting into Si through those Ag seeds, thus maintaining the directional dissolution of Si possessed by galvanic reaction. By contrast, as the initial Ag seeds were mediated with PVP, the Ag + ions far from the Si surfaces preferentially diffuse across the Ag dendrites and react with primary Ag seeds due to the strong coordinative force between Ag + ions and PVP.[26, 27] This implies that Ag dendrites are no longer Figure 5. SEM images of samples after 20 min chemical etching: a) Cross-sectional view and b) top view of effective to guide the directional a sample prepared without PVP addition. c) Cross-sectional view and d) top view of a sample prepared at RPVP = 0.1. The inset confirms the surface polishing of Si that occurs during the PVP-mediated MaCE process. hole transport onto Si. In such a case, excess holes are injected directly to the primary Ag seeds wire length on the etching time can be found in the black line, rather than through the Ag dendrites, and further spread into which is maintained by the continuous Ag + –Si electrochemical the vicinity of Si. Therefore, the surface polishing of Si takes reaction through Ag dendrites. In addition, the red line, explace to maintain the succeeding Si dissolution, as shown in tracted from a two-step process following the illustration in Figure 6 d. These features account for the distinct surface morFigure 6 a, is explicitly overlapped by the black line, revealing phologies (Figures 5 c,d) and etching dynamics (Figure 6 b) in the similar kinetic process involved. Evidently, the etching morPVP-mediated MaCE process, in contrast to the conventional Si phology via a two-step process was investigated by SEM, etching techniques, which provide the alternative way for the where the straight and vertical nanowires can be observed, as preparation of porous Si structures at room temperature. shown in Figure 6 c. These results suggest the functionality of In conclusion, both the formation of Ag dendrites and Si adding PVP to the MaCE process. Specifically, when AgNO3/HF/ etching via a PVP-mediated MaCE process were systematically investigated. We found that the introduction of PVP to the PVP etching electrolytes are employed directly on bare Si, surMaCE process favors the formation of faceted Ag dendrites. face polishing takes place and the etching depth is almost inThe summarized intensity ratios of the XRD peaks of Ag(111) dependent from the reaction time (green line in Figure 6 b). By to Ag(200) demonstrate their dependence on the PVP content. contrast, when AgNO3/HF/PVP etching electrolytes are used on Further explorations of the PVP-mediated effect on Si etching Ag-loaded Si, the addition of PVP does not affect the succeedare also revealed. In contrast to the aligned Si nanowires ing reaction of the Ag seeds with the underlying Si surface; formed by MaCE without PVP addition, only distributed nanonamely, preferential etching along the [100] orientation domipores with sizes of 200 to 400 nm appear on the Si surfaces. nantly takes place in such an etching process (red line in FigThe origin of surface polishing on Si in the PVP-mediated ure 6 b). It should be particularly noted that the molar ratio of MaCE process can be well elucidated by the distinct transport PVP versus AgNO3 used here is comparably lower than the pathways of holes, where the holes supplied by the Ag + ions value (normally > 1) in the reported references.[26, 27] The main reason is to avoid the unwanted decisive influence of Si on the are injected directly to the primary Ag seeds rather than catalytic dissolution when adding PVP to the aqueous electrothrough the Ag dendrites, further spreading into the vicinity of lytes. Therefore, the explicit effect of PVP mediation on the AgSi. These findings, along with systematic investigations, are anassisted chemical etching can be revealed in this work. ticipated to be beneficial for practical applications of the MaCE By combing systematic investigations on Ag morphology method to Si-based microelectronics, optoelectronics, and and Si etching, together with kinetic studies, the distinct etchother functional devices. ing mechanisms of the MaCE process with and without PVP were systematically studied and are presented in Figure 6 d. Experimental Section Without PVP addition, the existence of Ag dendrites behaves + as an efficient pathway for carrier transport between the Ag Investigations of Ag dendrites associated with Si etching were perions and Si, which inhibits the direct hole exchange of Ag + formed by dipping Si substrates into aqueous solutions containing

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Figure 6. a) Two-step process for Si etching. b) Plot of reaction time versus nanowire length: Black line: Sample prepared with a AgNO3/HF solution. Red line: sample prepared following the process presented in Figure 6 a. Green line: Sample prepared with a AgNO3/HF/PVP solution. It can be seen that the red and black lines approximately overlap and both demonstrate the linear relationship between etching time and nanowire lengths. c) SEM image of a sample prepared by a two-step process, following the process presented in Figure 6 a. d) Schematic illustrations of samples prepared through the MaCE process without and with PVP mediation.

various amounts of PVP (RPVP) in AgNO3 (0.02 m)/HF (4.5 m) electrolytes. Here, RPVP represents the molar ratio of PVP with respect to the concentration of AgNO3, calculated according to Equation (1): RPVP ¼ C PVP ðmol L1 Þ=CAgNO3 ðmol L1 Þ

croscopy (SEM, LEO 1530). To characterize the crystalline structures of the grown Ag dendrites, powder X-ray diffraction (XRD, Philips X’pert 1diffactometer) with Cu Ka radiation was applied.

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A control group was also prepared with solutions containing AgNO3 (0.02 m)/HF (4.5 m) electrolytes. In addition, a single-crystalline p-type Si(100) substrate with a resistivity of 1–10 W cm was utilized as the starting material. Prior to the etching process, the Si substrates were carefully cleaned several times with acetone, isopropyl alcohol (IPA), and deionized water, and dried with N2. The cleaned Si substrates were then immersed in the prepared solutions for 20 min. Notice that a high concentration of HF (4.5 m) was used for all the etching experiments, where the native oxide on Si surfaces was immediately removed prior to the catalytic dissolution of Si assisted by Ag. In the SI, the etching morphologies of Si with and without removing the native oxide layer on Si surfaces before performing the Ag-assisted etching process are presented, indicating no obvious differences in these two cases.

Acknowledgements This study was supported by the Ministry of Science and Technology, Taiwan (MOST 103-2218-E-260–001-). Keywords: metal-assisted chemical etching · morphology · reaction mechanism · silicon processing · surface treatment

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After etching, the as-prepared samples were rinsed with deionized water and dried with N2. The morphologies of the samples were investigated by field-emission scanning electron miChemPhysChem 0000, 00, 0 – 0



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COMMUNICATIONS The metal-assisted chemical etching (MaCE) of Si is systematically investigated. The morphology and crystallographic nature of silver dendrites grown by this method can be significantly modulated by the addition of polyvinylpyrrolidone (PVP).




C.-Y. Chen,* P.-H. Hsiao && – && Silver-Assisted Chemical Etching on Silicon with PolyvinylpyrrolidoneMediated Formation of Silver Dendrites

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