Substrate Symmetry Driven Endotaxial Silver Nanostructures by

Jun 6, 2013 - driven endotaxial nanostructures in the atmospheric pressure CVD system. A control over the shapes and sizes of the Ag nanostructures...
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Substrate Symmetry Driven Endotaxial Silver Nanostructures by Chemical Vapor Deposition R. R. Juluri, A. Rath, A. Ghosh, and P. V. Satyam* Institute of Physics, Sachivalaya Marg, Bhubaneswar, Odisha-751005, India ABSTRACT: We report a novel method of growth for endotaxial silver nanostructures on Si(100), Si(110), and Si(111) substrates using a chemical vapor deposition (CVD) method. Our procedure involves a low-temperature thermal etching of native oxide on silicon substrate using GeOx layer as an etchant to grow substrate symmetry driven endotaxial nanostructures in the atmospheric pressure CVD system. A control over the shapes and sizes of the Ag nanostructures has been obtained by using parameters such as substrate orientation and time of deposition during the growth. High-resolution electron microscopy has been used to elucidate the growth mechanism of these structures.

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INTRODUCTION The unusual physical, chemical, optical, and electronic properties of nanomaterials hold tremendous potential in the synthesis and their design aspects.1 Metal nanoparticles are drawing great attention due to their potential applications such as seed particles (Pt, Au, Ag) and as catalysts for nanowire growth2−5 These particles are also suitable as optical materials due to their Surface Plasmon Resonance (SPR) properties. Among these metals, Ag has attracted a great deal of attention due to its high electrical and thermal conductivities.2 In addition, Ag nanorods and nanowires have recently gathered great attention due to their interesting applications, which include photonic crystals,6 infrared polarizers,7 superlens and hyperlens applications due to its lowest on-resonance loss at optical frequencies.8,9 On earlier occasions, a solution phase polyol synthesis had been used to grow various shapes, such as, pentagonal nanowires, cuboctahedra, nanocubes, nanobars, etc.10 To the best of our knowledge, no reports on endotaxial growth of silver structures exist using a simple chemical vapor deposition (CVD) method. Endotaxial growth involves a coherent precipitate phase (in the present work, endotaxy of silver nanostructures) in the bulk matrix (substrate silicon) with coherent interfaces surrounding the precipitate.11 In simpler terms, epitaxy is coherent material growth on a crystalline substrate surface while endotaxy is material growth within a substrate. In this paper, we report on the thermal etching of native oxide using GeOx layer as an etchant to remove the native oxide at ∼800 °C in atmospheric conditions. Following this step, various shapes of Ag nanostructure were obtained on silicon substrates for orientations (100), (110), and (111). The planar transmission electron bright field micrograph depicts the various shapes as square, rectangular, and triangular Ag nanostructures for the substrate orientations of (100), (110), and (111), respectively. It should be noted that the TEM © XXXX American Chemical Society

micrographs are the projection images and the actual shapes of these structures inside the silicon structures would be governed by the interfacial energetics. Compared to conventional epitaxially grown nanostructures, the endotaxial structures may find it more advantageous in planar device applications as they simplify the fabrication process. These kinds of embedded or endotaxial silver nanostructures may be used in applications in the areas of plasmonics for enhancing light absorption and other opto-electronic applications. Incorporation of Ag nanostructures on the semiconductor surface and interfaces with various shapes would also help in integrating the optical properties of Ag nanostructures with those of semiconductor properties of silicon.12−15 It is established that the surface of the single crystalline silicon becomes oxidized by saturating its dangling bonds with oxygen and this results in the formation of a nanometer thick native oxide (SiOx). There are several methods to etch the native oxide, such as, chemical etching with hydrofluoric (HF) acid,16 thermal etching at high temperatures that is commonly used in the study of surface structures on silicon,17 and by ion beam sputtering process,17 etc. Russel et al. reported the vacuum thermal etching of silicon and germanium surfaces in the temperature regime of 900−1300 °C.18 Hopper et al. reported the removal of the oxide layer by flashing at ∼1200 °C and ion bombardment to obtain an optically flat silicon substrate under ultrahigh vacuum conditions.17 Futagami et al. reported the thermal etching of Si(100) at 1200 °C in argon atmosphere.19 Here, we report on low-temperature etching of silicon substrate using GeOx as an intermediate layer and thus helping in the growth of endotaxial silver nanostructures. To Received: February 15, 2013 Revised: June 6, 2013

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grown in Si(100). From the HR-X-TEM image an interplanar distance of 0.238 ± 0.005 nm corresponding to Ag(111) and an interplanar spacing of 0.315 ± 0.005 nm corresponding to Si(111) were found. Though the substrate is (100) oriented, a cross-section view allows us to image (111) planes in the system, as shown in Figure 2. The selected area electron diffraction (SAED) pattern taken from a single Ag nanostructure is shown in Figure 2c, and this confirms the presence of the single crystalline nature of Ag nanostructures. Now, we show the formation of well-defined shapes of Ag structures on silicon substrate by introducing a 17 nm thick GeOx on the native oxide covered silicon substrates with various orientations. Figure 3 depicts the SEM images of Ag nanostructures grown by the CVD method on GeOx/Si substrates at 800 °C. As shown in panels a−c of Figure 3, the SEM micrographs depict square, rectangular, and triangular structures grown on Si(100), Si(110), and Si(111) substrates, respectively. It is to be noted that the shapes of surface-unit-cell of Si(100), Si(110), and Si(111) are square, rectangular, and triangular corresponding to four-, two-, and threefold symmetry. SEM micrograph shown in Figure 3a depicts the square-shaped nanostructures on Si(100) with a coverage area of 2.3% with an average length 90 ± 9 nm, breadth of 85 ± 9 nm, and an aspect ratio of ∼1.0. The SEM micrograph shown in Figure 3b depicts the presence of ordered nanorods on Si(110) substrate, following its 2-fold symmetry of the surface unit cell of the substrate with the dimensions of the structures 650 ± 63 nm in length and 45 ± 6 nm in diameter with an aspect ratio of ∼14.3. Similarly, the SEM micrograph shown in Figure 3c reveals the formation of triangular nanostructures formed by following the 3-fold symmetry of Si(111) substrate with a coverage area of ∼0.5%. The error bars are the standard deviations obtained from averaging many frames, involving many particles. Though the shapes follow the corresponding unit-cell symmetry, the size distribution is not monodisperse. We now discuss more on the nature of the silver nanostructures that are grown on Si(100) using the aforementioned method (with the GeOx interfacial layer). Panels a and b of Figure 4 show planar and X-TEM images (low magnification bright field conditions) of silver nanostructures grown using a 17 nm GeOx/Si(100), using silver wire as a source material in the atmospheric CVD process. Many frames like the one shown in Figure 4a have been taken to determine the error bars in the sizes. Figure 4c shows a high-resolution (HR) X-TEM micrograph of the square nanostructures. From the HR-X-TEM micrograph, an interplanar distance of 0.238

the best of our knowledge, no reports of thermal etching of silicon oxide under atmospheric CVD conditions have been reported yet. Chemical vapor deposition is one of the wellknown experimental methods to grow materials and is sensitive to temperature, rate of gas flow, distance between precursor, and source.20 In the present work, a simple low-cost CVD system was used to carry out the growth of Ag nanostructures.

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RESULTS AND DISCUSSION In our work, the GeOx intermediate layer is found to be important for the growth of symmetric endotaxial silver nanostructure. To elucidate the use of the GeOx layer, first we report the growth of silver nanostructures without the intermediate GeOx layer. The silver wire has been kept in the heating zone of a tube furnace, on the side of a silicon substrate with different substrate surface orientations (the substrate is always covered with a 2 nm native oxide). Panels a and b of Figure 1 are the scanning electron microscope (SEM)

Figure 1. SEM micrographs of silver nanostructures deposited using Ag wire as a source on (a) SiOx/Si(100) and (b) SiOx/Si(110) at 800 °C in air for 30 min.

micrographs of silver nanostructures that are deposited using Ag wire as a source for Si(100) and Si(110) at 800 °C in air for 30 min, respectively. These images reveal that no regular shapes are formed. It is to be noted that, only a native oxide (SiOx) 2.0 nm thick is present on the silicon substrate. No substrate symmetry driven Ag nanostructures was formed for this case (i.e, when there is no GeOx layer on the silicon substrate). Corresponding to the Ag structures grown on Si(100), Figure 2a depicts a low-magnification cross-sectional transmission electron microscope (X-TEM) micrograph of silver nanostructures and Figure 2b depicts a high-resolution (HR) X-TEM micrograph, confirming the growth of endotaxial structures

Figure 2. X-TEM micrographs of silver nanostructures deposited using Ag wire as a source on SiOx/Si(100) at 800 °C in air for 30 min for (a) low mag X-TEM, (b) HR-X-TEM, and (c) SAED showing pure single crystalline silver. B

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Figure 3. SEM micrographs of silver nanostructures deposited using Ag wire as a source on 17 nm GeOx deposited (a) Si(100), (b) Si(110), and (c) Si(111) substrates at 800 °C in air for 30 min.

Figure 4. TEM micrographs of Ag nanostructure on Ge/SiO2/Si(100) @ 800 °C for 30 min for the (a) planar TEM image, (b) low magification XTEM image, (c) HR-X-TEM image, and (d) SAED on a single structure.

furnace up to 800 °C in air. On annealing of Ge oxides, it is known that the GeO2 transforms to GeO and desorbs for temperatures >420 °C. 22 Wang et al. 23 reported the disproportionation of GeO into GeO2 and Ge as per the following equation

nm corresponding to Ag(111) planes can be seen along with a spacing of ∼0.315 ± 0.005 nm corresponding to substrate silicon (111) reflection. It is to be noted that the substrate surface is in the (100) direction but when the XTEM measurements are carried out from this specimen, Si(111) structures can be observed. The lattice image depicts the presence of Moire fringes in the structures, which confirms the presence of the endotaxial nature of silver nanostructures in silicon. The coherent inclusion of Ag nanostructures inside the silicon has been confirmed by analyzing the Moire fringes. We deduce this relationship between the substrate lattice planes of both Ag and Si by further using the observed Moire fringes that are observed in Figure 4c. The Moire fringe spacing (dm) is determined by the formula

2GeO → GeO2 + Ge

(1)

Ge thus formed in the disproportionation of GeO would react with native SiOx to form volatile SiO around 750 °C, as shown by Yun et al.,24 Ge + 2SiO2 + Si → GeO(g) + 3SiO(g)

(2)

It is interesting to note that reaction between GeOx and SiOx is expected to be cyclic in nature and this helps in creating a clean surface for the deposition of Ag vapors. During the above process, silver vapors adsorbed on the silicon surface and the substrates allow it to grow the substrate symmetric silver nanostructures.20 Silver diffusion takes place through the interstitial-substitution mechanism.25,26 This interdiffusion might be one of the reasons for endotaxial formation. We also found that the time duration during the CVD process can be used as a parameter to increase the coverage area. Figure 5a depicts an SEM micrograph of silver nanostructures deposited on 17 nm GeOx/2 nm SiOx/Si(110) using Ag wire as a source for 60 min. Coverage of silver nanorods is increased from 2.3% to 9.6%. Similarly, an increase in the coverage from 0.5% to 7.0% is observed for 17 nm GeOx/2 nm SiOx/Si (111) with 60 min deposition time, figure 5b depicts an SEM micrograph. Controlled growth of substrate symmetric silver nanostructures with desired coverage and size can be obtained by varying the parameters of CVD.20

dm = d1d 2/[(d1 − d 2)2 + d1d 2β 2]1/2

where d1 and d2 are lattice spacings of the planes which are constituents of Moire fringes and β is the angle between the planes.21 The Moire fringe spacing of 0.96 nm was calculated between Ag(111) and Si(111) planes with β = 0. This matches well with the measured fringe spacing of 0.95 ± 0.01 nm confirming the coherence (endotaxy) of silver nanostructures. Figure 4d is the selected area electron diffraction (SAED) pattern taken on a single structure and indexed by following the symmetries of the diffraction spots. The SAED pattern confirms the single crystalline nature of the nanostructures as pure silver. For all three substrate orientations of Silicon, i.e., for Si(100), Si(110), and Si(111), with GeOx on Si, substrate symmetric endotaxial Ag nanostructures have been obtained. A plausible mechanism for the formation of such endotaxial nanostructures is discussed below. In obtaining the endotaxial structures, a 17 nm GeOx/2 nm SiOx/Si sample was annealed along with Ag wire separated by a distance of 2 mm in the C

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ACKNOWLEDGMENTS P.V.S. would like to thank the Department of Atomic Energy, Government of India for project No. 11-R&D-IOP-5.09-0100.

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Figure 5. SEM micrographs of silver nanostructures deposited using Ag wire as a source on 17 nm GeOx deposited for (a) Si(110) and (b) Si(111) substrates at 800 °C in air for 60 min.

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CONCLUSION

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METHODS

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AUTHOR INFORMATION

REFERENCES

Substrate symmetry driven silver nanostructures are grown on silicon substrates by using CVD method with silver as a source and GeOx layer as the interfacial layer. Endotaxial squares/ rectangle nanostructures are formed in the case of Si(100), nanorods are formed on Si(110), and endotaxial triangle nanostructures are formed for Si(111) substrate according to surface unit cell symmetries, respectively. Control over coverage can be obtained by varying the deposition time.

Silicon wafers were cleaned by the standard procedure of rinsing with alcohol and deionized water. Deposition of germanium was done by the physical vapor deposition (PVD) system. Under the growth condition (PVD), it was found that a GeOx has been formed. Confirmation of the presence of GeOx has been done with the help X-ray photoelectron spectroscopy (XPS) measurements (data not shown here). For all these systems, at first, a 17 nm GeOx was deposited on Si(100), Si(110), and Si(111). The synthesis of Ag nanostructures was done by an indigenous CVD system with a side entry furnace and a quartz boat of 80 cm length and 5 cm diameter; the source used was silver wire. Silver has been deposited in the CVD chamber on two different substrates, i.e., on (i) 17 nm GeOx/2 nm SiOx/ Si with (100), (110), and (111) orientations and (ii) silicon with (100), (110) orientations (without the GeOx interfacial layer). The distance between the source and sample was kept constant (2 mm) and heated to 800 °C, then kept for 30 min at 800 °C for deposition on the samples. The post growth characterizations were carried out ex situ, using a field emission gun based scanning electron microscope (FEG-SEM, Neon-40 Carl Zeiss GmbH). Planar and crosssectional TEM samples were prepared by mechanical polishing followed by low-energy Ar+ ion milling to achieve the electron transparency. Transmission electron microscope (TEM) micrographs were carried out with 200 keV JEOL JEM-2010.

Corresponding Author

*E-mail: [email protected] and [email protected]. Tel: +91-674-2306413. Notes

The authors declare no competing financial interest. D

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(22) Prabhakaran, K.; Maeda, F.; Watanabe, Y.; Ogino, T. Distinctly Different Thermal Decomposition Pathways of Ultrathin Oxide Layer on Ge and Si Surfaces. Appl. Phys. Lett. 2000, 76, 2244−2246. (23) Wang, S. K.; Liu, H. G.; Toriumi, A. Kinetic Study of GeO Disproportiantion into GeO2/Ge System Using X-ray Photoelectron Spectroscopy. Appl. Phys. Lett. 2012, 106, 61907−1−61907−4. (24) Yun, S. J.; Lee, S. C.; Lee, J. J.; Nam, K. S. Study of Interaction between Incident Silicon and Germanium Fluxes and SiO2 Layer Using Solid Source MBE. J. Vac. Sci. Technol., B 1994, 12, 1167−1169. (25) Nason, T. C.; Yang, G. R.; Park, K. H.; Lu, T. M. Study of Silver Diffusion into Si(111) and SiO2 at Moderate Temperature. J. Appl. Phys. 1991, 70, 1392−1396. (26) Rollert, F.; Stolwijk, N. A.; Mehrer, H. Solubility, Diffusion and Thermodynamic Properties of Silver in Silicon. J Phys. D: Appl. Phys. 1987, 20, 1148−1155.

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