Galvanic Deposition of Nanoporous Si onto 6061 Al Alloy from Aqueous HF Aarti Krishnamurthy, a Don H. Rasmussen, b,c and Ian I. Suni b,c, * ,z a Department of Chemistry and Biomolecular Science, b Department of Chemical and Biomolecular Engineering, and c Materials Science and Engineering PhD Program, Clarkson University, Potsdam, New York 13699-5705, USA We report galvanic deposition of Si onto 6061 Al alloy from dilute aqueous hydrofluoric acid HF at pH 2.5. The overall reaction involves reduction of SiF 6 2- to Si with simultaneous oxidation and dissolution of Al. The Si film is about 12 m thick after 6 h of deposition. High resolution scanning electron microscopy shows that these Si films are nanoporous, with pore sizes ranging from 3 to 8 nm. The nanoporous Si films oxidize rapidly upon sample emersion. Elemental analysis by energy dispersive X-ray spectroscopy demonstrates that the as-deposited film contains 1–3 atom % Al, 3–6 atom % Cu, and 90–95 atom % Si. We believe that this is the first report of electrochemical deposition of Si thin films that does not involve organic solvents or molten salt electrolytes. © 2010 The Electrochemical Society. DOI: 10.1149/1.3521290 All rights reserved. Manuscript submitted August 3, 2010; revised manuscript received November 4, 2010. Published December 3, 2010. While the market for photovoltaic cells is currently dominated by thick film Si solar cells, thin film crystalline, polycrystalline, and amorphous Si solar cells have also been intensively investigated. 1 Optical absorption in Si solar cells occurs mainly within the top micrometer of Si, so the rest of the Si wafer in thick film solar cells merely provides mechanical support. While thick film Si solar cells can directly employ Si wafer technology and materials from inte- grated circuit manufacturing, thin film Si solar cells provide obvious long-term cost advantages. In addition to photovoltaic applications, Si thin films are of interest for silicon-on-sapphire complementary metal oxide semiconductor technology, 2 for anode materials in Li ion batteries, 3,4 and for corrosion-resistant coatings. 5 Si thin films are typically deposited by expensive vacuum meth- ods such as chemical vapor deposition and plasma-enhanced chemi- cal vapor deposition. 1 In addition, Si thin films are typically depos- ited from silane, which is both highly pyrophoric and moisture sensitive. Electrochemical methods for depositing thin film solar cell materials are highly advantageous due to their low cost, scalability to large surface areas, and manufacturability. 6 However, Si is a highly active metal, so the standard reduction potential of SiO 2 -0.90 V vs normal hydrogen electrode NHE is more cathodic than the standard reduction potential of water -0.83 V vs NHE, 7 making electrochemical deposition of Si from aqueous electrolytes notoriously difficult. Si electrodeposition from molten salts at elevated temperatures   750°C has a long history. 8-11 However, Si electrodeposition at room temperature has only relatively recently been achieved from organic solvents 12-17 and from room temperature ionic liquids. 18-23 Here we report galvanic deposition of nanoporous Si onto Al from solutions of dilute aqueous hydrofluoric acid HF at pH 2.5, with 12 m thick Si films grown after 6 h. Energy dispersive X-ray spec- troscopy EDX measurements show that the as-deposited film con- tains mainly Si, Cu, and Al. Experimental Semiconductor grade 10 wt % HF and concentrated HNO 3 were obtained from J. T. Baker, Na 2 SiF 6 was obtained from Sigma- Alrdich, and 6061 Al alloy was obtained from McMaster Carr. For Al electrochemistry, all measurements were performed using a three-electrode setup with a 12 mm diameter 6061 Al alloy working electrode rotated at 850 rpm with a rotating disc electrode, Pt spiral counter electrode, and a reference SCE. A 6061 Al alloy typically contains 0.8–1.2 wt % Mg, 0.4–0.8 wt % Si, 0.70 wt % Mg, 0.15–0.40 wt % Cu, 0.04–0.35 wt % Cr, and smaller amounts of Mn, Ti, and Zn. For some of the galvanic deposition experiments, 99.99% pure Al was purchased from ESPI Metals. For Si electro- chemistry, B-doped  2  10 19 cm -3  degenerate Si100 wafers with a resistivity of 0.001–0.005  cm were purchased from Uni- versity Wafer. The electrical connection to the Si wafer’s back side was made using a Ga–In eutectic. Voltammetry experiments were controlled with an EG&G PAR model 273A potentiostat/galvanostat. Impedance measurements were made by coupling this potentiostat with a Solartron 1250B frequency response analyzer over the frequency range 0.01 Hz–10 kHz, using an ac probe voltage of 8 mV. The Si film thickness was measured with a JEOL model 7400F field emission scanning electron microscope at both 45 and 90°. Results and Discussion Figure 1 illustrates a voltammogram of the Al working electrode rotated at 850 rpm in 10 mM HF + 1 mM HNO 3 pH 2.5. An ad- dition of 20 mM Na 2 SiF 6 to this electrolyte had no discernable ef- fect on the voltammetry results. Figure 1 illustrates that this electro- lyte is quite corrosive to Al, with anodic currents from Al oxidation and dissolution observed at all potentials anodic to -1000 mV vs SCE. This is the reason why such a high scan rate  50 mV/s was employed. Immersion of the Al working electrode rotated at 850 rpm without potential control into 10 mM HF, 1 mM HNO 3 , and 20 mM Na 2 SiF 6 for 6 h results in the growth of a Si film about 12 m thick. The open circuit potential was measured during Si deposition and varied between -700 and -900 mV vs SCE, as shown in Fig. 2. The as-grown film is dark gray in solution but changes color to light gray after exposure to laboratory air for 1 h and then to white upon overnight exposure. Figures 3 and 4 present scanning electron microscopy SEM images following growth of a 12 m Si film as described above. Figure 3 illustrates the 12 m Si film middle atop the Al substrate bottom and appears to show a compact Si deposit. However, the higher resolution image in Fig. 4 shows that the Si deposit contains nanoscale porosity, with pore sizes ranging from 3 to 8 nm. Other methods that have been reported for room temperature Si elec- trodeposition similarly yield porous Si films, as discussed elsewhere. 15 X-ray diffraction studies of our galvanic Si films, both immediately after deposition and after overnight ambient exposure, show no diffraction peaks, indicating that our deposits are amor- phous. The Si deposit is grown atop Al by galvanic deposition, other- wise known as immersion plating, where a more noble metal is reduced and deposited onto a less noble substrate that is simulta- neously oxidized and dissolved. The cathodic 1, anodic 2, and overall 3 reactions for galvanic deposition of Si onto Al are 7 * Electrochemical Society Active Member. z E-mail: isuni@clarkson.edu Journal of The Electrochemical Society, 158 2 D68-D71 2011 0013-4651/2010/1582/D68/4/$28.00 © The Electrochemical Society D68 Downloaded 06 Dec 2010 to 128.153.13.75. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp