High Catalytic Activity of Palladium for Metal-Enhanced HF Etching of Silicon

Abstract

Metal-enhanced HF etching of Si is an electroless method used to produce porous Si. Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide or metal ions. Pd exhibits high activity in enhancing the HF etching of Si without an oxidizing agent even under dissolved-oxygen-free and dark conditions. Electrolessly deposited Pd particles on n-type Si enhance the HF etching of Si but produce no porous layer. Patterned Pd films localize the etching under the boundary of the Pd deposited areas, and thus Pd can produce a microetch pattern on Si with a simple immersion in the HF solution. This etching reaction is explained by electron injection into the conduction band of Si due to the Pd-enhanced anodic oxidization of Si with water and the cathodic hydrogen evolution on Pd with the injected electrons.Porous silicon ͑Si͒ is usually prepared by electrochemical etching under an anodic bias in a fluoride-containing solution. 1-3 Metalenhanced HF etching of Si has attracted considerable attention as a new electroless method that can produce porous Si by immersing metal-modified Si in an HF solution without bias. 4-15 Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide, 5,10,12-14 peroxodisulfate, 8 permanganate, 8 or metal ions. 7 We reported the preparation process 6,9,11,15 of porous Si using metal-particle-enhanced HF etching of Si without a particular oxidizing agent as well as its application for solar cells 6,9,16 and metal nanorod formation. 17 This etching generally proceeds by a local galvanic cell mechanism requiring photoillumination or dissolved oxygen. Palladium ͑Pd͒ exhibits high activity in enhancing etching under dissolved-oxygen-free and dark conditions. 15,18 In this paper, we describe the unique behavior of Pd on HF etching of Si and micropattern formation. 19 ExperimentalSingle-crystalline n-type Si wafers ͓Yamanaka Semiconductor, CZ ͑100͒, ca. 1 ⍀ cm͔ were cut into pieces ͑1 ϫ 1 cm͒, washed with acetone, and etched with CP-4A ͑a mixture of HF, nitric acid, acetic acid, and water͒ and a 7.3 M ͑M = mol dm −3 ͒ HF solution. Metal particles were deposited on Si wafers by electroless displacement deposition from a 1 mM metal-salt ͑PdCl 2 or H 2 PtCl 6 ͒ solution containing 0.15 M HF. 20 Pd thin films were deposited onto n-Si wafers by the electron-beam evaporation method. The thicknesses of the deposited films were measured using a quartz crystal microbalance ͑ULVAC, CRTM-5000͒ adjusted by gravimetry. The Pd films were patterned by attaching a mask ͑85 m square holes and 40 m space͒ to the n-Si wafers during deposition. The metaldeposited n-Si wafers were immersed in a 7.3 M HF aqueous solution at 298 K under dark conditions. The etching time was 24 h unless otherwise specified. Argon gas ͑Ͼ99.9999% purity, Taiyo Nippon Sanso, grade-1͒ bubbling was applied to the HF solution before and during etching to remove the dissolved oxygen from the solution. The average etch rate of Si was measured using a gravi...

Porous Silicon Formation by Metal Nanoparticle-Assisted Etching

“…The pore diameter is the same as that of Ag or Au particles, whereas for Pt the pores are cone shaped of micrometer size and lined with mesoporous Si (Tsujino and Matsumura 2005a, b). For Pd a polishing occurs when the etching is done in 7.3 M HF/molecular O 2 (Yae et al 2005(Yae et al , 2010a. When the distances between noble metals in the case of Ag and Au are small, the structures evolve from pores into wall-like or wirelike structures vertically standing with diameter between 100 and 200 nm and several microns long (Peng et al 2005b(Peng et al , 2006b (Chartier et al 2008).…”

“…The generation of helical holes is attributed to the microscopically difference in etching rate of silicon on a Pt particle (Tsujino and Matsumura 2005b). -When Ag, Au, and Pd/Pt metal particles are deposited in ordered arrays through a mask as a patterned thin film (~20-40 nm thick), quasi-ordered silicon micro-/ nanostructures (pores) are formed in silicon, whatever is the kind of deposited noble metal (Huang et al 2007;Yae et al 2010a;Asoh et al 2007aAsoh et al , b, c, 2008Asoh et al , 2009Bauer et al 2010;Peng et al 2007;Ono et al 2007Ono et al , 2009; Lévy-Clément Boarino et al 2011;Pacholski 2011;Geng et al 2011;Hildreth et al 2009Hildreth et al , 2013Rykaczewski et al 2011;Scheeler et al 2012;Geyer et al 2012Geyer et al , 2013G€ uder et al 2013;Wang et al 2013a). This observation is the basis of the fabrication of a rich variety of ordered Si structures with different length scales, as topographic lines, concentric rings, square arrays, etc., can be created at a micro-and nanoscale after etching the silicon and subsequent removal of the mask.…”

Solar to Chemical Conversion Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode

“…The metal-particle-assisted hydrofluoric acid etching of Si proceeds by a local galvanic cell mechanism requiring photoillumination onto Si or dissolved oxygen in the solution (Yae et al 2005(Yae et al , 2007d(Yae et al , 2009(Yae et al , 2010. Figure 5 shows a schematic diagram of n-Si and electrochemical reaction (equations (5), (6) and (7)) potential in a hydrofluoric acid solution.…”

Catalytic nanopores for electroless deposition of adhesive metal films on silicon: Applications to various silicon substrates including multi‐ and micro‐crystalline

“…The nanopore formation on Si, which was led by the metal particles, was caused by localizing the Si dissolution under the metal particles. The mechanism of the metal-particleassisted HF etching is described using a local galvanic cell consisting of the local cathode reduction of oxygen on the metal particles, which injects electric holes into the valence band of silicon, and the local anode oxidation of Si, which dissolves silicon with HF [6,10,11,21]. The absence of a photogenerated charge in Si under dark conditions localized the anode reaction in the vicinity of the metal particles.…”

“…1 Introduction Metal-assisted hydrofluoric acid (HF) etching of silicon (Si) can produce various porous structures, from nanometer to sub-millimeter size, on Si substrates [1][2][3][4][5][6][7][8][9][10]. One unique structure produced by this etching is Si nanopores, whose diameter resembles that of metal nanoparticles used as catalysts, that have metal nanoparticles at the bottom of each nanopore [7][8][9][10][11].…”