Catalytic activity of noble metals for metal-assisted chemical etching of silicon
© Yae et al; licensee Springer. 2012
Received: 28 April 2012
Accepted: 18 June 2012
Published: 27 June 2012
Metal-assisted chemical etching of silicon is an electroless method that can produce porous silicon by immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias. We have been studying the metal-assisted hydrofluoric acid etching of silicon using dissolved oxygen as an oxidizing agent. Three major factors control the etching reaction and the porous silicon structure: photoillumination during etching, oxidizing agents, and metal particles. In this study, the influence of noble metal particles, silver, gold, platinum, and rhodium, on this etching is investigated under dark conditions: the absence of photogenerated charges in the silicon. The silicon dissolution is localized under the particles, and nanopores are formed whose diameters resemble the size of the metal nanoparticles. The etching rate of the silicon and the catalytic activity of the metals for the cathodic reduction of oxygen in the hydrofluoric acid solution increase in the order of silver, gold, platinum, and rhodium.
KeywordsPorous silicon Metal nanoparticles Etching of semiconductor Displacement deposition Electroless plating Electrochemical catalysts Overpotential Galvanic etching Oxygen reduction Anodic dissolution of silicon 81.05.Rm; 82.45.Vp; 81.65.Cf
Metal-assisted chemical etching of silicon (Si) has attracted considerable attention as a new electroless method that can produce porous Si by immersing metal-modified Si in a hydrofluoric acid (HF) solution without electrical bias [1–5]. Such etching generally uses not only metal-modified Si but also an oxidizing agent. The common oxidizing agent is hydrogen peroxide [1, 2, 4, 5]. Several applications of this etching have been proposed, such as the surface-texturization (antireflection) of Si photovoltaics [6, 7] and the production of Si-nanowires  and Si-hole arrays . We have been studying the metal-assisted HF etching of Si using dissolved oxygen (O2) as an oxidizing agent and/or photoillumination onto Si for charge generation to promote etching [3, 10]. We reported that the structure of the produced porous Si can be controlled by changing the photoillumination intensity during etching, the dissolved oxygen concentration in the HF solution, and the size and distribution of metal particles on Si [3, 10–13]. This etching can be applied to the antireflection of Si photovoltaics [3, 10, 13–15], solar-to-chemical conversion using photoelectrochemical solar cells , metal nanorod formation , and the metallization of Si surfaces .
Single-crystalline n-type Si wafers (CZ, (100), ca. 1 Ω cm, 0.5-mm thick, Yamanaka Semiconductor Co. Ltd., Kyoto, Japan) were washed with acetone and etched with a CP-4A solution (a mixture of HF, nitric acid, acetic acid, and water (3:5:3:22 in volume)) and then with a 7.3-M (M = mol dm−3) HF solution. Nanoparticles of noble metals (silver (Ag), gold (Au), platinum (Pt), and rhodium (Rh)) were deposited on the Si by electroless displacement deposition . The Si wafers were immersed in a metal salt (silver nitrate (AgNO3), tetrachloroauric (III) acid (HAuCl4), hexachloroplatinic (IV) acid (H2PtCl6), potassium tetrachloroplatinate (II) (K2PtCl4), or rhodium chloride (RhCl3)) solution including 0.15-M HF. For metal-assisted HF etching, the metal particle-deposited n-Si wafers were immersed in a 7.3-M HF solution at 298 K of solution temperature for 24 h. The oxygen concentration of the HF solution was controlled by gas bubbling of oxygen and argon (Ar, >99.9999% purity, Grade-1, Taiyo Nippon Sanso Corp., Tokyo Japan). To avoid the influence of the photogenerated charges, the Si wafers were immersed in the HF solution under dark conditions. The rate of etching was measured by a gravimetric procedure using an analytical balance (XP2U, Mettler-Toledo International Inc., Columbus, OH, USA). Surface and cross-sectional inspections of the specimens were performed with scanning electron microscopes (SEM, JSM-7001 F, JEOL Ltd., Akishima, Tokyo, Japan). Electrochemical measurement was carried out with an electrochemical analyzer (VSP, Bio-logic Science Instruments, Claix, France) using a metal wire (0.12 mm in diameter) working electrode, a silver-silver chloride (Ag/AgCl) reference electrode, and a Pt counter electrode. The potential sweep at a rate of 20 mV s−1 started from the rest potential to the negative potential direction.
Results and discussion
Deposition conditions, particle density, and size of metal particles on Si wafers
Metal salt concentration(×10−3 mol dm−3)
Solution temperature (K)
Deposition time (s)
Average particle density (×1011 cm−2)
Average particle size (nm)
In this study, we investigated the catalytic activity of noble metals, Ag, Au, Pt, and Rh, for the metal-assisted HF etching of Si using dissolved oxygen as an oxidizing agent. The catalytic activity of the noble metals for the cathodic oxygen reduction in a HF solution increases in the order of Ag, Au, Pt, and Rh. The activity controls the local cathodic reaction of the metal-assisted HF etching of Si under dark conditions. Thus, the etching rate of Si is determined by the dissolved oxygen concentration, the kind of metals, and the metal coverage of the Si surfaces.
SY is an associate professor, YM is a graduate school student, NF is a research associate, and HM is a professor at Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo.
scanning electron microscope
The present work was partly supported by Grant-in-Aid for Scientific Research (C) (20560676 and 23560875) from Japan Society for the Promotion of Science (JSPS) and Research for Promoting Technological Seeds and FS stage of Exploratory Research of Adaptable and Seamless Technology Transfer Program through target-driven R&D (A-STEP) from Japan Science and Technology Agency (JST).
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