(Invited) Thin Porous Silicon Produced By Metal-Assisted Etching

Metal-assisted etching of silicon can produce porous silicon and silicon nanowires by simply immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias (1). Such etching proceeds by a local galvanic cell mechanism consisting of local anodic dissolution of silicon and local cathodic reduction of an oxidizing agent on metal. The structure of porous silicon is widely controlled from nm to sub-mm scale in pore size and from straight to an ants’ nest like in pore morphology by changing etching conditions (1-3). We have been studying the metal-assisted etching of silicon to produce porous structures for antireflection of solar cells (2), and for autocatalytic electroless formation of metal-nanorods and metal films on Si surfaces (4). In this paper, we present size-controlled thin porous silicon layers consisting of straight pores and those applications. Noble metal nanoparticles were deposited on crystalline silicon wafers by electroless displacement deposition immersing the wafers into a metal salt solution containing hydrofluoric acid. The metal-particle-deposited silicon wafers were immersed in a hydrofluoric acid solution including hydrogen peroxide as an oxidizing agent. Under suitable etching conditions, uniform porous silicon thin layers consisting of straight pores were formed on silicon wafers. The diameter of pores was consistent with the size of metal nanoparticles remained at the bottom of each pore. Thus, the diameter of pores, that is, the porosity of porous layers is easily controlled in the nm scale with the deposition time of nanoparticles. The thickness of porous layers, that is, the length of the straight pores is also easily controlled by the etching time. Optical properties of the porous layers thinner than 1 μm depends on those porosity and thickness (5). This is applicable for antireflection of solar cells. The strength of metal nanorods, which were produced by electrolessly filling the pores, increased with the diameter of pores from 10 to 25 nm. These nanorods work as nanoanchors of metal films on silicon wafers.  Acknowledgements The present work was partly supported by JSPS KAKENHI (26289276). References 1) Z. Huang, N. Geyer, P. Werner, J. de Boor, U. Gösele, Adv. Mater., 23, 285 (2011). 2) S. Yae, H. Tanaka, T. Kobayashi, N. Fukumuro, H. Matsuda, Phys. Stat. Sol. (c), 2(9), 3476 (2005). 3) S. Yae, M. Tashiro, M. Abe, N. Fukumuro, H. Matsuda, J. Electrochem. Soc., 157, D90 (2010). 4) S. Yae, K. Sakabe, N. Fukumuro, S. Sakamoto, and H. Matsuda, J. Electrochem. Soc., 158, D573 (2011). 5) K. Yamakawa, S. Sakamoto, N. Fukumuro, S. Yae, ECS Trans., 69(2), 185 (2015).