Nano-sized Taper Structure Formed by Wet Process Using Catalysis of Gold Nanoparticle
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1059-KK09-07
Nano-sized Taper Structure Formed by Wet Process Using Catalysis of Gold Nanoparticle Kensuke Nishioka, and Susumu Horita School of Material Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, 923-1292, Japan ABSTRACT This paper describes a new technique for fabrication of nano-sized taper structure formed by simple wet chemical etching using catalysis of gold (Au) nanoparticle. Single nano-sized Au particle dispersion solution was coated onto Si(111) substrate with polished surface. Then, the samples were soaked in an aqueous etching solution of hydrofluoric acid (HF) and hydrogen peroxide (H2O2). The reflectivity of the Si substrate was reduced to below 5% throughout the entire spectrum from 300 to 800 nm owing to nano-sized taper structure. The fractional area occupied by Si as a function of the depth across the textured layer showed a smooth increase of density up to a depth. The observed optical effects were explained by the formation of a nanoscale taper structure, representing an effective medium with a smooth transition of the refractive index from that of air to Si. This technique could be applied to both of singlecrystalline Si and multi-crystalline Si without reference to orientation and doping type. INTRODUCTION Polished flat silicon surfaces have a high natural reflectivity with a strong spectral dependence. The minimization of reflection losses is very important for high efficiency solar cells, and hence, a variety of approaches has been developed to this end. A subwavelength structured (SWS) surface, which is the surface relief grating with a period smaller than the wavelength of light, behaves as an antireflective surface. SWS surface with deep tapered shape grating suppresses the reflection over a wide spectral bandwidth and a large field of view [1]. It was reported that SWS that achieved low reflectance over a wide spectral bandwidth could be formed by using nano-sized mask of alumina and dry etching processes [1-3]. A texturization treatment of Si based on a noble metal assisted chemical etching has been reported recently [4-6]. It is performed in a solution containing hydrofluoric acid (HF) and hydrogen peroxide (H2O2) which does not etch Si except in presence of noble metal catalysts. Noble metal nanoparticles are deposited on Si surface by an electroless or evaporation method prior to HF/H2O2 etching. The localized Si dissolution results in the formation of textured surface on the Si surface. For the chemical etching of Si using noble metal catalyst and aqueous solution of HF and H2O2, the following mechanism is proposed [4]: At noble metal: H2O2 + 2H+ 2H2O + 2h+ , 2H+ + 2e- H2 ,
→ → ↑
At Si:
→
Si + 4h+ + 4HF SiF4 + 4H+ , SiF4 + 2HF H2SiF6 , Over all reaction: Si + H2O2 + 6HF 2H2O + H2SiF6 + H2 . In the ref. [4], it is explained that the generation of h+ from H2O2 and the reduction of H+ to form H2 are facilitated by the noble metals. The noble metals act as catalyst, and the neighboring Si is etched and excavated [6]. The structure and reflectiv
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