Evaluation of crystal structure of porous Si nanowires prepared by metal assisted etching
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Evaluation of crystal structure of porous Si nanowires prepared by metal assisted etching Takuya Yamaguchi1, Tomohiro Shimizu1, Chonge Wang1, Mitsuru Inada1, Shoso Shingubara1, Aleksandr Kuznetsov2, Johan Vanacken2 and Victor Moshchalkov2 1
Kansai University, Faculty of Engineering science, Yamate-cho 3-3-35, Suita, Osaka, 5648680, Japan 2 Institute for Nanoscale Physics and Chemistry (INPAC), Celestijnenlaan 200D, B-3001 Leuven ABSTRACT The macro and micro morphology of the Si surface, depending on noble metal ion concentration in etching solution, prepared by metal assisted etching were investigated. We defined the morphology of etched Si in four types, and developed the phase diagram of Si morphology. Mixture of silver nitrate (AgNO3) and hydrofluoric acid (HF) was used as an electroless-plating bath of Ag, as well as etching solution of Si. The morphology of the etched Si surface as function of concentration of AgNO3 in etching solution was observed by SEM. With increasing concentration of the AgNO3 in the etching solution, the surface of etched Si tended to be porous structure with very fine pores of a few nm (porous Si). When concentration of AgNO3 is low, single crystal Si nanowires with a smooth surface without pores were observed. INTRODUCTION Semiconductor nanowires have attracted interest due to their unique one-dimensional electric and optical properties. Among them, the Si nanowire is considered as a promising candidate of such an electronic device [1], biosensor [2], optoelectronic device [3], and energy storage [4] because of compatibility of conventional Si technology. When Si nanowire is used as a building block of these applications, morphology, size, crystal quality and crystal orientation of the nanowire would be crucial parameters for the device performance. Although several methods have been reported to prepare Si nanowires [4-9], the metal assisted chemical etching of Si substrate has advantages of low cost and mass production compared with others. In this method, using a noble metal as a catalyst on a Si substrate, the Si underneath the catalyst is selectively etched by the solution. Generally, there are two different types of etching solutions used for the metal assisted etching. Most of study used mixture of hydro fluoric acid (HF) and hydrogen peroxide (H2O2) as the etching solution [6-8]. In this solution, the H2O2 works as an oxidant, and the Si surface underneath the metal catalyst is selectively oxidized. Then the SiO2 is simultaneously etched away and the metal catalyst sinks into the substrate. The surface morphology of the etched Si is strongly depending on concentration of H2O2 [7]. On the other hand, the metal assisted etching of Si is known to occur with electroless plating solution of noble metals without H2O2, such as mixture of silver nitrate (AgNO3) and HF [8]. In this solution, oxidation of Si underneath metal catalyst is carried out by hole injection into Si, due to reduction of Ag1+ ion at the surface of catalyst. Therefore, concentration of Ag1+ ion would be a key parame
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