Pulsed Electrodeposition of Si Under Light Illumination in Ionic Liquid

Abstract

Electrodeposition of Si has attracted much attention for its possibility to apply various areas such as photovoltaic cells and MEMS. Si thin film can be electrochemically fabricated with precise microstructural control and in a relatively simple process. However, it requires nonaqueous solvents due to very negative reduction potential of Si(IV). Ionic liquids are promising solvents for Si electrodeposition because of its unique characteristics such as nonflammable, non-volatile, and relatively wide electrochemical window. We have reported the electrochemical processing on Si thin films, nanodots, and nanopillars in trimethylhexyl ammonium bis-(trifluorosulfonyl) imide (TMHATFSI) ionic liquid containing SiCl4 as Si precursor [1-3]. Moreover, our recent paper [4] reports relatively smooth and compact Si thin films obtained by applying pulsed deposition, and smoother ones by applying light illumination additionally. Here we present the results of our work in the electrochemical processing of Si thin film with the pulsed potential modulation and light illumination. These experiments were conducted in order to promote the diffusion of precursor and light-assisted electrodeposition respectively. In the preliminary experiments, it turned out that the composition of Si film in pulse potential modulation [4] contained relatively large amount of impurities than the films potentiostatically deposited [3]. This result suggests that the oxidation currents during the rest period of pulse deposition may be partly responsible for such a larger impurity. Then, pulse potential modulation scheduling was further examined. Controlling the potential and current density considerably suppressed oxidation current. The composition of Si thin films was improved to recover to the level of potentiostatic electrodeposition. However, some cracks were still observed the surface. Thus, light illumination only in deposition time was superimposed. As a result, large cracks disappeared, and continuous and relatively smooth Si thin films with relatively low impurity were obtained. [1] T. Homma, J. Komadina, Y. Nakano, T. Ouchi, T. Akiyoshi, Y. Ishibashi, Y. Nishimura, T. Nishida, and Y. Fukunaka, ECS Trans., 41 (2012) 9. [2] Y. Ishibashi, T. Akiyoshi, J. Komadina, Y. Fukunaka, T. Homma, ECS Trans., 50 (2012) 117. [3] Y. Tsuyuki, A. Pham, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta, 183 (2015) 49. [4] Y. Tsuyuki, H. Takai, Y. Fukunaka, T. Homma, Jpn. J. Appl. Phys. 57 (2018) 08RB11.