Electroless deposition of metal onto semiconductor is an efficient technique to fabricate metal nanostructures for electronic, photonic, and plasmonic applications. We previously reported a new area-selective electroless deposition of gold nanostructures on a crystalline silicon substrate. A simple tetrachloroauric(III) acid (HAuCl₄) solution using water or alcohol as a solvent is dropped on a crystalline silicon substrate preprocessed with a focused ion beam (FIB) or a femtosecond laser. Gold nanostructures selectively grow on the preprocessed area on the silicon substrate[1]. Hydrofluoric acid, which is commonly used for electroless deposition of metal on semiconductor, is unnecessary in our method. Our previous work revealed that silicon dangling bonds induced by the preprocessing reduce gold ions in the solution[2]:

3Si• + Au³⁺ + 3ROH → 3Si-OR + Au + 3H⁺  (R = H or C_nH_2n+1)

where Si• represents the silicon atom which has a single dangling bond and three covalent bonds with other silicon atoms. This reductive reaction requires hydroxyl groups of solvent molecules and initiates the nucleation of gold. However, the overall mechanism of the gold growth in our method is still unclear.

Here we focus on the growth stage of gold after the nucleation stage. For investigation of growth mechanism of gold, we quantitatively evaluate the amount of gold grown on FIB-irradiated silicon surface with a fixed microscopic area by measuring the thickness of gold. The quantitative evaluation indicates that once the gold/silicon interface is formed through the nucleation stage, electrons in crystalline silicon surrounding the FIB-irradiated silicon reduce gold ions in the solution across the interface. Therefore the difference in the Fermi level between silicon and the solution is the driving force in the growth stage. This mechanism is supported by a new two-step growth process introduced to our method, in which the exposure of the FIB-irradiated silicon substrate to HAuCl₄ solutions is separated into the first and second steps. Once the gold/silicon interface is formed in the first step, the gold growth proceeds in the second step with or without silicon dangling bonds and hydroxyl groups of solvent molecules. Furthermore, our new method enables us to fabricate gold nanostructures with various morphologies by selecting the reaction condition in the second step.

References

1. H. Itasaka, M. Nishi, Y. Shimotsuma, K. Miura, M. Watanabe, H. Jain, and K. Hirao, J. Ceram. Soc. Jpn. 122, 543 (2014).
2. H. Itasaka, M. Nishi, and K. Hirao, Jpn. J. Appl. Phys. 53, 06JF06 (2014).