(Invited) Evolutionary Applications of Functional Porous Silicon
Authors: Koshida Nobuyoshi , Kojima Akira , Nakamura Toshihiro 
Published: December 2020

Nanostructured porous silicon (PS) shows varied tunable functions leading to diversification of silicon technology. Among these functions, the emissions of photon, electron, and sound relate to the optical, electrical, and thermal properties of PS, respectively, as a quantum-confined material [1]. Some topics on further usefulness based on the functional combination is presented here.
PS is composed of thermally isolated Si quantum dots (Si QDs) with a very high packing density. Due to a strong phonon confinement and interfacial scattering in PS, the thermal conductivity of PS layers is drastically lowered in comparison to that of bulk single-crystalline Si (c-Si). At the same time, its volumetric heat capacity is also significantly decreased. When combined with the mechanically metastable nature, it becomes possible to produce Si QDs with a considerably low external thermal energy. Actually, PS layers prepared by a standard electrochemical anodization of c-Si wafers in ethanoic HF solutions followed by peeling off from the wafer were dipped into organic solvents mixed with HF aqueous solution, and then in-situ heat treatment was made. It has been confirmed that PS layers are almost completely transformed into colloidal Si QDs with a significantly high yield even at a moderate temperature. The surfaces of as-prepared colloidal Si QDs are autonomously and organically passivated and they exhibit brighter PL emission than original PS. The dispersibility of Si QDs in solvents can be controlled by using organic solutions with appropriate function groups. Being a practical, clean, and low-power approach, this process provides a way for the scalable use of colloidal Si QDs as a printable material in optoelectronic, photoelectric, and biological applications.
The thermal stability of PS with a controlled mesoporous surface structure, on the other hand, is useful as a substrate for epitaxial deposition of Si, Ge, SiC, diamond films, GaN, ZnO, and other III–V and II–VI semiconductors. The study has been extended to thin film deposition of ceramics like ferroelectric and ferrite materials. Recently [2], thin films of spinel ferrite (ZnFe2O4) were deposited on PS layers by pulsed laser deposition with an Nd:YAG laser of 266 nm for application to gas sensing. According to the structural and compositional characterizations, deposited thin films were polycrystalline and stoichiometric. Its average grain size and resistivity were smaller and higher, respectively, than those in the case of deposition on c-Si wafers. Obviously, the density of nucleation sites on the PS surface is much higher than that on the c-Si substrate. The measured sensitivity of a PS based device for liquefied petroleum gas was two-times higher than that of the c-Si based one. The sensitivity at high temperatures was comparable to that of ZnFe2O4 nanotubes and powder. It is important from a technological viewpoint that a reducing gas was detected effectively using the thin-film device configuration on c-Si wafer.
In the high-porosity PS layer, thermal constants become close to the lower limit of solid state materials. When a temperature fluctuation is produced by electrical input to the heater electrode formed on the PS layer, an acoustic wave is generated near the surface via the thermos-acoustic transfer effect. A significant sound pressure amplitude is produced without any mechanical vibrations. The PS thermos-acoustic emitter shows a sufficiently flat frequency response in a wide ultrasonic band. One possible application is to reproduce complicated ultrasonic vocalization (USV) calls for clarifying the mechanism of mice communication and male-female interactions. As previously demonstrated, mouse mothers were attracted by pup USVs reproduced by a PS emitter, while they did not respond to other synthesized sounds. It was also found that the response to pup USVs was enhanced by social experiences. Studies on mutual recognition between mother and infant suggest that pup USVs looks to have an individual signature used in pup differentiation by mouse mothers, similar to acoustic communication between human mothers and their infants. It has also been reported that female mice exhibit disassortative social preferences in response to male USVs, and that the acoustic features that affect these social preferences are different between mouse strains [3].
The author NK thank Prof. Wakiya, and Prof. Kikusui for their continuing cooperation.
[1] N. Koshida and T. Nakamura, Frontiers in Chemistry “Advances in Porous Semiconductors Science and Technology” Vol. 7, 273 (2019).
[2] H. Ishigami, T. Kawaguchi, N. Sakamoto, S. Che, N. Koshida, K. Shinozaki, H. Suzuki, and N. Wakiya, J. Ceramic Soc. of Japan 128, 2020.
[3] K. Nomoto, A. Hashiguchi, A. Asaba, M. Kato, N. Koshida, K. Mogi, and T. Kikusui, Experimental Animals, Published online, Feb 26, 2020.