Hydrosilylation of High Porosity Porous Silicon with 1-Hexene in Supercritical CO2 Fluid
Authors: Naoto Takura , Jin Lianhua , Kondoh Eiichi , Gelloz Bernard 
Published: December 2020

The surface of as-formed porous silicon (PSi) is terminated by Si-H bonds. Hydrogen-terminated PSi is generally unstable due to the high reactivity of these Si-H bonds. Thermally activated hydrosilylation of organic groups on hydride-terminated PSi enhances PSi stability because Si-C bonds and much more stable than Si-H bonds and also because of steric hindrance provided by the long organic molecules. In our previous work, gas-phase hydrosilylation for PSi with porosity of 68% was achieved by using 1-hexene, and highly stable luminescence was obtained by organic group termination [1].
The higher the porosity, the higher the luminescent efficiency of PSi. When the porosity is higher than ~75%, however, it is extremely difficult for the as-formed PSi to preserve its fine structure due to the surface tension induced during air-drying. The supercritical fluid is characterized for its zero surface tension [2]. In this work, the process of both drying and hydrosilylation in the supercritical fluid for high porosity PSi was developed. The effect of this process was investigated in terms of FTIR and photoluminescence (PL) measurement.
PSi layers were formed by anodization of p-type, (100)-oriented, and 5-10 Ω・cm silicon substrates with an Al back contact. Layers with porosity of 80% and thickness of approximately 1, 2, 5, and 10 mm were obtained by etching 200, 400, 1000, and 2000 s in the solution of HF (55 wt. %): ethanol = 1:1, at a constant current density of 10 mA/cm2. Following formation, PSi was rinsed in ethanol, then, without drying, placed inside a high-pressure reactor (volume: 5.5 ml) filled with ethanol. The drying and hydrosilylation were accomplished through the following procedures. First, liquid CO2 was flowed through the reactor to expel the ethanol inside and continuously purged until the reactor pressure was up to 10 MPa. Then, the reactor was heated at 80°C to yield the supercritical fluid CO2. The supercritical condition of 10 MPa and 80°C was maintained for 30 min to dry the PSi sample. After drying, the CO2 was continuously pumped into until the reactor pressure up to 15 MPa, and the reactor was heated at 150 °C. At this supercritical condition of 15 MPa and 150°C, CO2 - 1-hexene solution was supplied for 4 hours to achieve hydrosilylation. Finally, the reactor was cooled to the room temperature, and the supercritical CO2 was released.
FTIR spectra of PSi samples before and after the hydrosilylation process were investigated. The FTIR results showed substitution of hydrogen on the PSi surface by organic groups via hydrosilylation.
Photoluminescence of these samples were measured by exciting with a laser diode (wavelength is 405 nm) for 4 hours. Figure 1 illustrates the PL vs. time results of two samples (porosity 80%, thickness 2mm). Compared with the PL of the sample dried in the supercritical fluid, the sample after drying and hydrosilylation process shows good stability. The result indicates the availability of supercritical process for high porosity PSi.

Gelloz, T. Iriyama, N. Takura, and E. Kondoh, and L. Jin, ECS Journal of Sol. Stat. Sci. and Tech. 8 (9), R109-R113 (2019).

Kondoh and H. Kato, Microeletron, Eng. 64, 495-499 (2002).

Figure 1