(Invited) Photo-Assisted Etching of Porous Silicon Nanostructures in Hydrofluoric Acid Using Monochromatic Light
Authors: Gelloz Bernard , Fuwa Hiroki , Kondoh Eiichi , Jin Lianhua 
Published: February 2020

Silicon can be made porous by electrochemical (anodization) etching in hydrofluoric acid (HF). This is the most controlled and used technique for the preparation of porous silicon (PSi). The porous structure depends on several factors, such as silicon resistivity and type, as well as HF concentration and anodization current. In the case of lightly-doped p-type silicon, uniform sponge-like nano-structures can be formed. This type of PSi layer is characterized in particular by its porosity and thickness. PSi is interesting for a variety of properties, some related to quantum confinement in its nanostructure, such as luminescence and porosity-dependent refractive index and surface area. It has potential applications in optoelectronics, photovoltaics, medicine and sensing.
The post-anodization behavior of PSi in various environments is of interest for stability and degradation evaluation. The evolution of PSi is sometimes made intentional in order to further tailor the PSi properties after anodization. Typically, the effects of PSi oxidation (mainly for stabilization purposes), etching (to increase the porosity) or degradation in various environments (typically for biotechnology) are sometimes studied.
Recently, we have made use of a particular behavior of the PSi photoconduction in HF: when PSi is illuminated in HF, only the charge carriers photogenerated in the Si substrate contribute to the measured photocurrent, and thus, the photocurrent is a measure of the optical transmission of PSi. This behavior allowed us to measure the PSi optical constants for various optical wavelengths and PSi porosities in situ and thus for the most pristine form of PSi and avoiding all the problems related to the drying and handling of high-porosity PSi free-standing layers1. We have also used the photoconduction to study the chemical etching of PSi in HF. We could monitor the etching as it proceeded and developed a model that allowed us to derive the HF-dependent etching rates and the absorption coefficient of PSi for a large range of porosities not explored so far, from as-prepared to close to 100% porosity (PSi fully dissolved)2.
A small degree of tuning of the photoluminescence (PL) band of PSi may be achieved by adjusting anodization conditions, but fine tuning remains difficult. A few attempts aiming at controlling the PL or understanding PSi PL in HF have been attempted by using post-anodization photo-assisted etching in HF. The photoetching process itself was not studied in much detail, and were focusing on as-formed luminescent PSi. A better understanding of the photoetching process would be welcome to get better control and understanding of the effects.
In this paper, we show how the photoetching of PSi (initially luminescent or not) can be monitored and its rate evaluated using the photoconduction technique that we have used previously in other studies1
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2. The photocurrent was continuously measured during the illumination of PSi by monochromatic light, which was inducing photoetching. As PSi was etched, the PSi absorption was decreasing, more light was transmitted to the substrate and, as a result, the photocurrent was increasing.
Figure 1 shows results for an initial porosity of 62%, an illumination wavelength of 450 nm, and different PSi thicknesses. A saturation state is reached when the PSi layer does not absorb anymore at the illumination wavelength. The waves observed for 1 μm and 2 μm-thick layers were attributed to thin film optical interference. The curve for 16 μm-thick PSi was affected by reproducibility problems because of instabilities in the diffusion of species and hydrogen bubbles inside the pores. For clean photoetching study, 4-μm thick PSi layers were used.
Figure 2 shows the time evolution of the photocurrent for 4 μm-thick PSi layers, with different illumination powers at 450 nm. As expected, the photocurrent at saturation is proportional to the incident illumination power. Solid lines are fits obtained using a model that will be presented. Two regimes were characterized, one in which the photoetch rate is limited by the supply of photo-generated holes at the Si surface, and another one where it is limited by the photoetching reaction. The maximum photoetch rate was derived. The evolution of the in depth porosity profile can be obtained from the model as well. The effects of a range of parameters will be presented.

1 B. Gelloz, H. Fuwa, and L. H. Jin, Ecs Journal of Solid State Science and Technology 5 (3), P190 (2016).

2 B. Gelloz, K. Ichimura, H. Fuwa, E. Kondoh, and L. Jin, Ecs Journal of Solid State Science and Technology 6 (1), R1 (2017).

Figure 1