(Invited) Porous Silicon Dissolution Monitoring and Optical Constants Measurement Using in Situ Photoconduction in HF
Authors: Gelloz Bernard , Ichimura Kazuki , Fuwa Hiroki , Kondoh Eiichi , Jin Lianhua 
Published: February 2020

Porous silicon (PSi) is typically prepared by electrochemical etching of silicon substrates in hydrofluoric acid (HF). With lightly-doped silicon substrates, sponge-like nano-structures may be achieved and the PSi layer thickness and porosity can be rather well-controlled. Such silicon nanostructures have potential applications in particular in optoelectronics, photovoltaics, medicine and sensing.
(1) Determination of PSi optical constants.
For some applications such as optical sensing or photonics, the knowledge of PSi optical constants is often necessary. Most methods used to determine the optical constants of PSi use dried samples, sophisticated setups, and meticulous sample preparation, which may alter the PSi structure.
We have shown that the optical constants of PSi can be measured very easily using in-situ photoconduction of PSi in HF during PSi formation. The liquid-PSi-Si substrate junction was illuminated from the liquid side and reverse-biased, allowing the observation of a photocurrent, resulting from the reduction of protons in solution. These charge exchanges do not affect PSi. We have confirmed that only the charge carriers photogenerated in the Si substrate contribute to the photocurrent. Thus, the measured photocurrent is proportional to the optical transmission through PSi. The absorption coefficient and the refractive index can be obtained by fitting photocurrent-PSi thickness curves.
Figure 1 shows an example of determination of the absorption coefficient α of PSi of porosity 68% for three indicated illumination wavelengths. In this particular case, the photocurrent was studied for large thickness’s, so as to have a good precision of the absorption measurement. For the measurement of the refractive index, the photocurrent must be analyzed for much smaller thickness’s so as to observe interference patterns.
This technique has several advantages over conventional ones. No particularly sophisticated sample preparation or handling is necessary, no high-precision optical instruments are necessary, while still preserving a very good structural state of PSi layers, even for high porosities and for arbitrary layer thicknesses. Moreover, only one sample, in a single experiment, is sufficient for the determination of the optical constants. [1]
(2) Monitoring PSi dissolution in HF
For some applications in medicine or sensing, pore engineering may be required. After PSi formation, pores can be further enlarged by using chemical dissolution, which can be achieved in some cases by dipping PSi in HF for a period of time. Dissolution of PSi is accompanied by an increase in PSi porosity. In order to be able to, for example, stop the dissolution when the desired porosity is achieved, a technique of monitoring of the dissolution would be welcome. Such monitoring of PSi dissolution has been difficult to achieve and is usually based on post-treatment inspection studies and calibration.
Here, we show that it can be realized using the evolution of the PSi optical transmission in-situ in HF using the photoconduction technique described in (1). The liquid-PSi-Si substrate junction was illuminated from the liquid side with a monochromatic light probe and reverse-biased, and the photocurrent was measured during dissolution.
Figure 2 shows an example using a 5 μm-thick PSi layer of porosity 68% in 5% HF and a 405 nm illumination probe. During dissolution, the photocurrent increases due to the decreasing optical absorption of PSi. When the whole PSi layer was dissolved, the photocurrent reached a plateau (here after about 2000 s), reflecting the fact that the PSi optical absorption has become zero.
The results for a wide range of different HF concentrations, PSi porosities, and PSi thickness’s will be presented. In addition to monitoring, the analysis of the photocurrent as a function of time may be used to derive the optical constants of PSi as a function of porosity, for the wavelengths used as illumination probes.
[1] B. Gelloz et al. ECS Journal of Solid State Science and Technology, 5(3) P190-P196 (2016)

Figure 1