(Invited) In-Situ Monitoring of Luminescence and Oxidation of Porous Silicon in Liquid Electrolytes with Photoconduction
Authors: Gelloz Bernard , Souki Sakata , Jin Lianhua 
Published: December 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 optical absorption, refractive index and surface area. It has potential applications in optoelectronics, photovoltaics, medicine and sensing.
Recently, we have made use of the particular behavior of PSi photoconduction in an electrolyte: when PSi is illuminated, 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 in HF 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. Finally, we have also made use of the photoconduction technique to understand the photoetching of PSi in HF by monochromatic light.3

In this paper, we use the photoconduction technique to monitor the evolution of different types of oxidation processes of PSi in acid solutions. The photoluminescence is also acquired in-situ. The different oxidation processes studied here are: chemical oxidation in different solutions, photo-assisted oxidation by monochromatic light, and electrochemical oxidation. The photoconduction technique allowed for a good understanding of the oxidation progress and the evolution of the oxidation rates over reaction time. Moreover, the monitoring of the photoluminescence, coupled to the in-situ derivation of the optical absorption provides a way to optimize the reaction time for achieving maximum luminescence efficiency.
An example is shown in Fig. 1. It shows the photocurrent (at 405 nm illumination)-time plot for a 5-μm thick PSi layer of initial porosity 68% oxidized in H2SO4 0.5M. The photocurrent, signature of the optical absorption, increases as the oxidation progresses since the formed oxide is transparent at the 405 nm light. Here, the oxidation does not proceed up to full PSi oxidation, therefore the photocurrent does not reach full saturation. The photoluminescence was also recorded and reached its maximum at ~ 65 h (blue star on the plot).

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).

3 B. Gelloz, H. Fuwa, E. Kondoh, and L. Jin, Ecs Journal of Solid State Science and Technology 7 (12), P730 (2018).

Figure 1