Chapter 2. Transparent crystalline silicon substrates
2.4 Fabrication of microhole arrays to develop transparent crystalline silicon substrates
2.4.3 Fabrication of the transparent crystalline silicon substrate via wet etching process -38
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Third, the metal catalyst-coated crystalline silicon substrate is immersed in a mixed solution composed of hydrofluoric acid and an oxidant solution such as hydrogen peroxide (Figure 2.21). Then, the oxidant reduction on the surface of the metal catalyst generates electrical holes (h+), which subsequently leads to the dissolution of the crystalline silicon surrounding the catalyst by hydrofluoric acid. By this step, the transparent crystalline silicon substrates could be fabricated by employing metal-assisted chemical etching only where the catalyst metal is formed.
In order to fabricate a crystalline silicon microhole using metal-assisted chemical etching with a diameter of over 90 μm, 30-nm-thick metal catalyst (Au) was deposited on the particular area and then immersed in a 10 M hydrofluoric acid, 0.3 M hydrogen peroxide solution. However, as shown in Figure 2.22, almost no etching occurred until 4 hours. After that, only a very small amount of etching with a less than 20 μm was observed only when the etching time was 6 hours. This is because it is difficult for the etching solution to sufficiently penetrate between the metal catalyst and the crystalline silicon.
Therefore, we form the metal catalyst for nanoparticle shape, as shown in case 2 of Figure 2.23. Silver (Ag) nanoparticles as a metal catalyst were deposited uniformly on the crystalline silicon wafer via a galvanic displacement reaction for 120 seconds in an aqueous solution of hydrofluoric acid (4.8 M) and Silver nitrate (0.01 M). After the deposition of the silver nanoparticles, the crystalline silicon wafer was immersed in a mixed solution of hydrofluoric acid (4.8 M) and hydrogen peroxide (0.44 M). As a result, it was confirmed that etching occurred at an etching rate of around 30 μm/hour, and when 6-hour etching, over 180 μm was etched (Figure 2.24). After the etching, the sample was rinsed using deionized water and dried at room temperature. The silver nanoparticle residue was removed using dilute nitric acid (volume ratio of 1:1) for 30 min.
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Figure 2.20 The ratio of expected total fabrication cost for the transparent c-Si solar cells (Source:
http://pvinsights.com, Energy Procedia, 130, 43-49. (2017), Energy Environ, Sci., 5, 5874-5883. (2012), Microsyst Technol., 13, 85-95. (2007), Principles of Lithography, Second Edition (2005))
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Figure 2.21 The schematic illustration of the metal-assisted chemical etching process
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Figure 2.22 SEM images of the etched silicon from 1-hour etching to 6-hour etching (metal catalyst film)
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Figure 2.23 Schematic illustration of the possible model for metal-assisted chemical etching to fabricate transparent crystalline silicon substrates. Reprinted from Um et al.41 CC BY-NC-ND 4.0
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Figure 2.24 SEM images of the etched silicon from 1-hour etching to 6-hour etching (metal nanoparticle catalyst)
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Figure 2.25 (A) Photograph of the transparent crystalline silicon substrates fabricated via metal-assisted chemical etching, (B) tilted SEM image of the transparent crystalline silicon substrate, (C) Cross- sectional SEM image of the transparent crystalline silicon substrate.
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2.4.4 Optical properties of transparent crystalline silicon substrates
The thickness of a commercial crystalline silicon wafer is around 150-200 μm, and visible range light cannot be transmitted. On the other hand, the developed transparent crystalline silicon substrate shows perfect transparency like that of glass (Figure 2.26). This is because the light is transmitted through the microhole shaped light transmission windows, especially the visible wavelength region (380-780 nm). This can also be confirmed once more clearly through the transparent spectra of Figure 2.27. In addition, through the transmittance spectra, it is possible to verify that 100% of the visible region's light is transmitted through the light transmission windows, which means that this substrate shows a neutral color. On the other hand, In the case of the light absorption area, because the thickness of this region is thick, the incident light cannot penetrate this region, and light could be absorbed sufficiently. Therefore, the developed transparent crystalline silicon substrate can solve low light absorption problems in the long-wavelength region, which is one of the limitations of transparent substrates that have been mainly developed using thin-film technology.
We checked not only the transmittance properties of the transparent crystalline substrate but also the the transparent silicon substrate’s transmission haze ratio. The transparent crystalline silicon substrate also suffers the things behind it to be clearly visible without haziness. The reflected light from the items is not diffused during penetration through the silicon substrate. To calculate the transmission haze ratio, it is necessary to measure total transmittance (diffuse transmittance + specular transmittance) and diffuse transmittance. We measure the total transmittance and diffuse transmittance using a UV- Vis/NIR spectrophotometer. At this time, we use the equipment, which is a 110 nm integrating sphere, to account for total light. Total light includes diffused transmitted light and specularly transmitted light.
We calculated the transmission haze ratio of the transparent crystalline silicon substrate using Equation 1.5. The results clearly show that the transmission haze ratio of the transparent crystalline silicon substrate is 0.95%, like that of glass. The haze ratio of glass is around 0.89% (Figure 2.28).
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Figure 2.26 (Left) Photograph of the conventional polished crystalline silicon wafer with a thickness of 200 µm (Right) and the neutral-colored transparent crystalline silicon substrate with a thickness of 200 µm
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Figure 2.27 SEM image and transmittance spectra of the transparent crystalline silicon substrate.
Reprinted from Lee et al.14 Copyright 2020, Elsevier.
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Figure 2.28 Transmission haze ratio of the transparent crystalline silicon substrate and a glass substrate.
Reprinted from Lee et al.14 Copyright 2020, Elsevier.
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