Optical Property of Porous Silicon Produced by Metal-Assisted Etching

A number of ideas have been applied to improve the conversion efficiency of crystalline silicon (Si) solar cells. Antireflection of Si surface is an important technology to increase the photocurrent density of solar cells. Both random pyramid structures produced by anisotropic etching of crystalline Si wafers and antireflection coatings which have lower refractive index than Si are generally used for commercial solar cells. Common antireflection coatings using Si nitride have such problems as high manufacturing cost and limited antireflection effect only in a specific range of wavelength. Previously, antireflection coatings of porous Si formed by stain etching using a mixture solution of hydrofluoric acid and nitric acid or by anodic etching were reported^{1, 2}. This method can control the refractive index by changing the structure of the porous Si, so it can decrease the reflection of Si surface in a wide wavelength range. We reported that macro porous Si formed by a metal assisted etching, which is immersing metal catalyst modified Si in a hydrofluoric acid (HF) aqueous solution, can be used as a texture layer for antireflection of solar cells³. The macro porous layers consisting of pores in µm-size are expected to damage p-n junction and reduce the conversion efficiency of solar cells. In this study, we investigated the optical properties of porous Si thin films, which were thinner than p-n junction.

Single-crystalline n-Si wafers (CZ, (100), ca. 10 Ω　cm)　were used as substrates. The Si wafers were immersed in 1 mM (M = mol dm^-3) silver nitrate (AgNO₃) solution including 0.15 M HF at 278 K for electroless displacement metal deposition. Metal deposited Si wafers were immersed in a 6.6 M HF solution including hydrogen peroxide (H₂O₂) at 298 K for 1 ~ 60 s. The samples after etching were immersed in a 13 M nitric acid and then 7.3 M HF to remove silver nanoparticles remaining in the bottom of the pores and the Si oxide film, respectively. A multi-photometric system (Otsuka Electronics, MCPD-7000KA) with an integrating sphere was used to measure reflectance of Si surface. A spectroscopic ellipsometer (Otsuka Electronics, FE-5000) was used to analyze the optical constants of the porous Si thin film.

Fig. 1 shows that an uniform porous Si thin film consisting of straight pores, few tens nm in diameter and 250 nm in length, was formed by the etching for 15 s. The structure of porous Si was changed with HF and H₂O₂ concentrations of the etching solution and etching time. The pore size was consistent with the size of Ag nanoparticles as shown in Fig. 1. Thus, the porosity of porous Si can be evaluate as the coverage of Ag nanoparticles on Si before etching. The coverage, that is porosity, can be controlled by deposition time of Ag. The reflection of porous Si surface was much lower than mirror polished Si wafer before metal assisted etching (Fig. 2). As the porosity increased, interference waveform in the reflectance spectra appeared and the peak interval of reflectance spectra decreased. On the other hand, it was possible to control the thickness of the porous Si thin film by changing the etching time. As porous layer thickness increased, the reflectance decreased and the amplitude of the interference waveform decreased. The reflectance reached lower than 5.5%, which is much lower than 10% of a pyramidal structure, in the wavelength range of 400-1000 nm. From these results, we assumed that the porous Si functions as an optical thin film of a smooth single-layer. The refractive index of porous Si thin film at the porosity of 33 ~ 56% was observed as 1.1 ~ 2.2. The attenuation factor (factor that affects light absorption) of porous Si thin film was ~100 times higher than that of crystalline Si. The thin porous Si layers produced by the metal assisted etching are expected to work as suitable antireflection films for crystalline Si solar cells.

ACKNOWLEDGEMENT

The present work was partly supported by JSPS KAKENHI (26289276). The authers wish to express their thanks to Otsuka Elecronics Co., LTD for ellipsometric analysis.

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