Prof. Kentaro Imamura Profile

Prof. Kentaro Imamura

Assistant Professor at Osaka Univ

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Proceedings Article | 18 December 2018 Paper

Microstructure and optical properties of black silicon layers

Stanislav Jurečka, Martin Králik, Emil Pinčík, Kentaro Imamura, Taketoshi Matsumoto, Hikaru Kobayashi

Proceedings Volume 10976, 109760I (2018) https://doi.org/10.1117/12.2518368

KEYWORDS: Silicon, Reflectivity, Etching, Transmission electron microscopy, Refraction, Optical properties, Spectral models, Dielectrics, Platinum, Optical analysis

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Black silicon structures were formed by etching of silicon substrates based on the surface structure chemical transfer method. Formed structures show gradient of material density in the nanocrystalline Si layer leading to ultralow spectral reflectance below 3% in wide spectral region. In study of the development of microstructure properties during the forming procedure the TEM images were used. Information abut the microstructure observed in the TEM images was analysed by the Abbott-Firestone method. By using this approach limiting conditions for the black silicon layer formation were obtained. Spectral reflectances of studied samples were modelled by using the effective medium theory. Multilayer theoretical model based on splitting the black silicon layer into 20 sublayers was constructed. Optical properties of each individual sublayer were described by using Bruggeman effective media theory combining Si, SiO₂ and void fractions. Gradual development of real and imaginary part of complex index of refraction was observed in the volume of black silicon layers. Results of optical analysis correspond to the microstructure development during sample forming.

Proceedings Article | 1 December 2017 Paper

Reflectance analysis of porosity gradient in nanostructured silicon layers

Stanislav Jurečka, Kentaro Imamura, Taketoshi Matsumoto, Hikaru Kobayashi

Proceedings Volume 10603, 106031B (2017) https://doi.org/10.1117/12.2292723

KEYWORDS: Reflectivity, Nanostructuring, Silicon

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In this work we study optical properties of nanostructured layers formed on silicon surface. Nanostructured layers on Si are formed in order to reach high suppression of the light reflectance. Low spectral reflectance is important for improvement of the conversion efficiency of solar cells and for other optoelectronic applications. Effective method of forming nanostructured layers with ultralow reflectance in a broad interval of wavelengths is in our approach based on metal assisted etching of Si. Si surface immersed in HF and H2O2 solution is etched in contact with the Pt mesh roller and the structure of the mesh is transferred on the etched surface. During this etching procedure the layer density evolves gradually and the spectral reflectance decreases exponentially with the depth in porous layer. We analyzed properties of the layer porosity by incorporating the porosity gradient into construction of the layer spectral reflectance theoretical model. Analyzed layer is splitted into 20 sublayers in our approach. Complex dielectric function in each sublayer is computed by using Bruggeman effective media theory and the theoretical spectral reflectance of modelled multilayer system is computed by using Abeles matrix formalism. Porosity gradient is extracted from the theoretical reflectance model optimized in comparison to the experimental values. Resulting values of the structure porosity development provide important information for optimization of the technological treatment operations.

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