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. 2019 Sep 10;9:12988. doi: 10.1038/s41598-019-49324-5

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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SEM pictures of the device. (a) SEM top view of the grating coupler. (b) SEM cross-section of the stack of 18 layers. The insert shows the zoom-in image including 10 layers. The bright and dark strips are SiN_x and SiO₂ layers, respectively.