*Article* **Thin SiNC**/**SiOC Coatings with a Gradient of Refractive Index Deposited from Organosilicon Precursor**

#### **Hieronim Szymanowski 1, Katarzyna Olesko 1, Jacek Kowalski 1, Mateusz Fijalkowski 2, Maciej Gazicki-Lipman 1 and Anna Sobczyk-Guzenda 1,\***


Received: 13 July 2020; Accepted: 11 August 2020; Published: 17 August 2020

**Abstract:** In this work, optical coatings with a gradient of the refractive index are described. Its aim was to deposit, using the RF PECVD method, films of variable composition (ranging from silicon carbon-oxide to silicon carbon-nitride) for a smooth change of their optical properties enabling a production of the filter with a refractive index gradient. For that purpose, two organosilicon compounds, namely tetramethyldisilazane and hexamethyldisilazane, were selected as precursor compounds. The results reveal better optical properties of the materials obtained from the latter source. Depending on whether deposited in pure oxygen atmosphere or under conditions of pure nitrogen, the refractive index of the coatings amounted to 1.65 and to 2.22, respectively. By using a variable composition N2/O2 gas mixture, coatings of intermediate magnitudes of "n" were acquired. The optical properties were investigated using both UV-Vis absorption spectroscopy and variable angle spectroscopic ellipsometry. The chemical structure of the coatings was studied with the help of Fourier transform infrared and X-ray photoelectron spectroscopies. Finally, atomic force microscopy was applied to examine their surface topography. As the last step, a "cold mirror" type interference filter with a gradient of refractive index was designed and manufactured.

**Keywords:** thin films; silicon carbon-nitride; silicone carbon-oxide; PECVD method; inhomogeneous optical filters; gradient interference filters; organosilicon precursors
