NH₃-free PECVD silicon nitride for photonic applications

Abstract

Silicon Photonics has open the possibility of developing multilayer platforms based on complementary metal-oxide semiconductors compatible materials that have the potential to provide the density of integration required to fabricate complex photonic circuits. Amongst these materials, silicon nitride (SiN) has drawn attention due to its fabrication flexibility and advantageous intrinsic properties that can be tailored to fulfil the requirements of different linear and non-linear photonic applications covering the ultra-violet to mid-infrared wavelengths. Yet, the fabrication techniques typically used to grow SiN layers rely on processing temperatures > 400 C to obtain low propagation losses, which deem them inappropriate for multilayer integration. This thesis presents a systematic investigation that provided a comprehensive knowledge of a deposition method based on an NH₃-free plasma enhanced chemical vapour deposition recipe that allows the fabrication of low-loss silicon nitride layers at temperatures < 400 C. The results of this study showed that the properties of the studied SiN layers depend mostly on their N/Si ratio, which is in fact one of the only properties that can be directly tuned with the deposition parameters. These observations provided a framework to optimise the propagation losses and optical properties of the layers in order to develop three platforms intended for specific photonic applications. The first one comprises 300nm stoichiometric SiN layers with refractive index (n) of 2 that enable the fabrication of photonic devices with propagation losses < 1 dB/cm at l = 1310nm and < 1:5 dB/cm at l = 1550 nm, which are good for applications that require efficient routing of optical signals. The second one consists on 600nm N-rich layers (n = 1.92) that allow fabricating both devices with propagation losses < 1 dB/cm at l = 1310 nm, apt for polarisation independent operation and coarse wavelength division multiplexing devices with cross-talk < 20 dB and low insertion losses. Finally, the last platform consisted of suspended Si-rich layers (n = 2.54) that permits the demonstration of photonic crystal cavities with Q factors as high as 122 000 and photonic crystal waveguides capable of operating in the slow-light regime. Hopefully, the demonstration of these platforms will stimulate the development of more complex SiN devices for multilayer routing, wavelength division multiplexing applications and non-linear integrated photonics in the future.

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Identifiers

ORCID for Thalía Domínguez Bucio: orcid.org/0000-0002-3664-1403

ORCID for Goran Mashanovich: orcid.org/0000-0003-2954-5138

ORCID for Frederic Gardes: orcid.org/0000-0003-1400-3272

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