Deuterated Silicon-Rich Nitride (SRN:D): Design, Fabrication, and Applications in Nonlinear Integrated Optics

Xavier Xujie Chia

Silicon-nitride (SiN) has been established as a dominant material in nonlinear optics over the past decade. As a material with negligible nonlinear losses, modest nonlinearities and unrivalled propagation losses across the various integrated platforms, SiN has become an important cornerstone in the field. To enhance the efficiency of nonlinear processes, engineering an increase in the Si-N ratio beyond stoichiometric ratios to form Silicon-Rich Nitride (SRN) has also been a subject of intense interest in the silicon-photonics community. The increased silicon content augments the refractive index of the film, which also leads to a sharp increase in the film nonlinearity by virtue of Miller’s rule. The improved index contrast further confines the propagating modes to a smaller modal area, which – in tandem with improvements to the bulk film nonlinearity – results in significant improvements to the effective nonlinearity of the waveguides. In our group, the use of ultra-silicon rich nitride (USRN) has been highly effective in the demonstration of novel nonlinear effects that would be otherwise difficult to achieve in devices with lower nonlinearities. However, the use of silane (SiH₄) and ammonia gas (NH₃) in chemical-vapor deposition-based methods used to deposit SiNₓ (a term used to refer to stoichiometric and non-stoichiometric silicon nitride in general) films results in the incorporation of Si-H and N-H bonds. This leads to the presence of absorptive losses in the telecommunications bands near 1.55 μm, the elimination of which generally requires the use of high-temperature annealing methods that render the process incompatible with backend-complementary metal-oxide semiconductor (CMOS) processes due to the exceeding of the thermal budget (typically < 450°C). In this work, we leverage the use of deuterated silane (SiD₄) and nitrogen gas (N₂) to preserve the backend-CMOS compatibility of the deposition process of SRN films. We aim to develop SRN devices with low losses absent of Si-H and N-H bond related losses while retaining a low thermal budget and optical nonlinearity that is augmented compared to stoichiometric silicon nitride. We perform a first-cut analysis of the efficacy of these methods towards SRN chemistries by depositing SRN:D films with various gas ratios and characterizing their optical properties. Linear and nonlinear characterisation of the device properties shows a refractive index of 2.46 at 1550 nm and a sharply increased device nonlinearity of 95 W⁻¹m⁻¹, almost 2 orders of magnitude higher than that typically observed in stoichiometric SiN waveguides. When compared to a SiN:H film deposited using the same recipe, the SiN:D films show a ~2.5 dB/cm decrease in waveguide propagation losses, evidencing the effectiveness of H-bond elimination towards realising low-loss SRN devices. Subsequently, the tuneability of the film composition is evidenced by the deposition of a film with a different gas ratio, showing a variation in the refractive index and nonlinearity of the film. Ring resonators with intrinsic quality factors as high as 126,000 are demonstrated, and a conversion efficiency of -34 dB is achieved with a pump power of 4 mW over a signal-idler separation of 20 nm. This achievement underscores the potential of the SRN:D devices towards low-loss nonlinear optics as the high nonlinearity and quality factor of the devices work in tandem to manifest highly efficient nonlinear effects. Lastly, we leverage our experience with the ultra-silicon-rich nitride platform to create USRN:D, a highly nonlinear platform with a refractive index of 2.96. Using state-of-the-art fabrication techniques, we realise ring resonators with quality factors as high as 120,000, a 300% improvement over similar devices fabricated on the USRN:H platform. Devices present with a high nonlinearity of 321 W⁻¹m⁻¹, corresponding to a film nonlinearity of 2.37 × 10⁻¹⁸ m²/W, comparable to USRN:H. The improvements to the loss characteristics and high nonlinearity are harnessed through the realisation of a FWM wavelength conversion efficiency of -24 dB on a 1 cm long waveguide.

Deuterated Silicon-Rich Nitride (SRN:D): Design, Fabrication, and Applications in Nonlinear Integrated Optics

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