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NDIR CO 2 gas sensors using a 10-cm-long gas channel and CMOS-compatible 12% doped ScAlN pyroelectric detector have previously demonstrated detection limits down to 25 ppm and fast response time of ∼2 s. Here, we increase the doping concentration of Sc to 20% in our ScAlN-based pyroelectric detector and miniaturize the gas channel by ∼65× volume with length reduction from 10 to 4 cm and diameter reduction from 5 to 1 mm. The CMOS-compatible 20% ScAlN-based pyroelectric detectors are fabricated over 8-in. wafers, allowing cost reduction leveraging on semiconductor manufacturing. Cross-sectional TEM images show the presence of abnormally oriented grains in the 20% ScAlN sensing layer in the pyroelectric detector stack. Optically, the absorption spectrum of the pyroelectric detector stack across the mid-infrared wavelength region shows ∼50% absorption at the CO 2 absorption wavelength of 4.26 μm. The pyroelectric coefficient of these 20% ScAlN with abnormally oriented grains shows, in general, a higher value compared to that for 12% ScAlN. While keeping the temperature variation constant at 2 °C, we note that the pyroelectric coefficient seems to increase with background temperature. CO 2 gas responses are measured for 20% ScAlN-based pyroelectric detectors in both 10-cm-long and 4-cm-long gas channels, respectively. The results show that for the miniaturized CO 2 gas sensor, we are able to measure the gas response from 5000 ppm down to 100 ppm of CO 2 gas concentration with CO 2 gas response time of ∼5 s, sufficient for practical applications as the average outdoor CO 2 level is ∼400 ppm. The selectivity of this miniaturized CO 2 gas sensor is also tested by mixing CO 2 with nitrogen and 49% sulfur hexafluoride, respectively. The results show high selectivity to CO 2 with nitrogen and 49% sulfur hexafluoride each causing a minimum ∼0.39% and ∼0.36% signal voltage change, respectively. These results bring promise to compact and miniature low cost CO 2 gas sensors based on pyroelectric detectors, which could possibly be integrated with consumer electronics for real-time air quality monitoring.

Silicon nitride (SiN) has surged into prominence as a material for photonic-integrated circuits (PICs) in the past decade, well regarded for its broadband transparency, compatibility with complementary metal oxide semiconductor (CMOS) fabrication processes and high optical bandgap that avoids two-photon absorption. However, current fabrication methods result in users having to choose between low thermal budgets and low losses, which are suboptimal given that both are necessary to facilitate a wide range of applications. Recently, works have emerged featuring PICs fabricated using deuterated silicon nitride (SiNx:D) – SiNx films grown using deuterated precursors instead of conventional hydrogenated ones. This decreases material absorption near the telecommunications bands at 1.55 µm previously present due to parasitic silicon–hydrogen and nitrogen–hydrogen bonds, attaining low-loss PICs realised using a low temperature, back-end-of-line CMOS-compatible fabrication plasma-enhanced chemical vapour deposition process. These devices have shown promise for both linear and nonlinear applications and the platform has the potential to be instrumental in realising highly efficient chips with co-packaged electronics and photonics devices. This paper reviews recent developments on the SiNx:D platform and provides a glance at future advancements for this highly promising material.