Schottky Au-InSb Plasmonic Photodiode for Greenhouse Gas Detection

Precise detection of greenhouse gases (GHGs) is crucial for understanding their emission trends and developing mitigation strategies. The narrow bandgap of indium antimonide (InSb) allows it to be very responsive in the mid-wave infrared (MWIR) wavelength range (3-5 µm) and ideal for detecting gases like carbon dioxide (CO₂) and methane (CH₄), which have absorption peaks at 4.2 µm and 3.3 µm, respectively. Plasmonic metasurfaces offer a promising route to enhance the performance of InSb photodiodes by increasing their sensitivity, efficiency, and spectral tunability. We propose a ‘GHG plasmonic meta-absorber’ device that specifically targets fingerprint absorption peaks of CO₂ and CH_4. Our opto-electrical co-simulations show that a Au plasmonic grating over a sub-micron InSb film can be designed to achieve unity absorption at the target wavelengths with 6-fold and 10-fold improvement in photocurrent at 3.3 µm and 4.2 µm wavelengths, respectively. Moreover, even when the Au grating does not realise a metasurface, our numerical results still show photocurrent enhancement as long as the grating Au electrode separation is less than the minority carrier diffusion length (10 µm), attributed to collection of more photoexcited carriers. Thus, the proposed plasmonic grating serves a dual purpose - Absorption enhancement of MWIR light inside the InSb layer; and collection of more current through photoexcited carriers via multiple contacts. Thus, the superior opto-electrical performance of the proposed device offers a transformative approach to precise GHG detection, combining advanced materials with plasmonic engineering.