Composite Pyramid Structures for Localized Surface Plasmon Resonance Enhancement in Silicon-Based Mid-Wave Infrared Photodetectors

Silicon-based photodetectors are intrinsically limited in the mid-wave infrared region by the Si bandgap and the Schottky barrier. To overcome these constraints, we employed a CMOS-compatible inverted pyramid structure (IPS) that supports localized surface plasmon resonances (LSPR) and further introduced a composite pyramid formed by embedding an upright pyramid within the IPS. Three device architectures were fabricated and compared: a planar structure, an IPS, and the composite pyramid. At zero external bias under 3.46 μm illumination, the composite pyramid exhibited a responsivity of 17.4 μA/W, corresponding to enhancements of 55.9 times and 266 times relative to the IPS and planar devices, respectively. Notably, at 10 μm, the composite device still outperformed, with a responsivity 75 times higher than that of the IPS, while the planar structure yielded no detectable signal. COMSOL simulations confirmed that the composite design significantly enhances the local electric field, yielding a local optical intensity 1.54 × 10¹¹ times and 120 times higher than that of the planar device and the IPS at 3 μm, respectively, at 10 μm, the intensity remains 253 times higher than the IPS. These improvements are attributed to the composite configuration, which concentrates and reinforces the resonant electric fields, resulting in a stronger and more confined localized field distribution that sustains robust LSPR at longer infrared wavelengths. Consequently, optical absorption and hot-carrier generation are substantially improved, significantly boosting MWIR responsivity and overall optoelectronic performance. This composite design thus provides a promising pathway for efficient, CMOS-compatible silicon-based photodetectors operating in the mid-infrared region.