Plasmonically Enhancing 2D WSe₂ Light Absorption for Photodetection

2D material semiconductors, specifically transition metal dichalcogenides (TMDs), have gained attention in recent years due to their unique and tunable electronic and optical properties at the monolayer and few-layer thicknesses. Because these materials are thin, low loss, and lightweight, TMDs such as WSe₂ are good candidates for flexible photodetector technology. However, unlike traditional semiconductors, TMD films cannot simply be made thicker to increase their absorptivity because their spectral response is a function of their thickness. Surface plasmonics are a nanoscale, size-tunable solution for enhancing light absorption in certain materials. By incorporating a periodic pattern consisting of nanoplasmonic structures on top of WSe₂ and tuning its design parameters, the performance of a WSe₂ photodetector can be greatly improved at targeted wavelengths.<br/>In this work, Finite Element Method (FEM) simulations are first performed in CST Microwave Studio to design gold nanoellipse arrays as enhancement structures for 2D WSe₂ photodetectors. We simulate the coupling between the WSe₂ layer and the localized surface plasmon resonances of the patterned metal layer, and we show that the WSe₂ thickness (number of WSe₂ monolayers) strongly affects the behavior of these plasmonic resonances. By varying the Au nanoellipse size and lattice spacing in the plasmonic layer, we optimize the WSe₂ absorption enhancement at a targeted wavelength of 690 nm. We demonstrate an absolute theoretical WSe₂ absorption of 26% in a 5-layer WSe₂ film, which is over 3 times greater than the WSe₂ absorption without the plasmonic structures.<br/>In addition to investigating traditional nanoellipses in our simulations, we also investigate their inverses, elliptical nanohole arrays. At a wavelength of 690 nm, we show that inverted nanohole arrays achieve a theoretical WSe₂ absorption of 24%, which is comparable to the optical enhancement achieved by the inverse nanoellipse design. These designs can be directly incorporated into the interdigitated electrodes of WSe₂ photoconductive photodetectors. This multifunctional electrode strategy could allow for more efficient charge extraction due to the generation of hot carriers associated with plasmonic excitation precisely at the charge extraction point, causing increased photocurrent.<br/>We use our simulated designs to fabricate WSe₂ photoconductive photodetectors enhanced by plasmonic nanoellipse and nanohole arrays, and we investigate the impact of these designs on the light absorption, photoresponsivity and sensitivity in the devices. We fabricate devices consisting of Au nanoplasmonic arrays on epitaxially-grown WSe₂ using electron-beam lithography, and we characterize the photodetectors’ photocurrent response and polarization dependence at 690 nm.<br/>We demonstrate designs for plasmonic Au nanoellipse and nanohole arrays and calculate theoretical enhancement factors of up to 3 for absorption in 2D WSe₂. We use electron beam lithography to fabricate plasmonically-enhanced 2D WSe₂ photoconductive photodetectors and investigate these designs experimentally. These results should advance the field of next-generation sensors for flexible and wearable applications, and could be further applied to optical communications, photovoltaic, and environmental sensing technologies.