Integration of plasmonic gratings into optoelectronic devices

ABSTRACT This thesis deals with the control of light absorption in semiconductor devices by the plasmonic resonances of periodically arranged metallic nanostructures integrated on them. Metallic gratings support propagating (SPP) and localized (LSP) plasmonic excitations and surface plasmons-related phenomena, like Extraordinary Optical Transmission (EOT) and plasmonic band gaps, as well as conventional diffraction effects. We combine all the optical resonances outlined to tune the incoupling and distribution of incident photons in the absorbing semiconductor substrate. In particular we consider the application of these concepts to two typologies of optoelectronic devices: photovoltaic solar cells and phototransistors. In the case of photovoltaic devices the objective is to increase the energy conversion efficiency by enhancing light harvesting and re-shaping the absorption profile, in order to improve the collection of photo-generated charge carriers. We begin analyzing a case study, a one-dimensional lamellar grating placed on a silicon substrate, by numerical optical simulations. The aim is to find the coupling conditions of the resonances supported, by designing the geometric parameters of the nanostructures, and showing their impact on the generation profile. These findings are then applied for light trapping purpose to two realistic solar cell layouts. SPP and LSP resonances are able to provide high near field magnification and effectively enhance the absorption of ultrathin organic solar cells. On the other hand, EOT coupled to diffraction orders are more suited to wafer-based Si cells. Then we present the fabrication process developed to realize the designed nanostructures over the large surface area of Si photovoltaic devices. By experiments and simulation we show that an improvement of Internal Quantum Efficiency can be obtained compared to unpatterned devices. Concerning the phototransistors, the aim is use them as compact and scalable biosensors by integrating a plasmonic crystal on the active area. By simulations the grating is designed to maximize transmittance variation due the plasmon resonance shift related to the surface binding of bio-analyte molecules. This event is transduced into an electrical signal at device terminals, as confirmed by characterizations on the first prototypes fabricated. The metallic grating simultaneously works as plasmonic structure and as electronic gate of the transistor in a fully integrated architecture.