Plasmon-enhanced Hot Electron Effect And Photodetection Application

The plasmon enhanced hot electron effect is a phenomenon in which free electrons are excited to become hot electrons during surface plasmon relaxation inside a metal.The hot electrons have higher energy than the equilibrium electrons after being excited.This phenomenon can be utilized for photohydrolysis,photochemical catalysis,photoelectric conversion,etc.In photoelectric conversion,plasmon excited hot electrons can travel through the Schottky barrier at the metal-semiconductor contact interface,enter the semiconductor conduction band,and become a detectable photocurrent.The Schottky barrier height is generally lower than the semiconductor’s bandgap.Therefore,hot electron photoelectric conversion can be used to detect photons with energy lower than the semiconductor’s bandgap energy and has attracted widespread attention.Starting from the design of the plasmonic structures,this dissertation theoretically simulates the mutual coupling of different plasmon modes in two plasmonic structures and their application in hot electron optoelectronic devices.Three kinds of surface plasmon enhanced hot electron detector devices were studied by using the method of polystyrene（PS）nanosphere self-assembly and annealing process to prepare metal nanoparticles,and the silicon-based hot electron photoelectric conversion from visible to infrared band was realized.The specific research contents are as follows:（1）A structure of plasmonic cavity array for enhanced hot-electron photodetection is investigated by using FDTD simulation and the analytical probability-based electrical model.Apart from the Bragg-SPP mode in the periodic metal structure,there exist the Au-air and Au-Si CSP modes apart from the Bragg-SPP mode in the periodic metal structure.The Bragg-SPP and Au-air CSP modes are anticrossing at strong coupling,whereas the Au-Si CSP modes allow crossing with the other two types of modes.Hybrid modes formed with combination of these modes can strongly enhance absorption in metal and be exploited to design hot-electron photodetectors.With ease of fabrication,the structure can be adjusted to optimize photoresponse in single or dual bands of optical communication.（2）A plasmonic absorber for sub-bandgap photodetection based on the Au-fishnet/Ti O₂ spacer/Ag mirror structure is proposed,in which Fabry-perot resonance and localized surface plasmon can be excited to enhance hot electron generation.The absorbance is polarization insensitive and can reach up to 0.8-0.97.The peak wavelengths can be tuned by adjusting the Ti O₂ thickness and the width of the lines in the fishnet.Almost all absorbed photons are concentrated on the top Au fishnet layer,which facilitates the transmission and emission of hot electrons over the Schottky barrier.The theoretical responsivity and theoretical external quantum efficiency can approach 5m A/W and～1%,respectively,at the wavelength of 700 nm.（3）A silicon-based plasmon-induced hot-electron photodetector device that can operate in the near-infrared region is proposed and experimentally demonstrated.The nanopillar array structure was economically fabricated by using a self-assembled PS nanosphere monolayer with the hexagonal close-packed lattice as a mask for reactive ion etching.The photon absorption occurs mainly at sidewalls due to nonradiative dissipation of surface plasmon polaritons,thereby presenting a uniform spectrum in the wavelength detection.The curve shape of the responsivity is thus dominated by the wavelength-dependent internal quantum efficiency and is similar for devices fabricated with different conditions.At 1310 nm,a maximum responsivity of 1.8 m A/W was experimentally demonstrated,and the photoresponse wavelength range could be extended beyond 1500 nm.Such nanopillar array hot-electron infrared detector can be readily integrated in silicon photonics,and is suitable for low-cost mass production as it does not rely on expensive nanofabrication techniques.（4）A robust and novel architecture for visible-infrared photodetection based on hot electron injection from a plasmonic nanocone array is presented,formed by self-assembled PS nanosphere lithography.The device is based on a silica template which allows compatible fabrication with other Si-based devices.The photon absorption is mainly contributed by the Au layer at sidewalls of nanocones.This absorption is not due to local surface plasmon excitation but due to SPP at the Au-air interface.Responsivity under the short-circuit condition shows a peak of 180μA/W at about 620 nm,denoting that a significant fraction of excited carriers can be collected.Under reverse bias operation,the responsivity shows a tendency to increase with bias and red shift in peak value due to the tunneling effect.（5）An Au/Si Schottky diode on SOI substrate for hot electron photodetection is investigated.The Fabry-Perot resonance enhances the plasmon induced hot electron emission and extends the photo response to photon energies less than the bandgap energy of Si.The photo response is influenced by the morphology of the Au nanostructure which is controlled via annealing duration,with the highest photo response observed in the device with the Au film annealed for 90 s.This Schottky diode formed on SOI substrate is compatible with Si-based photonics and provides a solution for extending infrared detection at wavelengths beyond 1100 nm set by the band gap of Si.