Designing nanoparticle based plasmonically enhanced hot electron devices

Abstract

Plasmonically enhanced hot carrier (PEH) photodetectors are a new class of optoelectronic device based on the collection of energetic carriers generated by the decay of plasmonic resonances. Optical absorption, hot carrier generation, and hot carrier injection are the main processes that successively occur in PEH devices to enable light detection. So far, the efficiencies of such devices have been limited by low absorption in metal nanostructures, low hot electron generation rates, and/or poor hot electron injection rates over Schottky junction formed by the metal-semiconductor interface.

In this thesis, I investigate the electrical and optical properties of nanoparticle-based PEH devices, with the goal of developing design criteria for improved functionalities. I focus on two important design considerations: tuning the Schottky barrier height (SBH) to increase collection efficiency of hot electrons, and achieving strong and tuneable absorption in gold nanoparticles (AuNPs) to enhance both the absorption and the generation efficiency of hot electrons.

Firstly, I experimentally investigate the role of the SBH formed at the metal-semiconductor interface and its effect on the performance of PEH photodiodes. By studying the band alignment at the gold-TiO2 interface, I show that adjusting the deposition temperature of TiO2 changes the energy bands of the semiconductor, and hence the SBH. Further, I show that lowering the SBH increases the injection efficiency of hot electrons over the junction, indicating that this is an important design consideration for the improvement of PEH devices.

Secondly, I investigate optical methods to enhance and control the localized surface plasmon resonances (LSPRs) in absorption-dominated NPs. I focus on random NP arrays that are fabricated using inexpensive and large-scale fabrication techniques so that the structures can be directly integrated with photodetectors. I propose an NP array-cavity structure and theoretically investigate the coupling properties. I find that the coupling strength between LSPRs and Fabry-Perot (FP) modes is determined by the density of NPs, and the spectral and spatial overlaps between the resonances. This research demonstrates that strong absorption resonances in AuNPs can be controlled by a rational and facile design of the fabrication parameters, leading to PEH devices with tuneable wavelength selectivity.

Coupled NP array-cavity structures are then exploited in conjunction with exotic metamaterials to further tune absorption in NPs for enhanced functionality in PEH devices. Hyperbolic metamaterials (HMM) exhibiting epsilon-near-zero (ENZ) behaviour, are investigated as rear reflectors in optical cavities, to control the coupling between of AuNPs and FP modes. I investigate the optical characteristics of this structure, focusing on the coupling behaviours between the HMM-bounded cavity and the NP array.

In addition, I investigate active control of absorption resonances in NP arrays by introducing phase change materials (PCMs) into the coupled cavity system in different ways. By changing the phase of the PCMs with a stimulus, the wavelength of the NP resonances can be switched. These preliminary designs are a promising method to enhance absorption in small NPs and achieve spectral tunability in absorption for application in PEH devices.

Next, I develop design criteria that apply the learnings of the investigated optical behaviours for the realization of PEH devices. To assess the implementation of these findings for improving performance, I estimate the figures of merit that could be achieved for PEH devices. Then, I discuss how such optical and electrical designs could be integrated into devices.

Finally, I combine learnings from the whole thesis to design an optimised PEH device incorporating the PCM, which exhibits absorption resonances that can be actively and reversibly switched on and off with an external stimulus.