Tin disulfide (SnS2) is a two-dimensional (2D) material with excellent properties and high prospects for low-cost solutions to catalytic and optoelectronic applications. In this work, vertical nanoflakes of SnS2 have been synthesized using custom-designed close space sublimation (CSS) system and investigated for applications in photoelectrochemical (PEC) water oxidation and metal-semiconductor-metal (MSM) photodetector. For the PEC application, vertical SnS2 nanoflakes grown directly on transparent conductive substrates have been used as photoanodes, which produce record photocurrents of 4.5 mA cm−2 for oxidation of a sulfite hole scavenger and 2.6 mA cm−2 for water oxidation without any hole scavenger, both at 1.23 VRHE in neutral electrolyte under simulated AM1.5G sunlight, and stable photocurrents for iodide oxidation in acidic electrolyte. This remarkable performance has been attributed to three main reasons: (1) high intrinsic carrier mobility of 330 cm2 V−1 s−1 and long photoexcited carrier lifetime of 1.3 ns in the nanoflakes, (2) the nanoflake height that balances the competing requirements of light absorption and charge transport, and (3) the unique stepped morphology of these nanoflakes that improves photocurrent by exposing multiple edge sites in every nanoflake. In another application, these SnS2 nanoflakes have been used to enhance the performance of lead sulfide quantum dot (PbS QDs) photodetectors by providing a high-mobility channel for photoexcited charges from PbS QDs, which results in 2 orders of magnitude enhancement in responsivity. The physical models and experimental findings presented in this dissertation can help engineer more cost-effective solutions for PEC water splitting and optoelectronics based on 2D metal dichalcogenides.

In recent times, plasmonics has been a hallmark in improving optoelectronic device performance as well as in improving sensing. Confining light in dimensions below the diffraction limit and subsequently converting the incident photons into localized charge-density oscillations called localized surface plasmons, optical enhancements of the local fields by many orders of magnitude is possible. This dissertation explores the use of such surface plasmon resonances in core multishell nanostructures and demonstrates the values of such structures in energy harvesting and sensing. Additionally, it also shows the use of emerging plasmonic materials like metal nitrides (TiN, ZrN) instead of traditional plasmonic materials (Au, Ag) in the nanostructure designs. Utilizing the localized surface plasmon resonance (LSPR) in metallic components of core multishell nanowires, calculations of the local density of states as a measure of emission were made using a Green’s function method, while the absorption and scattering were simulated using the Mie formalism. Combining both the absorption and the emission, the quantum efficiency of white LEDs was calculated and the optimal material/dimensions for maximal performance was determined for different phosphor components in a white LED. Additionally, the use of ZrN as a plasmonic cloak for noise cancellation in Si photodetectors is shown and the performance is compared with an Au cloak. Using the developed methodology, it is proved that ZrN cloaks can outperform Au cloaks in a certain region of the visible spectrum, showing the benefit of using such plasmonic systems in place of traditional materials. The fabrication of the different components of the core multishell nanowires is also presented, and in particular, fabrication of ultra-thin (sub-10 nm) plasmonic TiN is achieved. Utilizing plasmon hybridization, a tunable double resonance feature is observed in Au/SiO2/Au core shell shell (CSS) nanoparticles, which have been then demonstrated to improve the photocatalytic performance in hematite. In particular, the double resonance peak allows absorption of light beyond the band gap of hematite and subsequent conversion into photocurrent through hot electron injection. Comparison has been made with Au nanoparticles, and it has been shown that the CSS nanoparticles outperform Au nanoparticles significantly. These CSS nanoparticles have also been used for bioimaging, in particular for Raman spectroscopy, with strong results at high densities of the nanoparticles. Utilizing stronger scattering SiO2/Au Nanoshells, it has been possible to work towards single particle imaging of molecules and demonstration of this phenomenon has been shown here through the use of coherent Raman scattering spectroscopy.

Computing as a service is rapidly becoming the new normal for many sectors of the economy. The widespread availability of broadband internet has allowed an extensive range of services to be delivered on-demand from centralized computing systems known as ‘data centers’. These systems have evolved to be enormously complex. Optical-based communication is desired to increase data center capability and efficiency, however traditional optical technologies are not feasible due to cost and size. Silicon photonics aims to deliver optical communications on an integrated and affordable platform for use in data centers by leveraging the existing capabilities of complementary metal-oxide semiconductor manufacturing. This thesis contains a description of the development of monolithic silicon photodiodes for use in photonic integrated circuits in, and beyond, the current telecommunications wavelength windows. The focus is on methods which are compatible with standard silicon processing techniques. This is in contrast to the current approaches which rely on hybrid material systems that increase fabrication complexity. Chapter 1 and 2 provide background information to place this work into context. Chapter 3 presents an experimental study of resonant devices with lattice defects which determines the refractive index change in silicon-on-insulator waveguides. High-speed operation of resonant photodiodes is demonstrated and is found to be limited by resonance instability. Chapter 4 demonstrates high responsivity avalanche photodetectors using lattice defects. The detectors are shown to operate error-free at 10 Gbit/s, thus confirming their capability for optical interconnects. Chapter 5 presents photodiodes operating with absorption through surface-state defects. These detectors show fast operation (10 Gbit/s) and have an extremely simple fabrication process. Chapter 6 demonstrates photodiodes operating beyond the traditional telecommunications window. Operation at 20 Gbit/s, at a wavelength of 1.96 µm is demonstrated, offering potential for their use in the next generation of optical communication systems which will exploit the thulium doped fiber amplifier. / Thesis / Doctor of Philosophy (PhD) / This thesis describes photodiodes constructed on silicon optical waveguides. The photodiodes are notable for their high-speed performance and simple fabrication methods. Such devices may find use within chip-integrated optical transceivers, which are desired for optical interconnects within large-scale computing systems such as data centers.

Quantum dot infrared photodetectors (QDIPs) have emerged as a promising technology in the mid- and far-infrared (3-25 μm) for medical and environmental sensing that have a lot of advantages over current technologies based on Mercury Cadmium Telluride (MCT) and quantum well (QW) infrared photodetectors (QWIPs). In addition to the uniform and stable surface growth of III/V semiconductors suitable for large area focal plane applications and thermal imaging, the three dimension confinement in QDs allow sensitivity to normal incidence, high responsivity, low darkcurrent and high operating temperature. The growth, processing and characterizations of these detectors are costly and take a lot of time. So, developing theoretical models based on the physical operating principals will be so useful in characterizing and optimizing the device performance. Theoretical models based on non-equilibrium Green's functions have been developed to electrically and optically characterize different structures of QDIPs. The advantage of the model over the previous developed classical and semiclassical models is that it fairly describes quantum transport phenomenon playing a significant role in the performance of such nano-devices and considers the microscopic device structure including the shape and size of QDs, heterostructure device structure and doping density. The model calculates the density of states from which all possible energy transitions can be obtained and hence obtains the operating wavelengths for intersubband transitions. The responsivity due to intersubband transitions is calculated and the effect of having different sizes and different height-to-diameter ratio QDs can be obtained for optimization. The dark and photocurrent are calculated from the quantum transport equation provided by the model and their characteristics at different design parameter are studied. All the model results show good agreement with the available experimental results. The detectivity has been calculated from the dark and photocurrent characteristics at different design parameters. The results shows a trade off between the responsivity and detectivity and what determines the best performance is how much the rate of increase of the photocurrent and dark current is affected by changing the design parameters. Theoretical modeling developed in the thesis give good description to the QDIP different characteristics that will help in getting good estimation to their physical performance and hence allow for successful device design with optimized performance and creating new devices, thus saving both time and money. / Thesis / Doctor of Philosophy (PhD)

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[pt] Os semicondutores III-V são amplamente investigados para a fabricação de fotodetectores de infravermelho baseados em pontos quânticos (QWIPs); no entanto, o comprimento de onda de operação é limitada pelo bandoffset dos materiais que permitem transições de infravermelho de comprimento de onda maior que 3,1 um. Para comprimentos de onda mais curto do que 1,7 um transições banda a banda são facilmente empregadas. Assim, em QWIPs III-V, o intervalo entre 1,7 e 3,1 um não pode ser alcançado tanto por transições banda-banda ou por transições intrabanda. Nesta tese uma estrutura de superrede especialmente desenhada é proposta a fim de detectar a radiação dentro desta faixa proibida. A estrutura proposta consiste numa superrede com um poço quântico central mais amplo, o qual gera uma modulação no contínuo criando minibandas e minigaps para energias acima da parte inferior da banda de condução do poço quântico, incluindo no contínuo. Com esta abordagem, a limitação de ter estados ligados apenas com energias abaixo a barreira não se mantém e é possível detectar energias mais elevadas do que o limite imposto pelo bandoffset dos materiais. Simulações teóricas para a estrutura foram realizados e medidas de absorção, corrente de escuro, e fotocorrente foram realizadas mostrando picos em 2,1 um, em estreita concordância com o valor teoricamente esperado. / [en] III-V semiconductors are extensively investigated for fabrication of quantum well infrared photodetectors (QWIPs); however the operation wavelength is limited by the bandoffset of the materials allowing infrared transitions for wavelength larger than 3.1 um. For wavelength shorter than 1.7 um band to band transitions are easily employed. Thus, in III-V QWIPs, the range between 1.7 and 3.1 um cannot be reached either by band-to-band or by intraband transitions. In this thesis a specially designed superlattice structure is proposed in order to detect radiation within this forbidden range. The structure proposed consists of a superlattice with a wider central quantum well, which generates a modulation in the continuum creating minibands and minigaps for energies above the bottom of the conduction band of the quantum well, including in the continuum. With this approach the limitation of having bound states only with energies below the barrier no longer holds and it is possible to detect energies higher than the limit imposed by the bandoffset of the materials. Theoretical simulations for the structure were performed and absorption, dark current, and photocurrent measurements were carried out showing peaks at 2.1 um, in close agreement with the theoretically expected value.

[pt] Fotodetectores de infravermelho possuem uma vasta gama de aplicações diretas em diversos setores, desde militar (e.g. visão noturna, orientação de mísseis) até lazer (aparelhos eletrônicos). Especificamente, os fotodetectores baseados em semicondutores III-V são dispositivos que podem ser construídos para selecionar e medir radiação em faixas específicas do espectro eletromagnético. Dentre as figuras de mérito dos fotodetectores um dos grandes desafios está na redução da corrente de escuro. Neste trabalho visamos produzir um dispositivo de InGaAs com geometria planar, que de acordo com a literatura tem como característica apresentar uma baixa corrente de escuro, nesse caso, há necessidade de difusão de um dopante. Estão reportadas neste trabalho todas as etapas, desde o crescimento das amostras até a caracterização do dispositivo final. Com o auxílio de um reator de MOVPE, foram feitas as calibrações das camadas que fazem parte do dispositivo final, bem como as calibrações do processo de difusão do dopante (Zn). Todas as camadas da amostra foram otimizadas individualmente, assim como a profundidade de difusão desejada (1 µm e nível de dopagem de 2x1018 cm−3). Diversas técnicas de processamento e caracterização foram utilizadas ao longo do trabalho para obter o melhor dispositivo possível. Podemos destacar os resultados de fotocorrente e de corrente de escuro, no qual as medidas foram realizadas com variação da temperatura de 77 até 300 K. Foi possível observar no resultado de espectro de fotocorrente um pico em 0,75 eV referente ao InGaAs a 300 K. Este resultado está de acordo com os de diodos de InGaAs feitos usando o método convencional de dopagem do Zn. / [en] Infrared photodetectors have a wide range of direct applications in various sectors, from military (e.g. night vision, missile guidance) to leisure (electronic devices). Specifically, III-V semiconductor-based photodetectors are devices that can be built to select and measure specific ranges of the electromagnetic spectrum. Among the photodetector figures of merit, one of the great challenges is reducing the dark current. In this work we aim to produce an InGaAs device with planar geometry, which according to the literature has the characteristic of presenting a low dark current. This geometry requires a dopant diffusion. This work reports all the steps, from sample growth to final device characterization. With the aid of an MOVPE reactor, the calibrations of the layers that are part of the final device were made, as well as the calibrations of the dopant diffusion process (Zn). All sample layers were individually optimized, as well as, the desired diffusion depth (1 µm and doping level of 2x1018 cm−3). Several processing and characterization techniques were used throughout the work to obtain the best possible device. We can highlight the photocurrent and dark current results, in which the measurements were performed with a temperature variation from 77 up to 300 K. It was possible to observe from the photocurrent spectrum result a peak at 0,75 eV relative to the InGaAs at 300 K. This result is in agreement with those of InGaAs diodes made using the conventional Zn doping method.

Semiconductor nanowires are nanostructures with lengths up to few microns and small cross sections (10ths of nanometers). In the recent years the development in the field of III-N nanowire technology has been spectacular. In particular they are consider as promising building in nanoscale electronics and optoelectronics devices; such as photodetectors, transistors, biosensors, light source, solar cells, etc. In this work, we present fabrication and the characterization of photodetector and light emitter based devices on III-N nanowires. First we present a study of a visible blind photodetector based on p-i-n GaN nanowires ensembles grown on Si (111). We show that these devices exhibit a high responsivity exceeding that of thin film counterparts. We also demonstrate UV photodetectors based on single nanowires containing GaN/AlN multi-axial quantum discs in the intrinsic region of the nanowires. Photoluminescence and cathodoluminescence spectroscopy show spectral contributions above and below the GaN bandgap according to the variation of the discs thickness. The photocurrent spectra show a sub-band-gap peak related to the interband absorption between the confined states in the large Qdiscs. Finally we present a study of photodetectors and light emitters based on radial InGaN/GaN MQW embedded in GaN wires. The wires used as photodetectors showed a contribution below the GaN bandgap. OBIC measurements demonstrate that, this signal is exclusively generated in the InGaN MQW region. We showed that LEDs based on this structure show a electroluminescence emission and a red shift when the In content present in the QWs increases which is in good agreement with photoluminescence and cathodoluminescence results.