Date of Award
Master of Science
Anupama B. Kaul
The conversion of light into electrical signals is at the basis of technologies that affect our daily lives. Applications, including video imaging, optical communications, biomedical imaging, security, night-vision, gas sensing and motion detection have reached a high level of maturity due to the development of high-performance materials, large-scale production, and integration technologies. Currently conventional photodetectors made of Silicon (Si) or III-V compounds are about to reach their maximum efficiency, and every time it is harder to get a noticeable improvement in performance of sensors based on these materials, not to mention the complicated fabrication methods to achieve just a few increments in efficiency. This is due to their intrinsic properties. For instance, Si has an indirect band gap, besides Si creates crystal lattice vibrations called "phonons" instead excitons or electron-hole pairs (aka "e-h pairs"), when photons are absorbed over the metal surface, these phonons make the switching slow between on and off states. GaAs is also more sensitive to light intensity, but high-performance Ga based photodetectors has reached values in responsivity around ~ 19.1mA/W and external quantum efficiency of 15% for a GaA nanosheet.
At the same time, the discovery of graphene in 2004 with its amazing properties, has turned attention to the development of revolutionary graphene based photosensors. In spite of the great improvements that graphene derives in photoconductivity, it is still difficult to achieve high values of efficiency due to the lack of natural bandgap on graphene. However, it has opened the door to a new functional class of 2D materials called Transition Metal Dichalcogenides (TMDs), which in contrast to graphene or silicon, where the electronic properties are based on hybridization of "s" and "p" orbitals, the electronic structure of TMDs depends on the filling of "d" orbitals of the transition metals. The natural bands of energy in 2D TMDCs due to the "d" orbital contribution, leads to sharp peaks in the density of states at particular energy (known as Hove Singularities). When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons, the photocurrent generated can be significantly enhanced up to ∼80 according to Yin et al. In this work, attention is focused on MoS2 (Molybdenum disulfide), which is one of the most promising TMDCs analogous to graphene. However, the main difference between these two 2D materials is the electronic properties. While graphene shows semimetal properties due to the lack of bandgap, MoS2 single layer (SL) has a natural semiconducting behavior having a direct bandgap of around 1.8 eV, which makes MoS2 SL suitable for transistor and photodetector applications. For being able to understand the capabilities of MoS2 and be able to notice the huge properties of this material, we fabricated a MoS2 based photodetector starting from the synthesis of the material. Then, some fabrication methods are applied in order to notice the reduction of processes compared with current heterostructure based photodetectors. The enhancement of this photodetector is supported by taking advantage of the Surface Plasmon Polarization, which are coherent oscillations in the electrons that activate when an electromagnetic wave (EMW) hits a metal structure that corresponds in size to the wavelength of the EMW which greatly enhances the absorption of light. Finally, some optical and electrical characterization are performed to make evident the enhancement before and after nanoparticles. In this work these measurements showed an increment in photocurrent of at least three orders of magnitude going from 2 × 10–7 Amperes before Au NP deposition, to 2 × 10–4 Amperes after Au NP deposition for broadband as light source. In the case of narrow-band measurements for laser at 660 nm an increment in photocurrent of 5 orders of magnitude was seen in going from 3.7 × 10–9 Amperes all the way to 1.6 × 10–4 Amperes. This work shows the enhancement in performance seen with Au quantum dots, as also validated by External Quantum Efficiency (EQE) measurements, which show an increment in EQE of 76 times at 550nm wavelength where the efficiency increased from 9.7% before adding Au NP to 735% efficiency after adding Gold NPs. These enhancements in optical absorption efficiency are attributed to the plasmonic effect which can prove to be vitally important for a number of applications platforms such as detectors or solar cells.
Received from ProQuest
Carlos Francisco de Anda Orea
De Anda Orea, Carlos Francisco, "Advances in Chemical Vapor Deposition Growth of Molybdenum Disulfide for Photodetectors and Flexible Electronics" (2017). Open Access Theses & Dissertations. 433.