Accurate quantum transport modelling and epitaxial structure design of high-speed and high-power In0.53Ga0.47As/AlAs double-barrier resonant tunnelling diodes for 300-GHz oscillator sources

Terahertz (THz) wave technology is envisioned as an appealing and conceivable solution in the context of several potential high-impact applications, including sixth generation (6G) and beyond consumer-oriented ultra-broadband multi-gigabit wireless data-links, as well as highresolution imaging, radar, and spectroscopy apparatuses employable in biomedicine, industrial processes, security/defence, and material science. Despite the technological challenges posed by the THz gap, recent scientific advancements suggest the practical viability of THz systems. However, the development of transmitters (Tx) and receivers (Rx) based on compact semiconductor devices operating at THz frequencies is urgently demanded to meet the performance requirements calling from emerging THz applications. Although several are the promising candidates, including high-speed III-V transistors and photo-diodes, resonant tunnelling diode (RTD) technology offers a compact and high performance option in many practical scenarios. However, the main weakness of the technology is currently represented by the low output power capability of RTD THz Tx, which is mainly caused by the underdeveloped and non-optimal device, as well as circuit, design implementation approaches. Indeed, indium phosphide (InP) RTD devices can nowadays deliver only up to around 1 mW of radio-frequency (RF) power at around 300 GHz. In the context of THz wireless data-links, this severely impacts the Tx performance, limiting communication distance and data transfer capabilities which, at the current time, are of the order of few tens of gigabit per second below around 1 m. However, recent research studies suggest that several milliwatt of output power are required to achieve bit-rate capabilities of several tens of gigabits per second and beyond, and to reach several metres of communication distance in common operating conditions. Currently, the shortterm target is set to 5−10 mW of output power at around 300 GHz carrier waves, which would allow bit-rates in excess of 100 Gb/s, as well as wireless communications well above 5 m distance, in first-stage short-range scenarios. In order to reach it, maximisation of the RTD highfrequency RF power capability is of utmost importance. Despite that, reliable epitaxial structure design approaches, as well as accurate physical-based numerical simulation tools, aimed at RF power maximisation in the 300 GHz-band are lacking at the current time. This work aims at proposing practical solutions to address the aforementioned issues. First, a physical-based simulation methodology was developed to accurately and reliably simulate the static current-voltage (IV ) characteristic of indium gallium arsenide/aluminium arsenide (In-GaAs/AlAs) double-barrier RTD devices. The approach relies on the non-equilibrium Green’s function (NEGF) formalism implemented in Silvaco Atlas technology computer-aided design (TCAD) simulation package, requires low computational budget, and allows to correctly model In0.53Ga0.47As/AlAs RTD devices, which are pseudomorphically-grown on lattice-matched to InP substrates, and are commonly employed in oscillators working at around 300 GHz. By selecting the appropriate physical models, and by retrieving the correct materials parameters, together with a suitable discretisation of the associated heterostructure spatial domain through finite-elements, it is shown, by comparing simulation data with experimental results, that the developed numerical approach can reliably compute several quantities of interest that characterise the DC IV curve negative differential resistance (NDR) region, including peak current, peak voltage, and voltage swing, all of which are key parameters in RTD oscillator design. The demonstrated simulation approach was then used to study the impact of epitaxial structure design parameters, including those characterising the double-barrier quantum well, as well as emitter and collector regions, on the electrical properties of the RTD device. In particular, a comprehensive simulation analysis was conducted, and the retrieved output trends discussed based on the heterostructure band diagram, transmission coefficient energy spectrum, charge distribution, and DC current-density voltage (JV) curve. General design guidelines aimed at enhancing the RTD device maximum RF power gain capability are then deduced and discussed. To validate the proposed epitaxial design approach, an In0.53Ga0.47As/AlAs double-barrier RTD epitaxial structure providing several milliwatt of RF power was designed by employing the developed simulation methodology, and experimentally-investigated through the microfabrication of RTD devices and subsequent high-frequency characterisation up to 110 GHz. The analysis, which included fabrication optimisation, reveals an expected RF power performance of up to around 5 mW and 10 mW at 300 GHz for 25 μm2 and 49 μm2-large RTD devices, respectively, which is up to five times higher compared to the current state-of-the-art. Finally, in order to prove the practical employability of the proposed RTDs in oscillator circuits realised employing low-cost photo-lithography, both coplanar waveguide and microstrip inductive stubs are designed through a full three-dimensional electromagnetic simulation analysis. In summary, this work makes and important contribution to the rapidly evolving field of THz RTD technology, and demonstrates the practical feasibility of 300-GHz high-power RTD devices realisation, which will underpin the future development of Tx systems capable of the power levels required in the forthcoming THz applications

Ferroelectric Tunnel Junctions

Ferroelectricity and quantum-mechanical electron tunneling are well-known physical phenomena that have been studied for as long as a century. During this long period, scientific research has been restricted either to ferroelectricity or to electron tunneling. Never before have these subjects been combined into a new phenomenon based on their interaction. Within this work, I present the novel concept of a ferroelectric tunnel junction, where the term ferroelectric refers to a property of the barrier material. This device consists of a ferroelectric layer sandwiched between metal electrodes. The thickness of the ferroelectric layer is thin enough to allow for electron tunneling. For the first time, the influence of macroscopic parameters, such as the spontaneous polarization and strain on quantum-mechanical electron tunneling through a ferroelectric tunnel barrier is studied experimentally. In addition, the experimental work is accompanied by theoretical ideas and predictions concerning the manifestation of piezoelectricity or ferroelectricity in direct electron tunneling

Integración de láminas delgadas de materiales feromagnéticos medio-metálicos en silicio por medio de un contacto túnel

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de Materiales, leída el 08-07-2019The use of magnetic tunnel contacts have been proved as one of the most efficient methods for the efficient injection and detection of spin polarized currents on non-magnetic semiconductors. In this kind of systems, the spin polarized carriers of the ferromagnetic electrode are transferred by tunneling through an insulator barrier into the semiconductor. The diusive transport directly through the ferromagnet/semiconductor interface is limited by the large impedance mismatch between the two materials. The introduction of a tunnel barrier allows to improve the efficiency of the system and increase spin accumulation in the semiconductor. However, in order to achieve high spin rates the system needs to ful ll certain requirements. One of the fundamental conditions is the formation of at and abrupt interfaces. High interface roughness can induce the presence of magnetostatic elds that might induce spin precession and reduce spin polarization. Moreover, the presence of defects on the interface can act as scattering centers and reduce even more spin accumulation in the semiconductor...Se ha demostrado que el uso de contactos tunel magneticos es uno de los metodos mas eficientes para la inyeccion y deteccion de corrientes polarizadas en espín en semiconductores no magneticos. En este tipo de sistemas, los portadores de carga polarizados en espín del electrodo ferromagnetico se transfieren al semiconductor por efecto tunel a traves de una barrera aislante. La inyeccion de espín por transporte difusivo de forma directa a traves de la intercara ferromagnetico/aislante es muy poco eficiente en terminos de polarizacion debido a la gran diferencia de impedancia electrica entre los dos materiales. La introduccion de una barrera tunel permite mejorar la eficiencia del sistema y aumentar la acumulacion de espín en el semiconductor. Sin embargo, para poder obtener una inyeccion eficiente el sistema debe ademas cumplir ciertos requisitos. Una de las condiciones principales es que las heteroestructuras presenten intercaras abruptas tanto morfologicamente como desde el punto de vista qumico. Esto se debe a que una rugosidad alta puede generar campos magnetoestaticos en la intercara que induzcan la precesion de los espines y reduzcan la polarizacion en el semiconductor. Ademas, los defectos presentes en la intercara pueden funcionar como centros de dispersion y reducir a su vez la polarizacion en espín de los portadores...Fac. de Ciencias FísicasTRUEunpu

Investigation of Si/SiO2 and Si/SiOx quantum well structures for applications as energy selective contacts and all-silicon tandem cells

In order to satisfy the world’s energy demands while simultaneously preserving the sustainability of the environment, it is inevitable to shift the reliance of fossil fuels to renewable energy sources. Photovoltaic is the fastest growing energy source in the world and the cost of production have reduced significantly over the past decade for it to be considered as a cost-competitive solution. Increasing the cell efficiency and bringing the cost down towards grid parity continues to be the primary motivation for research and development in photovoltaics technology. The third generation photovoltaics involve novel cell designs and concepts that have potentials in achieving very high efficiency and low cost solar cells. These include tandem solar cells, quantum well/dot solar cells, hot carrier solar cells and up-converters. Resonant tunnelling effect in Si/SiO2 quantum well structures could find potential applications in all-Si tandem cells in the form of superlattice and energy selective contacts in hot carrier solar cells in the form of double barrier structure. The fabrication of crystalline Si/SiO2 quantum well to achieve the desired confinement effect is no trivial task. In this thesis, the structures were deposited by RF magnetron reactive sputtering followed by post thermal treatment to crystallize the amorphous silicon layer. The enhancement of crystallization temperature has been observed experimentally for low dimensional Si well in the order of a few nanometres. The crystallinity has also been experimentally demonstrated to be strongly dependent on the annealing temperature rather than the duration. The size of silicon nanocrystals was calculated and compared using different analytical approaches. It was observed that the Si thickness and annealing temperature both plays a role in the size of the nanocrystals. The bandgap enhancement was evident from variation of luminescence energy between 1.3 to 1.8 eV as function of Si well thickness. The origin of this luminescence was studied. The crystallization and photoluminescence properties of Si/SiOx structures (x<2) were also investigated. Finally the feasibility of partially crystalline quantum well for energy selective contact application was discussed

Single-particle and collective excitations in quantum wires made up of vertically stacked quantum dots: Zero magnetic field

We report on the theoretical investigation of the elementary electronic excitations in a quantum wire made up of vertically stacked self-assembled InAs/GaAs quantum dots. The length scales (of a few nanometers) involved in the experimental setups prompt us to consider an infinitely periodic system of two-dimensionally confined (InAs) quantum dot layers separated by GaAs spacers. The the Bloch functions and the Hermite functions together characterize the whole system. We then make use of the Bohm-Pines' (full) random-phase approximation in order to derive a general nonlocal, dynamic dielectric function. Thus developed theoretical framework is then specified to work within a (lowest miniband and) two-subband model that enables us to scrutinize the single-particle as well as collective responses of the system. We compute and discuss the behavior of the eigenfunctions, band-widths, density of states, Fermi energy, single-particle and collective excitations, and finally size up the importance of studying the inverse dielectric function in relation with the quantum transport phenomena. It is remarkable to notice how the variation in the barrier- and well-widths can allow us to tailor the excitation spectrum in the desired energy range. Given the advantage of the vertically stacked quantum dots over the planar ones and the foreseen applications in the single-electron devices and in the quantum computation, it is quite interesting and important to explore the electronic, optical, and transport phenomena in such systems