Theory and optimisation of metamorphic photonic devices

Metamorphic growth of semiconductor materials – in which a “virtual” substrate with a desired lattice constant is obtained by growing a lattice-mismatched metamorphic buffer layer (MBL) on a conventional substrate such as InP or GaAs – is beginning to attract increasing interest due to its potential to facilitate the development of improved optoelectronic technologies. For example, by growing a relaxed InxGa1−xAs MBL on a GaAs substrate heterostructures can then be grown with a lattice constant intermediate between that of GaAs and InP, thereby providing enhanced scope for band structure engineering and semiconductor device design and optimization starting from a GaAs substrate. However, despite significant progress in material growth and device engineering, there has been very little theoretical analysis of metamorphic devices. We are particularly interested in the development of metamorphic AlInGaAs-based lasers operating at the technologically important 1.3 µm wavelength, as well as efficient AlInGaP-based 610 nm Light-Emitting Diodes (LEDs) for maximised white light efficiency. In this thesis we investigate the electronic and optical properties of these emitters and compare their performance with existing photonic devices. Using the continuum based multiband k·p model within the planewave expansion method we quantify the potential of lattice mismatched MBLs and identify the trends in device performance. We show that by employing an InGaAs MBL we can extend the ranges of strain and composition accessible for a direct band gap AlInGaAs or AlInGaP alloy, which allow the suppression of the amount of defects and CuPt atomic ordering created during the epitaxial growth. Using the model solid theory we demonstrate that the electron confinement strongly benefits from the use of an InGaAs MBL, bringing a reduced current leakage from the active region. After performing a detailed analysis over a series of metamorphic lasers and LEDs, which include such nanostructures in the active region as quantum wells, dots and wires, we identify the trends in electronic and optical properties which compare very favourably with existing devices, and we provide guidelines for the design of optimised devices. Using the experimental data available in the literature for metamorphic lasers we are able to estimate the defect-related current losses in such devices, and find that there remains opportunity to further improve laser performance. In addition, the micro-photoluminescence measurements performed on a prototype 610 nm metamorphic LED confirm our prediction of enhanced internal quantum efficiency compared to GaAsbased structures, suggesting that this novel type of LEDs is an excellent candidate for efficient white light emission

Well parameters of two-dimensional electron gas in Al0.88In 0.12N/AlN/GaN/AlN heterostructures grown by MOCVD

Resistivity and Hall effect measurements were carried out as a function of magnetic field (0-1.5 T) and temperature (30-300 K) for Al0.88In 0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two-dimensional electron gas (2DEG) channel located at the Al0.88In 0.12N/GaN interface with an AlN interlayer and a two-dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al 0.88In0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al 0.88In0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA

The 2019 materials by design roadmap

Advances in renewable and sustainable energy technologies critically depend on our ability to design and realize materials with optimal properties. Materials discovery and design efforts ideally involve close coupling between materials prediction, synthesis and characterization. The increased use of computational tools, the generation of materials databases, and advances in experimental methods have substantially accelerated these activities. It is therefore an opportune time to consider future prospects for materials by design approaches. The purpose of this Roadmap is to present an overview of the current state of computational materials prediction, synthesis and characterization approaches, materials design needs for various technologies, and future challenges and opportunities that must be addressed. The various perspectives cover topics on computational techniques, validation, materials databases, materials informatics, high-throughput combinatorial methods, advanced characterization approaches, and materials design issues in thermoelectrics, photovoltaics, solid state lighting, catalysts, batteries, metal alloys, complex oxides and transparent conducting materials. It is our hope that this Roadmap will guide researchers and funding agencies in identifying new prospects for materials design

Structural and Optical Characterization of III-V Nanostructures Monolithically Grown on Si Substrates

Group III-V semiconductor nanostructures have emerged as an important material platform over the past decades for wide-range device implementation in the field of electronics and optoelectronics. Among them, nanowires (NWs) are particularly attractive owing to the elastic strain relaxation through their sidewall facets which allows for the combination of lattice mismatched materials. Hence, optically active III-V materials become compatible with the mature and prevalent Si platform. Moreover, NWs are ideal for hosting quantum dots (QDs) ensuring their deterministic positioning and uniformity. This configuration opens the route for sophisticated applications including single photon emission, a crucial function in quantum information processing. In addition, another type of nanostructures that has attracted attention is the two-dimensional nanosheets, whose principal benefit is the band structure tuning from bulk to 2D by modulation of their thickness. Consequently, they are established as promising blocks for various optoelectronic devices and applications. In the current thesis, we reported the growth of self-catalysed AlGaAs NWs monolithically on Si (111) substrates via solid-source molecular beam epitaxy (MBE). The self-formation of an Al-rich shell is exhibited, which is thicker at the base and thins down towards the NW tip, while it demonstrates wide alloy fluctuations. The predominantly ZB structure presents twin defects and occasional WZ insertions, further increasing the intricacy of the NWs. The optical probing via photoluminescence reveals fully tuneable emission with the Al content of the alloy. Among the morphological variations of AlGaAs NWs, the branched NWs are of unique interest. The branching events increase with Al content, while the branches are confirmed to grow on the NW trunks epitaxially. In addition, complex compositional distribution in the branches is presented, as Ga-rich stripes along the growth direction of the branches, attributed to the different nucleation energies on different faces at the liquid/solid interface of the branch, intersect with Ga-rich stripes perpendicular to them, deriving from the rotation of the sample during growth. Moreover, self-catalysed, single GaAs/AlGaAs dot-in-wire structures have been designed and grown by inserting a short GaAs segment in each AlGaAs NW. The exhaustive optical probing reveals centrally localized peaks, with a decently narrow linewidth of roughly 490 μeV. The QD emission is comprised of an exciton and a biexciton transition, while a high degree of polarization is noticed when compared to the AlGaAs NW-related emission. The above characteristics are important steps towards achieving single photon emission. Finally, we optically inspect InAs nanosheets grown via MBE via photoluminescence measurements. Pristine nanosheets exhibit surface charge via carrier trapping mechanisms at the surface states, which is suggestive of the “memory effect”. The impact of sulphur passivation and core/shell configuration on the optical response of the nanosheets is evaluated. In addition, we fabricated an optoelectronic memory unit based on pristine InAs nanosheets, adopting a field-effect transistor configuration, which demonstrates negative photoresponse with good reproducibility and ultra-low power consumption

The 2019 materials by design roadmap

Advances in renewable and sustainable energy technologies critically depend on our ability to design and realize materials with optimal properties. Materials discovery and design efforts ideally involve close coupling between materials prediction, synthesis and characterization. The increased use of computational tools, the generation of materials databases, and advances in experimental methods have substantially accelerated these activities. It is therefore an opportune time to consider future prospects for materials by design approaches. The purpose of this Roadmap is to present an overview of the current state of computational materials prediction, synthesis and characterization approaches, materials design needs for various technologies, and future challenges and opportunities that must be addressed. The various perspectives cover topics on computational techniques, validation, materials databases, materials informatics, high-throughput combinatorial methods, advanced characterization approaches, and materials design issues in thermoelectrics, photovoltaics, solid state lighting, catalysts, batteries, metal alloys, complex oxides and transparent conducting materials. It is our hope that this Roadmap will guide researchers and funding agencies in identifying new prospects for materials design