Theory and optimisation of metamorphic photonic devices
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University College Cork
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.
Metamorphic , Laser , Light emitting diode , Quantum well , Quantum wire , Planewave , Algainas , Algainp
Bogusevschi, S. 2018. Theory and optimisation of metamorphic photonic devices. PhD Thesis, University College Cork.