Investigating GaAs metamorphic lasers

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Telecommunication is an important part of society and one of the ways data is transmitted is by sending light pulses down an optical fibre. Typically, semiconductor lasers emitting in the region between 1330nm and 1500nm are used with the current lasers being grown on InP substrates. However, GaAs substrates are preferred as they are less sensitive to temperature changes, available in larger sizes and cheaper in comparison to InP. Although GaAs is more favourable to use, optimal performance is usually limited to outside the telecommunication region due to differences between the lattice constants of the substrate and active region of the laser. One way to overcome this is to create a virtual substrate with a chosen lattice constant by growing a lattice mismatched InGaAs lattice buffer layer onto GaAs. This procedure is known as metamorphic growth and allows GaAs substrates to be used with lattice constants that are similar to InP. Metamorphic GaAs lasers have been known for a while but are now starting to show performances which are commercially viable. Furthermore, metamorphic growth could also be used for lasers in other fields such as medical and environmental monitoring.Recently, the Epitaxy and Physics of Nanostructures group at the Tyndall National Institute in Cork developed a novel metamorphic GaAs laser design. Grown by metalorganic vapour phase epitaxy, one heterostructure with active region of 3 quantum wells showed low threshold pulsed lasing with threshold density current of 127mA per QW at room temperature and continuous wave operation over 1300nm at 288K under various injection currents (further details in MOVPE metamorphic lasers and nanostructure engineering at telecom wavelengths). The next stage would be to find out more about the structure of the semiconductor laser as properties such as defects, strain and morphology can influence the performance of the laser.This project is being conducted in conjunction with the EPSRC Photonic Integration and Advanced Data Storage Centre for Doctoral Training, PIADS CDT. A collaboration between multiple academic and industrial organisations including Queen's University Belfast and Tyndall National Institute, the CDT's remit is to solve current challenges in the field of data storage and transmission. This research falls into the PIADS CDT research theme of Ultra-reliable semiconductor lasers operating in hostile environments and atomic scale analysis techniques.The project will use electron microscopy techniques such as Transmission Electron Microscopy, Scanning Electron Microscopy and Energy Dispersive electron spectroscopy. Computational software will be used to analyse the images. Overall, these techniques will provide information on the structures at the nanoscale, with the focus on chemical composition, defects and strain.The aim of this project is to investigate the GaAs metamorphic samples grown and use the techniques to provide a comprehensive analysis at the atomic scale. The findings can then be linked to the optical properties of the laser and the growth procedure for optimisation. In turn, we hope to improve the laser performance in the range of 1300nm to 1550nm.