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Title: ZnCdMgSe and AlGalnP multi-quantum well films for colour conversion and optically-pumped visible lasers
Author: Jones, Brynmor Edward
ISNI:       0000 0004 5357 3865
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2015
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II-VI semiconductor material ZnCdMgSe has the potential to enable optical devices emitting throughout the visible spectrum. While difficulties in doping of this material have hindered its development for conventional electrically-injected semiconductor lasers, the recent availability of efficient, high power InGaN-based laser diodes has created the opportunity for optically-pumped devices, and this work primarily focusses on the progression towards realising vertical external-cavity surface-emitting lasers (VECSELs) based on this material system. Challenges in the growth of a ZnCdMgSe distributed Bragg reflector (DBR), such as low refractive index contrast and limited growth thickness for maintaining material quality, lead to the design of novel thin-film VECSEL structures, and the development of epitaxial transfer techniques to overcome the absorptive InP growth substrate and buffer. Transfer of thin-film (few microns thick) multi-quantum well heterostructures is demonstrated for samples with areas of a few mm², successfully transferring and liquid-capillary-bonding the films to diamond heat-spreaders for thermal management. Continued challenging growth, namely heterostructure layer inaccuracies, mean that laser threshold is not yet reached, however extensive characterisation and analysis is carried out to inform future progress in realising the ZnCdMgSe thin-film VECSEL. The thin-film VECSEL architecture offers advantages beyond allowing for the use of novel materials, opening the potential for novel laser cavities and optical pumping schemes. The thin-film transfer method developed for the II-VI VECSEL is adapted for the transfer of III-V AlGalnP epitaxial structures from GaAs growth substrates, and AlGalnP thin-film VECSELs are demonstrated operating continuous wave at red wavelengths at room temperature. Laser operation is currently limited by pump-induced de-bonding from the diamond, with attempts made to counter this through the refinement of structre design (including strain balancing) and transfer method. Until thermal rollover occurs, performance is relatively comparable with the 'conventional'; gain-mirror AlGaInP VECSELs, with a maximum output power of 21 mW recorded at both 682 nm and 670 nm for low output coupling. Using the transfer method developed for the II-VI material, ZnCdMgSe multi-quantum well structures are used as colour conversion films for micron-size LED arrays. The resulting hybrid devices are demonstrated to have high modulation bandwidths, limited only by the LED modulation bandwidth, suitable for application in visible-light communication.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral