Fabrication and Characterisation of Microcavity Opto-electronic Devices via Heterogeneous Integration using Transfer Printing

This research involved the use of a novel fabrication approach for the development of optical microcavity devices by heterogeneous integration of various material systems using transfer printing. Owing to their exceptional ability of light confinement, micro cavities have gathered a lot of scientific interest both at academic and industry level. The photonic confinement leads to the modification of the photonic density of states and allows cavity mode coupling with quasiparticle excitons within the emitter leading to exciting phenomena such as spontaneous emission rate enhancement, emission spectrum width reduction and radiation spatial directional re-distribution. Optical microcavities are appealing as they offer the possibility of controlling light emission and for such specific design conditions need to be followed. The development of microcavities is usually by monolithic integration of materials using epitaxy, despite being a relatively old process with numerous benefits, direct epitaxy also present some challenges and limitations. These include high cost and limitation of types of emitters that can be integrated into them due to their extreme deposition conditions, scarcity of compatible DBR material configuration particularly in the GaN material series, and small reflectivity bandwidth due to the small refractive index contrast for existing compatible DBR configurations. Our new optical microcavity fabrication approach has as aim of mitigating some of the existing issues while using standard, simple and less costly processing. Standard dielectric DBRs with large refractive index contrast (wide reflectivity band gap and low cost PECVD deposition) were used. The platform involves the anchor undercutting of the DBR into suspended array of DBRs which can then be transfer printed and integrated heterogeneously in any material system to fit the reflecting purpose, for optical microcavity applications.
Successful anchor undercutting of DBR coupon array with yield of almost 100% was achieved. The transfer printing of high quality DBRs was also demonstrated with reflectivity and bandwidth of 99% and 95nm respectively. Three types of cavity were fabricated (using our developed platform) and characterised; GaN based RCLED device, light emitting polymer microcavity and QD microcavity. We demonstrated multiple cavity modes emission for the incavity RCLED device with the peak mode showing an emission linewidth reduction from 21nm to 3nm from outcavity and incavity devices. The multiple modes are due to the wide cavity length as the full LED epilayer is transfer printed into the cavity together with adhesion layers. The device showed high series resistance due to the non-optimised fabrication processing with turn on voltage of approximately 2.7V. Smaller cavity length were realised for the F8BT and QD microcavity. F8BT microcavity Purcell effect with a FWHM reduction from 80.6nm to 8.3nm due to the cavity effects was demonstrated. Carrier lifetime reduction of 68% from non-resonant cavity to resonant cavity was also demonstrated. The QD microcavity showed high quality factor of 1305 with FWHM reduction from 100nm to 0.4nm due to the microcavity effects. Power dependent non-linearity measurements were non-conclusive for lasing claims. Detailed process development including design, fabrication procedures and characterisation with relevant diagrammatic representations are provided. Discussion of the results obtained with routes for improvement are also provided.