Development of GaAs-based membrane lasers for heterogeneous integration to SiN platforms using hybrid techniques

A key aspect of the design was the introduction of an InGaP protective layer and an AlGaAs sacrificial layer beneath the epilayer to facilitate membrane release from the GaAs substrate. The entire membrane fabrication process was extensively optimized, with a focus on defining the lateral membrane trenches, protecting the epilayer during chemical etching, and improving the wet etching of the sacrificial layer. This thesis explores the development of Gallium Arsenide (GaAs)/Aluminium Gallium Arsenide (AlGaAs) membrane lasers for heterogeneous integration with waveguides fabricated on a silicon nitride (SiN) platform.

The quantum ‘magic wavelength’ of ∼813 nm was targeted by designing a single GaInAs quantum well epilayer structure. The magic wavelength refers to a phenomenon called "AC Stark shift", in essence, this is when a non-static electromagnetic field such as a laser, shifts the ground and excited states of an atom equally. Thereby imparting a near net zero shift in frequency allowing accurate measurements to be taken.

A key aspect of the design was the introduction of an In<br>GaP protective layer and an AlGaAs sacrificial layer beneath the epilayer to facilitate membrane release from the GaAs substrate. The entire membrane fabrication process was extensively optimized, with a focus on defining the lateral membrane trenches, protecting the epilayer during chemical etching, and improving the wet etching of the sacrificial layer. Following this fabrication process, several laser geometries were developed, including Fabry-Pérot cavities and tangled waveguides.

A significant part of the project involved optimizing the membrane release process, which included designing a new head for the chip die bonder and conducting multiple fabrication iterations to determine the optimal geometries for the supporting anchors and via holes. Despite numerous optimization efforts, however, membrane transfer success remained low, primarily due to an excessively high aspect ratio between the membrane area and the sacrii Abstract faicial layer thickness, leading to membrane collapse onto the substrate.

This research produced a number of successful milestones, including:<br>(i) The wafer material producing broad area lasers with lasing characteristics sufficient for<br>the fabrication of semiconductor devices.<br>(ii) Various membranes were successfully suspended.<br>(iii) The successful development, fabrication and suspension of a membrane laser.<br>(iv) Using a flip-chip bonder tool to release a membrane laser from its native substrate.<br>(v) The waveguide anchors broke as designed to create clean facets.<br><br>This demonstrated the accuracy of the process alongside the successful amalgamation of two different pick and place technologies.

Although most of the technological processes were optimized and fabrication successfully completed, the final step—thermal-compression bonding of the membranes onto the metallization pads of the SiN waveguide PIC—could not be accomplished. A successful outcome for this project would require only minor improvements of the epilayer design and membrane geometry, which are discussed at the end of this thesis.