Compound Semiconductor Oxidation for Single-Mode High-Index-Contrast Ridge Waveguide and Quantum Cascade Lasers

Advancing the development of cheaper and faster photonic integrated circuits (PICs) requires strong optical confinement, low bend loss waveguides in order to shrink component size and increase integration density. Addressing this need, a self-aligned process utilizing the oxygen-enhanced wet thermal oxidation (OEWTO) technique developed at the University of Notre Dame has been successfully applied to realize deeply-etched oxide-confined GaAs-based high-index-contrast (HIC) ridge waveguide (RWG) structures enabling lasers with promising performance attributes. In this work, especially uniform, low surface roughness (0.2 nm) and low interface roughness (2.3 nm) native oxides grown directly on GaAs are achieved, with highly specular optical properties demonstrated by the excellent performance of a quarter wave optical thickness antireflection layer. Through extensive optimization of lithography, etching and OEWTO processes, single mode HIC RWG diode lasers are demonstrated for the first time. Oxidation smoothing of the semiconductor/oxide interface sidewall roughness along the propagation direction of oxide-confined HIC RWG structures is directly observed through focused ion beam milling and electron microscopy. High external differential quantum efficiency (65%), stable single-mode, high beam quality (M^2~1.2), ~880 nm wavelength graded index separate confinement heterostructure (GRINSCH) HIC RWG laser diodes are fabricated and characterized. Notably, a circularly symmetric output beam HIC RWG laser is achieved for the first time, overcoming the traditional asymmetric, highly elliptical beam divergence limitation of edge-emitting lasers and well surpassing the ~2 mW maximum power of current symmetric single mode vertical cavity surface emitting lasers. This single mode device exhibits a total optical output power as high as 101 mW, with a threshold current of 81 mA and slope efficiency of 0.92 W/A (without heatsinking and under pulsed operation). The oxide-confined deeply-etched 3.25 µm wide ridge produces a single mode laser with a 1.88 µm 1/e^2 near-field mode width and full width half maximum (FWHM) far field divergence of 29.95° and 29.71° in the fast and slow axes, respectively. With detailed stripe width dependence studies, we highlight the performance improvements relative to our prior oxide-confined HIC RWG devices and fully characterize the multi-mode to single-mode transition behavior. Wider 4.5 µm ridge width devices are observed to operate with single spatial mode output powers up to 166 mW. Finally, significant optimization of the OEWTO process for InGaAs and InP-based laser heterostructures is also demonstrated. Most of the processes required to apply the OEWTO process to the fabrication of InP-based quantum cascade lasers (QCLs) have been fully explored and implemented, with issues regarding a remaining contact window etch process characterized and discussed.