Native-oxide-confined mid-IR quantum cascade lasers via non-selective oxygen-enhanced wet oxidation

In this work, a novel self-aligned process utilizing non-selective, 02-enhanced wet thermal oxidation is presented for fabricating InP-based, ridge waveguide mid-infrared (λ=5.4 μm) quantum cascade lasers (QCLs) with a strain-compensated, 30-stage (1.53 μm thick) InGaAs/AlInAs active region, grown via metal organic chemical vapor deposition. This process, previously used in GaAs-based diode lasers containing low-Al content AlGaAs or even Al-free III-As alloys, forms a highly-insulating native oxide layer while simultaneously smoothing and passivating the etch-exposed active region, resulting in low-loss, strongly-confining waveguides. Here we report the first application of this process for directly oxidizing the deeply-etched QCL InGaAs/AlInAs active region ridge waveguide sidewalls and field (outside the ridge), eliminating the need for a deposited dielectric for electrical isolation, thus allowing self-aligned device fabrication. An 8 hour, 500 °C wet oxidation with 7000 ppm added O₂ (relative to N₂ carrier gas) yields a uniform oxide of∼350 nm in the field outside the ridge to ∼500 nm on the ridge sidewall. Laser devices tested under room temperature, pulsed excitation exhibit a threshold current density of J_th∼3.2 kA/cm² for a 19.5 μm wide × 3 mm long stripe width.