In this work, a novel self-aligned process utilizing non-selective, O2-enhanced wet thermal oxidation is presented for
fabricating InP-based, ridge waveguide mid-infrared (λ=5.4 μm) quantum cascade lasers (QCLs) with a straincompensated,
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 Alfree
III-As alloys, forms a highly-insulating native oxide layer while simultaneously smoothing and passivating the etchexposed
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 Jth~3.2 kA/cm2 for a 19.5 μm wide x 3 mm
long stripe width.
The use of the non-selective, O2-enhanced wet thermal oxidation of deep-etched sidewalls in GaAs-based
heterostructures has enabled the fabrication of low-loss, high-index-contrast ridge waveguides suitable for ring resonator
laser devices. In a self-aligned process, the grown native oxide simultaneously provides excellent electrical insulation,
passivation of the etch-exposed bipolar active region, and smoothing of etch roughness. The resulting strong lateral
optical confinement at the semiconductor/oxide interface has enabled half-racetrack ring resonator (R3) lasers with bend
radii r as small as 6 μm. In this work we have experimentally characterized the loss due to the mode mismatch at the
straight to curved waveguide transition from analysis of efficiency data of half-R3 lasers with multiple cavity lengths.
Using an 808 nm InAlGaAs graded-index separate confinement heterostructure, the transition losses are extracted from
an inverse efficiency 1/ηd vs. length L plot for half-R3 lasers with r=150, 100, 50, 25 and 10 μm and 3 different ridge
widths, w. The round trip transition loss ranges from 11.5 to 37.0 dB (for w=7.3 μm), 6.7 to 27.0 dB (w=4.2 μm), and
1.8 to 16.2 dB (w=2.1 μm) with decreasing radii, showing a clear decrease with width and corresponding improved
mode overlap in the transition region. Simulation results elucidate the role of mode mismatch vs. radiative bend loss in
high-index-contrast racetrack ring resonator lasers. We demonstrate a full-ring laser having a tangential stripe output
coupler guide fabricated via e-beam lithography and non-selective oxidation with a threshold current density of 719
A/cm2 for an r=150 μm, w=6 μm ring.
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