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.
A nonselective wet thermal oxidation technique for AlGaAs-containing heterostructures has been shown to enable the
fabrication of a variety of novel high-efficiency, high-power GaAs-based in-plane laser devices. Applied in conjunction
with a deep anisotropic dry etch, nonselective oxidation yields a simple, self-aligned high-index-contrast (HIC) ridge
waveguide (RWG) structure. The native oxide grown directly on the waveguide ridge simultaneously provides excellent
electrical insulation, passivation of the etch-exposed bipolar active region, and a low refractive index cladding, leading
to numerous laser performance benefits. The resulting strong lateral optical confinement at the semiconductor/oxide
interface (with refractive index contrast &Dgr;n~1.7) enables half-racetrack ring resonator lasers with a record small 6 &mgr;m
bend radius. A nearly circularly-symmetric output beam is demonstrated on narrow w=1.4 &mgr;m aperture width straight
stripe-geometry lasers with single spatial and longitudinal mode total power output of ~180 mW at 228 mA (9x
threshold). With the complete structural elimination of lateral current spreading, the excellent overlap of the optical field
with the gain region provides high slope efficiency performance (ranging from >1.0 W/A at w=1.4 &mgr;m to 1.3 W/A for
w=150 &mgr;m broad area stripes) for 300 K cw operation of unbonded, p-side up 808 nm InAlGaAs graded-index separate
confinement heterostructure (GRINSCH) active region lasers. Using the direct thermal oxidation of a dilute nitride
GaAsP/InGaAsN MQW active region, 1.3 &mgr;m emission GaAs-based HIC RWG lasers exhibit a >2X threshold reduction
and kink-free operation relative to conventional low-confinement devices. Other recent progress on the application of
nonselective oxidation to GaAs-based semiconductor lasers will be reported.
A simple, novel self-aligned deep etch plus wet thermal oxidization process is demonstrated which enables high-index-contrast (HIC) ridge waveguide (RWG) lasers fabricated in a high-efficiency, high-power AlGaAs/InAlGaAs/GaAs graded-index separate confinement heterostructure to operate with a curved half-ring resonator geometry having a bend radius as low as 10 μm. A wet thermal oxidation process modified through addition of <1% O2 to the N2 carrier gas is shown to smooth the sidewall roughness of etched AlGaAs ridge structures 10-100 fold as the oxidation front progresses inward. The reduction of propagation scattering loss due to the reduced sidewall roughness is examined. The thermal oxide grown on the deeply-etched RWG sidewalls and base also provides electrical isolation from the contact metallization, resulting in a simplified, self-aligned process, and yields a RWG structure which effectively prevents current spreading. The thermal oxide appears to be of sufficiently high quality to passivate the etched active region surface based on a comparative analysis of straight RWG lasers of varying stripe widths (w=5 to 150 μm) passivated with native-oxide vs. PECVD-deposited SiO2. For example, at w<15 μm, the SiO2-insulated devices have ~2X higher threshold current densities than the native-oxide devices for comparable bar lengths. The resulting high lateral optical confinement factor at the semiconductor/oxide interface (Δn=1.69) significantly enhances the laser gain and efficiency. A native-oxide-defined straight laser (w=7 μm, L= 452 μm) operates cw (300 K, unbonded, p-side up) with a threshold current of Ith=21.5 mA (Jth=679.5 A/cm2) and slope efficiency of 1.19 A/W (differential quantum efficiency = 78%) at a wavelength of ~813 nm.