High-index-contrast ridge waveguide lasers fabricated via oxygen-enhanced wet thermal oxidation

Abstract

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 &#956;m. 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 &#956;m) passivated with native-oxide vs. PECVD-deposited SiO₂. For example, at w<15 &#956;m, 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 (&#916;n=1.69) significantly enhances the laser gain and efficiency. A native-oxide-defined straight laser (w=7 &#956;m, L= 452 &#956;m) operates cw (300 K, unbonded, p-side up) with a threshold current of I_th=21.5 mA (J_th=679.5 A/cm²) and slope efficiency of 1.19 A/W (differential quantum efficiency = 78%) at a wavelength of ~813 nm.