Silicon nitride encapsulation of sulfide passivated GaAs/AlGaAs microdisk lasers

Abstract

SiNx encapsulation of sulfide passivated GaAs/AlGaAs surfaces was examined with respect to the SiNx deposition conditions and applied to GaAs/AlGaAs microdisk lasers. The SiNx encapsulation is a method for preserving the positive effects (e.g., reduced surface recombination velocity, Sv) of the sulfide passivation by inhibiting the oxidation process of the air-exposed semiconductor. GaAs-based microdisks, with diameters of 2–10 μm, provide a very sensitive probe of the surface recombination since the resonator mode is in close proximity to the edge of the disk (∼<0.5 μm) and the carrier diffusion length is typically several microns. An ammonium sulfide solution was used for the S passivation. The SiNx was deposited by plasma-enhanced chemical vapor deposition (PECVD) at 50 or 300 °C, with and without post-deposition annealing (300–400 °C), or by electron cyclotron resonance chemical vapor deposition (ECR-CVD) at 100 °C. Photoluminescent measurements of GaAs/InGaP epitaxial structures provided an indication of the relative change in Sv. The GaAs/AlGaAs microdisks do not lase without sulfide treatment when optically pumped at 77 K. With the sulfide treatment alone, cw lasing was achieved but the lifetime was only several seconds. A dramatic increase in the laser lifetime was obtained using PECVD-deposited SiNx at 300 °C. The laser output for the PECVD SiNx (300 °C) encapsulated microdisks could be increased by nearly an order of magnitude by annealing at 400 °C for 300 s or by cw operation for several hours. Initial results using the ECR-deposited SiNx films suggest improved performance (lower threshold, stability) compared to the PECVD SiNx. The laser performance is correlated with the photoluminescence results.