980-NM ALUMINUM-FREE INGAAS/INGAASP/INGAP GRIN-SCH SL-QW LASERS

The design of 980-nm InGaAs/InGaAsP/InGaP GRIN-SCH lasers with aluminium-free GaAs-based materials is discussed. The first approach was successful MOCVD growth of InGaAsP alloy lattice matched to GaAs. It was found that an immiscible region existed, as determined by photoluminescence measurements. The main approach was to introduce the graded bandgap structure consisting of InGaAsP layers lattice matched to GaAs into GaAs/InGaP interfaces. The graded structure suppresses the heterojunction spikes, especially of the valence band at these interfaces. As a result, series resistance of GRIN-SCH lasers with the graded bandgap structure was reduced compared with simple SCH lasers with abrupt bandgap interfaces due to improved hole injection. Furthermore, the optimum graded structures for optical confinement region were investigated to improve the carrier injection efficiency, especially electron injection efficiency into a single quantum well active layer. Also, this graded bandgap structure formed the graded refractive index profile in an active region, which is the so-called GRIN-SCH waveguide. The GRIN-SCH profile could be controlled to narrow the transverse beam divergence for high coupling efficiency into a single-mode fiber and to reduce the optical power density at facets for high reliability. Finally the results of a life-test of GRIN-SCH lasers was shown, and the lifetime of GRIN-SCH lasers with immiscible InGaAsP layers was discussed.