THE EFFECTS OF STRAIN ON THE THRESHOLD CURRENT-DENSITY IN INGAASP/INP STRAINED-LAYER SINGLE-QUANTUM-WELL LASERS

Basic design principles are formulated for minimizing the threshold current density in InGaAsP/InP strained-layer single-quantum-well (SL-SQW) lasers. A quaternary InGaAsP active layer is shown to provide more freedom in design than a ternary InGaAs active layer because the amount of strain (both tension and compression) and quantum-well thickness can be independently determined in the InGaAsP system for a given emission wavelength. Strain-induced changes in the valence-band structures are analyzed within the framework of kp theory by taking into account the interaction with spin-orbit split-off bands as well as heavy-hole and light-hole bands. It is clarified that the quantum-well thickness plays a more significant role than the amount of strain when designing compressive-strained wells, while the situation is just the opposite in tensile-strained wells. It is shown that, although the application of biaxial tension reduces the threshold current density in bulk-like SL-SQW lasers more significantly than biaxial compression, the quantum-confinement effect has a pronounced impact on the reduction in the current density in compressive-strained wells. This makes either type of strain attractive for reducing the threshold current density in InP-based SL-SQW lasers.