The effects of strain on the threshold current density in InGaAsP/InP strained-layer single-quantum-well lasers

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Abstract
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 k⋅p 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.