Optical gain due to excitonic transitions in ZnCdSe/ZnMgSSe strained layer quantum well blue–green lasers: Prediction of low threshold under tensile strain

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Abstract
Gain coefficient of excitonic transitions is calculated for unstrained as well as strained ZnCdSe/ZnMgSSe multiple quantum well(MQW) lasers as a function of operating wavelength and injection current. Unlike the III–V strained layer quantum well lasers, the gain coefficient due to excitonic transitions is significantly higher in II–VI systems as the exciton binding energy is more than 5 times larger. This results in a primary role for excitons in lasing which has been verified experimentally. Strain induced changes in energy band gaps and effective masses of light and heavy holes are included in the gain and threshold current density calculations, which are in agreement with experimental data available for compressive strained layers grown on GaAs substrates. Significantly low threshold current density of about 180 A/cm2 are predicted for the tensile strained ZnCdSe–ZnMgSSe and ZnCdSe–ZnSeTe quantum well lasers using InP substrates.

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