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The authors measure the combined affect of strain and well width on the gain and recombination mechanisms in 635-nm laser structures containing three combinations of tensile strain and well width of 0.5%/10 nm, 0.6%/12.5nm, and 0.7%/15nm using the segmented contact method. They find an improvement in the intrinsic properties with increasing strain but the dominant effect in device performance is an extrinsic effect-the overall radiative efficiency, which is found to be less than 30% for all three samples even at 200 K. The authors attribute this to nonradiative recombination within the quantum well. The intrinsic gain-spontaneous current density characteristics of all three samples exhibit similar tangential gain parameters and a decreasing transparency current density from 116 to 87 to 83 Acm/sup -2/ with increasing strain and well width. They show that the reduction occurs because of a reduction in the TE polarized spontaneous recombination due to the increased splitting of light and heavy hole subbands. The quasi-Fermi level separation required to achieve a fixed value of gain is insensitive to the particular strain/well width combination. The authors use a microscopic laser theory to model the behavior of a larger range of combinations of tensile strain and well width than were examined experimentally, having first demonstrated that the model correctly describes the experimental gain spectra of the only sample exhibiting appreciable gain in both TM and TE polarizations. The calculated data suggest that using still larger values of strain and well width produces no further benefit in performance.
Selected Topics in Quantum Electronics, IEEE Journal of (Volume:9 , Issue: 5 )
Date of Publication: Sept.-Oct. 2003