Reliability of strained quantum well lasers

Abstract

Strained quantum well lasers have demonstrated remarkably improved characteristics compared to unstrained quantum well lasers. For extracting the highest level of performance, the required strain may be large. An important factor in the use of strained quantum wells is the long-term stability of the pseudomorphic active region and the associated reliability of the device. The effect of strain on reliability is investigated, in particular, for In/sub x/Ga/sub 1-x/As/GaAs (x=0.2, 0.25, and 0.3) multiple quantum well lasers in 64 mW/facet constant output power tests at 85/spl deg/C for 40 hours. Laser characteristics such as the operating currents (I/sub op/), the threshold currents (I/sub th/), and the slope efficiencies (dL/dI) are measured during the test and serve as useful degradation parameters. The average changes in I/sub op/ are 15, 9.9, and 0.22%, and the average changes in I/sub th/ at 85/spl deg/C are 21, 8.7, and -1.2% for x=0.2, 0.25, and 0.3, respectively. The average changes in dL/dI at 85/spl deg/C are -19, -14, 1.5%, respectively. Defect migration into the pseudomorphic active region is verified to be the dominant mechanism of degradation observed in these lasers. Hence, to account for the strain-induced reliability improvement, it is necessary to study the propagation of defects in semiconductor heterostructures. A theoretical model is constructed based on the the linear theory of elasticity, and relevant experiments are conducted for its support. Strain energy considerations show that defect propagation across a strained layer is unfavorable. The nonradiative defect densities in the GaAs-Al/sub 0.4/Ga/sub 0.6/As quantum wells with and without the surrounding pseudomorphic In/sub 0.2/Ga/sub 0.8/As layers are compared by measuring the photoluminescence intensities after intentionally creating defects and enhancing their diffusion. The structures with pseudomorphic In/sub 0.2/Ga/sub 0.8/As layers consistently show much higher quantum well photoluminescence intensity by as much as 130 times, thereby confirming our model. These results clearly account for the observed reliability improvement in quantum well lasers with increased strain in the well. >