Reliability of strained quantum well lasers

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_xGa_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°C for 40 hours. Laser characteristics such as the operating currents (I_op), the threshold currents (I_th), and the slope efficiencies (dL/dI) are measured during the test and serve as useful degradation parameters. The average changes in I_op are 15, 9.9, and 0.22%, and the average changes in I_th at 85°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°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_0.4 Ga_0.6As quantum wells with and without the surrounding pseudomorphic In_0.2Ga_0.8As layers are compared by measuring the photoluminescence intensities after intentionally creating defects and enhancing their diffusion. The structures with pseudomorphic In_0.2Ga_0.8As 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