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The influence of substrate misorientation on the quality of the active region with low AlAs portion barrier layers and the .performance of the GaAsIAlGaAs grade index separate-confinement triplequantum-well lasers grown under nonoptimal conditions have been investigated. It is found that the threshold current could be reduced from 32.5 mA to 15.7 mA by tilting the substrate 6" and the high quality lasers show a characteristic temperature as high as 260 "C, indicating a high output stability. The luminescence results reveal that carbon impurity, like oxygen, could also be incorporated and trapped at and near AlGaAsIGaAs interfaces during growth and cause roughness, resulting in excessive scattering and loss and thus a high threshold current. Substrate misorientation for 6" toward c111>A could reduce the incorporation of carbon impurity to an unobservable level.