Threshold current variations and optical scattering losses in (Al,Ga)As double-heterostructure lasers

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Abstract
Photoluminescence studies have revealed that the active‐layer waveguide of (Al,Ga)As double‐heterostructure laser material often contains significant thickness variations which are introduced during the epitaxial layer growth. Examples of defects which may exist over substantial portions of an epitaxial wafer (12 mm×12 mm) are growth terraces and meniscus‐dissolution lines. Observations of surface morphology alone are inadequate to gauge the actual active‐layer deformations due to these defects. Angle‐lap measurements permit a quantitative characterization of the growth defects. Both classes of defects can introduce several discrete optical‐scattering disturbances within the gain volume of the laser. The works of Kogelnik and Marcuse are used to theoretically model the defects and compute optical scattering losses as functions of the parameters which describe the severity of the local waveguide deformations. When representative characterizations of observed defects are used in the calculations for scattering loss, it is found: (i) that significant threshold variations (tens to hundreds of percent) are predicted; (ii) that the computed range of threshold scatter agrees with the experimentally observed range of threshold scatter; and (iii) that in one specific instance the relatively elevated threshold of a particular laser agreed reasonably with the computed threshold based upon a knowledge of the defects actually present in the device.