Surface migration of the group-III precursors and strain relaxation at the ridge sidewalls are compared for wide waveguides based on InGaAsP and InGaAlAs multiple-quantum-well (MQW) structures. The cross-sectional thickness and strain variations have been measured using synchrotron radiation-based x-ray diffraction with an angular resolution of and a beam size of . Indium-rich overgrowth has been observed for the InGaAsP-based waveguides, while InGaAlAs-based waveguides demonstrate thickness uniformity of the MQW active region with a strain relief of at the sidewalls.
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For MOVPE growth controlled by diffusion, the uniform thickness across a certain area of the wafer corresponds to a constant growth rate, which can be explained only by a uniform composition in such area. Otherwise (composition changes across the ridge, but thickness is uniform), in the diffusion model describing such an improbable situation, at least one of the precursors should migrate along the gradient of its own concentration.