GaAs/AlGaAs quantum wells (QWs) were grown by molecular beam epitaxy on GaAs (100) substrates patterned with ridges and grooves in the [011¯] direction. Low‐temperature cathodoluminescence was used to measure the Al fraction and QW thickness on top of the ridges and grooves as a function of ridge and groove width. Surface diffusion during growth depletes Ga from the side facets while increasing the incorporation of Ga on the (100) sections of ridges and grooves. The QW thickness on top of a ridge grown at 710 °C increases from 72 to 95 A˚, and the Al fractionxdecreases fromx=0.33 tox=0.29 as the ridge width is narrowed from 30 to 4 &mgr;m. Graded refractive index separate confinement heterostructure lasers with nominally 70 A˚ QWs and Al0.2Ga0.8As barriers were grown on patterned substrates at 695 and 725 °C. Lasers fabricated on the overgrown 4‐&mgr;m‐wide ridges have a 20 meV decrease in emission energy compared to laser diodes on 30 &mgr;m ridges.