We have grown InGaAs/InAlAs quantum wells and InGaAs, InAlAs single layers over InP nonplanar substrates. Photoluminescence, transmission electron microscopy, and energy‐dispersive x‐ray analysis give evidence for large lateral thickness and compositional variations, which lead toinsitumodification of the growth profile. These effects, which differ for InGaAs and InAlAs, reveal the importance of surface migration in (Al,Ga)InAs. Our results demonstrate a tool forinsitulateral patterning of this material system.