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(Al,Ga)As layers have rough surface morphologies when deposited under certain growth conditions in molecular beam epitaxy (MBE). This leads to poor interfaces between GaAs and (Al,Ga)As and degraded performance in heterojunction devices. We have observed that by misorienting the substrate slightly from (100), in a manner specific to the growth conditions, smooth (Al,Ga)As layers can be grown at 675 °C for an Al mole fraction of 0.15. Similar conditions for nominal (100) result in a rough, textured morphology. The results suggest that the roughness is due to an energetic instability at the growth surface with respect to the formation of features such as terraces and hillocks. To our knowledge, this is the first reported experimental verification of singular instabilities in (Al,Ga)As grown by MBE. Smooth layers obtained by using an optimal misorientation of 2° 45’ from (100) towards (111)A also exhibit superior optical properties as determined from low‐temperature photoluminescence measurements. These findings may have major implications for the performance of heterojunction device structures grown by MBE.