Spontaneously formed long-range Al-rich and Ga-rich AlxGa 1-xAs/AlyGa1-yAs superlattice in (111)A was demonstrated. This was observed by cross-sectional transmission electron microscopy (TEM) in 0.75 μm Al0.30Ga0.70As grown on (111)A GaAs substrates at 640°C. In contrast, none of the above superstructure was observed by TEM on a side-by-side grown (100) oriented substrate. 15 K photoluminescence (PL) showed a 31 meV redshift and six times peak intensity enhancement in layers grown on (111)A substrates to that on (100). A reduced long-range compositional modulation can be achieved by growth at higher temperatures and is shown in the finer and moderately modulating (111)A Al0.40Ga0.60As grown at 700°C. A 15 K PL linewidth of 17 meV was achieved in 700°C grown (111)A A 0.40Ga0.60As that is the narrowest reported linewidth for (111)A AlGaAs.