We have studied the 77 K photoluminescence (PL) of undoped-AlxGa1−xAs(0.21⩽x⩽0.83)grown by organometallic vapor phase epitaxy. A deep broad (DB) PL band is found at 1.6–1.7 eV at a range ofxfrom 0.21 to 0.63, with a maximum intensity at aroundx=0.5.Its large full width at half-maximum(∼200 meV)suggests that this emission originates from some impurity-defect complex. The Si- and C-doping dependencies of the PL reveal that the emission disappears in Si-dopedn-type samples while it increases in intensity superlinearly with the hole concentration, thus, the emission center is C acceptor related. Furthermore, annealingAl0.52Ga0.48Assamples inH2flow eliminates the emission, while annealing inAsH3flow increases the emission. Based on these results, we have considered the doping and V/III dependencies of various major point defects present inp-type GaAs and those of the combinations of the C acceptor and these defects. It is concluded that the DB band originates from the substitutional C–As antisite complex(CAs–AsGa).Mass-action rule analysis of the complex deduces a quadratic increase in the PL intensity with hole concentration, which generally explains the experimental results. ©1997 American Institute of Physics.