A substantial difference has been found in the initial incorporation mechanism of excess As into low temperature grown (LTG) (Al,Ga)As compared to LTG GaAs. In as-grown LTG GaAs the excess As initially incorporates as point defects (interstitials and/or antisite defects) resulting in an expansion of the lattice parameter. Only upon subsequent annealing does the excess As coalesce into precipitates, allowing the lattice parameter to relax back to its normal value. In contrast, as-grown LTG (Al,Ga)As shows no expansion of the lattice parameter until close to the maximum achievable excess As concentration has been reached, and most of the excess As is incorporated immediately as small precipitates. ©1997 American Institute of Physics.