Systematic studies have been made for the first time on the basic properties of AlxGa1−xAs (x=0.2–0.7) grown by molecular beam epitaxy in the wide growth temperature range of 540–780 °C with As4and As2. Theforbiddengrowth temperature region (FTR), where the specular smooth surface cannot be obtained, has been found to depend strongly upon both the As species and the AlAs mole fractionx. FTR does not change withxin the case of As4; however, in the case of As2, FTR does not exist forx=0.2 and it increases withxfrom 0.3–0.7. Photoluminescence ofn‐Al0.3Ga0.7As (Si=1×1018cm−3) grown with As2shows lower intensity and higher sensitivity to growth temperature than those of samples grown with As4. Deep level transient spectroscopy has been measured onn‐Al0.7Ga0.3As (Si=2×1016cm−3). New electron traps are found in layers grown with As2.