The effects of growth temperature and As4/Ga beam equivalent pressure ratio on molecular‐beam epitaxial Al0.25Ga0.75As were studied with deep level transient spectroscopy (DLTS), photoluminescence (PL), secondary ion mass spectrometry (SIMS), Hall effect, capacitance profiling, and Nomarski microscropy. The room temperature photoluminescence efficiency is controlled by a deep trap atEc−0.79 eV. Careful analysis shows that the trap concentration is independent of the oxygen concentration. The dependence of this trap on growth temperature and V/III ratio suggests that this trap is related to stoichiometric defects caused by arsenic rich growth. A strong correlation between trap concentration and surface roughness supports the idea that this trap forms as a result of nonstoichiometric growth. SIMS measurements show that the incorporation of oxygen, sulfur, and carbon decreases as the growth temperature increases. Finally, the Hall mobility is found to decrease as the growth temperature increases. This drop in mobility may be caused by material inhomogeneity caused by changes in the growth kinetics.