Reflection high energy electron diffraction (RHEED) has been used to track the evolution of both low temperature (LT) GaAs and LT-AlxGa1-xAs growth surfaces. LT-AlxGa1-xAs surfaces have been found to roughen by a pyramidal defect formation similar to that reported for LT-GaAs over the entire range of aluminum compositions. Correlations between RHEED and transmission electron microscopy (TEM) results indicate that the critical thickness found using the formation of extra fourth order diffraction spots as a marker correlate well to the nucleation of pyramidal defects. Additionally, results for the critical thickness of layers grown over the range of aluminum compositions indicate an exponential decrease in the critical thickness with increasing aluminum composition under similar growth conditions. While the mechanism for this dependence is still unclear, it does not appear to be related to an increase in the excess arsenic incorporation with increasing Al composition.