Epitaxial layers of GaAs and AlxGa1−xAs (0.2≤x≤0.5) have been grown directly on single‐crystal Si (100) substrates, without an intermediate Ge layer, by molecular beam epitaxy (MBE). To improve nucleation, after being chemically cleaned the Si substrates were preheated in the MBE system to reduce surface contamination and achieve ordered surface reconstruction prior to growth. Reflection electron diffraction, Rutherford backscattering channeling, and photoluminescence measurement were used to characterize the epitaxial layers. The AlGaAs layers were found to be superior to the GaAs layers.