Solid phase epitaxial (SPE) crystallization of amorphous GaAs on (100) Si tilted by 4° toward ⟨011⟩ formed by molecular beam deposition (MBD) was first achieved by cw Kr laser irradiation for short durations. The ratio of As to Ga (y/x) in deposited amorphous GaxAsyfilms was varied from 0.4 to 1.2. During the laser irradiation, movement of the amorphous/crystalline interface was measured using time‐resolved optical reflectivity (TROR). It was found from TROR and micro‐Raman scattering measurements that hetero‐SPE is attained in samples with As/Ga ratios ranging from 0.8 to 1.1 and that the interface roughness is larger than that observed in homo‐SPE (e.g., MBD GaAs on GaAs and P+ion‐implanted GaAs).