The epitaxial deposition of (111)-oriented GaAs on substrates consisting of BaF2 (111)/Si (100) heterostructures has been investigated. The temperature dependence (from 575 to 650 degrees C in 25 degrees C steps) of the initial stages of deposition was examined. Substrates were exposed to Ga and As fluxes simultaneously during these depositions. In situ x-ray photoelectron spectroscopy and reflection high-energy electron diffraction showed that the GaAs grows epitaxially between 575 and 625 degrees C. The amount of material adhering to the surface during deposition decreases as the substrate temperature increases. GaAs does not adhere to the surface at 650 degrees C. Analysis of the Ba 3d(5/2) photoelectron peak indicated a surface chemical reaction between Ba and the GaAs. In order to determine which species was reacting with the Ba atoms, a BaF2 (111) surface was exposed separately to Ga and As fluxes at 625 degrees C and to Ga at 650 degrees C. No chemical interactions were observed, indicating that the presence of both species is required before the chemical reaction and subsequent epitaxial deposition will occur. A chemical reaction that produces a gaseous GaF byproduct resulting in a Ba "template" layer is proposed. This Ba layer facilitates the epitaxial deposition of the GaAs layer.