The solid-state reaction between monocrystalline AlGaAs(100) and polycrystalline Fe films, having thickness in the 50-100 nm range, has been investigated using X-ray diffraction, conversion electron Mossbauer spectroscopy and transmission electron microscopy. The Fe films were deposited on ion-etched semiconductor substrates using an ion-beam-sputtering technique. It has been found that, in the as-deposited state, the thickness of the interfacial zone never exceeds 1.5 nm, a value comparable to that obtained for Fe films deposited by molecular-beam-epitaxy. The interface reaction of the Fe films was also studied after annealing in vacuum at 400 degreesC for 1 h. The orientation of the magnetization and the grain size of the deposited metallic layers are reported for the as-deposited, as well as for the annealed films. Interdiffusion after annealing mainly results in the generation of Fe2As grains. These grains were observed in the AlGaAs substrate, exactly under the Fe/AlGaAs interface, and they have a triangular shape; this is the first report on the formation of grains with such a shape. Additional analysis on the orientation relationship between the Fe2As grains and the substate was carried out and the results are also presented here. Studies concerning the diffusion of Al, Ga and As atoms into the Fe overlayer revealed the low mobility of As, as compared to Al and Ga.