MoO3 thin Films were prepared using the assisted laser evaporation technique. Samples were grown on glass
and silicon substrates at different substrates temperatures. The effect on structural and optical properties of
the substrate and on annealing temperatures was evaluated. A phase transition was found around 200 °C in all
samples from the amorphous to the β phase with a small percentage of α phase, and another one was found
around 500 °C from the α+β to the α phase. The percentage errors between the lattice parameter a0 of the
crystallographic index card for the MoO3 alpha phase and the indexed lattice parameters were 1.4% and 0.3%
for the samples deposited on glass and silicon respectively, indicating the crystalline structure of the silicon
substrate favors the formation of the MoO3 alpha orthorhombic phase. The spectral variation of the refractive
index and the absorption coefficient were theoretically determined. The amorphous samples presented a
constant gap of 3.2 eV while the optical properties critically depended on the substrate and annealing
temperatures.