Characteristics of MoO3 films grown by molecular beam epitaxy

Characterization of the MoO3 films grown by molecular beam epitaxy on c-plane sapphire substrates was conducted. X-ray diffraction and Raman scattering measurements revealed that amorphous, (100) β-phase, and (010) α-phase MoO3 films were preferentially grown at 150, 200, and 350 °C, respectively. Their optical bandgap energies were estimated to be ∼3.5 eV for the amorphous, ∼3.7 eV for the β-phase, and ∼4.1 eV for the α-phase films. Intense near-band-edge emission was observed from the α-phase films even at room temperature. Postgrowth annealing effect on the β- and the α-phase MoO3 films was also studied, and it was found that the β-phase films were completely transformed into stable α-phase films at 600 °C, accompanied by a bandgap increase to ∼4.1 eV.