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) beta-phase, and (010) alpha-phase MoO3 films were preferentially grown at 150, 200, and 350 degrees C, respectively. Their optical bandgap energies were estimated to be similar to 3.5 eV for the amorphous, similar to 3.7 eV for the beta-phase, and similar to 4.1 eV for the alpha-phase films. Intense near-band-edge emission was observed from the alpha-phase films even at room temperature. Postgrowth annealing effect on the beta- and the alpha-phase MoO3 films was also studied, and it was found that the beta-phase films were completely transformed into stable alpha-phase films at 600 degrees C, accompanied by a bandgap increase to similar to 4.1 eV. (C) 2014 The Japan Society of Applied Physics