Moo3 Films Grown On Stepped Sapphire (0001) By Molecular Beam Epitaxy

MoO3 films were grown on stepped c-plane sapphire substrates by molecular beam epitaxy using MoO3 vapor from a conventional Knudsen cell. Stepped sapphire (0001) substrates were prepared by ex situ annealing at 1100-1300 degrees C in dry air. Step bunching typically resulted in multistepped surfaces with wide atomically smooth terraces. Ex situ annealing at 1100 degrees C followed by in vacuo annealing at 700 degrees C provided clean substrates for growth. Ultrathin films were grown at 450 degrees C via a self-limiting process that represents a balance between the incident MoO3 flux and the desorption flux. Elongated bilayer islands (0.7-nm thick) were formed on sapphire (0001) terraces. Monocrystalline alpha-MoO3 (010) thin films [(010)(alpha-MoO3)parallel to(0001)(sapphire)] were grown at 450 degrees C using a higher incident MoO3 flux and characterized by atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and cross-sectional transmission electron microscopy. The step-terrace surface morphology of the monocrystalline films strongly suggests multilayer growth.