Ultrathin MoO3 layers have been grown on Si substrates at 120 degrees C by atomic layer deposition (ALD) using molybdenum hexacarbonyl [Mo(CO)(6)] and ozone (O-3) as the Mo- and O-source precursors, respectively. The ultrathin films were further annealed in air at T-ann = 550-750 degrees C for 15 min. Scanning-electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy have been employed to evaluate the morphological and elemental properties as well as their evolutions upon annealing of the thin films. They revealed an interfacial SiOx layer in between the MoO3 layer and the Si substrate; this SiOx layer converted into SiO2 during the annealing; and the equivalent thickness of the MoO3 (SiO2) layer decreased (increased) with the increase in Tann. Particles with diameters smaller than 50 nm emerged at T-ann = 550 degrees C and their sizes (density) were reduced (increased) by increasing T-ann to 650 degrees C. A further increase of T-ann to 750 degrees C resulted in telephone-cord-like MoO3 structures, initiated from isolated particles on the surface. These observations have been discussed and interpreted based on temperature-dependent atomic interdiffusions, surface evaporations, and/or melting of MoO3, which shed new light on ALD MoO3 towards its electronic applications. (C) 2018 Elsevier B.V. All rights reserved.