Lanthanum oxide thin films were deposited at deposition temperatures ranging from 170 to 370 °C using alternate injection of tris(isopropyl-cyclopentadienyl)lanthanum and various reactants (H2O, O2, and NH3). It was possible to deposit films containing a carbon impurity concentration ≪1 at. %. The residual carbon could be further reduced by postdeposition annealing. X-ray photoelectron spectroscopy (XPS) showed that after rapid thermal annealing (RTA) at 870 °C the films transformed to silicate which increased the band gap of the films (7.0±0.15 eV). With changing the RTA ambient from O2 to N2, a binding energy shift of the substrate’s Si peaks by 0.9 eV was observed in the XPS spectra relative to the signals from the oxide layer. The shift was attributed to the electric field in the oxide film caused by the fixed charge distribution. A flat band voltage shift indicating the change of the fixed charge was also observed according to the RTA ambient. From Pt/La-oxide/p-Si(100) structures, the dielectric constant and the interface trap density were estimated to be ∼16 and ∼4×1011 eV-1 cm-2, respectively. An equivalent oxide thickness of 0.9 nm was obtained after RTA.