X-ray diffraction and infrared spectroscopy measurements are conducted in order to assess the crystallographic structure and chemical purity of lanthanum oxide (La2O3) films grown by atomic layer deposition (ALD) on Si substrates. In situ capping with thin aluminum oxide (Al2O3) layer is proved to be beneficial in preventing the formation of lanthanum hydroxide phases. The effect of two process parameters, namely, La2O3 film growth temperature (260–500°C range) and postdeposition annealing temperature (600–1100°C range), on the chemical and structural evolutions of Al2O3∕La2O3∕Si stacks is discussed. This study enables the identification of the optimum ALD growth recipe yielding the highest hexagonal La2O3 phase content, which might be suitable for integration into innovative metal oxide semiconductor devices.