In this article, three novel cyclopentadienyl precursors are evaluated for the atomic layer deposition (ALD) of erbium oxide, with either ozone or water as the oxygen source. The erbium precursors evaluated are Er(iPrCp)3, Er(MeCp)2(iPr-amd), and Er(nBuCp)3. The films are deposited on silicon within the temperature range 200–400°C. Self-limiting growth is achieved with all three precursors, with both ozone and water. It is found that the water processes of all three precursors present significantly higher growth rates when compared to the ozone processes. An up to three-fold increase in the growth rate is observed for the water processes of Er(iPrCp)3 and Er(MeCp)2(iPr-amd) (amd: amidinate) when compared to their ozone processes. The films are smooth and uniform, as determined by atomic force microscopy (AFM) (rms roughness < 3% of film thickness). The composition of the films is investigated by means of X-ray photoelectron spectroscopy (XPS). It is found that the films contain small amounts of carbon as an impurity, especially in the case of ozone-processed films. Using Er(nBuCp)3 together with ozone as the oxygen source, a highly conformal Er2O3 thin film is deposited on a 1:60 high-aspect-ratio substrate. This is the first report of the conformal growth of Er2O3 thin films by ALD on a high-aspect-ratio structure.