Hexagonal orthoferrite h-ErFeO3 thin films are synthesized by Atomic Layer Deposition on SiO2(100 nm)/Si substrate, followed by rapid thermal annealing at 650–700 °C. Structural, chemical and morphological characterizations of as-deposited and annealed layers are performed by X-ray Reflectivity/Diffraction and Time-of-Flight Secondary Ion-Mass Spectrometry. The formation of the hexagonal phase, which is metastable compared to the more stable orthorhombic ErFeO3, is explained within a simple model considering the different activation energies for the nucleation of hexagonal and orthorhombic phases. The possibility to grow h-ErFeO3 in contact with SiO2/Si by chemical methods opens perspective for the inclusion of new multiferroics in silicon-based devices.