Growth of epitaxial oxides on silicon using atomic layer deposition: Crystallization and annealing of TiO₂ on SrTiO₃-buffered Si(001)

Epitaxial anatase titanium dioxide (TiO₂) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer without any amorphous SiO_x layer at the STO-Si interface. Four unit cells of STO grown by molecular beam epitaxy (MBE) serve as the surface template for ALD growth. To preserve the quality of the MBE-grown STO, the samples were transferred in situ from the MBE chamber to the ALD chamber. The growth of TiO₂ was achieved using titanium isopropoxide and water as the coreactants at a substrate temperature of 250 °C. In situ x-ray photoelectron spectroscopy analysis revealed that the ALD process did not induce Si-O bonding at the STO-Si interface. Slight improvement in crystallinity of the TiO₂ film was achieved through in situ annealing under vacuum (10^-9 Torr) at 450-600 °C. However, the amount of Si-O bonding increased following annealing at temperatures greater than 250 °C. X-ray diffraction revealed that TiO₂ films annealed at a temperature of 250 °C in vacuum (10^-9 Torr) for 1 h were the anatase phase and well crystallized. The results indicate that careful consideration of growth temperature and annealing conditions may allow epitaxial oxide films to be grown by ALD on STO-buffered Si(001) substrates without formation of an amorphous SiO_x layer.