Strain Dependence Of Dissociative Adsorption Of H₂O On Epitaxially Strained,out-of-plane Polarized,BaTiO₃（001） Thin Films

Ferroelectric material is a kind of spontaneous polarized material.Its spontaneous polarization is due to the shift of its positive and negative charge centers.Because of its excellent physical properties such as light,heat and electricity,it is widely used in electronic components and semiconductor devices.Barium titanate（BaTiO₃（BTO））is one of the ferroelectric materials,which demonstrates the properties of non-toxicity,non-pollution,piezoelectricity,thermoelectricity,etc.Thus it has been widely used in ceramic electronic devices and memories.The ferroelectric phases of BTO include tetragonal,orthogonal,and triangular phase.In this dissertation,the relationship between the lattice strain of tetragonal phase BTO（001）film and the desorption energy of H₂O molecules on its surface was studied.Since the lattice strain of BTO film has a close relation with the polarization,the increase of strain will also elevate the distortion of the ferroelectric material along the c-axis direction,and the positive and negative charge centers will shift more,resulting in stronger polarization.Compared to bulk ferroelectrics,the growth of high-quality BTO films can significantly increase the strain.The larger polarization of BTO film will strengthen its interaction with H₂O molecules on its surface.Here,by using molecular beam epitaxy（MBE）,BTO（001）thin films of 8,15 and 30nm in thickness were grown on strontium titanate（SrTiO₃（STO））（001）substrates.Different film thicknesses caused different strains.The strains of the BTO（001）films increased with the decrease of films’thickness.Water dissociated into OH^-and H⁺on the BTO（001）film with TiO₂ as the surface layer.OH^-can combine with oxygen vacancies or surface cations of the BTO（001）surface,while H⁺can combine with lattice oxygen atoms on BTO surface.In order to study the relationship between the desorption energy of H₂O molecules and strain on BTO（001）with different thicknesses,the following research process was adopted:（1）STO（001）substrate was etched with buffered NH₄-HF solution（BHF）and then annealed in O₂ to obtain a TiO₂-terminated surface composed of steps and atomically flat terraces.Then 8,15 and 30nm BTO thin film was grown on it.Afterwards 3 thin films with different thickness were exposed to purified H₂O vapor.（2）Characterization and analysis.By using X-ray diffraction（XRD）,the relationship between the thickness of BTO（001）film and lattice strain was obtained.The surface structure of BTO（001）before and after exposure to H₂O vapor was studied by low energy electron diffraction（LEED）.X-ray photoelectron spectroscopy（XPS）was used to confirm the adsorption state of H₂O on the surface of BTO（001）film.The desorption energies of H₂O on 8,15 and 30nm BTO（001）films were obtained by temperature programmed desorption（TPD）.According to the variation of desorption energy related to the strain of BTO（001）films with different thickness,it is found that the desorption energy of H₂O on the BTO（001）film increases significantly with the increase of strain.H₂O undergoes mainly dissociative adsorption on the polarized BTO（001）,and the chemisorbed OH^-species provide the screening,inverting the surface dipole layer and stabilizing the bulk polarization.