분무열분해법으로 성장된 BaTiO3 와 BaTiO3 : Eu3+ 박막의 구조와 광학적 특성

Abstract

BaTiO3 and BaTiO3:Eu3+ films were deposited on glass substrates using spray pyrolysis. The precursor solutions were obtained by varying the concentration of barium acetate, titanium oxide acetylacetonate, europium nitrate pentahydrate in twice-distilled water. A spray solution of 0.02M concentration nebulized by ultrasonic vibration was sprayed for 120min. at a flow rate of 2cc/min onto a glass substrate which was kept at a constant substrate temperature ranging from 350℃ to 500℃. The crystal quality and phase of BaTiO3 and BaTiO3:Eu3+ films were characterized by X-ray diffraction(XRD) spectroscopy in θ～2θ geometry. The surface morphology, microstructure and thickness of the films were determined by using a scanning electron microscope (SEM), and stoichiometry of the perovskite type BaTiO3 and BaTiO3:Eu3+ films were measured through energy dispersive X-ray spectroscopy (EDS). The optical transmission and absorption of BaTiO3 and BaTiO3:Eu3+ films were measured by using UV-VIS-NIR spectrophotometer in wavelength ranging from 200～1200nm. Photoluminescence (PL) of BaTiO3 and BaTiO3:Eu3+ films were characterized by a PL device consisting of a double spectrometer, a PM-tube, a Ge-detector and cryogenic system. Photoluminescence of BaTiO3:Eu3+ films were measured at 300K. The crystal structure of the BaTiO3 and BaTiO3:Eu3+ thin films prepared at various substrates temperatures and impurity concentrations of Eu3+ were identified with a polycrystalline phase with cubics of (100), (110), (111), (200), (210), (211), (220) planes and hexagonal of (101), (004), (202), (205), (214), (109) planes. The lattice parameter of the cubic BaTiO3 grain was a 4.031 Å and the lattice parameter and of the hexagonal BaTiO3 grain were 5.720 and 13.965 Å. The crystaline size of BaTiO3 thin films and annealed-BaTiO3 thin films were identified with 8.14～12.21nm and 8.14～8.88nm. The crystalline size of the non-annealed samples was larger than that after annealing. It can be inferred that annealing of the samples led to the tensile stress in the films because the interatomic spacing and lattice parameters were reduced. The surface morphologies of BaTiO3 films prepared at various substrate temperatures and the annealed BaTiO3 films at 650℃, as deduced from the SEM microstructure, show that the films consisted of hexagonal-like grains and cubic-like grains. The chemical composition of the BaTiO3 and BaTiO3:Eu3+ films obtained from the EDS peaks indicates that the atomic percentage of Ba, Ti and Eu of the as-prepared films is nearly the same as those of the spray solution in the limits of detector resolution. The chemical composition of the Ba and Ti of the BaTiO3 films annealed at 650℃ in the air were found to be 89.67 and 89.54 at.%, respectively. The chemical composition of the Ba, Ti and Eu of the BaTiO3:Eu3+ films deposited with Eu3+ impurity concentration of 3.0 mol% were identified as 15.86, 15.85 and 0.21 at.%, respectively, except for the Si due to the glass substrate. Since oxygen also enters in to the composition of the glass substrate, this element was not evaluated in order to avoid erroneous results. The optical transmission spectra of the BaTiO3 films deposited at various substrate temperatures was about 20～90% in the wavelength range of 200～1200nm. The optical absorption spectra of the BaTiO3:Eu3+ films deposited with Eu3+ impurity concentration above the 1.0mol% Eu3+ showed a fundamental absorption edge shift to the longer wavelength, whereas these edges shifted to the shorter wavelength for the samples prepared at Eu3+ concentration of 0.0～1.0mol%. The surface morphology, the grain size and the grain boundary width of the BaTiO3 films affected the lattice constant and the optical energy band gap of the samples. Direct band gap and indirect band gap of BaTiO3 were found to be 3.275∼3.545eV and 3.027～3.145eV, respectively. The shift of the optical energy band gap of as-prepared and annealed BaTiO3 are due to the changes in the lattice constant and the grain size of the samples. From the results of photoluminescence of BaTiO3:Eu3+ films, the emission line appearing at 580nm could be attributed to the 5D0 → 7F0 transition, peaks at 593nm to 5D0 → 7F1, the line at 615nm corresponds to 5D0 → 7F2 transition, and the lines at 661nm and 702nm to 5D0 → 7F3 and 5D0 → 7F4 transitions, respectively. The emission peak at 593nm (5D0 → 7F1) is known as a magnetic dipole transition, and the peak at 615nm (5D0 → 7F2) is an electric dipole transition. The 5D0 → 7F2 electric dipole transition, which is here much stronger than the 5D0 → 7F1 magnetic dipole transition, is independent of the symmetry site of the Eu3+ ions.