Ferroelectric Strontium Barium Niobate (sbn) Thin Film Optical Properties

In order to satisfy the need of modern optical communication and optical information processing in the application areas, optoelectronic devices are developing in the direction of miniaturization and integration. Strontium barium niobate (SBN) possesses excellent dielectric, piezoelectric, pyroelectric, electro-optic and photorefractive properties. SBN is an attractive material for many applications such as electro-optic modulators, pyroelectric infrared detectors, holographic storage. Especially the electro-optic (EO) properties of SBN are drawing much more attention in recent years. The designing, fabricating and characterizing of SBN films have become one of the key focuses of film optoelectronics due to their considerable application potential. In this paper, the linear eletro-optic properties of SBN films have been investigated theoretically and experimentally. The method of characterizing SBN film electro-optic properties has been put forward, established and optimized.EO coefficient and half-wave voltage are the key parameters to characterize the EO properties. The material with large EO coefficient and low half-wave voltage meets the requirements of integrated optoelectronic devices. SBN film has large longitudinal EO coefficient, as well as SBN doped with potassium sodium ions (KNSBN) has large transverse EO coefficient, as a result our research focuses on measuring EO coefficients of SBN and KNSBN fims.Based on the principle of tunable Fabry-Perot (F-P) filter, a new method used to measure the EO coefficient was designed. Compared with previous methods, this method doesn't need to assume the relationship between the longitudinal EO coefficient r₁₃ and r₃₃, the value of them can be calculated independently. By the means of simulation, it is found that the wavelength of transmittance is tunable when the thickness of MgO substate is 0.01mm and SBN film is 1μm. Unfortunately, restricted by preparation process, measuring method and laboratory conditions, it is found to infeasible for the current stage. The modified simple reflection technique is chosen to measure the longitudinal EO coefficients of SBN films, eventually. This method is simple to operate and don't need to apply high voltage. However, it has acertain limit because the birefringence is ignored and the ratio of r^lr\ 3 is assumed according to the literatures.In this paper, according to the simple reflection technique, the expressions of measuring longitudinal E-0 coefficient of SBN films were deduced and two different types of sample structures were designed, one is SBN75/KSBN75/Si, and the other is SBN60/LiNiO2/MgO. To the former, the films were deposited by Sol-Gel process. In order to produce the buffer layer to improve the lattice mismatch between the SBN75 film and the Si substrate, the KSBN75 layers were post-annealed at 1000°C. Then the top pure SBN75 films were heat-treated at 750°C. It is found the SBN films had a highly c-axis preferred orientation perpendicular to the Si (100) substrate by the analysis of X-ray diffractometer(XRD). The measured longitudinal electro-optic coefficient r33 of SBN film on Si substrate is 98.44pm/V. To the latter, the sample was obtained using pulsed laser deposition (PLD) method in Applied Physics Department of Hong Kong Polytechnic University. The measured r^ of SBN film on MgO substrate is 186.61pm/V. Because the electro-optic coefficient of SBN thin film is related to the orientation of films, preparing method and the electrode material, although the value of measured r33 is comparatively smaller than the theoretic value, it exhibits larger value than that of LiNbC>3 crystal (rc=18.9pm/V).Compared with the large longitudinal electro-optic coefficient, the transverse electro-optic coefficient r$\ of SBN is always small. The studies show that the value of r$\ can be improved by ions doping. In our research, fully occupied ferroelectric KNSBN thin films with stoichiometric proportions have been successfully grown on MgO (001) substrate by the sol-gel process. Microstructure of the films was studied with X-ray diffraction, X-ray rocking curve, X-ray phi scan, atomic force microscopy and scanning electron microscope. It is found that the KNSBN thin films were c-axis epitaxial grown on MgO single crystal substrates. The intensity of the XRD peaks becomes weaker and the surface roughness increases with increasing Sr2+/Ba2+ ratios. The electro-optic properties of KNSBN films were measured by Adachi method, the transverse electro-optic coefficient 7-51 of KNSBN films can be greatly improved byintroducing of alkali metal ions, when the molar ratio of Sr +/Ba2+ reaches 9:1, the value of rsi of KNSBN film is 126.96pm/v, it possesses three times the value of SBN60 single crystal.The main destination of developing SBN films is to make E-0 devices with high quality. Fabricating and characterizing of SBN films can provide useful data for the applications of SBN EO devices. In this paper, SBN films with preferred orientation on Si substrate and epitaxial KNSBN film on MgO substrate were fabricated. The electro-optic coefficients of SBN and KNSBN films were obtained. Our result might be significant for the integration and industrialization of SBN materials.