A Study On The Fabrication, Optical And Optoelectronic Behavior Of Barium Titanate-based Ferroelectric Thin Films

The study of ferroelectric thin film materials and devices is still facing a lot of problems, such as improvement and optimization problems of optical and optoelectronic conversion factors, integration and compatibility problems in large-scale optoelectronic integrated circuits(OEICs), interface problems caused by thin film, and size effect, processing and characterization problems brought by small size, etc. In this paper, we take barium titanate-based ferroelectric thin films as the research object and relatively systematically study the preparation and characterization, optical performance, application of barium titanate (BTO) and barium strontium titanate (BST) thin films.The main research results in this paper are as follows:Developments of the preparation process of ferroelectric thin film at low temperature (≤550℃) or highly preferred oriented growth have been implemented. We successfully prepare crystalline BST thin films at550℃by using Sol-Gel method and metal organic decomposition (MOD) method. The crystalline BST thin film with (100)-oriented growth have been fabricated at700℃by the magnetron sputtering. These processes are one of preparation processes at lower temperature reported so far. The preparation of ferroelectric thin film at low temperature can be very useful for integrated application of OEICs. Besides, we obtained crystalline BST thin film with highly (100)-oriented growth on SiO2/Si substrate at700℃by the magnetron sputtering. The preparation of ferroelectric thin films with highly oriented growth on amorphous SiO2layer has been a hot spot and one of the difficulties of researches in this field.We relatively systematically studied the optical and optoelectronic behaviors of polycrystalline BST thin film monolithically integrated on silicon substrate. The optical band gap Eg of crystalline Ba0.Sro.5TiO3film with highly oriented growth on SiO2/Si substrate showing great correlation with the deposition temperature and the thickness of film, which is mainly due to the quantum size effect and influenced by crystallinity of thin film, such as grain boundaries, size, orientation growth and the presence of amorphous phase, etc. It is believed that dependence of the band-gap energies of the thin films on the crystallinity such as deposition temperature and film thickness should find applications in integrated optical devices. We firstly prepare the monolithically integrated Mach-Zehnder electro-optic modulator on silicon substrate based on the polycrystalline Ba0.5Sr0.5TiO3(BST0.7) thin film. The modulation of light has been successfully achieved. The electro-optic coefficient of the polycrystalline BST0.7film is2.4pm/V.We successfully designed and prepared a ferroelectric thin film groove-buried optical waveguide. The90°bent structures with a small curvature of micrometers have been also designed on the basis of a double corner mirror structure. It is easy for the waveguides with the designed structures to be fabricated and integrated in OEICs. The quantitative standard measurement of the light transmission loss also becomes easy. The optical propagation losses were about17dB/cm for the5μm wide and300nm thick polycrystalline BST0.7film waveguides at the wavelength of632.8nm. It was found that the optical propagation losses were about12.8and9.4dB/cm and the90°bend losses were about1.2and0.9dB respectively for5and10μm wide waveguides at the wavelength of632.8nm., which are much smaller than that in high index contrast polycrystalline Si/SiO2waveguides for the optical interconnection in monolithic OEICs (34dB/cm) and30%smaller than the optical propagation losses of crystalline BST0.7thin film waveguide. It is obvious that amorphous BST0.7thin film groove-buried type waveguides can be promised in being applied not only for the optical interconnection in monolithic OEICs but also for active waveguide devices on the Si chip despite the optical losses are not very small at present.We proposed a new method for thin film optical constants measurement by theoretical analysis. For the amorphous BST0.7film with the thickness of about214nm, the optical band gap is4.27eV and refractive index n=1.94. The extinction coefficient of the film in the visible and near-infrared region is far below the polycrystalline BST film, approximately the magnitude order of10"3. We studied the room temperature photoluminescence (PL) properties and mechanism of the amorphous BST and BTO films. The PL phenomenon was well explained by using the model of band to band transition. Due to the PL performance of film, we firstly applied it to LED devices and obtained white LED devices based on the amorphous BTO film. The InGaN/GaN multi-quantum well LEDs based on the150nm thick amorphous BTO film display a blue-white light. The coordinates value of CIE is (0.2139,0.1627).Compared with the traditional process, it is more easy for the white LED devices based on the amorphous BTO film to implement the structure of conformal coating. The BTO thin film has high transparent performance in the visible wavelength. The binders were not used in the fabrication of the LEDs based on the amorphous BTO film. So, they could be used at higher temperature. It provides a new way to study LEDs.