Fabrication and optical characterization of epitaxial strontium barium niobate films for waveguide applications

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Fabrication and optical characterization of epitaxial strontium barium niobate films for waveguide applications

 

Author: Lang, Zhong
Title: Fabrication and optical characterization of epitaxial strontium barium niobate films for waveguide applications
Degree: M.Phil.
Year: 2007
Subject: Hong Kong Polytechnic University -- Dissertations.
Strontium barium niobate.
Pulsed laser deposition.
Optical wave guides.
Ferroelectric thin films.
Department: Dept. of Applied Physics
Pages: xi, 125 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2145869
URI: http://theses.lib.polyu.edu.hk/handle/200/1000
Abstract: LixNi1-xO (LNO) films with x = 0.15, 0.20, 0.25, 0.30, 0.40, 0.50 and 0.60 on (001) MgO substrates were fabricated by pulsed laser deposition (PLD). X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to characterize the structural properties of these films; while the optical transmittance (OT) measurement and prism coupling technique were used to measure their optical properties. Our results showed that, above the deposition temperature of 600 oC, all of the LNO films exhibited single phase crystalline structure. They were c-axis oriented with cubic structure, although some very weak peaks, which were identified as the Li2O phase, were observed from the XRD profiles of the LNO50 and LNO60 films. The LNO films were cube-on-cube grown on MgO substrates with an in-plane epitaxial relationship of (001) LNO ll (001) MgO. X-ray photoelectron spectroscopy (XPS) results showed that the Li contents of the films were proportional to the initial Li contents of the targets. The values of the Li content of the films were approximately 20% of the initial Li contents due to Li evaporation during the fabrication process. The LNO films with different Li concentrations showed no remarkable difference in the shape of the OT spectra. Transmittance of about 40% in the visible range was obtained from LNO40 films. The optical band-gap energy of LNO was found to be in the range of 3.16-3.21 eV. Refractive indices of 1.75-2.0 for the LNO films were measured by spectroscopic ellipsometry. Among all the LNO films, the LNO40 film deposited at 600 oC was found to possess the lowest resistivity of 0.132 Q-cm. Based on this consideration, LNO40 films were used as both a transparent as well as conducting electrode in this project. High quality strontium barium niobate (SBN)/LNO thin films were epitaxially grown on (001) MgO substrates using a two-step PLD process. Their structural properties and OT, waveguide properties and electrical conductivity were investigated. The excellent crystalline SEN and LNO films with heteroepitaxial relationship of (100) SBN ll (310) LNO/MgO and (100) SBN ll (350) LNO/MgO were obtained. SEM results showed that the heterostructure possessed good structural qualities both in the surface and cross-section morphologies. Both the OT and resistivity measurements indicated that our two-step PLD process efficiently increased the transmission of the LNO films from 40% to 65% meanwhile the resistivity was only slightly increased. The transverse electric and transverse magnetic guided modes of SBN films deposited on MgO substrates were measured by a coupling prism technique. The ordinary refractive index n0 (nTE) and the extraordinary refractive index ne (nTM) of SBN film were calculated to be 2.1843 and 2.1684, respectively. The thickness of SBN films obtained by the coupling prism technique was in a good agreement with our SEM results. A propagation loss of 3.45 dB/cm was determined in our SBN waveguide structures. Epitaxial SBN films have not been successfully grown directly on Si substrates due to a large lattice mismatch. In order to overcome this problem, thin layers of TiN and MgO were used as buffer layers between SEN and Si in our studies. In our work, SBN/MgO/TiN heterostructures were successfully grown on (001) Si substrates by PLD method. Single phase MgO, TiN and SBN layers were c-axis oriented. The MgO and TiN are cubic while SBN has a tetragonal structure. All of the films were grown with a cube-on-cube mode and the in-plane orientation relationship between the SBN films, MgO/TiN layers and Si substrates can be described as (110) SEN ll (100) MgO/TiN ll (100) Si. The SEM micrographies showed that the surfaces were smooth and crack-free. The film thicknesses of each layer were obtained from the SEM cross-sectional micrographs. The waveguide properties were measured by using a prism coupler. The ordinary refractive index n0 (nTE) and the extraordinary refractive index ne (nTM) of SBN film were calculated to be 2.0393 and 2.0025, respectively.

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