| As a prominent multifunctional material, lithium niobate (LiNbO3, LN) is well known for its excellent piezoelectric, ferroelectric, acousto-optical, electro-optical, photorefractive and nonlinear optical properties. LN crystal in large size has been extensively grown by Czochralski method, which is suitable for industrialization due to its easy processing, low-cost, mechanical and chemical stability. On the basis of above merits, LN has been widely used in various areas, such as surface acoustic wave (SAW) filter, optical waveguide, modulator and optoisolator, which also shows a good prospect for application in mass storage and optical integration. Recently, periodically poled lithium niobate (PPLN) has attracted considerable attention for use as an optical superlattice in the quasi-phase matching (QPM) technique, owing to its popular application in optical process, for instance, frequency doubling, sum frequency/difference frequency and optical parametric oscillator (OPO).LiNbO3 is a typical non-stoichiometry crystal. The congruent lithium niobate (CLN) has good consistency in component, grown from melt with 48.6 mol% Li2O by traditional Czochralski method. Doping with rare earth ions in CLN was used to obtain efficient self-frequency-doubling generation. The resistance of optical refractive damage can be effectively enhanced by doping with MgO in CLN crystals. However, the deficiency of lithium ions in the crystal induced a number of intrinsic defects, such as Li vacancy and anti-site Nb, and hence influenced some important physical properties seriously. Recently, near stoichiometric lithium niobate (SLN) crystal with low defect density has been grown and attracted great attention, which results in blue shift of the absorption edge and narrowing of OH- absorption band, compared with CLN. More important, SLN doped with 1 mol% MgO shows great potential in fabrication of PPLN super lattice, as a result of its higher resistance of laser damage and lower coercive field than CLN doped with 5 mol% MgO.Lithium tantalate (LiTaO3, LT) is an important multifunctional material, which has wide application in SAW, optical communication, laser and optoelectronics. LT wafers through mirror polishing are extensively used in manufacture of high frequency SAW devices which play an important role in the advanced communication field, such as mobile phone, interphone, satellite and aerospace. There is no better commercial material can substitute it at present under 2.5G or 3G standard. The birefringence of LT is very low and so the normal birefringence phase matching could not be realized in regular crystals. Periodically poled lithium tantalate (PPLT) has attracted considerable attention because it exploits the application in nonlinear optical field. Owing to its higher resistance of laser damage and ultraviolet transmittance, PPLT has been extensively used in laser frequency doubling, optical parametric amplification and OPO devices.In this thesis, a series of LN and LT crystals were studied through measuring their structure, composition, defect and basic properties, including SLN, MgO-doped SLN, Mg,Yb-doped CLN grown by Czochralski technique and black LT wafers prepared by chemical reduction. And some exploratory researches were made about the applications of PPLN and black LT wafers. The main content of this thesis is described as follows:1. crystal growth of SLN and MgOSLNOn the basis of summarizing several crystal growth methods of SLN in this field, a modified hanging double crucible technique with continuous powder feeding system was adopted for growth of different SLN crystals. The main factors that influence the growth process and quality of crystals during were discussed, from the viewpoint of thermodynamics and dynamics theories of crystal growth. Some defects, for example, dislocations, twin crystals and inclusions were observed through chemical etching and optical microscopy. By establishing suitable thermal field, employing the seed of high quality and selecting proper technological parameters, a variety of SLN crystals were grown with high quality, such as 2-inch SLN and MgOSLN,3-inch SLN, MgNdSLN, MgYbSLN, MgErSLN and ErYbSLN. The single domain structure of as-grown crystals was verified by the etching picture and measurement of the piezoelectric constant d33. 2. crystal structure, optical and thermal properties of SLN and MgOSLNThe structures of LN crystals were studied by the X-ray powder diffraction (XRD).The XRD patterns confirm that the grown crystals belong to hexagonal system. The calculated lattice parameters show that the increasing of Li2O and the doping of MgO may lead to slight change of lattice parameters, while not influence the basic structure. The crystalline quality was characterized by high resolution X-ray diffraction (HRXRD). The rocking curve exhibits that the crystals have high excellent quality and small stress.Optical properties were measured. The transmittance spectra in the range of 190～1100nm indicates that the transparent range covers ultraviolet, visible and infrared with a high transmittance close to 75%-80%. The reason is discussed for blue shift in the absorption edge of SLN and MgOSLN (1 mol%). The OH- infrared absorption peak for SLN and MgOSLN crystal was located at 3466 cm-1 and 3534 cm-1, respectively, which indicates indirectly that Mg2+ has replaced anti-site Nb entirely and the doping content has exceeded the threshold concentration of optical damage resistance. The refractive index no and ne were measured by prism coupling instrument. The optical homogeneity is up to 10-5 order of magnitude, tested by Veeco interferometer.Thermal properties and the influence on crystal growth and application were researched systematically. The Curie temperature of CLN, SLN and MgOSLN was 1145.0℃,1207.6℃and 1215.3℃, respectively, obtained by differential thermal analysis (DTA) method and the corresponding DDTA curve. The specific heat was measured from 300 K to 770 K by differential thermal scanning calorimeter. The almost identical value at room temperature implies that it is independent with the non stoichiometric defects and doping. The thermal expansion was measured by thermomechnical analysis apparatus. The thermal expansion coefficient along a-axis is several times than that along c-axis, and the anisotropy of SLN and MgOSLN is larger than that of CLN. The density was measured by buoyancy method at room temperature, and the relationship between density and temperature was calculated. The thermal diffusion coefficients in the range of 300～570 K was measured by laser flash method. Then the thermal conductivity was calculated, which is also anisotropic and decreased with increasing temperature.3. crystal growth, structure and physical properties of MgYbCLNBy traditional Czochralski method and using a large crucible to grow a small crystal, MgYbCLN crystals were grown, with MgO fixed at 5 mol% and Yb3+ doped with 0.8 mol% and 1.2 mol%, respectively.The structures were measured by XRD patterns, which confirm that the crystals belong to hexagonal system. The calculated lattice parameters show that the doping of Mg and Yb will induce slight change of lattice, while not influence the basic structure. The co-doping of Mg and Yb is the main reason for the extinct growth ridges. The crystalline quality was characterized by HRXRD and conoscopic interference figures. The rocking curve exhibits that the crystals have high excellent quality and small stress. The concentration of doped ions was measured by X-ray fluorescence technique and then the effective segregation coefficient was calculated.Optical properties were measured. The transmittance spectra in the range of 190～1100 nm indicates that the transparency region covers ultraviolet, visible and infrared with a high transmittance close to 75%-80%. The reason is discussed for red shift in the absorption edge of MgYbCLN, compared with MgOCLN. The OH-infrared absorption peak for MgYbCLN was located at 3535 cm-1, which indicates indirectly that Mg2+ has replaced anti-site Nb entirely and the doping content has exceeded the threshold concentration of optical damage resistance. The refractive index no and ne were measured by prism coupling instrument. The optical homogeneity is up to 10-5 order of magnitude, measured by Veeco interferometer. The Raman spectra of MgYbCLN is almost the same as that of SLN, except the obscure A1(TO2) band, measured with scattering geometry at room temperature.Thermal properties and the influence on crystal growth and application were studied systematically. The Curie temperature is obviously decreased when doped with Yb in CLN and MgOCLN. The specific heat was measured from 300 K to 770 K by differential thermal scanning calorimeter. The almost identical value at room temperature implies that it is independent with the non stoichiometric defects and doping. The thermal expansion was measured by thermomechnical analysis apparatus in the temperature region of 300～770 K. The thermal expansion coefficient along a-axis is several times than that along c-axis, and the anisotropy of MgYbCLN is larger than that of CLN. The density was measured by buoyancy method at room temperature which implies that Yb3+ ions will probably occupy Li sites, and the relationship between density and temperature was calculated. The thermal diffusion coefficients in the range of 300～570 K was measured by laser flash method. Then the thermal conductivity was calculated, which is also anisotropic and decreased with increasing temperature. The thermal conductivity of MgYbCLN is close to MgOCLN, which ensures its application in laser devices.4. crystal growth and chemical reduction of LTDifferent LT crystals were grown from congruent melt with 48.75 mol% Li2O by Czochralski method using Ir/Pt crucible. The conoscopic interference images manifest the high quality of crystals. The refractive index no and ne were measured by prism coupling instrument.Based on analyzing the influences of pyroelectric effect and summarizing several methods of eliminating pyroelectric response at present, a new simple chemical reduction technique was put forward. Lithium tantalate wafers of different colors have successfully been obtained by annealing of regular wafers while surrounded by a mixture of iron and lithium carbonate powders under a continuous flow of nitrogen.The heating cycle experiment from room temperature to 180℃and the simple measuring of pyroelectric response and body wave resonator were carried out, which indicates that the chemical reduction can effectively solve the problem induced by pyroelectric discharge while maintaining the piezoelectric property.SAW devices were fabricated on production line using wafers after chemical reduction and all the parameters meet the requirements. The chemical reduction technology will play an important role in SAW, especially in high frequency devices. 5. influence of chemical reduction on crystal structure, composition, electrical and optical properties and formation mechanism of BLTThe structures were measured by XRD patterns, which demonstrate that the chemical reduction does not influence the basic structure while the lattice parameters decreased slightly along with the increasing temperature. The Curie temperature measured by DTA was almost identical for the samples before and after processing, which indicates that the chemical reduction does not change the composition.The electric resistance was measured by high resistance meter and the electrical resistivity and conductivity were calculated. The electrical conductivity of wafers after reducing improves by 2-4 orders of magnitude, while the piezoelectric and dielectric properties remain almost the same.The OH- infrared absorption peak located at 3482 cm-1 in regular CLT wafers was not found in those after chemical reduction. The transmittance spectra in the range of 190～3200 nm indicates that chemical reduction can significantly increase the optical absorption of the crystal, causing its appearance to change from grey to dark black and to be almost non-transparent. The absorption edge (α=20 cm-1) of each sample is situated at 270nm for CLT,276nm for GLT and 810nm for BLT.The X-ray photoelectron spectroscopy (XPS) was measured by ESCALAB 250 system. All the peaks can be attributed to photoelectrons emitted from the constituent elements, Li, Ta and O, and C from residual hydrocarbons. The fitted results for Ta4f and O1s XPS spectra reveals that Ta5+ is converted to Ta4+ as CLT transforms to BLT through chemical reaction. Considering the result of XRD and density, the formation of black LT through chemical reduction is ascribed to the change in the chemical state of Ta, from Ta5+ to Ta4+. Evidence for the creation of oxygen vacancies during chemical reduction has also been observed. |
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