Structure And Properties Of Single-Crystal Lithium Niobate Thin Film

Since the end of 20th century, the integrated optics, which refers to the fabrication and integration of several optical components on a common planar substrate, is rapid developing. Integrated optics in modern optical communication and optical information processing application plays and irreplaceable role, and thin film material is an important part of integrated optics. Thin film materials combines the single crystal materials with different optical properties together in some way, in order to realize the special function of the single crystal cannot be achieved.Lithium niobate (LiNbO3, LN) is one of the most promising integrated optical materials known to humans and it has many applications because of its excellent piezoelectric, ferroelectric, electro-optic, pyroelectric, photo-elastic, photorefractive and non-linear optical properties. LN called "silicon of photonics" is widely used in optical function materials. In the past decade, there is a fabrication method for a high refractive index contrast of single crystal thin film. The single-cyrstal LN thin films on a SiO2 cladding layer of other substrates (lithium niobate on insulator, LNOI) had been fabricated using crystal ion slicing and wafer bonding technologies. The LNOI in this thesis is a sandwich structure of LN film/SiO2 layer/LN substrate. The upper layer is the single-crystal LN film with about 0.5μm, the middle lay is an about 2 μm thick amorphous SiO2 layer, and the bottom is LN substrate. High-refractive-index contrast between LN thin film and SiO2 layer enable high confinement of light in LN film. In such structure, the light is confined within a small spaceand optical density could reach a high level with low input power. As a result, the corresponding features of the bulk LN, such as nonlinear effect and optical effect, may be strengthening within LNOI. Based on the study of structure and properties, LNOI could be appropriately designed for integrated photonics devices with different function in various research fields. LNOI is an ideal platform for various integrated function optical devices, the performance of the photonic devices based on LNOI have been greatly improved. As we know, only several studies of optical and structural properties of LNOI have been reported in the literature so far. The optical and structural ananlyse of LNOI should be further studies and discussed. For the design, preparation, evalution of integrated optics in LNOI is helpful.The ion implantation is one of the core technologies of LNOI fabrication. However, this method still has some difficulties and challenges in optical applications, the lack of knowledge on material physics process is one of the factors limiting the application of this method, and the study on the defects and lattice damage induced by ion implantation is very important. It is generally considered that in the process of implantation, ions interact and collide with the electrons and crystal lattice atoms of target material, and result in crystal lattice damage. Ion implantation brings about crystal lattice damage, which will influence and weaken the opbical performance and physical properties LN thin film, such as its electro-optic property and and second order nonlinear optical coefficients. In this thesis, we studied the lattice structure of LNOI, measured the perpendicular and parallel strains of LNOI, and the rotational misalignment between the thin film and the substrate, determined the linear electro-optic coefficients γ13 and 733 in LNOI by prism coupling, etched microring resonators in LNOI, and think out the fabrication method of near stoichiometric LN thin film.Here list the main contents and results of this thesis:1. Lattice structure of LNOIThe lattice damage in the LNOI caused by ion implantation could be reduced by annealing in an oxygen atmosphere. Prism coupling, Rutherford backscattering spectroscopy/channeling, Raman spectroscopy, and high-resolution transmission electron microscopy were used to study the lattice damage properties in lithium niobate thin films with different annealing parameters. The crystalline quality of the LNOI was evaluated using HRXRD. The experimental results showed a good crystal lattice arrangement in the LNOI by annealing in an oxygen atmosphere at 520℃/5h. The refractive indices of LNOI were close to that of bulk LN; RBS/Channeling spectra showed that in the near-surface regionof LNOI was a perfect crystal; Raman spectra showed the curve of LNOI agreed well with that of bulk LN; the cross-section image of HRTEM showd an amorphous transition layer was very thin in the LN/SiO2 interface region, and the crystalline strips were clear. The selected area electron diffraction showed amorphous ring could not be found, and the diffraction pattern was the expected hexagonal pattern, HRXRD showed a good crystal lattice arrangement in the LNOI.2. Stress characterization of LNOIStress is an important factor which can’t be ingored among so many factors influencing the performance of thin film. The stresses will affect stability and reliability of thin film. The thickness of LNOI is a few hundred nanometers, but the strss induced at interfaces can locally distort the crystal lattice, and influence the optical and electrical properties of thin film, which may in turn affect device performance in an unpredictable way. A means of characterizing such stress fields with high spatial resolution and sensitivity is therefore highly desirable. The strains research of LNO is important for the fabrication of optical device.HRXRD was used to characterize the perpendicular and parallel strains of LNOI, because x-ray diffraction characteristics of non-destructive testing, high accuracy, and a much greater penetration depth. HRXRD profiles generated from lithium niobate thin film provided a direct measurement of the thin film’s strain components. The lattice constants were calculated by the change of the thin films, and characterized the strain values, such as compressive perpendicular and tensile parallel strains. We have demonstrated that the rotational misalignment (δφ) between the thin film and the substrate. This behavior was contrary to unconstrained strain relaxation but was consistent with layer constraint and the existence of finite stresses at the liNbO3/SiO2 interface, resulting from the difference in coefficients of thermal expansion between LiNbO3 and SiO2. Since both the overlying strained LiNbO3 and underling substrate maintained a stressed state in the buried SiO2, the compressively strained oxide retained the lattice expansion of the overlying strained LiNbO3 and resulted in increasing parallel strains after annealing. We have determined that the rotational misalignment (δφ) between between the thin film and the substrate was 0.98°3. Electro-optic coefficients of LNOI LiNbO3LN single-crystal has excellent electro-optic properties, and larger linear electro-optical coefficient (γ13=9.5 pm/V,γ33=31.2 pm/V). As the basic components of intergrated optics system, optical waveguide structures have been used to realize many functional devices in LiNbO3, including acousto-optic device, surface filter, electro-optic switch, electro-optic modulator and so on. For LNOI, the LN thin film is expected to have similar linear electro-optic coefficient as bulk LN. The lattice damage caused by ion implantation could affect linear electro-optic coefficients of LNOI. In this thesis the linear electro-optic coefficients were determined by a measuring method based on the prism coupling technique. We could measure the refractive indices of LNOI using prism coupling, and by applying an electric field and measuring the modulation close to one coupling angle, we measured the linear electro-optic coefficients γ13 and γ33 in LNOI at λ=632.8 nm. The results showed that the measured γ13 and 733 values of the LNOI by annealing in an oxygen atmosphere at 520℃ for 5 hour is in excellent agreement with the bulk LiNbO3 value.4. Microring resonators in LNOIThe optical components in LNOI, can greatly improve the performance of based on the nonlinear optical properties. Microring resonator, which is constructed by ring and straight waveguides, is a compact filter. It is a versatile element used for various applications, such as optical modulators, optical switches, opticaladd-drop multiplexers, optical routers and sersors. We designed in LNOI a microring resonator. After the sample was polished into wedges, the microring was fabricated on the tip in order to etch the entire device at the same view by focus ion beam. We have built a coupling system about microring resonator in LNOI and single-mode fiber. We are measuring the optical effect of microring resonator.5. Fabrication and characterization of near stoichiometric LN thin filmLN is a typical non-stoichiometry crystal, commercially available LN crystals usually have a congruent composition ([Li]/[Nb]=48.4/51.6). The deficiency of lithium ion in the crystal induced a number of vacancies reduction of some function properties. LNOI are split from the bulk LiNbO3, are no exception. There are many reports concerning near stoichiometric LiNbO3 single-crystal, the physical properties have be changed distinctly, for instance, reversal voltage remarkably decreased, the birefringence increased. Thus, it is obvious that fabrication of good-quality, near stoichiometric LN thin film (NSLNOI) is essential and can be identified by the changes of the physics properties such as the refractive index, lattice parameter and dipole moment.A lithium-rich vapor transport equilibration technique was used to increase the Li/Nb ratio in LNOI at low temperature (below 600℃). The extraordinary refractive index ne of the (NSLNOI was measured and found to be 2.1983 at 632.8 nm using the prism coupling method, which deviated from the congruent LN (2.2024) and moved towards that of the stoichiometric LN (2.1898). The lattice parameter (cr) of NSLNOI was determined to be 13.8604 A using HRXRD ω-2θ scan, which was between the values of congruent LN (13.8655 A) and stoichiometric LN (13.8562 A). The Raman spectra showed significant differences in the relative intensity and the FWHM of Raman lines between the NSLNOI and congruent LN.