Preparation And Properties Of La₃Ga₅SiO₁₄ Optical Waveguide By Pulsed Laser Deposition And Ion Implantation

Photonics technology is a newly high technology following the rise of microelectronics technology, and they constitute the two pillars of information technology. Photonics technology includes optical-fiber technology, integrated photonics, and other important branches. Optical waveguide not only provides the basic theory of optical fiber technology, and it is also regarded as the most fundamental part of the integrated photonics. So researchers have paid a lot of attentation to it and investigated many effective methods to fabricate waveguides, such as metal diffusion, ion exchange, ion implantation and film deposition etc. And the film deposition methods include sputtering, epitaxy, thermal evaporation and Pulsed laser deposition (PLD) etc. Compared with the above mentioned methods, PLD is a versatile technique that can be used to grow complex multicomponent oxide thin films. In addition, as a mature method for modifying surface properties of materials, ion implantation has been used to form waveguide structures in many optical materials. In this work, the PLD and ion implantation methods are taken to prepare the La3Ga5SiO14 (LGS) waveguide.Crystalline LGS was first reported in the eighties of last century. It is a multi-crystal blending piezoelectrical, electro-optic and laser properities. Until now, the studies of LGS are focused on the growth of crystal and the natures above mentioned. There are a few reports about the LGS films and the waveguide properities. Some researchers taken liquid phase epitaxy (LPE), sol-gel process, and radio frequency (RF) sputtering to produce LGS films, and studied the structures and morphology of the film. In addition, there is only one report about LGS optical waveguide formed by O ion implation. So it has some scientific significance and potential practical value to study the LGS optical waveguide.In this dissertation, we report the growth, thermal and optical propertities of LGS and Nd:LGS crystal. LGS waveguide films were prepared on different substrates by PLD, characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and fluorescence spectrophotometer etc. The prism coupling method was used to obtain the dark mode spectra, the refractive index and thickness of the LGS film. Moreover, the LGS crystal was implanted by H ions to form planar optical waveguide. The dark-mode spectra of the waveguide were measured by prism coupling method. The near-field profile and the propagation loss of the waveguide were got by end-fire coupling method. The refractive index profile was obtained by the simulation of the effective refractive indices of the waveguides. The TRIM'98 code was used to simulate the process of H ions penetrating into LGS crystal. At last, the mechanism of ion-implanted waveguide was analyzed. The outline is shown as follows:(1) Study on growth and properties of LGS and Nd:LGS crystal1 Polycrystalline materials of LGS compounds were prepared by solid-phase recation by mixing by mixing La2O3, Ga2O3 and SiO2 with 4N purity. Then, the crystal was grown by Czoaralski method. Adding appropriate masses of Nd2O3 in above raw materials, Nd:LGS crystal was grown by the same method.2 The Differential Scanning Calorometer (DSC) was used to measure the specific heat, which was 0.48 J/g℃at room temperature. Thermal-mechanical Analyzer was taken to obtain the thermal expansion coefficients, with the result ofα11=5.92×10-6/℃andα33=4.01×10-6/℃at the range of 30-500℃. Laser flash method was used to measure the thermal diffusion coefficient of LGS crystal, being 0.792mm2/s along Z axis at room temperature.3 The absorption spectra of LGS and Nd:LGS were measured by UV/NIS/NIR spectrometer. The transmittance data of LGS and Nd:LGS were～81.8% and～80.9% at the range of 580-2500nm, respectively. For Nd:LGS crystal, two absorption peaks could be observed, one centered at 882nm and the other one centered at 808nm with the full width at half maximum was 20nm. The coupling prism method was used to measure the refractive index of LGS crystal at 633nm:ne=1.9112, no=1.8995.(2) Preparation and properties of LGS films on SiO2/Si(100) substrateLGS thin films were successfully deposited on SiO2/Si (100) substrates by PLD, and the effects of oxygen pressure, growth and annealing temperature on the properties of the films were investigated.1 Effects of growth oxygen pressureXRD results indicated the intensity of (220) and (300) diffrication peaks of the LGS film increased with increase in oxygen pressure, which should be related to the slower growth rate at lower oxygen pressure. TEM results shown there are amorphous parts for the thin film. The deposited films exhibit smooth surface and dense structure without layer separation as observed by AFM and SEM. PL measurements with 260nm excitation shown that Ga ions occupy more octahedrally coordinated sites at higher pressures.2 Effects of growth and annealing temperatureAnnealed above 800℃, the LGS thin film grown at room temperature became polycrystalline from amorphous structure. And, the films exhibited large numbers of defects, such as cracks and holes. For the film prepared at 400℃, LGS thin film shown strong (200) and (400) preferred orientations after annealing at 1000℃. And the morphological properties studies indicated that the film deposited at 400℃had larger grain size as the annealing temperature increasing. The phenomena of the interdiffusion reaction between the LGS film and Si substrate could be observed on the TEM images. By analysis, it should be LaxSiyOz which was formed by high temperature reaction of La2O3 and SiO2.3 Effects of the same annealing temperature on LGS thin films grown at different oxygen pressureAlthough the films deposited at different oxygen pressure showed the same crystal orientation, the films prepared at lower pressure (5Pa, 10Pa) shown worse orientation after annealing at 1000℃. This indicates that the initial growth conditions directly affect the structure of films after annealing.4 Waveguide propertiesThe dark-mode spectra of the LGS films grown at different oxygen pressure and annealed at 750℃were measured by using prism coupler. And the refractive index and thickness of the LGS film were calculated by using the effective refractive index. Modes with higher effect refractive indexes than substrate index were found for the films before and after annealing. This means the LGS thin film waveguide structure on SiO2/Si substrate was successfully prepared. The refractive index of the LGS film was closer to the bulk material as the improvement of crystalline quality.(3) Preparation and properties of LGS films on sapphire substrates by PLD techniqueThe amorphous LGS film grown at 400℃and 20Pa converted to polycrystalline structure after annealing at 1000℃, containing impurities in the form of Ga2O3 and LaxAlyOz. The film surface was smooth with RMS (root mean square) roughness 2.73nm. The properties of the optical waveguide were measured by prism coupling method and the refractive index of the LGS film was calculated. The extraordinary refractive index ne is 1.8030 and 1.8915 respectively before and after annealing, which is 1.8114 and 1.8926 for no respectively. After annealing, the refractive indexes of the as deposited LGS films are close to that of bulk material.(4) Preparation and properties of LGS films on silica glass substrates by PLD techniqueLGS thin film on silica glass deposited at 400℃and 20Pa by PLD was amorphous, without obvious crystalline diffraction peaks even after annealing at 1000℃. The RMS (root mean square) roughness for the film before annealing was 3.29nm. The SEM image shown cracks for the film after annealing at 1000℃, which was caused by the large difference of the thermal expansion coefficients between LGS film and the substrate. Prism coupling method had been used to investigate the properities of the film, and the refractive index was calculated based on the dark-mode spectroscopy. The result indicated that the LGS/silica glass optical waveguide structure have been fabricated successfully. After annealing, the refractive index of the film was no=1.8197 and ne= 1.8206, much less than bulk crystal. This was related to the poor crystalline quality of the film grown on silica glass.(5) H ion-implanted planar waveguide in LGS and Nd:LGS crystalsThe optical planar waveguides in LGS crystals were fabricated by H ions implantation at energy of 500keV and doses of 9.6×1016ions/cm2 at room temperature. The refractive index profiles were reconstructed by Reflectivity Calculation Method (RCM) based on the dark-mode spectroscopy, which was measured by coupling prism method. The result shown the optical waveguide owned a typical "enhanced well+barrier" distribution for ne. According to the SRIM 2006 calculated result, the ne increase was assumed to be related to electronic-enenrgy-depsition-induced effects in the electronic damage region, while the reduced refractive indices for both ne and no came from the nuclear damage caused by the collision of incident ions with LGS atoms. The near-field mode profiles of the waveguide were obtained by an end-fire coupling arrangement, agreement with the simulation by using BeamPROP code based on the finite-difference beam propagation method. The propagation loss of the waveguide could be reduced by thermal annealing treatment, measured by Back-Reflection end-fire coupling method. And the effective refractive index approached to the substrate as the annealing temperature increased.The Nd:LGS planar optical waveguide was also prepared by H ion implantation (500keV,9.6×1016ions/cm2). And the near-field mode profiles of the waveguide were investigated by end-fire coupling method.