单晶铌酸锂薄膜的结构和属性研究

发布时间：2018-01-04 06:11

本文关键词：单晶铌酸锂薄膜的结构和属性研究　出处：《山东大学》2016年博士论文　论文类型：学位论文

【摘要】：从20世纪末开始集成光学得到了迅速的发展,它可以实现将多个光学元件集成在同一块芯片材料上,形成一个结构复杂、功能强大的微型/小型器件,以实现一种或多种光学功能,在传感、通信/信号传输、环境检测等领域都有广泛的应用。集成光学在现代光通信和光信息处理应用中起了不可替代的作用,而薄膜则是集成光学系统一个关键的组成部分。将具有不同光学性质的单晶材料通过一定方式结合在一起形成薄膜材料,可以实现单一晶体所无法实现的功能。铌酸锂(LiNbO3, LN)晶体是集成光学应用最多的一种晶体材料,因为其自身具有多种优良的光学性能,如压电、铁电、光电、光弹、热释电、光折变和非线性等光学性质。LN晶体有光学“硅”材料之称,成为少数经久不衰、源源不断开辟应用新领域的光学功能材料。近十几年以来,出现了一种制作高折射率差单晶薄膜的方法。离子注入技术和晶体键合技术相结合,将单晶铌酸锂薄膜键合到沉积有SiO2(也可以是别的材料)的绝缘体衬底,制备出单晶铌酸锂薄膜(lithium niobate on insulator, LNOI)。本论文中用的LNOI材料是三明治结构：LN薄膜/SiO2层/LN衬底。最上面是一层LN薄膜,厚度大约0.5μm,中间一层是非晶的SiO2,厚度约2μm,最下面一层是LN衬底。LN薄膜和SiO2层形成的高折射率差对光形成限制作用。光被限制在一个很小的空间内,在很低的入射能量下,LNOI制作的波导中光的能量密度就可以达到很高的量级。LN体材料中的某些光学性质,例如非线性光学效应和电光效应,在LNOI中可以得到一定程度的加强。LNOI作为一种制作集成光学器件的理想平台,制作的光学器件的性能可能有大幅度的提高,迄今已经有不少在LNOI上制作的器件的报道。但是器件的性能在很大程度上取决于材料的性能,所我们所知,关于LNOI光学和结构属性的报道并不多。因此,对于LNOI的结构和光学属性应该进一步的探讨,这对于设计、制备、评价LNOI上的集成光学器件有帮助和推动作用。LNOI样品制备中用到的核心技术之一是离子注入。离子注入过的光学晶体在应用上还存在着一些困难和挑战,关于离子注入的材料物理性质的研究和认识的缺乏是限制LNOI应用的主要原因之一,其中因为离子注入引起的缺陷和损伤的性质是至关重要的。一般认为在离子注入的过程中,注入离子与靶中的电子和晶格原子发生碰撞等相互作用,对晶格结构有一定程度的破坏。离子注入引起的晶体损伤会降低LN的电光系数、二阶非线性系数等光学性质。论文研究了LNOI中的晶格结构,测量了LNOI的平行方向和垂直方向的应力和LN薄膜层与衬底层在晶面上的取向差,基于棱镜耦合技术测量了线性电光系数γ13和γ33,在LNOI上制备了微环谐振器,还提出了近化学计量比LN薄膜的制作方法。本论文研究的主要内容与结果如下：1.LNOI晶格结构的研究在LNOI制作过程中离子注入过程能导致晶格损伤,在论文中我们通过在氧气的气氛下退火来恢复晶格结构。实验中使用棱镜耦合技术、卢瑟福背散射／沟道技术、共聚焦微拉曼散射和高分辨透射电镜来研究在不同退火条件下LNOI的晶格损伤,用高分辨X射线评估LNOI的晶格结构。实验证明在氧气的环境中经过520℃下5个小时的退火,离子注入的损伤得到了很大程度的恢复。LNOI的双折射接近于体材料；沟道谱中显示在近表面LNOI是一个完美晶体；拉曼谱中显示LNOI和LN体材料的谱线几乎吻合；透射电镜横截面图中显示在LiNbO3薄膜/Si02层界面上仅仅有一层非常薄的非晶过渡层,其余区域晶格条纹清晰；选区电子衍射的图中没有出现非晶环,并且衍射图案是六边形格子图案,高分辨X射线显示LNOI很好的晶格排列。2.LNOI应力的研究在影响薄膜材料的诸多因素中,应力是不可能忽视的一个重要因素。应力的存在会直接影响薄膜材料的稳定性和可靠性,严重的情况下可能会造成薄膜的脱落。LNOI的结构虽然只有几百纳米的厚度,但是应力的存在能导致界面的晶格扭曲,从而影响薄膜的光学性能和电学性能,还可能诱导不可预知的破坏力影响器件性能。LNOI应力的研究对于光学器件的制备非常重要。高分辨X射线(HRXRD)的ω-20扫描可以用来表征LNOI的平行和垂直方向的应力,因为x-ray表征的时候是无损的,而且具有高精度和很强的穿透力。HRXRD从LN薄膜层中得到的图像提供了直接测量薄膜应变分量的一种办法。根据薄膜峰的位置变化计算得出晶格常数,再用晶格常数表征应力,包括垂直方向的压力和平行方向的拉力。HRXRD的φ扫描还可以测出在某个晶面上LN薄膜层和衬底在键合过程对齐时的取向差(δφ)。由于LN薄膜和SiO2层的热膨胀系数不同,形成了LN薄膜的垂直方向和水平方向的应变分量。实验结构表明,由于LN薄膜的热膨胀系数远大于Si02,退火后晶格常数水平方向a,变大,垂直方向cr变小。HRXRD的φ扫描探测的在(104)上LN薄膜层和衬底的取向差是0.98°3.LNOI线性电光系数的研究LN晶体有优良的电光特性,具有较大电光线性系数(γ13=9.5 pm/V,γ33=31.2pm/V)。LN晶体作为集成光学系统的基础部分,被用来实现许多光学器件,包括声光器件、表面滤波器、电光开关、电光调制器等。对于LNOI来说,希望LN薄膜层跟体材料一样,也具有较大的电光系数。离子注入导致的晶格损伤,可能会影响到LN薄膜层电光系数。论文中改进了一种基于棱镜耦合技术的平面波导电光系数的测量方法。使用棱镜耦合技术测量出LNOI的折射率,通过上下电极加上电场后,再测量调制后折射率,通过折射率的变化来确定LNOI的线性电光系数。利用此方法在入射光波长是632.8 nm的时候测量了LNOI的电光系数γ13和γ33。结果显示,在氧气的气氛下,经过520℃/5h退火处理的LNOI的线性电光系数γ13和γ33和LN体材料的很接近。4. LNOI上微环谐振腔的研究在LNOI上面制备的光学器件,将会大幅度提高基于非线性效应的光学性能。由环形的光波导和直的光波导耦合而成的微环谐振器是一个结构紧凑的光学滤波器,它可以用来构建多种不同用途的集成光学器件,包括光调制器、光开关、光插分复用器、光路由器、光传感器等。论文中在LNOI上面设计了微环谐振腔。将样品抛光成楔形,微环谐振腔刻蚀在楔形样品的尖上,实现了聚焦离子束刻蚀时不用换视野,大大提高了刻蚀的精度。我们搭建了LNOI微环谐振腔与单模光纤的耦合系统。有关微环谐振器与直波导耦合效果的测试工作正在进行中。5.近化学计量比LiNbO3薄膜的制备和表征LN晶体是一种典型的非化学计量比1：1的晶体,商业用的LN晶体具有同成分的配比([Li]/[Nb]=48.4/51.6)。锂离子的缺失造成晶体中存在大量的空位缺陷,致使LN晶体的许多物理性能受到一定的影响。LNOI是从LN晶体上剥离下来的,成分也不例外。近化学计量比的LN晶体已经出现,其中一些物理属性得到了的改变,如反转电压明显降低、双折射差增大等。因此很有必要制作高品质的近化学计量比的LN薄膜,探索折射率差、晶格常数和偶极矩等物理性质。论文中采用富锂气相传输平衡技术在氧气的气氛下520℃退火5小时提高LNOI中的Li/Nb比,制备近近化学计量比铌酸锂薄膜(NSLNOI)。使用棱镜耦合仪测量折射率,在激光波长632.8 nm时测得NSLNOI异常光的折射率是2.1983,小于同成分LN的2.2024,向化学计量比LN的2.1898靠近。使用高分辨X射线的的ω-20扫描测得NSLNOI的晶格常数(cr)是13.8604 A,小于体材料LN的13.8655A,向近化学计量比LN的13.8562A靠近。共聚焦微区拉曼散射对于LiNbO3晶格结构的微小变化都很敏感,通过实验测得NSLNOI和LNOI在共聚焦微区拉曼散射下的散射峰,其中一部分峰的相对强度明显降低,半高宽(FWHM)也有变化。
[Abstract]:From the end of twentieth Century began to integrated optics has been rapid development, it can achieve a plurality of optical components are integrated on the same chip material, forming a complex structure, powerful micro / small devices, to achieve one or more optical functions in communication / sensing, signal transmission, environmental monitoring and other fields have a wide range of applications. Integrated optics plays an irreplaceable role in modern optical communication and optical information processing applications, while the film is an integrated optical system, a key component. Single crystal materials with different optical properties together to form thin film materials through a certain way, can achieve a single crystal can the realization of the function. The lithium niobate (LiNbO3, LN) crystal is a kind of integrated optical crystal materials most widely used, because its have many excellent optical properties, such as piezoelectric, ferroelectric, optical, photoelastic, heat release Electric, photorefractive and nonlinear optical properties of.LN crystal optical silicon that became one of the few Jingjiubushuai, Everfount optical functional materials, open up new areas of application. In recent years, the emergence of a manufacturing method of high refractive index films. Ion implantation technology and crystal bonding technology the combination of the single crystal Lithium Niobate Thin film bonded to the deposition of SiO2 (also other materials) of the insulator substrate prepared by single crystal Lithium Niobate Thin Film (lithium niobate on insulator, LNOI). The LNOI material in this paper is a sandwich structure: /SiO2 layer LN film /LN substrate. The top layer is LN film thickness is about 0.5 m, the middle layer is amorphous SiO2, thickness of about 2 m, and the bottom layer is a high refractive index and LN substrate.LN film and SiO2 layer formation rate difference of light form limitations. Light is confined in a very small space, in a The low incident energy, LNOI produced waveguide light energy density can reach some optical properties of different.LN materials is very high in, such as nonlinear optical effect and electro-optic effect,.LNOI can be enhanced to some extent as an ideal platform for the production of integrated optical devices in LNOI, the performance of optical devices production may be greatly improved, so far there have been a lot of production on the LNOI devices is reported. But the performance device performance depends largely on the material, we know not much about LNOI optical and structural properties reported. Therefore, the structural and optical properties of LNOI should be further, for the design, preparation and evaluation of integrated optical devices on the LNOI help and promote the use of the core technology of.LNOI sample preparation is ion implantation ion beam through the light. Study of crystal in the application there are still some difficulties and challenges, research on the physical properties of materials by ion implantation and the lack of knowledge is one of the main reasons to limit the application of LNOI, which because of the nature of defects and damage caused by ion implantation is essential. Generally during ion implantation, injected electrons and lattice atoms ion and target the collision interaction, with a certain degree of damage to the crystal lattice structure. Damage will reduce the electro-optic coefficients of LN ion implantation, two order nonlinear coefficient of optical properties. The research of the crystal structure of LNOI, LNOI and parallel to the direction perpendicular to the direction of the measurement of stress and LN thin film layer and the substrate layer on the crystal face orientation difference, the prism coupling technique to measure the linear electro-optic coefficient gamma 13 and gamma 33 based on LNOI prepared by micro ring resonator, is proposed in Chemistry Stoichiometric method of preparation of LN film. The main contents and results are as follows: 1.LNOI lattice structure in the fabrication of LNOI during ion implantation process can lead to lattice damage, in this paper we through annealing in oxygen atmosphere to restore the lattice structure. Using the prism coupling technique experiment, Rutherford backscattering / channel technology, confocal micro Raman scattering and high resolution transmission electron microscopy to study the lattice damage of LNOI under different annealing conditions, the lattice structure with high resolution X ray evaluation LNOI. Experimental results show that the oxygen environment under 520 DEG C for 5 hours after the annealing of ion implantation damage birefringence recovery.LNOI much closer to the body material; channel spectrum display in the near surface LNOI is a perfect crystal; Raman spectra of LNOI and LN in the display materials are almost identical; TEM cross Cross section is displayed in the LiNbO3 film /Si02 layer interface is only a very thin layer of amorphous transition layer, the clear lattice fringes; no amorphous ring electron diffraction diagram, and the diffraction pattern is hexagonal pattern, high resolution X ray showed that.2.LNOI stress on many factors affecting the film the materials in LNOI good lattice, stress is an important factor that can not be ignored. The presence of stress will directly affect the stability and reliability of thin film materials, severe cases may cause the shedding of.LNOI thin film structure although only a few hundred nanometers in thickness, but the presence of stress can lead to distortions the interface lattice, thus affecting the optical properties and electrical properties of the films, the research damage may induce unpredictable stress influences the performance of.LNOI for the preparation of non ordinary optical devices Important. High resolution X ray (HRXRD) stress Omega -20 scanning can be used to characterize the LNOI parallel and vertical direction, because when X-ray characterization is lossless, and images with high accuracy and strong penetration of.HRXRD obtained from the LN film layer provides a way to directly measure the thin film strain component the change of Bo Mofeng's position. According to the calculated lattice constant and lattice constant characterization of stress, including the vertical pressure and parallel to the direction of the tensile force.HRXRD Phi scans can also be measured in a plane orientation LN thin film layer and the substrate during the process of alignment in key difference (delta phi). Due to the LN film the SiO2 layer and thermal expansion coefficient of different strain components formed LN film in vertical and horizontal directions. Experimental results show that, due to the thermal expansion coefficient of LN films is greater than Si02, the direction of the lattice constant level after annealing a, large vertical The direction of smaller Cr.HRXRD with scanning probe in (104) LN film layer and the substrate misorientation of LN crystal is 0.98 DEG 3.LNOI linear electro-optic coefficient has excellent electro-optical properties, with large electro-optic coefficient of linear (R 13=9.5 pm/V, gamma 33=31.2pm/V) based on the part of the.LN crystal as the integrated optical system. Is used to implement many optical devices, including acousto-optic devices, surface filter, electro-optical switch, electro-optical modulator. For LNOI, I hope LN film like material, also has a large electro-optic coefficient. The lattice damage caused by ion implantation, may affect the LN film layer. The electro-optic coefficient is improved a measurement method of planar waveguide electro-optic coefficients of the prism coupling technique based on the use of the prism coupling technique to measure the refractive index of LNOI, the upper and lower electrodes coupled with the electric field, and then measuring refractive index modulated by refractive index To determine the change of the linear electro-optic coefficients of LNOI. By using this method in the wavelength of incident light is 632.8 nm when measuring the electro-optic coefficient of 13 and 33. LNOI of gamma gamma results in oxygen atmosphere, the linear electro-optic coefficient gamma 520 C /5h annealed LNOI 13 and gamma 33 and LN bulk materials are to study.4. LNOI micro ring resonator in the LNOI above the preparation of optical devices, it will greatly improve the performance of optical nonlinear effect based on micro ring resonator by optical waveguide ring and straight waveguide coupled into the optical filter is a compact, integrated optical devices which can be used to build a variety of for different purposes, including light modulator, optical switch, optical add drop multiplexer, optical routers, optical sensors. The above in the LNOI design of the micro ring resonator. The sample is polished into a wedge, micro ring resonator etched in the wedge-shaped sample On the tip of the focused ion beam etching without changing the view, greatly improving the etching precision. We set up a coupling system of LNOI micro ring resonator and single-mode fiber. The test work related to micro ring resonator and straight waveguide coupling effect is.5. in the near stoichiometric LiNbO3 film preparation and characterization of LN the crystal is a kind of typical non stoichiometric 1:1 crystal, LN crystal has the same commercial component ratio ([Li]/[Nb]=48.4/51.6). The lack of lithium ions are caused by the large number of vacancy defects in the crystal, resulting in many physical properties of LN crystal can be affected by certain.LNOI from stripping down on LN crystals, components is no exception. LN crystal near-stoichiometric has emerged, some physical properties has been changed, such as reversal voltage decrease, double refraction difference increases. So it is necessary to produce high quality products LN near stoichiometric films, to explore the difference of refractive index, lattice constants and dipole moments of physical properties. The oxygen atmosphere annealing at 520 DEG C for 5 hours to improve the LNOI Li/Nb ratio of lithium rich vapor transport equilibration technique in this paper, the preparation of near stoichiometric lithium niobate thin film (NSLNOI). The use of measurement the refraction prism coupler, the laser wavelength of 632.8 nm is measured 2.1983 NSLNOI abnormal rate of light refraction, less than 2.2024 of the same component of LN, to the near stoichiometric LN 2.1898. Using the lattice constant measured by -20 scanning, NSLNOI high resolution X ray of the omega (CR) is 13.8604 A, less than the body LN 13.8655A, close to the stoichiometric ratio LN near 13.8562A. Confocal micro Raman scattering is very sensitive to small changes in the LiNbO3 lattice structure, scattering peak measured NSLNOI and LNOI in the confocal micro Raman scattering, a part of the peak The relative strength decreased obviously, and the half width (FWHM) also changed.

【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O484

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