Titania and hybrid titania - silica sol-gel thin films and their applications in integrated optical devices

Sol‐gel materials are prepared by the generation of colloidal suspensions ("sols") which are subsequently converted to viscous gels by polymerization and further into solid materials upon densification. By controlled mixing of sols of different materials and concentrations, organic/ inorganic composite materials with useful optical properties have been synthesized for a variety of photonic applications. Inorganic titanium dioxide (titania) and silicon dioxide (silica) thin films have been developed using the sol‐gel process and used as anti‐reflection coatings for solar‐cells. Nevertheless, most of the integrated optical devices reported in literature are primarily based on the silica sol‐gel process. ❧ By doping titania sol‐gel with controlled amounts of silica sol‐gel the refractive index of the material obtained can be varied from ~2.5 to ~1.45 at the optical communication wavelengths. Moreover, being a large bandgap material (~ 3.5 eV for rutile phase) titania sol-gel is optically transparent over a large spectral region spanning the visible and near infrared wavelengths like silica. However, the titania sol‐gel process is relatively complicated compared to the established silica process mainly due to the reactive (hygroscopic) nature of the titanium starting compound. Apart from their use in antireflection coatings which require sub 100 nanometer film thicknesses it has been difficult to realize their potential in integrated optical device applications due to the lack of a scalable and reliable process. ❧ This work involves a modified synthesis process and densification route to obtain variable refractive index titania, titania – silica and titania - zirconia hybrid sol‐gel films in the range of 1.6 – 2.5 at the telecommunication wavelengths. We present a synthesis process involving a chelating agent by which stable viscous solutions of titania sol were obtained giving ~ 650 nm crack free thin films on silicon substrates with a single spin and bake cycle. On addition of silica sol (45% by molar ratio) as the dopant, films with thicknesses in the range of 1 μm (refractive index ~ 2.1) after a single spin and bake cycle were achieved which is the largest combination reported to date. ❧ We investigate the properties of thin films densified at high temperatures (> 700 °C) both under vacuum (< 100 mTorr) and nitrogen atmospheres. The vacuum based baking process provides a reliable technique to obtain crack free hybrid sol‐gel films. Titania, titania – silica and titania - zirconia hybrid sol‐gel films were characterized by various techniques including variable angle spectroscopic ellipsometry, ultra violet – visible – near infrared spectrophotometry, Fourier transform infrared spectroscopy, scanning electron microscopy and X‐ray diffraction to confirm their thickness and refractive index, optical transparency, morphology and crystallinity respectively. ❧ In order to truly determine its optical quality we fabricated ridge waveguides using hybrid titania‐silica sol‐gel and performed waveguide cut‐back measurements on 2.5 cm long waveguides. Reactive ion etch (RIE) recipes using CF₄ and O₂ gases were developed for the various sol‐gel materials and RIE induced roughness was measured using AFM measurements and alleviated using wet etching. The optical propagation losses recorded for the fundamental TM like mode were between 8 – 13 dB/cm at 1550 nm for waveguides of widths 4 μm – 2 μm. From 2D and 3D scattering loss calculations the estimated absorption losses are in the range of 6 – 8 dB/cm and waveguide width dependent scattering losses are between 2 – 7 dB/cm. ❧ We also demonstrate integrated micro ring resonator passive filters made of hybrid titania - silica sol‐gel coated on 4 μm silica/ silicon substrates with a group index of 1.91, FSR of 12 GHz, Q factor ~ 16,000 and extract an optical loss of ~18 dB/cm which is scattering dominated. The straight waveguide and microring resonator characterization are the first demonstration of its kind using hybrid titania – silica sol‐gel containing relatively large titania content (~ 90% molar). ❧ Another possible application of these materials in fabricating microdisk / microtoroidal resonators was explored. Using tapered fiber coupling a microdisk with Q factor of ~ 5500 was recorded. CO₂ laser ablation experiments to melt the microdisk preforms into microtoroids were unsuccessful. ❧ The experimental results presented in this dissertation suggest that the proposed synthesis process and densification route are promising but need improved fabrication processes (RIE induced roughness) to fully realize this hybrid material system for photonic applications.

溶胶-凝胶（sol-gel）材料通过制备胶体悬浮液（sols，即溶胶）得到，该悬浮液可经聚合反应转化为粘性凝胶，再经致密化过程最终形成固体材料。通过可控混合不同材料与浓度的溶胶，可合成具备优异光学性能的有机/无机复合材料，适用于各类光子学应用。科研人员已采用溶胶-凝胶工艺制备出无机二氧化钛（titania）与二氧化硅（silica）薄膜，并将其用作太阳能电池的抗反射涂层。然而，文献中报道的多数集成光学器件主要基于二氧化硅溶胶-凝胶工艺。
通过向二氧化钛溶胶-凝胶中掺入可控量的二氧化硅溶胶-凝胶，所得材料的折射率在光通信波段可在约2.5至1.45范围内调节。此外，作为宽禁带材料（金红石相带隙约3.5 eV），二氧化钛溶胶-凝胶与二氧化硅类似，在可见光至近红外的宽光谱区域内均具有光学透明性。但相较于成熟的二氧化硅工艺，二氧化钛溶胶-凝胶工艺相对复杂，这主要源于钛基起始原料的反应性（吸湿性）特质。除了用于厚度低于100纳米的抗反射涂层外，由于缺乏可规模化且可靠的制备工艺，该材料在集成光学器件应用中的潜力难以充分发挥。
本研究采用改性合成工艺与致密化路径，制备出在电信波段折射率范围为1.6~2.5的二氧化钛、二氧化钛-二氧化硅及二氧化钛-氧化锆杂化溶胶-凝胶薄膜。本文提出一种引入螯合剂的合成工艺，通过该工艺可获得稳定的二氧化钛溶胶粘性溶液，单次旋涂-烘烤循环即可在硅衬底上制备出厚度约650 nm的无裂纹薄膜。当掺入摩尔比为45%的二氧化硅溶胶作为掺杂剂时，单次旋涂-烘烤循环即可制备出厚度约1 μm（折射率约2.1）的薄膜，这是目前已报道的最大组分组合。
本研究对高温（>700℃）致密化的薄膜进行了表征，致密化过程分别在真空（<100 mTorr）与氮气氛围下开展。真空烘烤工艺为制备无裂纹杂化溶胶-凝胶薄膜提供了可靠的技术路径。科研人员采用多种表征技术对二氧化钛、二氧化钛-二氧化硅及二氧化钛-氧化锆杂化溶胶-凝胶薄膜进行分析：变角度光谱椭偏仪用于测定薄膜厚度与折射率，紫外-可见-近红外分光光度法用于表征光学透明性，扫描电子显微镜用于分析薄膜形貌，X射线衍射则用于确定薄膜结晶度。
为准确评估该材料的光学性能，本研究采用杂化二氧化钛-二氧化硅溶胶-凝胶制备了脊形波导，并对长度为2.5 cm的波导开展了波导截断测量。针对各类溶胶-凝胶材料，科研人员开发了采用四氟化碳与氧气的反应离子刻蚀（RIE）工艺，通过原子力显微镜（AFM）测量了RIE诱导的表面粗糙度，并采用湿法刻蚀对粗糙度进行了优化。在1550 nm波长下，宽度为2 μm~4 μm的波导的类基横磁模传输损耗为8~13 dB/cm。通过二维与三维散射损耗计算可知，估算的吸收损耗范围为6~8 dB/cm，随波导宽度变化的散射损耗范围为2~7 dB/cm。
本研究还演示了在4 μm二氧化硅/硅衬底上制备的杂化二氧化钛-二氧化硅溶胶-凝胶集成微环谐振器无源滤波器，其群折射率为1.91，自由光谱范围（FSR）为12 GHz，品质因数（Q factor）约为16000，测得的光学损耗约为18 dB/cm，该损耗以散射为主。本次报道的直波导与微环谐振器表征结果，是首次采用二氧化钛摩尔占比约90%的高含量杂化二氧化钛-二氧化硅溶胶-凝胶实现此类器件的演示。
本研究还探索了该材料在制备微盘/微圆环谐振器中的潜在应用。通过锥形光纤耦合，测得微盘谐振器的Q值约为5500。采用二氧化碳激光烧蚀将微盘预制体熔融制备微圆环的实验未获成功。
本学位论文呈现的实验结果表明，本文提出的合成工艺与致密化路径具备应用前景，但仍需优化制备工艺（如改善RIE诱导的表面粗糙度），才能充分发挥该杂化材料体系在光子学应用中的潜力。