Graphene optoelectronic devices on silicon nitride waveguide platform

Ph.D.The rapid development of silicon photonics largely boosts the on-chip integration of an optical communication system for an extremely powerful computer and data center interconnects. To match this need, ultrafast optoelectronic devices like modulators and photodetectors have received increasing attention. However, the incompatibility of micrometer-scale photonic and nanometer-scale electronic devices leads to huge power consumption, latency, and noise during the optical-to-electronic conversion. On the receiver side, traditional III-V compound and germanium-based photodetectors have been well developed. Nevertheless, the dense integration of the devices with complementary metal-oxide semiconductor (CMOS)-compatible materials is still challenging due to the demanding integration technology and high cost.Alternatively, graphene, made up of a single layer of carbon atoms with excellent electrical and optical properties, is regarded as a potential solution. As the typical representative of two-dimensional materials, graphene can be densely integrated with CMOS-compatible materials like silicon and silicon nitride (Si3N4). These dielectric layers can be used both as waveguides and photodetectors, making integration possible within the back-end process for interconnects and avoiding the integration problems arising from the huge mismatch in the height of an optical waveguide with state-of-the-art CMOS transistors. On-chip optical interconnects can solve the thermal bottleneck of high-speed electrical interconnects and thus potentially enable computers to run faster by over 1 order of magnitude. Besides, owing to the zero bandgap and high carrier mobility of graphene, the photodetector has a broadband operation from visible to mid-infrared with a potential to reach a bandwidth exceeding 500 GHz.The thermo-optic effect in graphene-on-Si3N4 ring resonators was firstly analyzed. Then, a graphene-on-Si3N4 waveguide photodetector with a G- S-G structure was demonstrated. It has a responsivity of 15 mA/W and a bandwidth of 30 GHz at ~1550 nm. Next, a carrier dynamics model was built to analyze the carrier diffusion and recombination in detectors. Based on the theoretical model, asymmetric photodetectors with grating-like metal contacts were demonstrated. A calculated transit-time-limited bandwidth of 111 GHz and a resistance-capacitance-limited bandwidth of 227 GHz were derived. A 38-GHz electro-optic bandwidth and a 13- mA/W intrinsic responsivity within a 6-μm detection length were experimentally obtained.However, the previous detectors suffer from huge dark current operating at voltage bias due to the absence of an electrical bandgap in graphene. The dark current gives rise to detrimental noise in communication systems. To improve the on/off current ratio in graphene photodetectors while simultaneously remain a high bandwidth and a large responsivity, we designed Van der Waals heterostructure tunneling photodiodes integrated on Si₃N₄ waveguides. The heterostructure is made up of molybdenum disulfide, graphene, boron nitride, and graphene from bottom to top. A pi- n-like photodiode is formed in the vertical direction. At ~1550 nm, high on/off current ratios larger than 10,000 were measured. A 0.24-A/W responsivity and a 56-GHz bandwidth were also obtained. Compared to previous high-speed graphene photodetectors, this device has three orders of magnitude improvement in on/off current ratio. Its wafer-scale fabrication process and room-temperature operation also pave the way for high-performance photodetectors in optical interconnects and data centers.矽光子學的快速發展促進了集成光通信系統的發展，為了能實現超級計算機和數據中心互聯。為了滿足這壹需求，對於高速光電器件像調制器和光電探測器的研究受到了很多關註。但是，微米量級的光子器件和納米量級的電子器件的不兼容會在光電轉換過程中帶來高功耗，延遲和噪聲。對於探測器，傳統的基於三五族材料和鍺的探測器已經廣泛應用。但是受到集成技術和高價格的限制，如何將這些器件與互補式金屬氧化物半導體集成在壹起仍然是個挑戰。石墨烯，單層碳原子組成並且有優秀的電學和光學性質，被視為壹種解決方案。作為二維材料的典型代表，石墨烯可以將這些器件與互補式金屬氧化物半導體比如氮化矽集成在壹起。這些介電層可以同時作為光波導和光電探測器。這樣集成可以在光學互聯中實現，還可以避免光波導和現有金屬氧化物半導體高度不匹配的問題。集成光學互聯可以解決高速電互聯的散熱瓶頸，因此集成光學互聯可以使計算機運行速度提升壹個數量級。此外，由於石墨烯沒有能帶隙且有很高載流子遷移速率，石墨烯光電探測器從可見光到中紅外波段都可以工作，且有潛力達到大於500 GHz 的帶寬。本文首先研究基於石墨烯氮化矽微環諧振腔的熱光效應。然後，我們研究壹個G-S-G 結構的石墨烯氮化矽波導光電探測器。在1550納米波長，此探測器的響應有15 mA/W，帶寬有30 GHz。隨後，我們建了壹個載流子動態模型，此模型分析了探測器中載流子的遷移和復合。基於這個理論模型，我們實現了壹個非對稱結構的光電探測器。在載流子渡越時間限制下，它有111 GHz 的帶寬。在器件RC 常數在載流子渡越時間限制下，它有227 GHz 的帶寬。 在壹個6 微米長的器件中，我們測到38 GHz 的帶寬和13 mA/W 的響應。然而，由於石墨烯沒有能帶隙，當工作在偏壓下基於石墨烯的光電探測器通常有很高的暗電流。這個暗電流在通信系統中會導致噪聲。為了增加石墨烯光電探測器的電流開關比，同時保證高速和高響應，我們設計了壹個集成在氮化矽光波導上的範德華異質結邃穿光電探測器。這個異質結從上到下由二硫化鉬，石墨烯，氮化硼和石墨烯組成。在垂直方向會形成壹個類似PIN 的光電二極管。在1550 納米波長，我們測到大於壹萬的電流開關比，0.24 A/W 的響應和56 GHz的帶寬。相比於之前的高速光電探測器，這個器件在電流開關比上有三個數量級的提升。這個器件的制備過程和室溫工作條件也為高性能光電探測器在光學互聯和數據中心中的應用提供了新思路。Gao, Yun.Thesis Ph.D. Chinese University of Hong Kong 2019.Includes bibliographical references.Abstracts also in Chinese.Title from PDF title page (viewed on 17, December, 2020)