Browsing by Author "Gu, Qing"
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Item A Steep-slope Threshold Switching Selector Using Silver-doped Polycrystalline Zinc Oxide: Fabrication, Characterization, & Application for 3D X-point Memory & Neuromorphic Devices(December 2021) Sahota, Akshay; Kim, Jiyoung; Farago, Andras; Gu, Qing; Lee, Jeong-Bong; Young, Chadwin D.An assortment of emerging non-volatile memory (NVM) devices has displayed a surge of interest in being investigated for their implementation in energy-efficient bio-inspired neuromorphic computing. The intrinsic device physics of NVMs give them the capability to be employed for emulating the dynamics of a biological neuron and synapse. NVM devices are connected in a dense cross (X)-point circuit architecture thus enabling massive system-level parallelism necessary for a neural network. However, the leakage/sneak current that typically arises from neighboring unselected memory cells is considered as a stumbling block in enlarging X-point arrays. Metalfilament threshold switch has been suggested as a selector device, demonstrated on low leakage characteristics, that holds potentiality due to its straightforward metal-insulator-metal structure, superior performance, and excellent CMOS process compatibility. This dissertation demonstrates research study on the electrical and surface characterization of nano-polycrystalline silver-doped zinc oxide (ZnO) thin films for threshold switching selector device, to propose a way for amending the prevalent selector drawbacks: threshold voltage (Vth) variabilities i.e., intercell and cycle-to-cycle shifts and lousy DC cycling endurance. The current work demonstrates a novel approach to subside system variabilities by uniformly doping a crystalline selector medium i.e., ZnO with Ag metal atoms, rather than incorporating an Ag active metal layer/electrode. First, electrochemical deposition (ECD) process has been employed to slightly dope ZnO with Ag, because of its admirable dopant concentration controllability having atomic percent precision. ECD process helps in demonstrating the proof-of-concept experiment and provides an understanding of volatile switching behavior when ZnO is lightly doped with Ag. Next, “supercycle ALD” technique has been evaluated, where alternating ZnO ALD and Ag metal ALD was employed for lightly doping/delta doping ZnO with Ag. To fend off the shortcomings/drawbacks associated with both the ECD and ALD processes, RF magnetron co-sputtering process is the last fabrication method put to evaluation. Co-sputtering technique provides the wherewithal to control Ag doping levels when lightly doped composite targets (ZnO/Ag 100-x/x at. %, x=1,3,10) are employed. The switching parameters were observed to significantly improve and the trends have been explained based on surface characterizations with XPS, GIXRD, AFM, SEM, EDAX, ICP-MS, HR-TEM, and semiconductor parameter analyzer.Item Design and Applications of Nanoscale Light Sources(2022-08-01T05:00:00.000Z) Li, Xi; Gu, Qing; Ng, Vincent; Henderson, Rashaunda; Lee, Mark; Friedman, JosephFast and efficient nanoscale light sources are at the heart of on-chip optical communication and computation systems. With the rapid development of advanced fabrication techniques and the use of metal in cavity designs, light confinement, and manipulation at the nanoscale, far below the diffraction limit of light, have become possible. Over the years, various nanoscale lasers and LEDs have been analytically or experimentally demonstrated. From the modulation bandwidth perspective, nanolasers are ultimately limited by gain compression at high injection currents. From the energy efficiency perspective, nanolasers are inefficient due to the required high injection current to reach the lasing threshold. In contrast, nanoLEDs can simultaneously support large modulation bandwidth due to the Purcell effect, and high energy efficiency because they can be operated at low injection currents without the need to reach the lasing threshold. This dissertation is focused on the design and applications of nanoscale light sources towards the realization of nanoLEDs that can support high speed modulation and efficient operation. Firstly, we present an optically pumped version of a shifted-core coaxial nanoLED, with a footprint of merely 1/3 of its emission wavelength in all three dimensions at telecommunication wavelengths. By shifting the metallic core off the center of the coaxial cavity, the effective mode volume can be reduced to 0.0078×(λ0/na)3, resulting in a Purcell factor over 390 and a modulation bandwidth exceeding 60 GHz. Furthermore, this nano-emitter features improved emission directivity, which increases its coupling efficiency to an on-chip waveguide. As this nano-emitter supports only one TEM-like mode over the entire material gain spectrum, the spontaneous emission factor becomes close to unity, which greatly improves its internal quantum efficiency. In order to calculate the Purcell factor precisely, we exhaustively studied the effective modal volume, Veff. We found that for cavities with poor confinement and low quality factors, the choice of a correct field normalization method is crucial to adequately describe the diverging behavior of the cavity’s effective modal volume. Secondly, we present the design of an electrically pumped shifted-core coaxial nanoLED. We design the multiple quantum well III-V gain material to achieve high internal quantum efficiency and an impedance transformer to improve the injection efficiency into the nanoLED. Lastly, we propose a biochemical sensor based on plasmonic nanofocusing phenomenon in a pair of coupled shifted-core coaxial nano-cavities. By placing a fluidic channel between the two cavities in close vicinity to the hotspots created by the coupled modes, the sensitivity of this biochemical sensor can be greatly enhanced. In our simulation, this biochemical sensor shows an ultra-high sensitivity up to 1.5179×104 nm/RIU.Item Effective Modal Volume in Nanoscale Photonic and Plasmonic Near-Infrared Resonant Cavities(MDPI AG) Li, Xi; Smalley, J. S.; Li, Zhitong; Gu, Qing; 0000-0003-2707-4969 (Li, X); 0000-0003-3855-3690 (Gu, Q); Li, Xi; Li, Zhitong; Gu, QingWe survey expressions of the effective modal volume, Veff, commonly used in the literature for nanoscale photonic and plasmonic cavities. We apply different expressions of V_{eff} to several canonical cavities designed for nanoscale near-infrared light sources, including metallo-dielectric and coaxial geometries. We develop a metric for quantifying the robustness of different V_{eff} expressions to the different cavities and materials studied. We conclude that no single expression for V_{eff} is universally applicable. Several expressions yield nearly identical results for cavities with well-confined photonic-type modes. For cavities with poor confinement and a low quality factor, however, expressions using the proper normalization method need to be implemented to adequately describe the diverging behavior of their effective modal volume. The results serve as a practical guideline for mode analysis of nanoscale optical cavities, which show promise for future sensing, communication, and computing platforms.Item Extended-gate MOSFET for High Sensitivity Photodetectors and pH Sensors(2021-12-01T06:00:00.000Z) Liu, Jinbo; Young, Chadwin D.; Hu, Walter; Anderson, William; Frensley, William R.; Zakhidov, Anvar A.; Gu, QingOver the past years, semiconductors have been greatly used in sensors. With the development semiconductor technology, the semiconductor sensors showed high sensitivity, large integration and reliable stability. Ion-Sensitive Field Effect Transistor (ISFET) changed the gate electrode of Metal-oxide-semiconductor Field Effect Transistor (MOSFET) from metals to electrolyte. In this dissertation, the perovskite, which is a kind of material with large light absorption coefficient, is used to replace the electrolyte in ISFET based on the structure of ISFET to create high sensitivity photodetector. The perovskite is deposited on a silicon wafer and physically separated with MOSFET. Besides taking both advantages of perovskite with excellent optoelectrical property and silicon as a single crystal with good electrical property to get high responsivity, this extended-gate structure provides convenience for changing the capacitance of perovskite and removing the influence of light on MOSFET. The frequency of electrical signal on perovskite can modulate the capacitance of perovskite, which can be used when the capacitance of perovskite is too high compared with MOSFET. The ionic movement influence, which degrees the performance of this photodetector, can be reduced by adding another MOSFET served as current source at the gate of original MOSFET. Inspired by the ionic movement of perovskite, this dissertation also proves ionic movement in pH electrolyte deteriorates the sensitivity of ISFET by electrical measurement. The extended gate structure is utilized to separate the MOSFET and pH capacitance so the MOSFET is free from changing of temperature. Low temperature can decrease the mobility of ions in pH electrolyte especially after the phase change from liquid to solid. The ions in electrolyte can’t follow the high frequency bias voltage so the ionic movement is less at high frequency. Our results show that the ISFETs have larger sensitivity in low temperature and high frequency since the ionic movement can be suppressed by low temperature and high frequency.Item Halide Perovskite Light-emitting Devices: Ionic Doping and Nanostructuring in Single Layer LEC and Laser(2022-05-01T05:00:00.000Z) Alahbakhshi, Masoud; Gu, Qing; Zakhidov, Anvar; Harabagiu, Sanda; Slinker, Jason D.; Fumagalli, AndreaMetal halide perovskites, as a new type of hybrid semiconductors, have demonstrated promising optoelectronic properties for state-of-the-art and emerging photonic technologies such as pure color light-emitting diodes, cost-effective nano-lasers, and efficient photovoltaic devices. Owing to highly tunable emission wavelengths, high absorption coefficient, high exciton binding energy, narrow emission linewidth, and less expensive fabrication methods, perovskite materials are excellent choices for the next generation of optoelectronic applications. In this dissertation, we mainly focus on introducing and understanding the physics and processing of the perovskite lightemitting devices regarding their dynamic behavior associated with ionic doping and nanopatterning effects in perovskite materials. We begin by investigating a novel and facile approach to overcome some important limitations of Perovskite Light-Emitting Electrochemical Cells (PeLECs) such as intrinsic ion motion degradation, low brightness, and short operational lifetime. In this method, we leverage the advantages of new nanocomposite with an electrolyte polymer along with a lithium salt additive (LiPF6) incorporated into the CsPbBr3 perovskite structure in order to passivate and suppress the traps, defects, and pin-holes in perovskite thin films aiming to improve the morphology and achieve high-performance single layer PeLEC for green emission. By implementing the material characterization techniques, we scrutinize the optimization process for lithium salt additive and demonstrate the advantages of LiPF6 additive including high photoluminescence quantum yield (PLQY), and stable photoluminescence (PL) dynamics, electroluminescence (EL) stability, low hysteresis, and high efficiency of devices. Inspired by the successes of ionic additives in these types of PeLECs, we further investigate the operational stability of devices and reach 100 hours of operational lifetime which is a 5.6-fold improvement over devices with no LiPF6 additive. We further develop our research by utilizing a new synthesized ionic iridium complex to build a HostGuest system in PeLEC structure in order to effectively tune the color emission, improve the morphology and consequently increase the efficiency of PeLECs for future display applications. In the next part of this dissertation, we provide a unique method to construct a multilayer blue Perovskite Light-Emitting Diode (PeLED) by utilizing the electron and hole transport layers as well as Quasi-2D perovskite composition. We successfully show that implementing two long and small ligands into the 3D perovskite precursor can beneficially form both small and large n phases perovskite layers, for the selective energy transfer process, and eventually provide an extremely efficient blue PeLED device. The maximum 10% EQE, maximum luminance 5500 cd m-2 , and 170 min half lifetime (T50) operational stability have been demonstrated. In the last section, we present the novel nanoimprint lithography method in order to perform direct nanopatterning on halide perovskite thin films to create laser cavities. With a meticulous approach that includes a practical encapsulation method, we have exhibited the first demonstration of quasi-CW lasing from directly patterned perovskites with a high-quality cavity design.Item Metal Halide Perovskite Light Sources for On-chip Applications(2021-11-10) Moon, Jiyoung; Gu, QingMetal halide perovskites have attracted much attention as economical gain materials for light emitting diodes (LEDs) and on-chip lasers in photonic integrated circuits. However, the potential commercialization of these perovskite light sources has been hindered by the lack of a systematic approach to overcome perovskite instability, and in the case of lasers, the challenge to directly pattern perovskites into resonators. The goals for this dissertation are to establish systematic approaches to stabilize perovskites, and to demonstrate economical perovskite lasers. To understand the importance of the abovementioned goals, the requirements and fundamentals of on-chip light sources are first discussed. Next, a detailed study of the degradation mechanisms in perovskites is presented. Perovskites degrade by exposure to high temperature, ambient atmosphere, polar solvents (e.g., water), and high photo-/e-beam energy which are used in conventional lithography (e.g., photo-/e-beam lithography). Because of the instability of perovskites, we face not only the short shelf life of perovskite devices, but also the difficulty in the direct patterning of perovskites into an optical cavity. In order to improve the shelf life of perovskite devices, a practical encapsulation method as well as its comparison to other common encapsulation methods are presented. To create laser cavities, a novel direct-patterning method of perovskite — nanoimprint lithography — is presented. We show that nanoimprint lithography improves the quality of perovskites and explain the mechanism behind the positive effects of nanoimprint lithography. With a synergistic approach which includes a practical encapsulation method, an efficient patterning method that also improves material morphology, and a high-quality cavity design, the first demonstration of quasi-CW lasing from directly patterned perovskites is shown. Finally, future directions for improving the efficiency of optically pumped perovskite lasers and the realization of electrically pumped perovskite lasers are discussed.Item Nanoimprinted Perovskite Metasurface for Enhanced Photoluminescence(Optical Soc Amer, 2018-11-05) Wang, Honglei; Liu, Shih-Chia; Balachandran, Balasubramaniam; Moon, Jiyoung; Haroldson, Ross; Li, Zhitong; Ishteev, Artur; Gu, Qing; Zhou, Weidong; Zakhidov, Anvar A.; Hu, Wenchuang (Walter); 0000 0003 5287 0481 (Zakhidov, AA); 0000-0003-3983-2229 (Zakhidov, AA); Wang, Honglei; Balachandran, Balasubramaniam; Moon, Jiyoung; Haroldson, Ross; Li, Zhitong; Gu, Qing; Zakhidov, Anvar A.; Hu, Wenchuang (Walter)Recently, solution-processed hybrid halide perovskite has emerged as promising materials for advanced optoelectronic devices such as photovoltaics, photodetectors, light emitting diodes and lasers. In the mean time, all-dielectric metasurfaces with high-index materials have attracted attention due to their low-loss and high-efficient optical resonances. Because of its tunable by composition band gap in the visible frequencies, organolead halide perovskite could serve as a powerful platform for realizing high-index, low-loss metasurfaces. However, direct patterning of perovskite by lithography-based technique is not feasible due to material instability under moisture. Here we report novel organolead halide perovskite metasurfaces created by the cost-effective thermal nanoimprint technology. The nanoimprinted perovskite metasurface showed improved surface morphology and enhanced optical absorption properties. Significantly enhanced optical emission with an eight-fold enhancement in photoluminescence (PL) intensity was observed under room temperature. Temperature-dependent PL of perovskite nanograting metasurface was also investigated. Based on our results, we believe that thermal nanoimprint is a simple and cost-effective technique to fabricate perovskite-based metasurfaces, which could have broad impact on optoelectronic and photonic applications.Item Perovskite Nanophotonic Devices and Topological Photonic Devices(2021-05-01T05:00:00.000Z) Li, Zhitong; Gu, Qing; Minary, Majid; Zhang, Chuanwei; Lee, Jeong-Bong; Zakhidov, Anvar A.Solution processed organic-inorganic lead halide perovskites have rapidly emerged as a promising gain material for development of the next generation of nanophotonic device ranging from nanolasers, nano LEDs, and solar cells. Here, continuous-wave operation of MAPbI3 perovskite nanolaser is achieved at room temperature with ultralow threshold, which is enabled by thermal nanoimprint lithography that directly patterns perovskite into laser cavities and improves perovskite’s emission characteristics. In the meantime, hyperbolic metamaterials and metasurfaces (HMMs), a special class of anisotropic media, has drawn tremendous research attention recently owing to its remarkable ability to manipulate electromagnetic waves at the subwavelength scale. However, the inevitable metal loss hinders the development of HMMs. Here, a luminescent perovskite HMM operating at 760 nm is achieved using alternating layers of MAPbI3 perovskite and Au, where the loss in Au is maximally compensated by MAPbI3. Simultaneously, topological photonics is a rapidly emerging field, aiming to apply topological physics in photonic systems. The topological protected photonic edge mode is immune to the system disorders and imperfections. However, all photonic edge modes reported in the pioneering works are from lattice systems. Here, a topological band theory is developed in continuous HMM through a nonHermitian Hamiltonian formulated Maxwell’s equations. Two types of edge mode can be induced by including gyromagnetic and chiral effect in HMM and can be numerically observed. Finally, a topological micro ring laser array that possesses edge mode lasing is designed and experimentally achieved on the III-V semiconductor platform.Item Room Temperature Operation of Directly Patterned Perovskite Distributed Feedback Light Source under Continuous-Wave Optical Pumping(Institute of Electrical and Electronics Engineers Inc.) Gharajeh, Abouzar; Haroldson, Ross; Li, Zhitong; Moon, Jiyoung; Balachandran, Balasubramaniam; Hu, Wenchuang (Walter); Zakhidov, Anvar A.; Gu, Qing; 0000 0003 5287 0481 (Zakhidov, AA); 0000-0003-3983-2229 (Zakhidov, AA); 0000-0003-3855-3690 (Gu, Q); Gharajeh, Abouzar; Haroldson, Ross; Li, Zhitong; Moon, Jiyoung; Balachandran, Balasubramaniam; Hu, Wenchuang (Walter); Zakhidov, Anvar A.; Gu, QingWe report the first directly patterned perovskite distributed feedback (DFB) resonator with a narrow amplified spontaneous emission (ASE) at pump powers as low as 0.1W/cm², under continuous-wave (CW) optical pumping condition at room temperature.Item Ultrafast Shifted-Core Coaxial Nano-Emitter(Optical Soc Amer) Li, Xi; Gu, Qing; 0000-0003-3855-3690 (Gu, Q); Li, Xi; Gu, QingWe present an ultrafast nanoscale light source utilizing a shifted- core coaxial nanocavity, with a footprint of merely one-third of its emission wavelength in all three dimensions at telecommunication wavelengths. We show that, by shifting the metallic core off center of the coaxial structure, the effective mode volume of the cavity can be as small as 0.0078 x (λ₀/n_a)³, resulting in a Purcell factor over 390 and a modulation bandwidth exceeding 60GHz. We further show that the evolution trend of the cavity Q factor as a function of core- shifting distance can be engineered by choosing proper substrate material. Compared to its symmetric counterpart, this shifted-core coaxial nano-cavity features not only higher Q factor, Purcell factor, and modulation bandwidth but also an improved emission directivity that is essential in its coupling with other on-chip components. The proposed nano-emitter also features robust single mode operation over the entire core-shifting range, resulting in a near-unity spontaneous emission factor. Therefore, this device can be a good candidate for low power optical interconnect applications.