Browsing by Author "Chen, Tsung-Cheng"

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Bidirectional All-Optical Synapses Based on a 2D Bi2O2Se/Graphene Hybrid Structure for Multifunctional Optoelectronics
(1970-01-01)
Yang, Chia-Ming
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Chen, Tsung-Cheng
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Verma, Dharmendra
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Li, Lain-Jong
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Liu, Bo
Neuromorphic computing has been extensively studied to mimic the brain functions of perception, learning, and memory because it may overcome the von Neumann bottleneck. Here, with the light-induced bidirectional photoresponse of the proposed Bi2O2Se/graphene hybrid structure, its potential use in next-generation neuromorphic hardware is examined with three distinct optoelectronic applications. First, a photodetector based on a Bi2O2Se/graphene hybrid structure presents positive and negative photoresponsibility of 88 and -110 A W-1 achieved by the excitation of visible wavelength and ultraviolet wavelength light at intensities of 1.2 and 0.3 mW cm(-2), respectively. Second, this unique photoresponse contributes to the realization of all optically stimulated long-term potentiation or long-term depression to mimic synaptic short-term plasticity and long-term plasticity, which are attributed to the combined effect of photoconductivity, bolometric, and photoinduced desorption. Third, the devices are applied to perform digital logic functions, such as "AND" and "OR," using full light modulation. The proposed Bi2O2Se/graphene-based optoelectronic device represents an innovative and efficient building block for the development of future multifunctional artificial neuromorphic systems.
Dimensionally anisotropic graphene with high mobility and a high on-off ratio in a three-terminal RRAM device
(2020-06-01)
Liu, Bo
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Tai, Han Hsiang
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Liang, Hanyuan
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Zheng, En-Yan
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Sahoo, Mamina
In the past decade, graphene has aroused worldwide academic passion in the electronics community, due to its unique two dimensional lattice structure and high mobility properties. Meanwhile, the utilization of graphene for next-generation nano-electronic materials is still under debate due to its intrinsic zero band gap nature. To meet the requirement of logic applications, diversified academic attempts have been carried out to modify its intrinsic band structure, such as doping, bilayer stacking, nano-patterning, or heterostructures. In this work, a tailor made strategy for graphene is proposed and experimentally demonstrated: a dimensionally anisotropic graphene based three terminal resistive random-access memory (RRAM). This device could be operated in two modes, which realized not only high carrier mobility (10(3) cm(2) V-1 s(-1)) and low gate leakage in field effect mode, but also a high on-off ratio (10(7)) and ultra-low off state current (1 pA) in resistive switching mode. This device also exhibits high reliability in logic data storage: long retention up to 8000 s, endurance of 400 DC operation cycles, and read immunity up to 1 million cycles. To guarantee the dimensional uniformity and high quality of the graphene, surface polishing and passivation techniques were carried out on Cu foil before graphene CVD synthesis. As demonstrated in this study, the current work paves the way for graphene toward practical logic and memory applications with high performance and low steady state power consumption.
Enhancement for Potential-Induced Degradation Resistance of Crystalline Silicon Solar Cells via Anti-Reflection Coating by Industrial PECVD Methods
(2018-12-01)
Chen, Tsung-Cheng
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Kuo, Ting-Wei
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Lin, Yu-Ling
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Ku, Chen-Hao
;
Yang, Zu-Po
The issue of potential-induced degradation (PID) has gained more concerns due to causing the catastrophic failures in photovoltaic (PV) modules. One of the approaches to diminish PID is to modify the anti-reflection coating (ARC) layer upon the front surface of crystalline silicon solar cells. Here, we focus on the modification of ARC films to realize PID-free step-by-step through three delicate experiments. Firstly, the ARC films deposited by direct plasma enhanced chemical vapor deposition (PECVD) and by indirect PECVD were investigated. The results showed that the efficiency degradation of solar cells by indirect PECVD method is up to -33.82%, which is out of the IEC 62804 standard and is significantly more severe than by the direct PECVD method (-0.82%). Next, the performance of PID-resist for the solar cell via indirect PECVD was improved significantly (PID reduced from -31.82% to -2.79%) by a pre-oxidation step, which not only meets the standard but also has higher throughput than direct PECVD. Lastly, we applied a novel PECVD technology, called the pulsed-plasma (PP) PECVD method, to deal with the PID issue. The results of the HF-etching rate test and FTIR measurement indicated the films deposited by PP PECVD have higher potential against PID in consideration of less oxygen content in this film. That demonstrated the film properties were changed by applied a new control of freedom, i.e., PP method. In addition, the 96 h PID result of the integrated PP method was only -2.07%, which was comparable to that of the integrated traditional CF method. In summary, we proposed three effective or potential approaches to eliminate the PID issue, and all approaches satisfied the IEC 62804 standard of less than 5% power loss in PV modules.
Enhancing extracted electroluminescence from light-emitting electrochemical cells by employing high-refractive-index substrates
(2017-12-01)
Jang, Yea-Fen
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Lin, Tzu-Chun
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Guo, Jhih-Yan
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Chiang, Chien-Ming Fan
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Wu, Ming-Lun
Solidstate lightemitting electrochemical cells (LECs) show several advantages over conventional organic lightemitting devices (OLEDs) such as simple device structure compatible with solution processes, low operation voltage and capability of utilizing inert cathode metals. However, device performance of LECs must be improved, e.g. enhancing light extraction, to meet the requirements for practical applications. Among the optical modes trapped in LECs, light trapped in substrate mode is easier to be extracted, e.g., by simply roughing the output surface. Therefore, increasing the percentage of substrate mode is beneficial in improving light extraction. In this work, the contributions of optical modes in LECs employing substrates with various refractive indices are analyzed. Higherrefractiveindex substrates are shown to trap more light in the substrates. Smaller refractive index difference between higher-refractive-index substrate and indium tin oxide (ITO) layer also increases the cutoff spectral range of light waveguided in ITO layer. Furthermore, light intensity in surface plasmon mode significantly reduces as the refractive index of the substrate increases. Reducing the percentage of surface plasmon mode facilitates light extraction since it requires more complicated methods for outcoupling light in this mode. With commercially available unpolished sapphire substrates, light output of LECs is enhanced by 56%. When a scattering layer was inserted between ITO and sapphire substrate, more light in substrate mode can be extracted and 71% enhancement in light output is realized. High external quantum efficiency up to 5.5% is consequently obtained in LECs based on a ruthenium complex. Such device efficiency is among the highest reported values for redemitting LECs and thus confirms that utilizing higher-refractive-index substrates would offer a simple and feasible approach to improve light output of LECs. In comparison to OLEDs, increased EL trapped in substrates of LECs mainly comes from surface plasmon mode rather than waveguide mode. (C) 2017 Elsevier B.V. All rights reserved.
Single material TiO2 thin film by atomic layer deposition for antireflection and surface passivation applications on p-type c-Si
(2018-09-01)
Chen, Tsung-Cheng
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Yang, Tsuo-Chuan
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Cheng, Hsyi-En
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Yu, Ing-Song
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Yang, Zu-Po
A thin film deposited on the front surface of solar cell plays an important role in reducing the reflection of incident light and providing surface passivation. Although ultrathin TiO2 films have shown excellent performance of surface passivation, simultaneously serving both functions is hindered by the crystallization issue, which can degrade the passivation quality of TiO2 film, as the film thickness is over certain value (few nanometer only). Here we showed that both functions can be satisfied by single material of titanium oxide film through atomic layer deposition at low temperatures. By varying deposition temperatures from 80 to 200 degrees C, the TiO2 film deposited at 80 degrees C can maintain its amorphous phase up to the thickness of 114 nm. In addition, the optimal thickness for antireflection calculated by OPAL 2 is 61 nm, meaning that amorphous TiO2 film to provide antireflection and surface passivation can be achieved. The passivation quality is verified by minority carrier lifetime measurement using photo-conductance decay method and presented the ultralow surface recombination velocity of 8.7 cm/s. Furthermore, the passivation mechanism is investigated by X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS), in which the absence of crystalline issue is confirm and both chemical passivation (due to H termination) and field effect passivation (due to residual Cl atoms) are associated with the improvement of passivation quality. A post deposition anneal is conducted on the TiO2 film deposited at 80 degrees C. The results present that extra thermal budgets can ruin the passivation quality, which is explicated by the TiO2 crystallization as the temperature exceeding 350 degrees C from the XRD measurements, and by the descending of Cl atom (also field effect passivation) as increasing of the annealing temperature from the SIMS measurements. (C) 2018 Elsevier B.V. All rights reserved.