Publication: Flexible Resistive Memory Devices for Eco_friendly Electronics: Fabrication, Modeling, and Circuit Implementation
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Date
2025-06-20
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Indian Institute of Technology, Jodhpur
Abstract
The domain of flexible electronics has experienced an unprecedented rise in research and development activities, owing to its wide applications such as wearable devices, flexible displays, and sensors for biomedical purposes. This has motivated researchers to actively explore a suitable flexible memory device for storing the data generated or received by the flexible electronic circuit(s), to achieve monolithic integration i.e., fabrication of different flexible devices on the same substrate. Resistive random access memory (RRAM), a two terminal metal-insulator-metal structure device, with its various advantages such as ease of fabrication, low cost, high speed, ease of integration, and high scalability has emerged as a promising memory device for flexible electronic systems. Moreover, in view of ever increasing electronic waste, nature originated materials are being explored for the fabrication of different organic devices, and flexible electronics provide an added advantage of incorporating the organic materials in fabrication process, as most of these materials have properties such as solution processability and mechanical flexibility, along with non-toxicity, biocompatibility, and biodegradability which further instill eco-friendliness in devices. In this work, initially, natural proteins gelatin and egg-albumen, were investigated separately as a switching layer of RRAM devices. The prepared solutions of gelatin and albumen were deposited on the Indium-doped tin oxide coated polyethylene terephthalate substrate, and silver was used as the top electrode. The fabricated flexible RRAM devices with gelatin switching layer, have exhibited excellent switching behavior with Ion=Io f f ratio of greater than 105 and retention time of more than 104 seconds. Similarly, the albumen switching layer devices have also shown excellent resistive switching with high current on/off ratio of around 105 and memory retention time of 103 s without showing relevant degradation. The devices were then subjected to a mechanical bending of radius 7.5 mm, and it was observed that the device maintained the memory window of greater than 103 for more than 103 seconds. These results suggested that these organic proteins should further be explored for fabrication of flexible organic memory devices. Furthermore, to enhance the multiple cycle switching (endurance) of the devices, these proteins were investigated in hybrid bilayer combination with an ultrathin (5nm) layer of HfO2 as switching layer. Hybrid bilayer Ag/Gelatin/HfO2/ITO devices have shown a very high memory window of greater than 105 and data retention of 104 s without any degradation in a pristine state. Moreover, after bending the devices at a 12 mm radius followed by 7 mm, it was observed that the devices have maintained the memory window of 105 without any degradation in data retention, indicating excellent electromechanical stability of the devices. Similarly, Ag/Albumen/HfO2/ITO devices have demonstrated excellent switching characteristics with a current on/off ratio of greater than 104, stable retention of both low resistance and high resistance states, reliable multiple cycle switching, and very low switching power (with set power as 0.5 mW and reset power as 3.1 mW). The devices have also shown excellent electro-mechanical stability, with bending radii of 7.5 mm, 5 mm, and 2.5 mm. Additionally, to enable simulation based study of the fabricated device, a mathematical model of RRAM device has been studied and investigated. The model is then calibrated with the median of experimental results to extract the values of fitting parameters with less than 5% rms error. This ensures that the model can be utilized to simulate the switching characteristics of the fabricated device with greater accuracy. The model was further improved by introducing parameters for multiple layers of insulator, and multiple cycle variation parameters to make the model more robust. This improved RRAM model was then utilized to design a simple flexible hybrid electronic (FHE) circuit that is capable of generating 4 bit random numbers by utilizing intrinsic randomness of the device. The circuit is designed with 65nm technology node and the RRAM model with cyclic variation is utilized as the source of entropy. To verify the randomness of the proposed FHE circuit, the generated outputs have been test with NIST SP 800-22 test suits, a widely accepted set of tests to verify randomness of a bitstream. The outputs of this circuit have passed all the applicable tests of NIST SP 800-22, indicating the presence of randomness. This not only represents taht the output bitstreams are random in nature but also reflects that the improved model is capable of taking care of cyclic variations to a larger extent. Recently, Pectin was extracted from orange peel, and utilized as a switching layer. The fabricated flexible devices have demonstrated good resistive switching behavior with high current on/off ratio of 104 and retention time of 103 seconds. The fabricated device has then been investigated for synaptic behavior, probably for the first time, and it demonstrated depression characteristics with 10 ms input pulse, and the PPF relaxation time constants t1 = 0.3 ms and t2 = 4.1 ms are obtained. The results show a similar trend as that of a biological synapse, and thus it may be concluded that the fabricated device with pectin can be utilized for neuromorphic applications.
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Dwivedi, Anurag (2019).Flexible Resistive Memory Devices for Eco_friendly Electronics: Fabrication, Modeling, and Circuit Implementation(Doctor's thesis).Indian Institute of Technology, Jodhpur