Realization and Stability Analysis of Perovskite Oxide-Based Thin Films for Memory Applications

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Shringi, Amit Kumar
Kumar, Mahesh
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Indian Institute of Technology Jodhpur
In the past few decades, with the rapid digitalization and extensive use of the internet of things and artificial intelligence, non-volatile memories have received a lot of attention for their use in electronic devices and logic storage units as current silicon-based memories are hitting the downscaling limits. In such a scenario, resistive switching random access memory (RRAM) seems to be a good candidate for the next-generation memory technology due to its low power consumption, high-density integration, and fast switching operation. Resistive switching random access memory has been studied for different types of memory characteristics such as write once read many (WORM), dynamic random-access memory (DRAM), and flash memory. Various perovskite oxides have been investigated for memory applications and neuromorphic computing. This work includes the synthesis of perovskite oxides like Barium Titanate (BaTiO3 i.e., BTO), Strontium Titanate (SrTiO3 i.e., STO) and Lanthanum Orthoferrite (LaFeO3 i.e., LFO) using solid-state reaction method, deposition of thin films and device fabrication for memory applications. BTO is a very well-explored ferroelectric material for different applications. Initially, we synthesized the BTO powder to prepare a sputtering target. We deposited the BTO thin film on FTO coated glass substrate using RF sputtering. The circular silver (Ag) top contacts were deposited to form memory cells to analyze the resistive switching properties. The Ag/BTO/FTO devices were characterized to optimize the thickness of the BTO layer in the memory device using BTO layers of different thickness. The memory characteristics are investigated as a function of the thickness of the amorphous-BTO layer, which is determined by varying the deposition time. Devices with all deposited thicknesses show data retention for more than 4000 seconds and 300 reading cycles. We get a high on-off ratio of the order of 106 of the devices having a thickness of 180 nm. The fabricated WORM devices exhibit good reading endurance and data-retention characteristics. Further, the switching performances are investigated for different devices with different deposition temperatures, varying from room temperature (RT) to 450°C. All the devices are characterized for a voltage sweep of ?1.5 V to 1.5 V with 12,000 pulses for retention and 300 write-read erase read cycles. Shifting in the switching voltage to a lower value from 1.31 to 0.49 V (450°C) is recorded with the increased deposition temperature. To verify the radiation stability of BTO thin film WORM devices, we irradiated the RT-deposited 180 nm BTO thin film with a different fluence of Ar+6 ions. The characterizations show a negligible shift in forming voltage and read current after irradiation which confirms that the BTO WORM devices show very high tolerance towards radiation and can be used in a harsh atmosphere. RRAMs have been explored for a long back due to their potential to replace the existing non-volatile memory technologies. We synthesized SrTiO3 (STO) for the realization RRAM using these layers. The SrTiO3 based RRAM devices were exposed to swift heavy ions and the stability and performance of the devices were analyzed. We observed a reduced memory window with reduced read current. The abrupt switching behaviour of the devices change to soft switching as the fluence of Au ions increased. Further, the LFO thin film-based memory devices were investigated for RRAM characteristics and synaptic behaviour analysis. The LFO powder was synthesized using a conventional solid-state reaction method to prepare the sputtering target. Ag/LFO/FTO structures were fabricated and investigated for RRAM for different thickness and deposition temperatures of LFO thin film. The resistive switching properties are investigated using macroscopic I?V measurements, showing low voltage switching with a high on-off ratio (?300) and long retention (?9000 seconds). The fabricated devices show stable, low voltage and high switching speed operation. Furthermore, in this work, we demonstrate the synaptic behaviour of the LFO thin film memory devices, as it exhibits analogue memory characteristics, potentiation and depression. Further, the LFO thin films were irradiated to 180 MeV Ar+6 ion for different fluence and analysed the effect of radiation. The increment in reset voltage was observed as we increased the fluence of Arions while the devices were able to maintain the RRAM characteristics. These studies show, that the BTO, STO and LFO-based memory devices have the potential to be used as memory devices.
Shringi, Amit Kumar. (2023).Realization and Stability Analysis of Perovskite Oxide-Based Thin Films for Memory Applications (Doctor's thesis). Indian Institute of Technology Jodhpur, Jodhpur.