Investigation of electrical properties of leading-edge scaled CBRAM devices using novel materials for future memory technologies

The global datasphere is predicted to grow from 33 zettabytes (ZB) in 2018 to 175 ZB by 2025. Ever increasing data is accelerated by the Internet of Things (IoT) and hyper personalisation of consumer life by behavioural modelling. Therefore, the development of novel data storage solutions has become more important than before. Among the different emerging memories, Conducting Bridge Random Access Memory (CBRAM) is a prime candidate for high density storage class memory (SCM) applications due to its ease of fabrication, scalability and high cycling endurance. The memory device consists of a solid electrolyte placed between an active and inert electrode constituting a Metal- Insulator-Metal (MIM) structure. The information is stored as device resistance, which can be switched between high and low values. The change in resistance is brought about by the voltage-controlled formation and dissolution of the active electrode metal filament in the electrolyte. A low operating current (≤ 50 μA) is favourable for SCM devices in order to reduce current density requirements of the selector element. This enables 1-selector 1-resistor (1S1R) integration in large memory array blocks. However, at such low currents, the retention and variability of CBRAM devices are adversely affected by the small filament size controlled by the operating current. The primary goal of this thesis is to optimise CBRAM single cells for stand- alone low power SCM. Different active electrode-electrolyte combinations are tested at an operating current of 50 μA. The impact of electrolyte composition, electrolyte thickness, tantalum buffer layer and copper supply alloying on the reliability properties of traditional copper CBRAM are investigated. Based on the findings, a novel Cu-GeSe CBRAM is developed for SCM applications. These devices can be switched with 10 μs pulses and show an extrapolated retention of > 10 years at 85°C, with no tailed distributions. In-depth studies using conduction and kinetic models are conducted to understand the origins of switching voltage and resistance variability in Cu-GeSe CBRAM devices. In order to reduce the switching pulse widths further, we investigate Cu-CBRAM based on oxide electrolytes. We unravel the crucial role played by the electrically generated intrinsic oxygen vacancies on the switching and reliability of these devices. Finally, we develop a novel Co-based CBRAM through extensive active electrode screening based on theoretical considerations. Reliability investigations of Co-CBRAM with both oxide and chalcogenide electrolytes were performed for the first time, to our knowledge, in this thesis. The influence of multiple factors like thermal anneal, electrolyte alloying and electrolyte thickness on the electrical characteristics of Co devices are thoroughly analysed and pathways to improve the reliability are identified. Based on the findings, we develop a novel Co-GeSe CBRAM stack which shows an extrapolated retention of > 10 years at 85°C and can be switched with 1 μs pulses, ten times faster than Cu-GeSe CBRAM.status: publishe