Characterisation of Novel Resistive Switching Memory Devices

Resistive random access memory (RRAM) is widely considered as a disruptive technology that will revolutionize not only non-volatile data storage, but also potentially digital logic and neuromorphic computing. The resistive switching mechanism is generally conceived as the rupture/restoration of defect-formed conductive filament (CF) or defect profile modulation, for filamentary and non-filamentary devices respectively. However, details of the underlying microscopic behaviour of the resistive switching in RRAM are still largely missing. In this thesis, a defect probing technique based on the random telegraph noise (RTN) is developed for both filamentary and non-filamentary devices, which can reveal the resistive switching mechanism at defect level and can also be used to analyse the device performance issues. HfO2 is one of the most matured metal-oxide materials in semiconductor industry and HfO2 RRAM shows promising potential in practical application. An RTN-based defect extraction technique is developed for the HfO2 devices to detect individual defect movement and provide statistical information of CF modification during normal operations. A critical filament region (CFR) is observed and further verified by defect movement tracking. Both defect movements and CFR modification are correlated with operation conditions, endurance failure and recovery. Non-filamentary devices have areal switching characteristics, and are promising in overcoming the drawbacks of filamentary devices that mainly come from the stochastic nature of the CF. a-VMCO is an outstanding non-filamentary device with a set of unique characteristics, but its resistive switching mechanism has not been clearly understood yet. By utilizing the RTN-based defect profiling technique, defect profile modulation in the switching layer is identified and correlated with digital and analogue switching behaviours, for the first time. State instability is analysed and a stable resistance window of 10 for >106 cycles is restored through combining optimizations of device structure and operation conditions, paving the way for its practical application. TaOx-based RRAM has shown fast switching in the sub-nanosecond regime, good CMOS compatibility and record endurance of more than 1012 cycles. Several inconsistent models have been proposed for the Ta2O5/TaOx bilayered structure, and it is difficult to quantify and optimize the performance, largely due to the lack of microscopic description of resistive switching based on experimental results. An indepth analysis of the TiN/Ta2O5/TaOx/TiN structured RRAM is carried out with the RTN-based defect probing technique, for both bipolar and unipolar switching modes. Significant differences in defect profile have been observed and explanations have been provided

Bismuth halide thin films for resistive random access memory device

Resistive random-access memory (RRAM) is a kind of highly promising non-volatile memory technology. Recently, halide perovskites have aroused attention worldwide because of their outstanding resistive switching performance and ease of fabrication. The advantages of the halide perovskite devices include high ON/OFF ratio and low operation voltage, enabling excellent device performance with low power consumption. Currently, the most widely studied halide perovskites contain lead, which is a toxic element that may incur serious environmental problems and significant harm to human health. In order to address these issues, there is a pressing need to develop lead-free halide perovskites and their derivatives possessing comparable functional properties to their lead-based counterparts. Bismuth-based halide perovskites have emerged as a promising lead-free alternative for applications in RRAM. A great advantage of bismuth-based halide perovskites lies in their high solubility for various elements, thus offering the possibility of the formation of modified compositions to tailor the resistive switching behaviours including ON/OFF ratio, endurance and retention. Cs3Bi2I9 and MA3Bi2I9 (MA = methylammonium) are two common lead-free perovskite halides that have been widely studied for RRAM. However, doping in Cs3Bi2I9 and MA3Bi2I9 is normally conducted on a single chemical site (either A-site or X site) and the impact of co-doping on their resistive switching properties remains less explored. In this project, thin films of several co-doped compositions namely MA2CsBi2BrxI9-x (x=2, 3, 4, 5, 6, 7, 8) were prepared to investigate the double doping (Cs on A-site, Br on X-site) effects on their structural, morphological and electrical properties. In addition, the effect of different top electrodes (Ag and Au) on the electrical performance of the MA2CsBi2BrxI9-x thin films was also studied. It was found that more uniform and denser thin films could be obtained with an increase in Br content. Among the several compositions under investigation, MA2CsBi2Br8I-based thin film with Au top electrodes exhibited typical resistive switching behaviour and an interface-type conduction mechanism. When the perovskites layer was covered by Ag top electrodes, the distinct resistive switching behaviour could be observed with the increase of I content, which could be attributed to the redox reaction of Ag electrodes and iodide ions at the interface between electrodes and the active layer. Compared to other compositions, MA2CsBi2Br2I7-based thin film with Ag electrodes exhibited an outstanding ON/OFF ratio of around 105. Since the MA2CsBi2Br8I perovskite had good endurance and full-coverage surface, the MA2CsBi2Br8I perovskite was employed for further study. Au/MA2CsBi2Br8I/ITO devices with different thicknesses (290 nm, 307 nm, 341 nm and 435 nm) showed stable bipolar resistive switching behaviours. With the increasing thickness, the SET electric field remains around 6.5 V/μm, which is nearly independent of film thickness. When the thickness of the MA2CsBi2Br8I perovskite layer increased from 136 nm to 307 nm, the device demonstrated better stability over 100 cycles and a higher ON/OFF ratio (~10) at a low reading voltage of 0.27 V

Processos de condução eletrónica em células de memória ReRAM do tipo VCM com óxidos metálicos

New applications, such as neuromorphic computing, and the limitations of current semiconductor technologies demand a revolution in electronic devices. As one of the key enablers of a new electronics paradigm, redox-based resistive switching random access memory (ReRAM) has been the focus of much research and development. Among the ReRAM research community, Ta2O5 has emerged as one of the most popular materials, for enabling high endurance and high switching speed. Ta2O5-based ReRAM rely on the nonvolatile change of the resistance via the modulation of the oxygen content in conductive filaments, as it is described in the valence change mechanism. However, the filaments’ structure and exact composition are currently under intense debate, which hinders the development of better device design rules. The two current models in the literature consider filaments composed of oxygen vacancies and those containing metallic Ta. This work attempts to solve this dispute by reporting a detailed study of the electrical transport through the conductive filaments inside Ta2O5-based ReRAM. In parallel, the electrical transport and structure of substoichiometric TaOx thin films, grown to try and match the material of the filaments, was studied in detail. A strong correlation between the transport mechanisms in the conductive filaments inside the Ta2O5 ReRAM and in the TaOx thin films with x 1 was found. This clearly links the physical properties of the materials composing the filaments and the substoichiometric TaOx thin films. Structural analysis performed on the TaOx films reveals the presence of Ta clusters inside the films. Moreover, the electrical transport of metallic Ta films shows the same transport mechanism as TaOx with x 1, for most of the measured temperature range, from 2 K to 300 K. Beyond the transport mechanisms, both cases share a carrier concentration on the order of 1022 cm−3 and a positive magnetoresistance associated with weak antilocalization at T < 30 K. Therefore, it is concluded that the transport in the TaOx films with x 1 is dominated by a percolation chain of Ta clusters embedded in an insulating Ta2O5 matrix. These clusters exhibit disordered metal-like behaviour, where quantum corrections to the Boltzmann transport dominate the conduction. In conclusion, the electrical transport in the conductive filaments inside Ta2O5-based ReRAM devices is determined by percolation through Ta clusters, which is in line with independent observations of metallic Ta in the filaments. This work strongly supports the metallic Ta filament model.Novas aplicações, tais como computação neuromórfica, e as limitações da tecnologia de semicondutores atual exigem uma revolução nos dispositivos eletrónicos. Sendo uma peça chave para um novo paradigma da eletrónica, a memória ReRAM (redox-based resistive switching random access memory) tem sido alvo de muita investigação e desenvolvimento. O Ta2O5 é um dos materiais mais populares para usar em dispositivos ReRAM, permitindo alta durabilidade e velocidades de comutação elevadas. As ReRAM com Ta2O5 baseiam-se na mudança não volátil da resistência elétrica através da modulação da quantidade de oxigénio em filamentos condutores, como é descrito no mecanismo de alteração de valência (valence change mechanism). No entanto, a estrutura dos filamentos e a sua composição química exata, são ainda alvo de intenso debate, limitando o desenvolvimento de melhores receitas de fabricação de dispositivos. Os dois modelos atuais na literatura consideram filamentos compostos por lacunas de oxigénio e filamentos com Ta metálico. Este trabalho procura resolver esta disputa ao reportar um estudo detalhado do transporte elétrico através de filamentos condutores em dispositivos ReRAM de Ta2O5. Paralelamente, foi estudado em detalhe o transporte elétrico e a estrutura de filmes finos de TaOx subestequiométrico, depositados de forma a emular o material dos filamentos. Foi encontrada uma forte correlação entre os mecanismos de transporte nos filamentos condutores dentro dos dispositivos ReRAM de Ta2O5 e nos filmes finos de TaOx com x 1. Isto estabelece uma ligação clara entre as propriedades físicas dos materiais que compõem tanto os filamentos como os filmes finos de TaOx. A análise estrutural efetuada nos filmes de TaOx revela a presença de aglomerados de Ta. Por outro lado, o transporte elétrico em filmes finos de Ta é dominado pelos mesmos mecanismos de condução observados nos filmes de TaOx com x 1, para a maior parte da gama de temperatura de 2 K a 300 K. Ambos os casos partilham ainda uma concentração de portadores da ordem de 1022 cm−3 e uma magnetoresistência positiva associada a anti-localização fraca para T < 30 K. Portanto, é concluído que o transporte em filmes de TaOx com x 1 é dominado por uma cadeia de percolação de aglomerados de Ta embutidos numa matriz isoladora de Ta2O5. Estes aglomerados exibem um comportamento típico de metais desordenados, para os quais a condução é dominada por correções quânticas ao transporte de Boltzmann. Em conclusão, o transporte elétrico em filamentos condutores dentro de dispositivos ReRAM baseados em Ta2O5 é dominado pela percolação de aglomerados de Ta, o que corrobora observações independentes de Ta metálico nos filamentos. Assim, este trabalho suporta o modelo baseado no filamento metálico de Ta.Programa Doutoral em Engenharia Físic