Novel amorphous carbon thin films for resistive switching applications

Carbon is an abundant element with allotropes that exhibit a wide range of physical, chemical and electrical properties that depend strongly on the material microstructure. Amorphous carbon (a-C) is durable and chemically inert with electrical and mechanical properties that depend on the ratio of graphite-like to diamond-like bonding (or sp2- to sp3-hybridised bonding). The conditions of growth can be selected to produce electrically conducting a-C with ~80 % sp2 bonding or tetrahedrally-bonded a-C with ~80 % sp3 bonding, exceptional hardness and extremely high electrical resistance. These carbon materials can readily be synthesized as thin films, a vital aspect for electronic applications. The bonding `landscape' between the diamond-like and graphitic endpoints offers intermediate and undiscovered properties. Hence, there is potential for new applications and carbon is widely tipped as an enabling material in the next generation of electronic devices. One device that has been identified as a potential application for a-C is the memristor. Resistive switching and emulation of synaptic characteristics have been reported from a variety of a-C device structures. As current computing methods and materials begin to near fundamental limits, intense research activities are focused on finding alternatives. The memristor is poised to enable a next generation of computing with promise of massively parallel processing allowing for lower energy consumption coupled with an exponential increase in processing power. Beyond the benefits to traditional computing, the biomimicry exhibited by memristors could enable a new platform for true artificial intelligence. The potential of the memristor and the aforementioned benefits of carbon as a material have ensured carbon-based memristors have attracted increasing interest from the research community. This thesis describes the fabrication, characterisation and implementation of a-C-based resistive switching devices. Devices were fabricated via physical vapour deposition (PVD) methods with a filtered cathodic vacuum arc (FCVA) used as the primary deposition method for a-C. The project work described in this thesis set out to achieve two aims. The first aim was to investigate the relationship between the sp2:sp3 ratio and the electrical characteristics in wholly carbon devices. The second aim was to explore differing device structures and compositions and implement these devices to reveal the wide variety of memristive effects exhibited by a-C. To satisfy the first aim, an all-carbon memristor was fabricated and tested. The devices consisted of an a-C active layer with mixed sp2/sp3 bonding separating two low resistance sp2-rich contacts. Repeatable bipolar switching was observed with repeatability over >10k cycles.  Unlike conventional metal-oxide memristors, this device did not rely on the formation and rupturing of conductive metal filaments or valence change/chemically altered filament (e.g. O vacancies). The switching was attributed to reversible electric field-induced carbon bond re-hybridisation within the active layer. The current-voltage characteristics were modelled assuming the formation of conducting pathway(s) either at the interface regions where barriers could be present or within the bulk of the active layer where Poole-Frenkel conduction occurred. To satisfy the second aim, oxygenated a-C (a-COx) and hydrogenated a-C (a-C:H) were investigated. a-COx films were used to explore switching and neuromorphic functionality of bilayer a-C devices. The bilayer utilised an interfacial a-COx layer with a ta-C layer forming the bulk. The bilayer was sandwiched between Pt and Ag contacts to form an asymmetric device structure. Multi-state amorphous carbon-based memory devices were demonstrated with devices exhibiting both bipolar and unipolar resistive switching behaviour. Independent use of these modes enabled access to multiple resistance states, enabling higher memory density than conventional binary non-volatile memory technologies. Significantly, these devices displayed various neuromorphic functionalities. Notably, paired-pulse facilitation and paired-pulse inhibition (PPI) were observed at bio-realistic timescales (<100 ms). a-C:H films were synthesised in a FCVA system in the presence of various CH4 partial pressures. Through-film devices were fabricated and comprised two Pt contacts separated by an a-C:H layer deposited with a CH4 partial pressure of 1.0 mTorr. These devices exhibited repeatable bipolar switching characteristics with a low energy requirement for operation. Notably, the devices exhibited temperature-dependent negative differential resistance (NDR). These characteristics were observed in nominally homogeneous devices with hetero layers not required. It was proposed that the switching and NDR behaviour exhibited by the devices was due to trapping and de-trapping of charge carriers by interfacial defects causing potential barrier height modulation.