Über die Entwicklung von Memsensoren

Since the postulation of the experimental realization of memristive devices in 2008, a broad variety of concepts for the fabrication of memristive devices has been pursued and the underlying switching mechanisms have been studied in detail. The unique electronic properties of memristive devices inspire applications that go beyond conventional electronics, such as using memristive devices as programmable interconnects, to realize logics for in-array-computing or in neuromorphic engineering. A particularly interesting aspect of biological neural networks is the close connection between signal detection and processing at the neuron level, which is an essential contribution to their outstanding efficiency. This work evolves around the concept of memsensors, which unify the characteristic features of memristive devices and sensor devices and as such appear as promising candidates to realize a close connection between signal detection and processing on the device level. Memsensors are a highly interdisciplinary topic, bridging research in the fields of material science and electrical engineering and relating to insights from biology and medicine through neuromorphic engineering. The major objective of this thesis is to provide tools and building blocks and showcase pathways to incorporate memristive and sensitive properties into memsensor devices. For this purpose, motivated by an experimental point of view, a nanoparticle-based memristive device with diffusive memristive switching characteristics was developed and characterised in detail and sensors relying on semiconducting metal oxide thin films and nanostructures were thoroughly studied. In addition, in terms of modelling of memsensor circuits, emerging features such as amplitude adaptation are discussed, showcasing the particular eligibility of memsensors in the context of neuromorphic engineering

Fabrication, characterisation and modelling of electronic devices based on amorphous metal-oxide and carbonaceous materials

This thesis describes the deposition, characterisation and device applications of amorphous zinc tin oxide (a-ZTO) and carbon-based materials using energetic deposition techniques. The study aimed to explore the suitability of this growth method in producing highquality carbon and a-ZTO films for device applications. Technology Computer Aided Design (TCAD) simulations were also conducted in tandem with experiments to identify critical parameters affecting device behaviour and to provide guidance for improved device performance. Firstly, electrical carbon contacts to n-type 6H-SiC were energetically deposited from a filtered cathodic vacuum arc (FCVA) at room temperature and elevated temperature with low and high biases. Lower energy (< 100 eV) in the carbon flux resulted in resistive amorphous carbon contacts. As the deposition energy and sp2 bonding (graphitic) fraction were increased, an oriented graphitic microstructure developed and rectifying electrical characteristics emerged. TCAD simulations revealed the effects of interfacial layers and contact work functions on the device performance and suggested that the rectification ratios of C/6H-SiC Schottky diodes could be increased by improving the lateral homogeneity of the junctions and/or by controlling the thickness of interfacial layers. Secondly, thin films of amorphous zinc tin oxide (a-ZTO) were energetically deposited using high power impulse magnetron sputtering (HiPIMS). HiPIMS and DC magnetron sputtering modes were enabled to co-deposit an a-ZTO layer with Zn:Sn ratio that varied laterally across a 4-inch diameter sapphire substrate. Electrical, structural and optical properties of the films were investigated as a function of composition. The as-deposited films were found to be amorphous, transparent and highly resistive with little variation in the Zn:Sn ratios. Annealing in the presence of hydrogen yielded improved film conductivity and measured carrier concentrations of ~ 1017 cm-3 . Hall mobilities of up to 13 cm2 /V.s were also measured in the ntype films. These findings suggest that HiPIMS can produce dense, high quality a-ZTO suitable for device applications. As a transparent amorphous conducting oxide with high transparency and good electron mobility, a-ZTO has proven applications in interconnects and thin film transistors. In this thesis, the potential for this material in ‘next-generation’ signal processing devices is discussed. Specifically, the ability of the material to support resistive switching and memristive phenomena was investigated in lateral memristors on HiPIMS a-ZTO. The transport mechanisms and conductance of Ag/a-ZTO memristors were found to depend on prior activity and on the imposed current limit, mimicking biology synaptic plasticity. After microscopy, the switching mechanism was attributed to nanoscale filaments formed between the electrodes. These filaments were subject to Rayleigh instability and exhibited relaxation times determined by their effective radii

Probing the resistance switching mechanisms in SiOₓ/Ag RRAM devices

Resistive random access memory (RRAM) devices represent promising candidates for emerging non-volatile data storage applications and neuromorphic computing. In those devices, the resistance of a dielectric -often a binary oxide- is switched between a low resistance state (LRS) and one or more high resistance states (HRS) by the application of an appropriate external electrical bias. This resistance switching could be filamentary, i.e., involves the formation of a conductive filament. This filament can be thought of as chains of conductive oxygen vacancies (intrinsic resistance switching) or metallic atoms from an active device electrode (extrinsic resistance switching). In this thesis, the relationship between device electrode material and its resistance switching mechanism in SiOx (x∼1.9)-based RRAM devices was studied. Although it’s widely reported that RRAM devices with electrochemically active top electrodes, such as Ag, switch extrinsically, I show that both mechanisms and their associated conductive filaments can be triggered during device switching in ambient conditions. Resistance vs temperature measurements and conduction mechanism analysis were used to probe the nature of the formed filaments within device oxide layer. Results show that the two mechanisms can coexist within the device during switching. The type of filament generated by the initial electroforming of the device, however, depends on the polarity of the applied voltage during the electroforming step. This finding could help in optimising those RRAM devices for the different storage applications. Although the two mechanisms were observed under ambient conditions, SiOx/Ag devices showed extrinsic switching behaviour only under vacuum. In such an oxygen-poor environment, the contribution of intrinsic resistance switching mechanism appears to be reduced or probably eliminated. In extrinsically electroformed RRAM devices with Ag top electrodes, a metallic filament is likely to form within the switching layer. Using conductance tomography technique, the metallic filament of those SiOx/Ag devices was partially imaged using a conductive AFM (CAFM) tip

연성 및 생재흡수성 전자소자용 비휘발성 메모리 소자와 집적센서 구현

학위논문 (박사)-- 서울대학교 대학원 : 화학생물공학부, 2015. 8. 김대형.Over years, major advances in healthcare have been made through research in the fields of nanomaterials and microelectronics technologies. However, the mechanical and geometrical constraints inherent in the standard forms of rigid electronics have imposed challanges of unique integration and therapeutic delivery in non-invasive and minimally invasive medical devices. Here, we describe two types of multifunctional electronic systems. The first type is wearable-on-the-skin systems that address the challenges via monolithic integration of nanomembranes fabricated by top-down approach, nanotubes and nanoparticles assembled by bottom-up strategies, and stretchable electronics on tissue-like polymeric substrate. The system consists of physiological sensors, non-volatile memory, logic gates, and drug-release actuators. Some quantitative analyses on the operation of each electronics, mechanics, heat-transfer, and drug-diffusion characteristic validated their system-level multi-functionalities. The second type is a bioresorbable electronic stent with drug-infused functionalized nanoparticles that takes flow sensing, temperature monitoring, data storage, wireless power/data transmission, inflammation suppression, localized drug delivery, and photothermal therapy. In vivo and ex vivo animal experiments as well as in vitro cell researches demonstrate its unrecognized potential for bioresorbable electronic implants coupled with bioinert therapeutic nanoparticles in the endovascular system. As demonstrations of these technologies, we herein highlight two representative examples of multifunctional systems in order of increasing degree of invasiveness: electronically enabled wearable patch and endovascular electronic stent that incorporate onboard physiological monitoring, data storage, and therapy under moist and mechanically rigorous conditions.Contents Abstract Chapter 1. Introduction 1.1 Organic flexible and wearable electronics.................................................. 1 1.2 Inorganic flexible and wearable electronics............................................... 14 1.3 Flexible non-volatile memory devices.......................................................... 25 1.4 Bioresorbable materials and devices........................................................... 34 References Chapter 2. Multifunctional wearable devices for diagnosis and therapy of movement disorders 2.1 Introduction ................................................................................. 45 2.2 Experimental Section ......................................................................... 49 2.3 Result and Discussion ........................................................................ 65 2.4 Conclusion ................................................................................... 95 References Chapter 3. Stretchable Carbon Nanotube Charge-Trap Floating-Gate Memory and Logic Devices for Wearable Electronics 3.1 Introduction ................................................................................ 101 3.2 Experimental Section ........................................................................ 104 3.3 Result and Discussion ....................................................................... 107 3.4 Conclusion .................................................................................. 138 References Chapter 4. Bioresorbable Electronic Stent Integrated with Therapeutic Nanoparticles for Endovascular Diseases 4.1 Introduction ................................................................................ 148 4.2 Experimental Section ........................................................................ 151 4.3 Result and Discussion ....................................................................... 173 4.4 Conclusion .................................................................................. 219 References 국문 초록 (Abstract in Korean) .................................................................. 230Docto

Novel electrical and chemical findings on SIOx-based ReRAM devices

Existing non-volatile flash memory technologies are characterised by slow access time, high power consumption and a quickly approaching scaling limit. Filamentary resistive RAM (ReRAM) is an emerging type of storage device that relies on the electrically driven change in resistance of a thin film sandwiched between two electrodes. The active region is often a binary oxide that develops a restorable conductive filament thanks to the electrically driven movement of oxygen. This technology offers potential sub-10 nm scalability, nanosecond programming with direct overwriting (unlike FLASH) and an appealing sub pJ/bit power consumption (compared to nJ/bit of FLASH). In this thesis, metal-insulator-metal ReRAM devices with a TiN/SiOx/TiN structure are used. While other binary oxides have been used in the literature, SiOx must be used in its amorphous form allowing for easier fabrication, and is an extremely well-studied material as its CMOS compatibility dates back 40 years. Using the above devices, it was possible to observe data storage performance comparable to the one of other types of ReRAM. More interestingly, it was observed that the resistance states of this family of devices may be programmed using nanosecond pulses of identical magnitude, possibly leading to simple programming circuits. Consequently, it is shown that this programming method may also be used to gradually increase or decrease the device resistance state as well as have devices enter states that relax over time. These types of behaviour mean that SiOx devices may be used in neuromorphic networks that require components whose behaviour resembles the one of the neuronal synapsis or the mammalian brain’s forgetting process. The literature reports on endurance-hindering electrode deformation phenomena during the operation of oxide-based ReRAM devices. A residual gas analyser (RGA) was used to detect that oxygen species are emitted during operation and therefore confirmed that such phenomena are caused by oxygen emission. Using SIMS (secondary ion mass spectroscopy) analysis on devices switched in atmospheres containing isotopically labelled oxygen, it was observed that, under deformed regions, it is possible to find incorporated atmospheric oxygen. Additionally, reducing atmospheric pressure had negative impact on device reliability. SiOx-based filamentary ReRAM is a strong candidate in the search for alternatives to flash memory. Moreover, these devices display behaviour that may be useful in applications trying to emulate the mammalian brain. Having observed device dependence on its atmosphere, endurance issues may now be addressed using electrodes capable of either adsorbing oxygen without bubbling or letting it go through without cracking

Biasing Plasmonic Nanocavities

Molecular electronics promises a new generation of ultralow-energy information technologies, based around functional molecular junctions where two electrodes are bridged by just a few molecules. How molecules exactly behave in a real junction however is still not well understood, since interactions with the electrode materials and neighbouring molecules at the nanoscale is difficult to model and probe experimentally. Many studies so far have characterised in detail the electrical response of molecular junctions by statistical analysis of many repeated measurements, but without monitoring in real time the behaviour of individual devices. The main difficulty of in situ measurements is the absence of characterisation tools that can provide direct access to the dynamics of nm-sized junctions during device operation. This thesis explores two novel approaches used to create optically accessible nanoscale junctions. One method is based on graphene electrodes, used to fabricate extended junctions with a metal oxide spacer. These graphene junctions are found to behave as memristive devices, where a solid state redox reaction releases gas from the oxide spacer that remains trapped under the graphene, resulting in an actuating mechanism. Displacement of the surface layers can be optically tracked in real time using metallic nanoparticles deposited on the sample, whose plasmonic coupling with the bottom electrode is modulated by the actuation mechanism. The second method used to construct nanoscale junctions is more appropriate for molecular junctions, and is based on electrical contacting of individual metallic nanoparticles with a conductive transparent cantilever. Nanoparticles are deposited on a molecular monolayer and represent one electrode of a molecular junction, and at the same time allow to optically probe the junction itself by enabling plasmonic confinement of light to volumes <100nm$^3$. Darkfield and Raman spectroscopy are performed on single nanoparticle junctions in real time while voltage is applied, and a modulation of the optical response with voltage is observed, revealing that molecules undergo conformational changes during device operation

Filamentary Threshold Switching In Niobium Oxides

Two-terminal metal/oxide/metal (MOM) structures exhibit characteristic resistance changes, including non-volatile memory and volatile threshold switching responses when subjected to electrical stress (i.e., voltage or current stimuli), which are of interest as active elements in non-volatile memory arrays and neuromorphic computing. Recently, the threshold switching response in MOM devices based on vanadium oxides and niobium oxides have attracted particular attention due to their simple structure and reliability. Interestingly, specific phases of these oxides (e.g., VO2, NbO2 etc.) exhibit a metal-insulator transition (MIT) which causes dramatic changes in their intrinsic properties, including electrical and thermal conductivities, and often arguably reported as the dominant cause of the observed threshold switching response. While this response has been extensively studied for VO2, but the low transition temperature (~ 340K) limits their use only to low temperature microelectronics applications. In contrast, NbO2 has a much higher transition temperature ~ 1070 K, and NbO2 and other NbOx phases have drawn recent attention due to their reliable threshold switching characteristics. The resistance changes in MOM structures are often initiated by a one-step electroforming process that forms a filamentary conduction path. Knowledge about the structure, composition and spatial distribution of these filaments is essential for a full understanding of filamentary resistive/threshold-switching and for effective modelling and optimisation of associated devices. Additionally, NbOx-based devices exhibit a wide range of resistive and threshold switching responses that critically depend on operating condition, composition and device geometry. Thus, a proper understanding of these factors is important for achieving reliable switching with desired characteristics. This thesis focuses on understanding the electroforming process and subsequent threshold switching responses in NbOx by employing different techniques, including electrical testing, and thermo-reflectance imaging. At first, a simple means of detecting and spatially mapping conductive filaments in metal/oxide/metal cross-point devices is introduced and the utility of this technique is demonstrated to identify distinct modes of electroforming in low- and high-conductivity NbOx films. After that, the role of metal/oxide interface reactions on the post-forming characteristics of reactive-metal/Nb2O5/Pt devices is demonstrated. Specifically, devices are shown to exhibit stable threshold switching under negative bias but the response under positive bias depends on the choice of metal. Then, the threshold-switching and current-controlled negative differential resistance (NDR) characteristics of cross-point devices fabricated from undoped Nb2O5 and Ti-doped Nb2O5 are compared. In particular it is shown that doping offers an effective means of engineering the device response. Based on temperature dependent current-voltage characteristics and lumped-element modelling, these effects are attributed to doping-induced reductions in the device resistance and its rate of change with temperature. Finally, the physical origin of the discontinuous 'snapback' NDR is investigated. Specifically, it is shown that the snapback response is a direct consequence of current localisation and redistribution within the oxide film. Furthermore, it is demonstrated that material and device dependencies are consistent with predictions of a two-zone parallel memristor model of NDR which is based on a non-uniform current distribution after electroforming. These results advance the current understanding of threshold switching response in amorphous NbOx films, and provide a strong basis for engineering devices with specific NDR characteristics. Significantly, these results also resolve a long-standing controversy about the origin of the snapback response which has been a subject of considerable debate

Development of Green Synthetic Approaches for the Potential Application of Carbon and Semiconductor Nanomaterials for Emerging Applications

The increasing interest towards the synthesis and modification of different nanomaterials is attributed to their outstanding mechanical, physical and electrical properties that allow their use in different fields. In the last decades, novel nanomaterials have been successfully synthesized in order to provide materials with improved performances to be employed for water treatment, photocatalysis, to replace silicon–based devices in electronics and so on. For example, carbon-based materials are promising candidates for the fabrication of conductive inks and future non-volatile memory devices. However, the absence of an eco-sustainable, straightforward and time effective process for their production has hindered their large-scale application in electronics. The aim of this thesis is to explore alternative synthetic approaches for the synthesis of different materials and their structural modification in order to gain a better understanding how the processes could be controlled to have desired structure and hence materials with improved performances. In particular, laser ablation in liquids (PLA) and electrochemical processes will be the focus of this study. It has been shown that pulsed laser ablation of carbon materials and TiO2¬ nanoparticles can be used for the synthesis of new materials and/or modification of their structure. The laser ablation compared to other common synthetic approaches has many advantages. One of which is the eco-sustainability of the process, since the synthesis is performed in water without the use or production of products harmful for the environment. The second advantage is the versatility of the technique that allows the synthesis and modification of different nanomaterials depending on the target material employed. In this thesis it will be demonstrated that laser ablation of a dispersion of graphene oxide can be employed as a straightforward technique to induce structural modifications of the material, i.e. reduction of the graphene oxide sheets and synthesis of graphene quantum dots varying laser ablation time and ablation power. The nanomaterials obtained can be mixed with silver nanoparticles for the fabrication of hybrid conductive inks, which have a resistivity lower than inks made with only silver nanoparticles. The versatility of the laser ablation is demonstrated by extending the study to titanium dioxide powders. It will be discussed that the laser ablation of TiO2 nanoparticles leads to nanoparticles with different crystalline structures. Indeed, with a proper control over the laser ablation parameters, such as ablation time and laser power, it is possible to induce a phase transformation of TiO¬2 nanoparticles whether they are dispersed in water or deposited onto a substrate. Similar to the laser ablation, the electrochemical processes such as the electrophoretic deposition (EPD) allows the synthesis and deposition of different type of materials. In particular, in this thesis this technique will be employed for the straightforward synthesis of carbon nanowalls (CNWs). These carbon-based materials are usually synthesized by chemical vapor deposition, which requires the use of precursor gases and high temperatures and pressures. Whereas, the method developed during my research allows a time-effective synthesis of these nanomaterials; moreover, the deposition of the CNWs directly onto conductive substrate permits for the first time the fabrication of carbon-based resistive switching memory devices. This technique could be used for the development on a large scale of this type of devices, whose broad fabrication has been hindered due to the complex production mechanisms. Another advantage of the electrochemical processes is the possibility of modifying the chemical composition of the materials. In this thesis, the anodic oxidation has been used for the first time to oxidize the carbon structures obtained by EPD in order to engineer their electrical performances. In literature, the anodic oxidation has been used to study the redox processes in electronic devices or to increase the electrochemical capacitance of carbon materials, but never as a specific technique to tailor the materials properties. As aforementioned EPD, like PLA, is a versatile technique and in this study it has been used for the growth of ZnO rods. ZnO rods are usually grown by hydrothermal processes, which can be time consuming. In this thesis, the growth of the rods has been conducted directly on conductive substrates, which were then patterned for the fabrication of electronic devices

High performance CMOS-compatible perovskite oxide memristors: compositional control and nanoscale switching characteristics

Nanoscale memristive devices have been dubbed as one of the main contenders for the next generation nonvolatile memories (NVM) and alternative logic architectures. Passive two-terminal metal-insulator-metal (MIM) memristive crossbar configurations based on functional transition metal-oxides (e.g. TiO2, SrTiO3) offer great potential for ultimate integration in contemporary electronic industry. This thesis focuses on the realization and nanoscale characterization of high performance CMOS-compatible memristive devices utilizing functional perovskite oxides. A PVD based synthesis route for the realization of functional perovskite oxides with control over their composition and structure has been established. Utilizing the synthesis approach, first realization of memristive devices based on oxygen deficient amorphous SrTiO3 (a-STO) oxides has been demonstrated and their resistive switching performance has been studied in detail utilizing micro-scale crossbar MIM arrays and a sophisticated conductive nano-contact technique based on in situ electrical nanoindentation. RF magnetron sputtering has been used in this work to synthesis perovskite oxide thin films on conventional silicon substrates. Firstly, a lead-free ferro/piezoelectric perovskite oxide (KxNa1‑xNbO3) was chosen to study the effects of sputtering parameters and post-deposition treatments on the composition and the structure of sputtered thin films. This study demonstrates that the crystal orientation, thickness and the elemental composition of the thin films sputtered from the same ceramic target can be effectively and reliably controlled via tuning the sputtering parameters (process gas, substrate temperature, etc.) and the oxide structure and secondary phases can be engineered through post-annealing treatments. The same procedure was employed for the synthesis of SrTiO3 thin films as a reliable resistive switching perovskite oxide. A low temperature synthesis of amorphous SrTiO3 (a-STO) thin films with precise control over the thickness, oxygen deficiency and A‑site/B-site dopants has been demonstrated for the first time. The switching characteristics of a-STO cross-point devices suggest the possibility of fine tuning the memristive performance through tailoring the oxide composition and device structure. Outstanding switching performance (high switching ratios, excellent endurance and retention) is demonstrated in oxygen deficient a-STO devices. Also, it is shown that niobium doping through low temperature co-sputtering of Nb: a-STO result in significant improvements in device energy requirements. Furthermore, nanoscale conduction and resistive switching mechanisms of these devices have been studied in detail utilizing a sophisticated in situ electrical nanoindentation technique, capable of forming nano‑contacts with controlled size and mechanical force. To this end, a unique empirical model has been developed that allows for a complete characterization of the electrical properties of the load controlled nano‑contact and therefore yields quantified insights into the conduction and switching mechanisms of a‑STO based memristive device at nanoscale. The results exhibit ultimately scalable and isolatedly controllable switching characteristics in these devices and also suggest the possibility of mechanically modulated nanoscale resistive switching in a‑STO based devices. Overall, this thesis highlights a‑STO based devices as strong candidates for the ongoing development of the alternative memory technologies as well as applications in MEMS/NEMS devices