29 research outputs found
Optimization of niobium oxide-based threshold switches for oscillator-based applications
In niobium oxide-based capacitors non-linear switching characteristics can be observed if the oxide properties are adjusted accordingly. Such non-linear threshold switching characteristics can be utilized in various non-linear circuit applications, which have the potential to pave the way for the application of new computing paradigms. Furthermore, the non-linearity also makes them an interesting candidate for the application as selector devices e.g. for non-volatile memory devices. To satisfy the requirements for those two areas of application, the threshold switching characteristics need to be adjusted to either obtain a maximized voltage extension of the negative differential resistance region in the quasi-static I-V characteristics, which enhances the non-linearity of the devices and results in improved robustness to device-to-device variability or to adapt the threshold voltage to a specific non-volatile memory cell. Those adaptations of the threshold switching characteristics were successfully achieved by deliberate modifications of the niobium oxide stack. Furthermore, the impact of the material stack on the dynamic behavior of the threshold switches in non-linear circuits as well as the impact of the electroforming routine on the threshold switching characteristics were analyzed. The optimized device stack was transferred from the micrometer-sized test structures to submicrometer-sized devices, which were packaged to enable easy integration in complex circuits. Based on those packaged threshold switching devices the behavior of single as well as of coupled relaxation oscillators was analyzed. Subsequently, the obtained results in combination with the measurement results for the statistic device-to-device variability were used as a basis to simulate the pattern formation in coupled relaxation oscillator networks as well as their performance in solving graph coloring problems. Furthermore, strategies to adapt the threshold voltage to the switching characteristics of a tantalum oxide-based non-volatile resistive switch and a non-volatile phase change cell, to enable their application as selector devices for the respective cells, were discussed.:Abstract I
Zusammenfassung II
List of Abbrevations VI
List of Symbols VII
1 Motivation 1
2 Basics 5
2.1 Negative differential resistance and local activity in memristor devices 5
2.2 Threshold switches as selector devices 8
2.3 Switching effects observed in NbOx 13
2.3.1 Threshold switching caused by metal-insulator transition 13
2.3.2 Threshold switching caused by Frenkel-Poole conduction 18
2.3.3 Non-volatile resistive switching 32
3 Sample preparation 35
3.1 Deposition techniques 35
3.1.1 Evaporation 35
3.1.2 Sputtering 36
3.2 Micrometer-sized devices 36
3.3 Submicrometer-sized devices 37
3.3.1 Process flow 37
3.3.2 Reduction of the electrode resistance 39
3.3.3 Transfer from structuring via electron beam lithography to structuring via
laser lithography 48
3.3.4 Packaging procedure 50
4 Investigation and optimization of the electrical device characteristic 51
4.1 Introduction 51
4.2 Measurement setup 52
4.3 Electroforming 53
4.3.1 Optimization of the electroforming process 53
4.3.2 Characterization of the formed filament 62
4.4 Dynamic device characteristics 67
4.4.1 Emergence and measurement of dynamic behavior 67
4.4.2 Impact of the dynamic device characteristics on quasi-static I-V
characteristics 70
5 Optimization of the material stack 81
5.1 Introduction 81
5.2 Adjustment of the oxygen content in the bottom layer 82
5.3 Influence of the thickness of the oxygen-rich niobium oxide layer 92
5.4 Multilayer stacks 96
5.5 Device-to-device and Sample-to-sample variability 110
6 Applications of NbOx-based threshold switching devices 117
6.1 Introduction 117
6.2 Non-linear circuits 117
6.2.1 Coupled relaxation oscillators 117
6.2.2 Memristor Cellular Neural Network 121
6.2.3 Graph Coloring 127
6.3 Selector devices 132
7 Summary and Outlook 138
8 References 141
9 List of publications 154
10 Appendix 155
10.1 Parameter used for the LT Spice simulation of I-V curves for threshold
switches with varying oxide thicknesses 155
10.2 Dependence of the oscillation frequency of the relaxation oscillator circuit
on the capacitance and the applied source voltage 156
10.3 Calculation of the oscillation frequency of the relaxation oscillator circuit 157
10.4 Characteristics of the memristors and the cells utilized in the simulation of
the memristor cellular neural network 164
10.5 Calculation of the impedance of the cell in the memristor cellular network 166
10.6 Example graphs from the 2nd DIMACS series 179
11 List of Figures 182
12 List of Tables 19
Electronic properties of Mn-Phthalocyanine - C bulk heterojunctions: combining photoemission and electron energy-loss spectroscopy
The electronic properties of co-evaporated mixtures (blends) of manganese
phthalocyanine and the fullerene C (MnPc:C) have been studied as
a function of the concentration of the two constituents using two supplementary
electron spectroscopic methods, photoemission spectroscopy (PES) as well as
electron energy-loss spectroscopy (EELS) in transmission. Our PES measurements
provide a detailed picture of the electronic structure measured with different
excitation energies as well as different mixing ratios between MnPc and
C. Besides a relative energy shift, the occupied electronic states of
the two materials remain essentially unchanged. The observed energy level
alignment is different compared to that of the related CuPc:C bulk
heterojunction. Moreover, the results from our EELS investigations show that
despite of the rather small interface interaction the MnPc related electronic
excitation spectrum changes significantly by admixing C to MnPc thin
films
Unveiling the double-well energy landscape in a ferroelectric layer
The properties of ferroelectric materials, which were discovered almost a century agoÂč , have led to a huge range of applications, such as digital information storageÂČ , pyroelectric energy conversionÂł and neuromorphic computingâŽâ»â” . Recently, it was shown that ferroelectrics can have negative capacitanceâ¶â»ÂčÂč, which could improve the energy efficiency of conventional electronics beyond fundamental limitsÂčÂČâ»ÂčâŽ. In LandauâGinzburgâDevonshire theoryÂčâ”â»Âčâ·, this negative capacitance is directly related to the doublewell shape of the ferroelectric polarizationâenergy landscape, which was thought for more than 70 years to be inaccessible to experimentsÂčâž. Here we report electrical measurements of the intrinsic double-well energy landscape in a thin layer of ferroelectric Hfâ.â
Zrâ.â
Oâ. To achieve this, we integrated the ferroelectric into a heterostructure capacitor with a second dielectric layer to prevent immediate screening of polarization charges during switching. These results show that negative capacitance has its origin in the energy barrier in a double-well landscape. Furthermore, we demonstrate that ferroelectric negative capacitance can be fast and hysteresis-free, which is important for prospective applicationsÂčâč. In addition, the Hfâ.â
Zrâ.â
Oâ used in this work is currently the most industry-relevant ferroelectric material, because both HfOâ and ZrOâ thin films are already used in everyday electronicsÂČâ°. This could lead to fast adoption of negative capacitance effects in future products with markedly improved energy efficiency
New genetic loci link adipose and insulin biology to body fat distribution.
Body fat distribution is a heritable trait and a well-established predictor of adverse metabolic outcomes, independent of overall adiposity. To increase our understanding of the genetic basis of body fat distribution and its molecular links to cardiometabolic traits, here we conduct genome-wide association meta-analyses of traits related to waist and hip circumferences in up to 224,459 individuals. We identify 49 loci (33 new) associated with waist-to-hip ratio adjusted for body mass index (BMI), and an additional 19 loci newly associated with related waist and hip circumference measures (PÂ <Â 5Â ĂÂ 10(-8)). In total, 20 of the 49 waist-to-hip ratio adjusted for BMI loci show significant sexual dimorphism, 19 of which display a stronger effect in women. The identified loci were enriched for genes expressed in adipose tissue and for putative regulatory elements in adipocytes. Pathway analyses implicated adipogenesis, angiogenesis, transcriptional regulation and insulin resistance as processes affecting fat distribution, providing insight into potential pathophysiological mechanisms
Global variations in diabetes mellitus based on fasting glucose and haemogloblin A1c
Fasting plasma glucose (FPG) and haemoglobin A1c (HbA1c) are both used to diagnose
diabetes, but may identify different people as having diabetes. We used data from 117
population-based studies and quantified, in different world regions, the prevalence of
diagnosed diabetes, and whether those who were previously undiagnosed and detected
as having diabetes in survey screening had elevated FPG, HbA1c, or both. We developed
prediction equations for estimating the probability that a person without previously
diagnosed diabetes, and at a specific level of FPG, had elevated HbA1c, and vice versa.
The age-standardised proportion of diabetes that was previously undiagnosed, and
detected in survey screening, ranged from 30% in the high-income western region to 66%
in south Asia. Among those with screen-detected diabetes with either test, the agestandardised
proportion who had elevated levels of both FPG and HbA1c was 29-39%
across regions; the remainder had discordant elevation of FPG or HbA1c. In most low- and
middle-income regions, isolated elevated HbA1c more common than isolated elevated
FPG. In these regions, the use of FPG alone may delay diabetes diagnosis and
underestimate diabetes prevalence. Our prediction equations help allocate finite
resources for measuring HbA1c to reduce the global gap in diabetes diagnosis and
surveillance.peer-reviewe
Pattern Formation With Locally Active S-Type NbOâ Memristors
The main focus of this paper is the evolution of complex behavior in a system of coupled nonlinear memristor circuits depending on the applied coupling conditions. Thereby, the parameter space for the local activity and the edge-of-chaos domain will be determined to enable the emergence of the pattern formation in locally coupled cells according to Chua's principle. Each cell includes a Niobium oxide-based memristor, which may feature a locally active behavior once it is suitably biased on the negative differential resistance region of its DC current-voltage characteristic. It will be shown that there exists a domain of parameters under which each uncoupled cell may become locally active around a stable bias state. More specifically, under these conditions, the coupled cells are on the edge-of-chaos, and can support the static and dynamic pattern formation. The emergence of such complex spatio-temporal behavior in homogeneous structures is a prerequisite for information processing. The theoretical results are confirmed b
Graph Coloring via Locally-Active Memristor Oscillatory Networks
This manuscript provides a comprehensive tutorial on the operating principles of a bio-inspired Cellular Nonlinear Network, leveraging the local activity of NbOx memristors to apply a spike-based computing paradigm, which is expected to deliver such a separation between the steady-state phases of its capacitively-coupled oscillators, relative to a reference cell, as to unveal the classification of the nodes of the associated graphs into the least number of groups, according to the rules of a non-deterministic polynomial-hard combinatorial optimization problem, known as vertex coloring. Besides providing the theoretical foundations of the bio-inspired signal-processing paradigm, implemented by the proposed Memristor Oscillatory Network, and presenting pedagogical examples, illustrating how the phase dynamics of the memristive computing engine enables to solve the graph coloring problem, the paper further presents strategies to compensate for an imbalance in the number of couplings per oscillator, to counteract the intrinsic variability observed in the electrical behaviours of memristor samples from the same batch, and to prevent the impasse appearing when the array attains a steady-state corresponding to a local minimum of the optimization goal. The proposed Memristor Cellular Nonlinear Network, endowed with ad hoc circuitry for the implementation of these control strategies, is found to classify the vertices of a wide set of graphs in a number of color groups lower than the cardinality of the set of colors identified by traditional either software or hardware competitor systems. Given that, under nominal operating conditions, a biological system, such as the brain, is naturally capable to optimise energy consumption in problem-solving activities, the capability of locally-active memristor nanotechnologies to enable the circuit implementation of bio-inspired signal processing paradigms is expected to pave the way toward electronics with higher time and energy efficiency than state-of-the-art purely-CMOS hardware
Anti-ACSA-2 defines a novel monoclonal antibody for prospective isolation of living neonatal and adult astrocytes
International audienceAstrocytes are the most abundant cell type of the central nervous system and cover a broad range of functionalities. We report here the generation of a novel monoclonal antibody, anti-astrocyte cell surface antigen-2 (Anti-ACSA-2). Flow cytometry, immunohistochemistry and immunocytochemistry revealed that Anti-ACSA-2 reacted specifically with a not yet identified glycosylated surface molecule of murine astrocytes at all developmental stages. It did not show any labeling of non-astroglial cells such as neurons, oligodendrocytes, NG2+ cells, microglia, endothelial cells, leukocytes, or erythrocytes. Co-labeling studies of GLAST and ACSA-2 showed largely overlapping expression. However, there were also notable differences in protein expression levels and frequencies of single-positive subpopulations of cells in some regions of the CNS such as cerebellum, most prominently at early postnatal stages. In the neurogenic niches, the dentate gyrus of the hippocampus and the subventricular zone (SVZ), again a general overlap with slight differences in expression levels were observed. ACSA-2 was unlike GLAST not sensitive to papain-based tissue dissociation and allowed for a highly effective, acute, specific, and prospective purification of viable astrocytes based on a new rapid sorting procedure using Anti-ACSA-2 directly coupled to superparamagnetic MicroBeads. In conclusion, ACSA-2 appears to be a new surface marker for astrocytes, radial glia, neural stem cells and bipotent glial progenitor cells which opens up the possibility of further dissecting the characteristics of astroglial subpopulations and lineages