14 research outputs found
Resistive Switching in Transition Metal Oxides for Integrated Non-volatile Memory
Transition metal oxides (TMOs) exhibit characteristic resistance
changes when subjected to high electric fields due to the
creation, drift and diffusion of defects, and this
resistive-switching response is of interest for future
non-volatile memory applications. Indeed, resistive random access
memories (ReRAM) are considered promising alternatives to
conventional charge storage-based devices because of their low
production cost, simple fabrication, and excellent scalability.
However, the realization of reliable ReRAM devices and their
integration in large-scale arrays requires further understanding
of the switching mechanisms and the development of new strategies
for improving integrated device functionality. The aim of this
work is to understand the role of the material structure on
device reliability and to investigate the integration of passive
selector elements with memory devices for use in memory cross-bar
arrays. The thesis begins by investigating the properties of
relevant oxide films (ALD HfO2 and plasma deposited NbOx) and
then addresses three technologically relevant problems.
Specifically these include: 1) understanding how the roughness of
metal/dielectric interfaces affects dielectric breakdown and
switching behaviour; 2) exploring methods for reducing the
operating current of selector and memory/selector devices and 3)
investigating the effect of operating conditions on the switching
response of devices.
The first of these studies is based on Pt/Ti/HfO2/Pt devices and
combines experimental methods and finite element modelling to
understand the effect of the Pt/HfO2 interface roughness on the
electroforming and switching response. Atomic force microscopy
(AFM) showed that the roughness of Pt electrodes deposited by
electron-beam evaporation increased with film thickness due to
facetted grain growth. Results show that roughness leads to a
reduction in the electroforming voltage of HfO2, an increase in
the failure rate of devices, and a corresponding reduction in
resistive switching reliability. Conventional wisdom suggests
that these effects result from local electric field enhancement
in the vicinity of electrode asperities. However, the effect on
electroforming voltage is much less than estimated from simple
geometric considerations. Comparison with finite-element modelled
showed high-aspect-ratio asperities can produce field
enhancements of more than an order of magnitude but that the
generation and redistribution of defects moderates this effect
prior to dielectric breakdown. As a consequence, the effect of
field enhancement is less than anticipated from the initial
electric-field distribution alone. It is argued that the increase
in the device failure rate with increasing electrode roughness
derives partly from an increase in the film defect density and
effective device area and that these effects contribute to the
reduction in breakdown voltage.
The second study showed that the leakage current in NbO2-x
selector (1S) elements is shown to be reduced by the properties
of an adjacent memory (1M) element when integrated into a hybrid
selector-memory device structure. This is shown to result from
current confinement in conductive filaments formed in the memory
layer. Finite element modelling of the selector-memory structures
is used to confirm the observations and to explore material
dependencies. The thermal and electrical conductivities of the
memory layer are shown to influence the threshold current, but
the dominant effect is due to current confinement.
The final study explores the effect of device operating
conditions on its operation and identifies an alternative
approach for reducing the forming and RESET current in integrated
memory/selector devices. This study is based on Pt/Nb/HfO2/Pt
devices which require a very "soft" electroforming process. Such
devices are shown to undergo configurable switching controlled by
the SET compliance current. When operated at a low
compliance-current (~100 µA), devices show uniform bipolar
resistive switching behaviour. As the compliance current is
increased (~500 µA), the switching mode changes to integrated
threshold-resistive (1S1M) switching, and at still higher
currents (~1 mA), it changes to symmetric threshold switching
(1S) characteristic of threshold switching in NbO2-. These
switching transitions are shown to be consistent with the
development of an NbO2- interlayer at the Nb/HfO2 interface
that is limited by the set compliance current due to its effect
on oxygen transport and local Joule heating. The proposed
mechanism is supported by finite element modelling of the 1S1M
response assuming the presence of such an interlayer. These
findings help to understand role of interface reactions in
controlling device performance and provide a means for the
self-assembly of integrated 1S1M resistive random access memory
structures
Current localisation and redistribution as the basis of discontinuous current controlled negative differential resistance in NbOx
In-situ thermo-reflectance imaging is used to show that the discontinuous,
snap-back mode of current-controlled negative differential resistance (CC-NDR)
in NbOx-based devices is a direct consequence of current localization and
redistribution. Current localisation is shown to result from the creation of a
conductive filament either during electroforming or from current bifurcation
due to the super-linear temperature dependence of the film conductivity. The
snap-back response then arises from current redistribution between regions of
low and high current-density due to the rapid increase in conductivity created
within the high current density region. This redistribution is further shown to
depend on the relative resistance of the low current-density region with the
characteristics of NbOx cross-point devices transitioning between continuous
and discontinuous snap-back modes at critical values of film conductivity,
area, thickness and temperature, as predicted. These results clearly
demonstrate that snap-back is a generic response that arises from current
localization and redistribution within the oxide film rather than a
material-specific phase transition, thus resolving a long-standing controversy.Comment: 21 Page
A balanced Memristor-CMOS ternary logic family and its application
The design of balanced ternary digital logic circuits based on memristors and
conventional CMOS devices is proposed. First, balanced ternary minimum gate
TMIN, maximum gate TMAX and ternary inverters are systematically designed and
verified by simulation, and then logic circuits such as ternary encoders,
decoders and multiplexers are designed on this basis. Two different schemes are
then used to realize the design of functional combinational logic circuits such
as a balanced ternary half adder, multiplier, and numerical comparator.
Finally, we report a series of comparisons and analyses of the two design
schemes, which provide a reference for subsequent research and development of
three-valued logic circuits.Comment: 15 pages, 30 figure
A wind-PV-battery hybrid power system at Sitakunda in Bangladesh
The measured wind data of Local Government Engineering Department (LGED) for 2006 at 30Â m height shows a good prospect for wind energy extraction at the site. For a few months and hours the speed is below the cut in speeds of the available turbines in the market. The predicted solar radiation data from directly related measured cloud cover and sunshine duration data of Bangladesh Meteorological Department (BMD) for 1992-2003 indicates that a reliable power system can be developed over the year if the solar energy technology is merged with the wind energy technologies for this site. This research work has studied on optimization of a wind-photovoltaic-battery hybrid system and its performance for a typical community load. The assessment shows that least cost of energy (COE) is about USD 0.363/kWh for a community using 169Â kWh/day with 61Â kW peak and having minimum amount of access or unused energy. Moreover, compared to the existing fossil fuel-based electricity supply, such an environment friendly system can mitigate about 25Â t CO2/yr. The analysis also indicates that wind-PV-battery is economically viable as a replacement for conventional grid energy supply for a community at a minimum distance of about 17Â km from grid.Solar energy Wind energy Green house gas
Room temperature synthesis of HfO2/HfO x heterostructures by ion-implantation
Implantation of Hf films with oxygen ions is shown to be an effective means of fabricating high-quality HfO2/HfO x heterostructures at room temperature, with the layer composition and thicknesses determined by the ion energy and fluence. Implantation with 3 keV O+ ions to a fluence of 1 × 1017 ions cm−2 produces a polycrystalline (monoclinic-) HfO2 layer extending from the surface to a depth of ~12 nm, and an underlying graded HfO x layer extending an additional ~7 nm, while implantation with 6 keV O to a similar fluence produces a near-stoichiometric surface layer of 7 nm thickness and a graded substoichiometric layer extending to depth of ~30 nm. These structures are shown to be broadly consistent with oxygen range data but more detailed comparison with dynamic Monte Carlo simulations suggests that the near-surface region contains more oxygen than expected from collisional processes alone. The bandgap and dielectric strength of the HfO2 layer produced by 3 keV; 1 × 1017 ions cm−2 implant is shown to be indistinguishable from those of an amorphous film deposited by atomic layer deposition at 200 °C. The utility of these layers is demonstrated by studying the resistive switching properties of metal-oxide-metal test structures fabricated by depositing a top metal contact on the implanted film. These results demonstrate the suitability of ion-implantation for the synthesis of functional oxide layers at room temperature
PHYTOCHEMICAL ANALYSIS, ANTIMICROBIAL ACTIVITY AND ASSESSMENT OF POTENTIAL COMPOUNDS BY THIN LAYER CHROMATOGRAPHY OF ETHANOL FRACTION OF ASPARAGUS RACEMOSUS ROOTS
Objectives: Asparagus racemosus (AR) Willd, is commonly known as Shatavari, belongs to family Liliaceae, and has potent medicinal values. The present study deals with detail Pharmacological evaluation of ethanol fraction of AR.
Methods: Preliminary phytochemical screening, antimicrobial and antioxidant activity, qualitative analysis of antioxidant and flavonoid detection, total phenol and flavonoid content were measured.
Results: Phytochemical screening showed the presence of various phytoconstituents like phytosterols, phenols, flavonoids, saponins and carbohydrates. Ethanol fraction of Asparagus racemosus roots have maximum relative percentage inhibition against gram (-) bacteria. The root displayed significant DPPH free radical scavenging activity against standard ascorbic acid. Thin layer chromatography of the ethanol fraction was also performed to determine the active principles. Four major spots obtained with the Rf values 0.20, 0.22, 0.24 and 0.58. Total flavonoid and phenol content was 188.3 mg/g quercetin equivalent and 165.22mg/g gallic acid equivalent respectively.
Conclusion: The findings of the current study shows that Asparagus racemosus is a good source of various phytochemicals and can be useful to progress and surge further scientific investigation to prevent and management of different diseases by their medicinal properties