WAVES IN 2D ANISOTROPIC L-C LATTICE METAMATERIALS:PHENOMENOLOGY AND PROPERTIES

Anisotropic metamaterials composed of 2D periodic infi- nite and finite periodic lattices of lumped inductor (L) and capacitor (C) circuits have been explored. The unique features of wave channeling on such anisotropic lattices and scattering at their interfaces and edges are reviewed and illustrated by the examples of the specific arrangements. The lattice unit cells composed of inductors and capacitors (basic mesh) as well as of assemblies comprised of double series, double parallel, and mixed parallel-series L-C circuits are discussed.

Germanium MOS Capacitors with Hafnium Dioxide and Silicon Dioxide Dielectrics

Germanium (Ge) does not grow a suitable oxide for MOS devices. The Ge/dielectric interface is of prime importance to the operation of photo-detectors and scaled MOSTs. Therefore there is a requirement for deposited or bonded dielectric materials. MOS capacitors have been formed on germanium substrates with three different dielectric materials. Firstly, a thermally grown and bonded silicon dioxide (SiO2) layer, secondly, SiO2 deposited by atmospheric pressure CVD ‘silox’, and thirdly a hafnium oxide (HfO2) high-k dielectric deposited by atomic layer deposition (ALD). Ge wafers used were p-type 1 0 0 2 O cm. C–V measurements have been made on all three types of capacitors to assess the interface quality. ALD HfO2 and silox both display acceptable C–V characteristics. Threshold voltage and maximum and minimum capacitance values closely match expected values found through calculation. However, the bonded SiO2 has non-ideal C–V characteristics, revealing the presence of a high density of interface states. A H2/N2 post metal anneal has a detrimental effect on C–V characteristics of HfO2 and silox dielectrics, causing a shift in the threshold voltage and rise in the minimum capacitance value. In the case of hafnium dioxide, capacitor properties can be improved by performing a plasma nitridation of the Ge surface prior to dielectric deposition.

Low temperature surface nitridation processes for dielectric-Ge interfaces

Germanium MOS capacitors have been fabricated with a high-? HfO dielectric using ALD. An in-situ low temperature (250°C) nitrogen plasma treatment on the germanium surface prior to the deposition of HfO was found to be beneficial to the electrical properties of the devices. Germanium MOS capacitors have also been fabricated with a SiO dielectric deposited by an atmospheric pressure CVD 'silox' process. The same low temperature plasma nitridation was found to degrade the electrical properties of the silox devices. The effect of a post-metal anneal in H and N on both types of capacitor structure was also found to degrade device electrical properties. copyright The Electrochemical Society.

Phase transitions in epitaxial Ba0.5Sr0.5TiO3 thin films

Ba0.5Sr0.5TiO3 (BST) thin-film capacitor structures with various thicknesses, (50-1200 nm) and different strain conditions (on lanthanum strontium cobalt oxide La0.5Sr0.5CoO3 and strontium ruthenate SrRuO3 buffer layers) were made using pulsed laser deposition, and characterized by x-ray diffraction. The out-of-plane lattice parameter was followed as a function of temperature within the 100-300 K temperature interval. The phase sequence (cubic-tetragonal-orthorhombic-rhombohedral) known to exist in the bulk analog is shown to be strongly affected by both the stress conditions imposed by the buffer layer and the thickness of the BST film itself. Thus, no phase transition was found for the in-plane compressed BST films. On the stress-free BST films, on the contrary, more phase transitions were observed. It appeared that the complexity of structural phase transitions increased as the film thickness in this system was reduced.

Studies on polypyrrole film in room temperature melt

A bluish-black shining free standing polypyrrole film (PPy) of electronic conductivity 130 S cm-1 has been prepared by electrochemical oxidative polymerization of pyrrole on Pt/transparent glass conducting electrode resistance 15 O cm-1, using a room temperature melt as an electrolyte, composed of 1:3 stoichiometric ratio of cetyl pyridinium chloride and anhydrous aluminum chloride at 0.58 V versus Al wire as a reference electrode. The film possessed a charge transfer resistance of 132 O, and showed two absorption peaks at 457 and 1264 nm in the UV-vis–NIR diffused reflectance spectra. The morphology of the film was hexagonal. The potential step technique suggested a layered structure. This thin film can easily be peeled off from the electrode surface after three cycles and can be used for various applications like dissipation of electrostatic charge, battery electrode materials, solid electrolytic capacitor, electrochromic windows and displays, microactuators etc. It was also characterized by IR, thermal and SEM studies.

Electrode field penetration: A new interpretation of tunneling currents in barium strontium titanate (BST) thin films

This paper shows that penetration of the applied electric field into the electrodes of a ferroelectric thin film capacitor produces both an interfacial capacitance and an effective mechanism for electron tunneling. The model predictions are compared with experimental results on Au-BST-SrRuO3 capacitors of varying thicknesses, and the agreement is excellent.

The effect of tungsten trioxide thin films at ferroelectric-electrode boundaries on fatigue behaviour

A conventional thin film capacitor heterostructure, consisting of sol-gel deposited lead zirconium titanate (PZT) layers with sputtered platinum top and bottom electrodes, was subjected to fatiguing pulses at a variety of frequencies. The fatigue characteristics were compared to those of a similarly processed capacitor in which a ~20nm tungsten trioxide layer had been deposited, using pulsed laser deposition, between the ferroelectric and upper electrode. The expectation was that, because of its ability to accommodate considerable oxygen non-stoichiometry, tungsten trioxide (WO3) might act as an efficient sink for any oxygen vacancies flushed to the electrode-ferroelectric boundary layer during repetitive switching, and hence would improve the fatigue characteristics of the thin film capacitor. However, it was found that, in general, the addition of tungsten trioxide actually increases the rate of fatigue. It appears that any potential benefit from the WO3, in terms of absorbing oxygen vacancies, is far outweighed by it causing dramatically increased charge injection in the system.

Liquid source misted chemical deposition process of three-dimensional nano-ferroelectrics with substrate heating

A modification of liquid source misted chemical deposition process (LSMCD) with heating mist and substrate has developed, and this enabled to control mist penetrability and fluidity on sidewalls of three-dimensional structures and ensure step coverage. A modified LSMCD process allowed a combinatorial approach of Pb(Zr,Ti)O-3 (PZT) thin films and carbon nanotubes (CNTs) toward ultrahigh integration density of ferroelectric random access memories (FeRAMs). The CNTs templates were survived during the crystallization process of deposited PZT film onto CNTs annealed at 650 degrees C in oxygen ambient due to a matter of minute process, so that the thermal budget is quite small. The modified LSMCD process opens up the possibility to realize the nanoscale capacitor structure of ferroelectric PZT film with CNTs electrodes toward ultrahigh integration density FeRAMs.

Characteristics of the interfacial capacitance in thin film Ba0.5Sr0.5TiO3 capacitors with SrRuO3 and (La, Sr)CoO3 bottom electrodes

Thin film Ba0.5Sr0.5TiO3 (BST) capacitors of thickness similar to75 nm to similar to1200 nm, with Au top electrodes and SrRuO 3 (SRO) or (La, Sr)CoO3 (LSCO) bottom electrodes were fabricated using Pulsed Laser Deposition. Implementing the "series capacitor model," bulk and interfacial capacitance properties were extracted as a function of temperature and frequency. 'Bulk' properties demonstrated typical ceramic behaviour, displaying little frequency dependence and a permittivity and loss peak at 250 K and 150 K respectively. The interfacial component was found to be relatively temperature and frequency independent for the LSCO/BST capacitors, but for the SRO/BST configuration the interfacial capacitance demonstrated moderate frequency and little temperature dependence below T similar to 300 K but a relatively strong frequency and temperature dependence above T similar to3 00 K. This was attributed to the thermal activation of a space charge component combined with a thermally independent background. The activation energy for the space charge was found to be E-A similar to 0.6 eV suggesting de-trapping of electrons from shallow level traps associated with a thin interfacial layer of oxygen vacancies, parallel to the electrodes.

Understanding thickness effects in thin film capacitors

Pulsed Laser Deposition (PLD) was used to make Au/(Ba0.5Sr0.5)TiO3/(La0.5Sr0.5) CoO3/MgO thin film capacitor structures. Functional properties were studied with changing BST thickness from similar to1265 nm to similar to63 nm. The dielectric constant was found to decrease, and migration of T-m (the temperature at which the dielectric constant is maximum) to lower temperatures occurred as thickness was reduced. Curie-Weiss plots of the as-obtained dielectric data, indicated that the Curie temperature was also systemmatically progressively depressed. Further, fitting to expressions previously used to describe diffuse phase transitions suggested increased diffuseness in transformation behaviour as film thickness decreased. This paper discusses the care needed in interpreting the observations given above. We make particular distinction between the apparent Curie-temperature derived from Curie-Weiss plots of as-measured data, and the inherent Curie temperature determined after correction for the interfacial capacitance. We demonstrate that while the apparent Curie temperature decreases as thickness decreases, the inherent Curie temperature is thickness independent. Thickness-invariant phase transition behaviour is confirmed from analysis of polarisation loops, and from examination of the temperature dependence of the loss-tangent. We particularly note that correction of data for interfacial capacitance does not alter the position of T-m. We must therefore conclude that the position of T-m is not related simply to phase transformation behaviour in BST thin films.

The Influence of Point Defects and Inhomogeneous Strain on the Functional Behaviour of Thin film Ferroelectrics

An attempt has been made to unequivocally identify the influence that inhomogeneous strain fields, surrounding point defects, have on the functional properties of thin film ferroelectrics. Single crystal thin film lamellae of BaTiO3 have been integrated into capacitor structures, and the functional differences between those annealed in oxygen and those annealed in nitrogen have been mapped. Key features, such as the change in the paraelectric-ferroelectric phase transition from first to second order were noted and found to be consistent with mean field modeling predictions for the effects of inhomogeneous strain. Switching characteristics appeared to be unaffected, suggesting that point defects have a low efficacy in domain wall pinning.

Ferroelectrics at the Nanoscale

There are several factors which make the investigation and understanding of nanoscale ferroelectrics particularly timely and important. Firstly, there is a market pressure, primarily from the electronics industry, to integrate ferroelectrics into devices with progressive decreases in size and increases in morphological complexity. This is perhaps best illustrated through the roadmaps for product development in FeRAM (Ferroelectric Randorn Access Memory) where the need for increases in bit density will require a move from 2D planar capacitor structures to 3D trenched capacitors in the next few years. Secondly, there is opportunity for novel exploration, as it is only relatively recently that developments in thin film growth of complex oxides, self-assembly techniques and high-resolution 'top-down' patterning have converged to allow the fabrication of isolated and well-defined ferroelectric nanoshapes, the properties of which are not known. Thirdly, there is an expectation that the behaviour of small scale ferroelectrics will be different from bulk, as this group of functional materials is highly sensitive to boundary/surface conditions, which are expected to dominate the overall response when sizes are reduced into the nanoscale regime. This feature article attempts to introduce some of the current areas of discovery and debate surrounding studies on ferroelectrics at the nanoscale. The focus is directed primarily at the search for novel size-related properties and behaviour which are not necessarily observed in bulk.

Electrical characterization of ALD Al2O3 and HfO2 films on germanium

Al2O3 and HfO2 films were deposited on germanium substrates by atomic layer deposition (ALD) and analyzed by MOS capacitor electrical characterization. In-situ plasma nitridation performed prior to ALD was found to improve the stability of the interface. For Al 2O3/GeON/Ge capacitors, a 450°C anneal in nitrogen ambient reduced hysteresis and oxide fixed charge to 90 mV and 1012 cm-2 respectively, with low leakage current density. On the contrary, degradation was observed for un-nitrided Al2O3/Ge capacitors after 300 and 400°C post-metal anneals. HfO2/GeON/Ge capacitors benefitted from a 400°C densification anneal but exhibited degradation after post-metal anneals at temperatures greater than 300°C. This degradation is attributed to the influence of Al electrodes on the HfO 2 gate stack. HfO2 is considered to be a suitable material for the gate stack and Al2O3 for the buried dielectric in a GeOI structure. ©The Electrochemical Society.

The effects of temperature on the performance of electrochemical double layer capacitors

An electrochemical double layer capacitor test cell containing activated carbon xerogel electrodes and ionic liquid electrolyte was tested at 15, 25 and 40 OC to examine the effect of temperature on electrolyte resistance (RS) and equivalent series resistance (ESR) measured using impedance spectroscopy and capacitance using charge/discharge cycling. A commercial 10F capacitor was used as a comparison. Viscosity, ionic self-diffusion coefficients and differential scanning calorimetry measurements were used to provide an insight into the behaviour of the 1,2-dimethyl-3-propylimdazolium electrolyte. Both RS and ESR decreased with increasing temperature for both capacitors. Increasing the temperature also increased the capacitance for both the test cell and the commercial capacitor but proportionally more for the test cell. An increase in temperature decreased the ionic liquid electrolyte viscosity and increased the self diffusion coefficients of both the anion and the cation indicating an increase in dissociation and increase in ionic mobility.

Energy storage in electrochemical capacitors: designing functional materials to improve device performance

Electrochemical capacitors, also known as supercapacitors, are becoming increasingly important components in energy storage, although their widespread use has not been attained due to a high cost/ performance ratio. Fundamental research is contributing to lowered costs through the engineering of new materials. Currently the most viable materials used in electrochemical capacitors are biomassderived and polymer-derived activated carbons, although other carbon materials are useful research tools. Metal oxides could result in a step change for electrochemical capacitor technology and is an exciting area of research. The selection of an appropriate electrolyte and electrode structure is fundamental in determining device performance. Although there are still many uncertainties in understanding the underlying mechanisms involved in electrochemical capacitors, genuine progress continues to be made. It is argued that a large, collaborative international research programme is necessary to fully develop the potential of electrochemical capacitors.