Ferroelectric : CNTs structures fabrication for advanced functional nano devices

This work is about the combination of functional ferroelectric oxides with Multiwall Carbon Nanotubes for microelectronic applications, as for example potential 3 Dimensional (3D) Non Volatile Ferroelectric Random Access Memories (NVFeRAM). Miniaturized electronics are ubiquitous now. The drive to downsize electronics has been spurred by needs of more performance into smaller packages at lower costs. But the trend of electronics miniaturization challenges board assembly materials, processes, and reliability. Semiconductor device and integrated circuit technology, coupled with its associated electronic packaging, forms the backbone of high-performance miniaturized electronic systems. However, as size decreases and functionalization increases in the modern electronics further size reduction is getting difficult; below a size limit the signal reliability and device performance deteriorate. Hence miniaturization of siliconbased electronics has limitations. On this background the Road Map for Semiconductor Industry (ITRS) suggests since 2011 alternative technologies, designated as More than Moore; being one of them based on carbon (carbon nanotubes (CNTs) and graphene) [1]. CNTs with their unique performance and three dimensionality at the nano-scale have been regarded as promising elements for miniaturized electronics [2]. CNTs are tubular in geometry and possess a unique set of properties, including ballistic electron transportation and a huge current caring capacity, which make them of great interest for future microelectronics [2]. Indeed CNTs might have a key role in the miniaturization of Non Volatile Ferroelectric Random Access Memories (NVFeRAM). Moving from a traditional two dimensional (2D) design (as is the case of thin films) to a 3D structure (based on a tridimensional arrangement of unidimensional structures) will result in the high reliability and sensing of the signals due to the large contribution from the bottom electrode. One way to achieve this 3D design is by using CNTs. Ferroelectrics (FE) are spontaneously polarized and can have high dielectric constants and interesting pyroelectric, piezoelectric, and electrooptic properties, being a key application of FE electronic memories. However, combining CNTs with FE functional oxides is challenging. It starts with materials compatibility, since crystallization temperature of FE and oxidation temperature of CNTs may overlap. In this case low temperature processing of FE is fundamental. Within this context in this work a systematic study on the fabrication of CNTs - FE structures using low cost low temperature methods was carried out. The FE under study are comprised of lead zirconate titanate (Pb1-xZrxTiO3, PZT), barium titanate (BaTiO3, BT) and bismuth ferrite (BiFeO3, BFO). The various aspects related to the fabrication, such as effect on thermal stability of MWCNTs, FE phase formation in presence of MWCNTs and interfaces between the CNTs/FE are addressed in this work. The ferroelectric response locally measured by Piezoresponse Force Microscopy (PFM) clearly evidenced that even at low processing temperatures FE on CNTs retain its ferroelectric nature. The work started by verifying the thermal decomposition behavior under different conditions of the multiwall CNTs (MWCNTs) used in this work. It was verified that purified MWCNTs are stable up to 420 ºC in air, as no weight loss occurs under non isothermal conditions, but morphology changes were observed for isothermal conditions at 400 ºC by Raman spectroscopy and Transmission Electron Microscopy (TEM). In oxygen-rich atmosphere MWCNTs started to oxidized at 200 ºC. However in argon-rich one and under a high heating rate MWCNTs remain stable up to 1300 ºC with a minimum sublimation. The activation energy for the decomposition of MWCNTs in air was calculated to lie between 80 and 108 kJ/mol. These results are relevant for the fabrication of MWCNTs – FE structures. Indeed we demonstrate that PZT can be deposited by sol gel at low temperatures on MWCNTs. And particularly interesting we prove that MWCNTs decrease the temperature and time for formation of PZT by ~100 ºC commensurate with a decrease in activation energy from 68±15 kJ/mol to 27±2 kJ/mol. As a consequence, monophasic PZT was obtained at 575 ºC for MWCNTs - PZT whereas for pure PZT traces of pyrochlore were still present at 650 ºC, where PZT phase formed due to homogeneous nucleation. The piezoelectric nature of MWCNTs - PZT synthesised at 500 ºC for 1 h was proved by PFM. In the continuation of this work we developed a low cost methodology of coating MWCNTs using a hybrid sol-gel / hydrothermal method. In this case the FE used as a proof of concept was BT. BT is a well-known lead free perovskite used in many microelectronic applications. However, synthesis by solid state reaction is typically performed around 1100 to 1300 ºC what jeopardizes the combination with MWCNTs. We also illustrate the ineffectiveness of conventional hydrothermal synthesis in this process due the formation of carbonates, namely BaCO3. The grown MWCNTs - BT structures are ferroelectric and exhibit an electromechanical response (15 pm/V). These results have broad implications since this strategy can also be extended to other compounds of materials with high crystallization temperatures. In addition the coverage of MWCNTs with FE can be optimized, in this case with non covalent functionalization of the tubes, namely with sodium dodecyl sulfate (SDS). MWCNTs were used as templates to grow, in this case single phase multiferroic BFO nanorods. This work shows that the use of nitric solvent results in severe damages of the MWCNTs layers that results in the early oxidation of the tubes during the annealing treatment. It was also observed that the use of nitric solvent results in the partial filling of MWCNTs with BFO due to the low surface tension (<119 mN/m) of the nitric solution. The opening of the caps and filling of the tubes occurs simultaneously during the refluxing step. Furthermore we verified that MWCNTs have a critical role in the fabrication of monophasic BFO; i.e. the oxidation of CNTs during the annealing process causes an oxygen deficient atmosphere that restrains the formation of Bi2O3 and monophasic BFO can be obtained. The morphology of the obtained BFO nano structures indicates that MWCNTs act as template to grow 1D structure of BFO. Magnetic measurements on these BFO nanostructures revealed a week ferromagnetic hysteresis loop with a coercive field of 956 Oe at 5 K. We also exploited the possible use of vertically-aligned multiwall carbon nanotubes (VA-MWCNTs) as bottom electrodes for microelectronics, for example for memory applications. As a proof of concept BiFeO3 (BFO) films were in-situ deposited on the surface of VA-MWCNTs by RF (Radio Frequency) magnetron sputtering. For in situ deposition temperature of 400 ºC and deposition time up to 2 h, BFO films cover the VA-MWCNTs and no damage occurs either in the film or MWCNTs. In spite of the macroscopic lossy polarization behaviour, the ferroelectric nature, domain structure and switching of these conformal BFO films was verified by PFM. A week ferromagnetic ordering loop was proved for BFO films on VA-MWCNTs having a coercive field of 700 Oe. Our systematic work is a significant step forward in the development of 3D memory cells; it clearly demonstrates that CNTs can be combined with FE oxides and can be used, for example, as the next 3D generation of FERAMs, not excluding however other different applications in microelectronics.

Growth and Characterisation of Radio Frequency Magnetron Sputttered Indium Tin Oxide Thin Films

The increasing interest in the interaction of light with electricity and electronically active materials made the materials and techniques for producing semitransparent electrically conducting films particularly attractive. Transparent conductors have found major applications in a number of electronic and optoelectronic devices including resistors, transparent heating elements, antistatic and electromagnetic shield coatings, transparent electrode for solar cells, antireflection coatings, heat reflecting mirrors in glass windows and many other. Tin doped indium oxide (indium tin oxide or ITO) is one of the most commonly used transparent conducting oxides. At present and likely well into the future this material offers best available performance in terms of conductivity and transmittivity combined with excellent environmental stability, reproducibility and good surface morphology. Although partial transparency, with a reduction in conductivity, can be obtained for very thin metallic films, high transparency and simultaneously high conductivity cannot be attained in intrinsic stoichiometric materials. The only way this can be achieved is by creating electron degeneracy in a wide bandgap (Eg > 3eV or more for visible radiation) material by controllably introducing non-stoichiometry and/or appropriate dopants. These conditions can be conveniently met for ITO as well as a number of other materials like Zinc oxide, Cadmium oxide etc. ITO shows interesting and technologically important combination of properties viz high luminous transmittance, high IR reflectance, good electrical conductivity, excellent substrate adherence and chemical inertness. ITO is a key part of solar cells, window coatings, energy efficient buildings, and flat panel displays. In solar cells, ITO can be the transparent, conducting top layer that lets light into the cell to shine the junction and lets electricity flow out. Improving the ITO layer can help improve the solar cell efficiency. A transparent ii conducting oxide is a material with high transparency in a derived part of the spectrum and high electrical conductivity. Beyond these key properties of transparent conducting oxides (TCOs), ITO has a number of other key characteristics. The structure of ITO can be amorphous, crystalline, or mixed, depending on the deposition temperature and atmosphere. The electro-optical properties are a function of the crystallinity of the material. In general, ITO deposited at room temperature is amorphous, and ITO deposited at higher temperatures is crystalline. Depositing at high temperatures is more expensive than at room temperature, and this method may not be compatible with the underlying devices. The main objective of this thesis work is to optimise the growth conditions of Indium tin oxide thin films at low processing temperatures. The films are prepared by radio frequency magnetron sputtering under various deposition conditions. The films are also deposited on to flexible substrates by employing bias sputtering technique. The films thus grown were characterised using different tools. A powder x-ray diffractometer was used to analyse the crystalline nature of the films. The energy dispersive x-ray analysis (EDX) and scanning electron microscopy (SEM) were used for evaluating the composition and morphology of the films. Optical properties were investigated using the UVVIS- NIR spectrophotometer by recording the transmission/absorption spectra. The electrical properties were studied using vander Pauw four probe technique. The plasma generated during the sputtering of the ITO target was analysed using Langmuir probe and optical emission spectral studies.

Multicamadas Ferromagnéticas Nanométricas Periódicas e Quasiperiódicas

We present a study of nanostructured magnetic multilayer systems in order to syn- thesize and analyze the properties of periodic and quasiperiodic structures. This work evolved from the deployment and improvement of the sputtering technique in our labora- tories, through development of a methodology to synthesize single crystal ultrathin Fe (100) films, to the final goal of growing periodic and quasiperiodic Fe/Cr multilayers and investi- gating bilinear and biquadratic exchange coupling between ferromagnetic layer dependence for each generation. Initially we systematically studied the related effects between deposition parameters and the magnetic properties of ultrathin Fe films, grown by DC magnetron sput- tering on MgO(100) substrates. We modified deposition temperature and film thickness, in order to improve production and reproduction of nanostructured monocrystalline Fe films. For this set of samples we measured MOKE, FMR, AFM and XPS, with the aim of investi- gating their magnocrystalline and structural properties. From the magnetic viewpoint, the MOKE and FMR results showed an increase in magnetocrystalline anisotropy due to in- creased temperature. AFM measurements provided information about thickness and surface roughness, whereas XPS results were used to analyze film purity. The best set of parame- ters was used in the next stage: investigation of the structural effect on magnetic multilayer properties. In this stage multilayers composed of interspersed Fe and Cr films are deposited, following the Fibonacci periodic and quasiperiodic growth sequence on MgO (100) substrates. The behavior of MOKE and FMR curves exhibit bilinear and biquadratic exchange coupling between the ferromagnetic layers. By computationally adjusting magnetization curves, it was possible to determine the nature and intensity of the interaction between adjacent Fe layers. After finding the global minimum of magnetic energy, we used the equilibrium an- gles to obtain magnetization and magnetoresistance curves. The results observed over the course of this study demonstrate the efficiency and versatility of the sputtering technique in the synthesis of ultrathin films and high-quality multilayers. This allows the deposition of magnetic nanostructures with well-defined magnetization and magnetoresistance parameters and possible technological applications

Crescimento de filmes nanométricos monocristalinos de Fe/MgO(100) por "SPUTTERING" DC

The research behind this master dissertation started with the installation of a DC sputtering system, from its first stage, the adaptation of a refrigerating system, passing by the introduction of a heating system for the chamber using a thermal belt, until the deposition of a series of Fe/MgO(100) single crystal nanometric film samples. The deposition rates of some materials such as Fe, Py and Cu were investigated through an Atomic Force Microscope (AFM). For the single crystal samples, five of them have the same growth parameters and a thickness of 250Å, except for the temperature, which varies from fifty degrees from one to another, from 100ºC to 300ºC. Three other samples also have the same deposition parameters and a temperature of 300ºC, but with thickness of 62,5Å, 150Å, and 250Å. Magneto-optical Kerr Effect (MOKE) of the magnetic curves measurements and Ferromagnetic Resonance (FMR) were made to in order to study the influence of the temperature and thickness on the sample s magnetic properties. In the present dissertation we discuss such techniques, and the experimental results are interpreted using phenomenological models, by simulation, and discussed from a physical point of view, taking into account the system s free magnetic energy terms. The results show the growth of the cubic anisotropy field (Hac) as the sample s deposition temperature increases, presenting an asymptotic behavior, similar to the characteristic charging curve of a capacitor in a RC circuit. A similar behavior was also observed for the Hac due to the increase in the samples thicknesses. The 250˚A sample, growth at 300°C, presented a Hac field close to the Fe bulk value

Propriedades fisicas do monocristal fe/mgo(100) e estudo da expansao termica da superficie da ag(100)

In this work we have developed a way to grow Fe/MgO(100) monocrystals by magnetron sputtering DC. We investigated the growing in a temperature range among 100 oC and 300 oC. Structural and magneto-crystalline properties were studied by different experimental techniques. Thickness and surface roughness of the films were investigated by atomic force microscopy, while magneto-crystalline properties were investigated by magneto-optical Kerr effect and ferromagnetic resonance. Our results show that as we increase the deposition temperature, the magneto-crystalline anisotropy of the films also increases, following the equation of Avrami. The best temperature value to make a film is 300 oC. As the main result, we built a base of magnetoresistence devices and as an aplication, we present measurements of Fe/Cr/Fe trilayer coupling. In a second work we investigated the temperature dependence of the first three interlayer spacings of Ag(100) surface using low energy electron diffraction. A linear expansion model of crystal surface was used and the values of Debye temperatures of the first two layers and thermal expansion coefficient were determinated. A relaxation of 1% was found for Ag(100) surface and these results are matched with faces (110) and (111) of the silver. iv

Resistive-Switching Behavior in Polycrystalline CaCu3Ti4O12 Nanorods

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Ferroelectric SBN thin films grown by an SBN/Bi2O3 PLD sequential process

Ferroelectric SrBi2Nb2O9 (SBN) thin films were prepared by pulsed laser deposition (PLD) on Pt/Ti/SiO2/Si(100) using a sequential deposition process from two SBN and Bi2O3 targets. This route allows for bismuth enrichment of the film composition in order to improve the ferroelectric characteristics. Structural and microstructural characterizations were performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The composition of films and targets was determined by energy dispersive X-ray spectrometry (EDX). The deposition temperature, which provided well-crystallized layered perovskite SBN phase films in situ, was found to be 700°C. The results were compared with those obtained for SBN films deposited at 400°C and then crystallized ex situ. For an ex situ annealing temperature of 750°C, a remanent polarization value (Pr) of 23.2 μc/cm2 and a coercive field (Ec) of 112 kV/cm were measured. © 2001 Elsevier Science Ltd. All rights reserved.

Influência da temperatura e do tipo de substrato em filmes de GaN depositados por magnetron sputtering reativo

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Síntese de nanofios de óxidos semicondutores para aplicações em dispositivos ópticos e eletrônicos

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Influence of the Substrate and Precursor on the Magnetic and Magneto-transport Properties in Magnetite Films

We have investigated the magnetic and transport properties of nanoscaled Fe3O4 films obtained from Chemical Vapor Deposition (CVD) technique using [(FeFe2III)-Fe-II(OBut)(8)] and [Fe-2(III)(OBut)(6)] precursors. Samples were deposited on different substrates (i.e., MgO (001), MgAl2O4 (001) and Al2O3 (0001)) with thicknesses varying from 50 to 350 nm. Atomic Force Microscopy analysis indicated a granular nature of the samples, irrespective of the synthesis conditions (precursor and deposition temperature, T-pre) and substrate. Despite the similar morphology of the films, magnetic and transport properties were found to depend on the precursor used for deposition. Using [(FeFe2III)-Fe-II(OBut)(8)] as precursor resulted in lower resistivity, higher M-S and a sharper magnetization decrease at the Verwey transition (T-V). The temperature dependence of resistivity was found to depend on the precursor and T-pre. We found that the transport is dominated by the density of antiferromagnetic antiphase boundaries (AF-APB's) when [(FeFe2III)-Fe-II(OBut)(8)] precursor and T-pre = 363 K are used. On the other hand, grain boundary-scattering seems to be the main mechanism when [Fe-2(III)(OBut)(6)] is used. The Magnetoresistance (MR(H)) displayed an approximate linear behavior in the high field regime (H > 796 kA/m), with a maximum value at room-temperature of similar to 2-3 % for H = 1592 kA/m, irrespective from the transport mechanism.

Präparation von Bismutsulfid-Absorberschichten für Dünnschichtsolarzellen mittels physikalischer Gasphasenabscheidung

Angesichts der sich abzeichnenden Erschöpfung fossiler Ressourcen ist die Erforschung alternativer Energiequellen derzeit eines der meistbeachteten Forschungsgebiete. Durch ihr enormes Potential ist die Photovoltaik besonders im Fokus der Wissenschaft. Um großflächige Beschichtungsverfahren nutzen zu können, wird seit einigen Jahren auf dem Gebiet der Dünnschichtphotovoltaik intensiv geforscht. Jedoch sind die gegenwärtigen Solarzellenkonzepte allesamt durch die Verwendung giftiger (Cd, As) oder seltener Elemente (In, Ga) oder durch eine komplexe Phasenbildung in ihrem Potential beschränkt. Die Entwicklung alternativer Konzepte erscheint daher naheliegend.rnAufgrund dessen wurde in einem BMBF-geförderten Verbundprojekt die Abscheidung von Dünnschichten des binären Halbleiters Bi2S3 mittels physikalischer Gasphasenabscheidung mit dem Ziel der Etablierung als quasi-intrinsischer Absorber in Solarzellenstrukturen mit p-i-n-Schichtfolge hin untersucht.rnDurch sein von einem hochgradig anisotropen Bindungscharakter geprägtes Kristallwachstum war die Abscheidung glatter, einphasiger und für die Integration in eine Multischichtstruktur geeigneter Schichten mit Schichtdicken von einigen 100 nm eine der wichtigsten Herausforderungen. Die Auswirkungen der beiden Parameter Abscheidungstemperatur und Stöchiometrie wurden hinsichtlich ihrer Auswirkungen auf die relevanten Kenngrößen (wie Morphologie, Dotierungsdichte und Photolumineszenz) untersucht. Es gelang, erfolgreich polykristalline Schichten mit geeigneter Rauigkeit und einer Dotierungsdichte von n ≈ 2 1015cm-3 auf anwendungsrelevanten Substraten abzuscheiden, wobei eine besonders starke Abhängigkeit von der Gasphasenzusammensetzung ermittelt werden. Es konnten weiterhin die ersten Messungen der elektronischen Zustandsdichte unter Verwendung von Hochenergie-Photoemissionsspektroskopie durchgeführt werden, die insbesondere den Einfluss variabler Materialzusammensetzungen offenbarten.rnZum Nachweis der Eignung des Materials als Absorberschicht standen innerhalb des Projektes mit SnS, Cu2O und PbS prinzipiell geeignete p-Kontaktmaterialien zur Verfügung. Es konnten trotz der Verwendung besonders sauberer Abscheidungsmethoden im Vakuum keine funktionstüchtigen Solarzellen mit Bi2S3 deponiert werden. Jedoch war es unter Verwendung von Photoemissionspektroskopie möglich, die relevanten Grenzflächen zu spektroskopieren und die Ursachen für die Beobachtungen zu identifizieren. Zudem konnte erfolgreich die Notwendigkeit von Puffermaterialien bei der Bi2S3-Abscheidung nachgewiesen werden, um Oberflächenreaktionen zu unterbinden und die Transporteigenschaften an der Grenzfläche zu verbessern.rn

Synthesis and characterization of turbostratic carbons prepared by catalytic chemical vapour decomposition of acetylene

The turbostratic mesoporous carbon blacks were prepared by catalytic chemical vapour decomposition (CCVD) of acetylene using Ni/MgO catalysts prepared by co-precipitation. The relationship between deposition conditions and the nanostructures of resultant carbon black materials was investigated. It was found that the turbostratic and textural structures of carbon blacks are dependent on the deposition temperature and nickel catalyst loading. Higher deposition temperature increases the carbon crystallite unit volume V-nano and reduces the surface area of carbon samples. Moreover, a smaller V-nano is produced by a higher Ni loading at the same deposition temperature. In addition of the pore structure and the active metal surface area of the catalyst, the graphitic degree or electronic conductivity of the carbon support is also a key issue to the activity of the supported catalyst. V-nano is a very useful parameter to describe the effect of the crystalline structure of carbon blacks on the reactivity of carbon blacks in oxygen-carbon reaction and the catalytic activity of carbon-supported catalyst in ammonia decomposition semi-quantitatively. (C) 2006 Elsevier B.V. All rights reserved.

Properties of ultra fast deposited diamond-like hydrogenated carbon films

Hydrogenated amorphous carbon films with diamond like structures have been formed on different substrates at very low energies and temperatures by a plasma enhanced chemical vapor deposition process employing acetylene as the precursor gas. The plasma source was of a cascaded arc type with Ar as carrier gas. The films were grown at very high deposition rates. Deposition on Si, glass and plastic substrates has been studied and the films characterized in terms of sp3 content, roughness, hardness, adhesion and optical properties. Deposition rates up to 20 nm/s have been achieved at substrate temperatures below 100°C. The typical sp3 content of 60-75% in the films was determined by X-ray generated Auger electron spectroscopy. Hardness, reduced modulus and adhesion were measured using a MicroMaterials Nano Test Indenter/Scratch tester. Hardness was found to vary from 4 to 13 GPa depending on deposition conditions. Adhesion was significantly influenced by the substrate temperature and in situ DC cleaning. Hydrogen content in the film was measured by a combination of the Fourier transform infrared and Rutherford backscattering techniques. Advantages of these films are: low ion energy and deposition temperature, very high deposition rates, low capital cost of the equipment and the possibility of film properties being tailored according to the desired application.

Electrical characterization of amorphous silicon MIS-based structures for HIT solar cell applications

A complete electrical characterization of hydrogenated amorphous silicon layers (a-Si:H) deposited on crystalline silicon (c-Si) substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) was carried out. These structures are of interest for photovoltaic applications. Different growth temperatures between 30 and 200 °C were used. A rapid thermal annealing in forming gas atmosphere at 200 °C during 10 min was applied after the metallization process. The evolution of interfacial state density with the deposition temperature indicates a better interface passivation at higher growth temperatures. However, in these cases, an important contribution of slow states is detected as well. Thus, using intermediate growth temperatures (100–150 °C) might be the best choice.

High-temperature growth of graphene films on copper foils by ethanol chemical vapour deposition

Chemical vapor deposition (CVD) is widely utilized to synthesize graphene with controlled properties for many applications, especially when continuous films over large areas are required. Although hydrocarbons such as methane are quite efficient precursors for CVD at high temperature (∼1000 °C), finding less explosive and safer carbon sources is considered beneficial for the transition to large-scale production. In this work, we investigated the CVD growth of graphene using ethanol, which is a harmless and readily processable carbon feedstock that is expected to provide favorable kinetics. We tested a wide range of synthesis conditions (i.e., temperature, time, gas ratios), and on the basis of systematic analysis by Raman spectroscopy, we identified the optimal parameters for producing highly crystalline graphene with different numbers of layers. Our results demonstrate the importance of high temperature (1070 °C) for ethanol CVD and emphasize the significant effects that hydrogen and water vapor, coming from the thermal decomposition of ethanol, have on the crystal quality of the synthesized graphene.