The scaling of integrated circuits requires the use of alternative dielectric materials as the replacement for silicon dioxide in the submicron devices. The scaling limit for silicon dioxide used in MOSFETs is 1.2nm and the Oxide Nitride Oxide (ONO) stack used in flash memory applications is 13.0nm. The use of alternative dielectrics with high- κ value will alleviate the problem of charge retention and also would help to decrease the programming voltage in case of flash memory cells. Many alternative high- κ dielectric materials such as TaO2, TiO2, Al2O3 etc., have been examined for this purpose previously. Recently the metal oxides such as ZrO2 and HfO2 have been found to be viable replacements for the existing oxide. The high- κ value along with high bandgap motivates this replacement. A complete modeling of the reactively sputtered HfO2 films in the thickness range of 294Å to 480Å is attempted using the data obtained by one of the group members at the Sharp Laboratories of America, Inc. The IV and CV data is used to characterize the material properties and conduction mechanism in HfO2 films used as a control dielectric. The slope of the Poole-Frenkel plot is close to the theoretical value in the intermediate region however it starts to deviate at high field regions. Temperature dependent data also suggests that there are two types of vii traps active in the intermediate and high field regions. However the origin of these traps is not known. Temperature dependent data indicates that there is a rapid increase in the leakage current at elevated temperatures in the high field region further suggesting that the charge retention capability of the device would be adversely affected under such conditions.

Habilitacinei procedūrai teikiamų darbų apžvalgoje yra pateikiama autoriaus atlikta ir kitų autorių įvairių kristalinių (ZnS, ZnSe) ir organinių junginių diodų, nanovamzdelių, plonų plėvelių, nanofibrų, ir kitų nanodarinių mokslinėje literatūroje skelbtų eksperimentinių elektrinio laidumo tyrimų rezultatų analizė bei šių rezultatų sugretinimas su fononais stimuliuoto tuneliavimo (FST) teorijų pagrindu paskaičiuotomis tuneliavimo tikimybių priklausomybėmis. Tyrimo rezultatai leido išsiaiškinti tai, kad krūvininkų generacijos procesas yra jų tuneliavimas iš gaudiklių dalyvaujant fononams. Įrodyta, kad organiniuose dariniuose (jų tarpe ir nanodariniuose) stebimą virsmą iš puslaidininkinio laidumo į metalinį laidumą (kai virsmo temperatūra TC) galima paaiškinti krūvininkų tunelinio proceso metu susidariusiu balansu tarp fononų absorbcijos ir jų generacijos, kai proceso metu elektrinis laukas krūvininkus išlaisvina iš lokalizuotos būsenos į laidumo būseną. Toje temperatūrų srityje, kur T < TC, tuneliavime dominuoja fononų absorbcija, ir todėl ten tunelinio perėjimo tikimybė, temperatūrai augant, padidėja. Priešingai, tose srityse, kur T > TC, procese dominuoja fononų emisija ir todėl ji sumažina krūvininkų tuneliavimo tikimybę, tuo pačiu iššaukdama elektrinio laidumo sumažėjimą kylant temperatūrai. Tokio FST modelio pagrįstumą įrodo ir tai, kad tiek plačiame elektrinių laukų, tiek ir temperatūrų intervale, tuneliavimo procesas yra vieningai aprašomas tų pačių parametrų... [toliau žr. visą tekstą] / Habilitation procedure review is for to clarify the conductance mechanism in polymers and other conducting nanodevices. The model based on the phonon-assisted tunnelling (PhAT) process, initiated by an electric field. An advantage of this PhAT model over the other models is the possibility to describe the behaviour of conductivity data measured at both low and high temperatures with the same set of parameters: εT - the energetic depth of the center, m* - the effective mass, T- temperature, E- the electric field, ħω - the phonon energy and a- the electron-phonon interaction constant. From these ones only the electron-phonon coupling constant a, and the phonon energy ħω are the fitting parameters estimated from the best fitting of the experimental data and the theory. Other parameters are from experiments or from literary sources known. Therefore, the PhAT mechanism could be dominant in the conductance of the MWNT, so as was shown, also in the case of the single-wall nanotubes. On the basis of this proposed model, the phenomenon of the crossover from non-metallic to metallic behavior of the conductivity is also explained. The decrease of conductivity in the framework of this model occurs at the temperatures T > TC when the phonon emission in the process of tunnelling dominates over the phonon absorption. The obtained agreement between the theoretical and experimental results, is not accidental but is due to the fact that the proposed model includes ultimate pictures of charge... [to full text]

abstract: This thesis discusses the evolution of conduction mechanism in the silver (Ag) on zinc oxide (ZnO) thin film system with respect to the Ag morphology. As a plausible substitute for indium tin oxide (ITO), TCO/Metal/TCO (TMT) structure has received a lot of attentions as a prospective ITO substitute due to its low resistivity and desirable transmittance. However, the detailed conduction mechanism is not fully understood. In an attempt to investigate the conduction mechanism of the ZnO/Ag/ZnO thin film system with respect to the Ag microstructure, the top ZnO layer is removed, which offers a better view of Ag morphology by using scanning electron microscopy (SEM). With 2 nm thick Ag layer, it is seen that the Ag forms discrete islands with small islands size (r), but large separation (s); also the effective resistivity of the system is extremely high. This regime is designated as dielectric zone. In this regime, thermionic emission and activated tunneling conduction mechanisms are considered. Based on simulations, when "s" was beyond 6 nm, thermionic emission dominates; with "s" less than 6 nm, activated tunneling is the dominating mechanism. As the Ag thickness increases, the individual islands coalesce and Ag clusters are formed. At certain Ag thickness, there are one or several Ag clusters that percolate the ZnO film, and the effective resistivity of the system exhibits a tremendous drop simultaneously, because the conducting electrons do not need to overcome huge ZnO barrier to transport. This is recognized as percolation zone. As the Ag thickness grows, Ag film becomes more continuous and there are no individual islands left on the surface. The effective resistivity decreases and is comparable to the characteristics of metallic materials, so this regime is categorized as metallic zone. The simulation of the Ag thin film resistivity is performed in terms of Ag thickness, and the experimental data fits the simulation well, which supports the proposed models. Hall measurement and four point probe measurement are carried out to characterize the electrical properties of the thin film system. / Dissertation/Thesis / M.S. Materials Science and Engineering 2012

Os processos envolvidos em condução eletrônica de polímeros condutores são muito complicados devido à intricada morfologia estrutural de tais materiais. Realizamos medidas de condutividade alternada em filmes de polianilina em uma grande faixa de freqüência, variando o grau de dopagem dos filmes e temperatura. Ao mesmo tempo, desenvolvemos um modelo estatístico de rede de resistores para descrever a estrutura polimérica e para simular as componentes real e imaginária de sua resistividade alternada. Leva-se em conta a polidispersividade do material, assim como os mecanismos de transportes de carga intracadeia e intercadeia. Pela aplicação de uma técnica de matriz de transferência, o modelo reproduz medidas de resistividade alternada realizadas em filmes de polianilina em diferentes graus de dopagem e em diferentes temperaturas. Nossos resultados indicam que os mecanismos intercadeias governam o comportamento da resistividade em regiões de baixa freqüência enquanto que, para altas freqüências, mecanismos intracadeia são dominantes. Essa simulação foi desenvolvida para redes bi e tridimensional. Aplicamos, também, esse método para estudar sistemas isoenergéticos de estrutura desordenada (poliacetileno estirado), sistemas isoenergéticos de estrutura desordenada (poliacetileno não-estirado) e sistemas não-isoenergéticos de estrutura desordenada (polianilina). Finalmente, uma comparação entre esses três materiais, permitiu-se discutir, em detalhes, a distribuição de barreiras de energia potencial e a diferença dos níveis de energia que controlam o mecanismo de salto dos portadores eletrônicos. / The processes involved in electronic conduction of conducting polymers are very complicated due to the intricate morphological structure of such materials. We carried out alternating conductivity measurements in polyaniline films in a large frequency range, varying the doping degree of the films and temperature. At the same time, we developed a statistical model of resistor networks to describe a polymer structure and to simulate the real and imaginary components of their ac resistivities. This model takes into account the polydispersiveness of the material as well as intrachain and interchain charge transport mechanisms. By the application of a transfer-matrix technique, it reproduces ac resistivity measurements carried out with polyaniline films in different doping degrees and temperatures. Our results indicate the interchain mechanisms govern the resistivity behavior in the low frequency region while, for higher frequencies, intrachain mechanisms are dominated. These simulations were developed in bi and tridimensional lattice. We also applied this method to study ordered structure in isoenergetic systems (stretched polyacetylene), disordered structure in isoenergetic systems (normal polyacetylene), and disordered structure in non-isoenergetic systems (polyaniline). Finally, a comparison between these three materials, allowed us to discuss in detail the energy barriers distribution and the difference in energy levels, which control the hopping mechanisms of the electronic carriers.

Des manganites de formule La0.67Ba0.33Mn1-xFexO3 et Pr0.67Ba0.33Mn1-xFexO3 ont étésynthétisés par voie céramique et leurs propriétés structurales et physiques ont été étudiées. Leséchantillons présentent une transition paramagnétique-ferromagnétique pour un taux de Fe £ 10%(resp. 5%) pour la série de La (resp. Pr) alors qu’un comportement type verre de spin est observéau delà de ces taux. Une transition métal-semiconducteur est associée aux transitionsferromagnétiques et un comportement semiconducteur sur toute la gamme de température estobservé lorsqu’il n’y a pas de transition ferromagnétique. Les propriétés physiques peuvents’interpréter par le mécanisme de double échange qui s’affaiblit pour des taux croissants desubstitution. La conduction métallique est due à la diffusion des électrons (par les phonons et lesjoints des grains) alors que le caractère semiconducteur est dû à la présence de polarons.L’étude magnétocalorique sur Pr0.67Ba0.33MnO3, Pr0.67Ba0.33Mn0.95Fe0.05O3, La0.67Ba0.33MnO3et La0.67Ba0.3Mn0.95Fe0.05O3 montre que nos matériaux sont prometteurs pour être utilisés dans latechnologie de la réfrigération magnétique. / Manganites of formula La0.67Ba0.33Mn1-xFexO3 and Pr0.67Ba0.33Mn1-xFexO3 were synthesizedby ceramic route and their structural and physical properties were studied. The samples exhibit aparamagnetic-ferromagnetic transition for Fe rate £ 10% (resp. 5%) for the La (resp. Pr) series,while a typical behavior of spin glass is observed beyond this rate. This ferromagneticparamagnetictransition is associated to a metallic-semiconductor transition and a semiconductorbehavior throughout the whole temperature range is observed when there is no ferromagnetictransition. The physical properties can be interpreted by the double exchange mechanism whichweakens for increasing rates of substitution. The metallic conduction is due to electron scattering(by phonon and grain boundaries), while the semiconductor character is due to the presence ofpolarons.Magnetocaloric study for Pr0.67Ba0.33MnO3, Pr0.67Ba0.33Mn0.95Fe0.05O3, La0.67Ba0.33MnO3 andLa0.67Ba0.3Mn0.95Fe0.05O3 shows that our materials are promising to be used in magneticrefrigeration technology.

Os processos envolvidos em condução eletrônica de polímeros condutores são muito complicados devido à intricada morfologia estrutural de tais materiais. Realizamos medidas de condutividade alternada em filmes de polianilina em uma grande faixa de freqüência, variando o grau de dopagem dos filmes e temperatura. Ao mesmo tempo, desenvolvemos um modelo estatístico de rede de resistores para descrever a estrutura polimérica e para simular as componentes real e imaginária de sua resistividade alternada. Leva-se em conta a polidispersividade do material, assim como os mecanismos de transportes de carga intracadeia e intercadeia. Pela aplicação de uma técnica de matriz de transferência, o modelo reproduz medidas de resistividade alternada realizadas em filmes de polianilina em diferentes graus de dopagem e em diferentes temperaturas. Nossos resultados indicam que os mecanismos intercadeias governam o comportamento da resistividade em regiões de baixa freqüência enquanto que, para altas freqüências, mecanismos intracadeia são dominantes. Essa simulação foi desenvolvida para redes bi e tridimensional. Aplicamos, também, esse método para estudar sistemas isoenergéticos de estrutura desordenada (poliacetileno estirado), sistemas isoenergéticos de estrutura desordenada (poliacetileno não-estirado) e sistemas não-isoenergéticos de estrutura desordenada (polianilina). Finalmente, uma comparação entre esses três materiais, permitiu-se discutir, em detalhes, a distribuição de barreiras de energia potencial e a diferença dos níveis de energia que controlam o mecanismo de salto dos portadores eletrônicos. / The processes involved in electronic conduction of conducting polymers are very complicated due to the intricate morphological structure of such materials. We carried out alternating conductivity measurements in polyaniline films in a large frequency range, varying the doping degree of the films and temperature. At the same time, we developed a statistical model of resistor networks to describe a polymer structure and to simulate the real and imaginary components of their ac resistivities. This model takes into account the polydispersiveness of the material as well as intrachain and interchain charge transport mechanisms. By the application of a transfer-matrix technique, it reproduces ac resistivity measurements carried out with polyaniline films in different doping degrees and temperatures. Our results indicate the interchain mechanisms govern the resistivity behavior in the low frequency region while, for higher frequencies, intrachain mechanisms are dominated. These simulations were developed in bi and tridimensional lattice. We also applied this method to study ordered structure in isoenergetic systems (stretched polyacetylene), disordered structure in isoenergetic systems (normal polyacetylene), and disordered structure in non-isoenergetic systems (polyaniline). Finally, a comparison between these three materials, allowed us to discuss in detail the energy barriers distribution and the difference in energy levels, which control the hopping mechanisms of the electronic carriers.

NASCION-type lithium-aluminum-germanium-phosphate (LAGP) glass-ceramic is one of the most promising solid electrolyte (SEs) material for the next generation Li-ion battery. Based on the crystallization of glass-ceramic material, the two-step heat treatment was designed to control the crystallization of Li-ion conducting crystal in the glass matrix. The results show that the LAGP crystal is preferred to internally crystalize, Tg + 60%∆T is the nucleation temperature that provides the highest ion conductivity. The compositional investigation also found that, pure LAGP crystal phase can be synthesized by lowering the amount of GeO2. To fill gap of atomic structure in LAGP glass-ceramic, molecular dynamic (MD) simulation was used to build the crystal, glass, and interfacial structure LAGP. The aliovalent ion substitution induced an simultaneously redistribution of Li to the 36f interstitial site, and the rapid cooperative motion between the Li-ions at 36f can drop the activation energy of LAGP crystal by decreasing the relaxation energy; furthermore, an energy model was built based on the time-based analysis of Li-ion diffusion to articulate the behavior. The glass and interfacial structure show and accumulation of AlO4, GeO4 and Li at the interface, which explains the Li-trapping on the intergranular glass phase. An in-situ synchrotron X-ray study found that, by using two-step heat treatment, the nucleation of Li-ion conducting crystal in the glass-matrix induced large strain from interfacial tension, which can also promote the incorporation of aliovalent ion substitution in the NASICON crystal and enhances the ion conductivity.

Ceramic-matrix composites of alumina and silicon carbide whiskers have recently found novel commercial application as electromagnetic absorbers. However, a detailed understanding of how materials issues influence the composite electrical response, which underpins this application, has been absent until now. In this project, such composites were electrically measured over a wide range of conditions and modeled in terms of various aspects of the microstructure in order to understand how they work. For this purpose, three types of composites were made by different methods from the same set of ceramic powder blends loaded with different volume fractions of whiskers. In doing so, the interfaces between whiskers, the preferred orientations of whiskers, and the structure of electrically-connected whisker clusters were varied; the whisker aspect-ratio distributions were the same for all methods. At the electrode interfaces, Schottky barriers at the junctions of the electrically-percolating wide-bandgap semiconductor whiskers on the surface were responsible for a significant portion of the total measured impedance. The associated electrical response was studied on the microscopic and macroscopic level, and the gap between these different scales was bridged. Also, a modeling approach was developed for the non-linear behavior of the composite which results from these barriers. In regards to the whiskers within the composite bulk, the effects of various factors on the wide-band frequency dependence of the dielectric response and dc conductivity were explained and contextualized for the electromagnetic absorber application. Such factors include whisker preferred orientation, electrical percolation and cluster structure, the interfaces between electrically-connected SiC whiskers, and porosity. A quantitative correlation between the anisotropy of the microstructure and that of the conductivity was found, and was understood in terms of the interfacial SiC-Al2O3-SiC conduction mechanism. This behavior was shown to differ from the behavior commonly observed for other disordered mixtures of relatively conductive particles dispersed inside insulating polymer hosts. A description of this new mechanism was developed based on an observed correlation between the temperature dependencies of the static and radio-frequency electrical responses. Also, the aforementioned non-linear response model was expanded upon to describe conduction through and across electrically-percolated clusters. The model demonstrates how loading and interface behavior influence the topology and the strength of the non-linear response of the clusters.

No description available.

Les matériaux ferroélectriques suscitent beaucoup d’intérêt du fait de leurs propriétés physiques telles que la piézoélectricité, la ferroélectricité ou encore leur permittivité élevée. Ainsi, on cherche à les intégrer dans les micro- et nano-systèmes dans lesquels on les retrouve généralement sous forme de couche mince dans une configuration de type capacité plane. En particulier, l’oxyde de plomb, titane et zirconium (PZT) est un matériau très attractif pour les applications RF capacitives du fait de sa grande permittivité. Son intégration sur des électrodes métalliques, i.e. les lignes coplanaires constituant le guide d’onde, implique de maîtriser sa croissance en film mince. L’application d’une tension dans un dispositif RF actif impose également de contrôler les propriétés électriques : nature des courants de fuite et comportement ferroélectrique du PZT. Dans ce contexte, les couches minces de PZT sont déposées par ablation laser (PLD) sur un empilement La₀.₆₇Sr₀.₃₃MnO₃ (LSMO) / Pt (111) déposé sur un substrat monocristallin de saphir. La couche d’accroche conductrice LSMO est nécessaire afin d’éviter la formation d’une phase pyrochlore paraélectrique. Le contrôle de l’orientation cristalline de la couche de LSMO permet de contrôler la texturation de la couche de PZT. Les courants de fuite au travers de l’empilement Pt/PZT/LSMO/Pt ont ensuite été étudiés dans l’intervalle de température 220-330K de façon à déterminer les mécanismes de conduction. Une transition a été mise en évidence entre, autour de la température ambiante, un mécanisme contrôlé par la diffusion des charges en volume et, à basse température, un mécanisme contrôlé par l’injection des charges aux interfaces électrode/PZT. Un mécanisme par sauts a été identifié au-dessus de 280K en cohérence avec la présence de défauts étendus et la structure colonnaire du PZT. Afin de contrôler ces courants de fuite, différentes stratégies ont été utilisées. La première consiste à insérer une couche d’oxyde isolante à l’interface supérieure Pt/PZT modifiant ainsi l’injection des charges et permettant de réduire les courants de fuite. La seconde stratégie consiste, quant à elle, à modifier la structure de la couche de PZT en volume en élaborant des composites diélectrique/PZT multicouches ou colonnaires. Ainsi, une couche d’oxyde isolante a été insérée au milieu de la couche de PZT et a permis de réduire les courants de fuite. Le contrôle de la nucléation du PZT a également permis par nanofabrication d’élaborer un composite colonnaire pérovskite PZT/pyrochlore. La densité de piliers de pyrochlore dans la phase ferroélectrique permet de moduler la densité de courant dans la structure. Le PZT et les hétérostructures permettant de réduire les courants de fuite ont ensuite été intégrés dans une structure RF capacitive avec des lignes coplanaires d’or. Les performances RF en termes d’isolation et de pertes par insertion ainsi que la compatibilité de ces différents matériaux ont été étudiées et ont montré que les solutions développées dans le cadre du contrôle des courants de fuite sont prometteuses pour être intégrées dans les dispositifs RF capacitifs. En outre, on a cherché à extraire la permittivité à haute fréquence du PZT lorsque celui-ci est inséré dans une structure capacitive. Cette étude a notamment permis de mettre en évidence les points techniques à modifier concernant la structure du dispositif afin de parvenir à exploiter les propriétés physiques du PZT à haute fréquence. / Ferroelectric materials are raising a lot of interest due to their physical properties such as piezoelectricity, ferroelectricity or high dielectric constant. Thus, they are generally integrated in micro- and nano-systems as thin films in a capacitive configuration. Especially, the lead zirconate titanate oxide (PZT) is an attractive material for capacitive RF applications due to its high dielectric constant. The growth of the PZT thin film has to be controlled on metallic electrodes for its integration on coplanar transmission lines. Moreover, electrical properties such as leakage current and ferroelectric behavior of PZT have to be monitored upon application of a dc voltage bias for RF device operation. In this context, PZT thin films were grown by the pulsed laser deposition technique (PLD) on a La₀.₆₇Sr₀.₃₃MnO₃ (LSMO) / Pt (111) electrode on a monocrystalline sapphire substrate. The LSMO buffer layer is mandatory to avoid the formation of the paraelectric pyrochlore phase. The control of the crystalline orientation of the LSMO layer allows for the control of the PZT layer texture. Leakage currents through the Pt/PZT/LSMO/Pt stack were then studied in the 220-330K temperature range to determine the conduction mechanisms. A transition is evidenced between a bulk-controlled mechanism near room temperature and an interface-controlled mechanism at low temperature. A hopping mechanism is identified above 280K in line with the presence of extended defects and the columnar structure of the PZT layer. Several strategies were tested to control leakage currents. The first one consists in inserting an insulating oxide layer at the top Pt/PZT interface. In this way, charge injection was modified and leakage currents were reduced. The second strategy consists in changing the PZT layer bulk structure by elaborating a layered or columnar dielectric/PZT composite. Thus, an insulating oxide layer was inserted in the middle of the PZT layer and permitted to reduce leakage currents. Moreover, the control of the PZT nucleation allowed for the elaboration of a columnar PZT/pyrochlore composite. The leakage currents in this composite can be tuned through the pyrochlore pillars density among the ferroelectric matrix. Then, PZT and the heterostructures for leakage current control were integrated in a capacitive RF structure with gold coplanar transmission lines. RF performances in terms of isolation and insertion loss of these materials were studied and gave good results. In particular the heterostructures developed to control the leakage currents are promising for their integration in capacitive RF devices. Besides, I tried to extract the permittivity of PZT at high frequency with the PZT layer in a capacitive configuration. This study highlighted the essential modifications of the capacitive structure that have to be made in order to be able to exploit PZT properties at high frequency.