CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / Os semicondutores magnÃticos atraÃram a atenÃÃo de muitos cientistas nos Ãltimos anos devido, principalmente, a aplicaÃÃes tecnolÃgicas no ramo da spintrÃnica. Esses semicondutores podem ser desenvolvidos por meio de um processo chamado dopagem, onde alguns Ãtomos da matriz semicondutora sÃo substituÃdos aleatoriamente por Ãtomos magnÃticos. Esta propriedade possibilita a fabricaÃÃo de uma variedade de dispositivos eletrÃnicos a partir do mesmo material semicondutor. Neste trabalho utilizou-se a tÃcnica de moagem mecÃnica de altas energias para realizar a dopagem dos Ãxidos TiO2, SnO2 e In2O3 com Fe2O3, que foram caracterizados estruturalmente por difraÃÃo de raios-x e espectroscopia MÃssbauer. A sÃntese foi realizada em trÃs tipos de ferramentas de moagem que influenciaram no desenvolvimento da reaÃÃo. Foram formados compostos com fÃrmulas Sn(1-x)Fe(x)O2, Ti(1-x)Fe(x)O2 e {(In(1-x)Fe(x)}2O3, com valores de concentraÃÃo do dopante magnÃtico x de 2, 5 e 10% em Ãtomos. Utilizando jarra de poliacetal e esferas de zircÃnia foram sintetizadas amostras de TiO2 dopadas com Fe2O3. No entanto, a energia produzida por estas ferramentas nÃo foi suficiente para completar a formaÃÃo do composto. Com uma jarra e esferas de aÃo inox foram produzidas amostras de TiO2 dopadas com Fe2O3, que por sua vez foram contaminadas com impurezas de ferro metÃlico proveniente das ferramentas utilizadas. AlÃm disso, essas impurezas contribuÃram para a formaÃÃo de outra fase, a ilmenita (FeTiO3). Para alcanÃar um grau de pureza, essas amostras foram submetidas à lavagem com soluÃÃo de HCl para a retirada do ferro metÃlico, porÃm a fase referente a ilmenita nÃo foi eliminada. Compostos formados por TiO2, SnO2 e In2O3 dopados com Fe2O3 foram sintetizados em uma jarra de alumina com esferas de zircÃnia. Estas amostras nÃo apresentaram impurezas indesejÃveis e nÃo houve formaÃÃo de outras fase. Enquanto os compostos baseados em TiO2 e SnO2 mantiveram suas estruturas cristalinas originais, o composto {In(1-x)Fe(x)}2O3 sofreu uma mudanÃa de fase cristalina, da estrutura cÃbica original do In2O3 para hexagonal do tipo corundum. Resultados obtidos das medidas de espectroscopia MÃssbauer e difraÃÃo de raios-x mostraram que, com o aumento do tempo de moagem, o Fe^{3+} entra na matriz dos compostos semicondutores substituindo o Ti^{4+}, Sn^{4+} ou In^{3+} em sÃtios octaÃdricos. Foi observado tambÃm a formaÃÃo de um sÃtio com deficiÃncia de oxigÃnio nos compostos finais que pode ser atribuÃdo ao processo e longos tempos de moagem ou ainda ao desbalanÃo estequiomÃtrico dos compostos precursores usados na moagem. / Magnetic semiconductors have attracted the attention of scientists in recent years due, mainly, to technological applications in the field of spintronics. These semiconductors can be developed by a process called doping, where some atoms of the semiconductor matrix are randomly replaced by magnetic atoms. This property enables the fabrication of a manifold of electronic devices from the same semiconductor material. In this work the technique of high energy mechanical milling was applied to synthesize TiO_2, SnO2 and In2O3 doped with Fe2O3. The samples were structurally characterized by x-ray diffraction and MÃssbauer spectroscopy. The synthesis was performed using three types of milling tools which influenced the outcome of the reactions. Compounds with formulas Sn(1-x)Fe(x)O2, Ti(1-x)Fe(x)O2 and {In(1-x)Fe(x)2}O3 were formed, with values of magnetic dopant concentration x of 2, 5 and 10% in atoms. Samples of Fe2O3-doped TiO2 were processed using a jar of polyacetal and zirconia spheres. However, the energy produced by these tools was not sufficient to complete the formation of the compound. Samples of TiO2 doped with Fe2O3 were successfully produced using stainless steel jar and spheres. The final compound was find to be contaminated with metallic iron impurities from the tools used. Furthermore, these impurities contributed to the formation of another phase, ilmenite (FeTiO3). To achieve purity, the samples were HCl washed for removal of metallic iron, but the phase related to ilmenite was not eliminated. Moreover, samples of Fe2O3-doped TiO2, SnO2, and In2O3 were synthesized using an alumina jar and zirconia spheres. These simples showed no undesirable impurities and no formation of other phases. While compounds based on TiO2 and SnO2 maintained their original crystalline structures, the compound {In(1-x)Fe(x)2O3} underwent a change in crystal phase, from its original cubic structure to a hexagonal corundum structure type. Results obtained from X-ray diffraction and MÃssbauer spectroscopy showed that with increasing milling time Fe^{3+} enters in the semiconductor matrices substituting Ti^{4+}, Sn^{4+} or In^{3+} in octahedral sites. It was also observed the formation of oxygen deficient sites in the final compounds that may be attributed to long milling times or to stoichiometric imbalance between the precursor compounds used in the milling processes.

Using photoelectron spectroscopy, we show measurements of energy level alignment of organic semiconducting layers. The main focus is on the properties and the influence of doped layers. The investigations on the p-doping process in organic semiconductors show typical charge carrier concentrations up to 2*10E20 cm-3. By a variation of the doping concentration, an over proportional influence on the position of the Fermi energy is observed. Comparing the number of charge carriers with the amount of dopants present in the layer, it is found that only 5% of the dopants undergo a full charge transfer. Furthermore, a detailed investigation of the density of states beyond the HOMO onset reveals that an exponentially decaying density of states reaches further into the band gap than commonly assumed. For an increasing amount of doping, the Fermi energy gets pinned on these states which suggests that a significant amount of charge carriers is present there. The investigation of metal top and bottom contacts aims at understanding the asymmetric current-voltage characteristics found for some symmetrically built device stacks. It can be shown that a reaction between the atoms from the top contact with the molecules of the layer leads to a change in energy level alignment that produces a 1.16eV lower electron injection barrier from the top. Further detailed investigations on such contacts show that the formation of a silver top contact is dominated by diffusion processes, leading to a broadened interface. However, upon insertion of a thin aluminum interlayer this diffusion can be stopped and an abrupt interface is achieved. Furthermore, in the case of a thick silver top contact, a monolayer of molecules is found to float on top of the metal layer, almost independent on the metal layer thickness. Finally, several device stacks are investigated, regarding interface dipoles, formation of depletion regions, energy alignment in mixed layers, and the influence of the built-in voltage. We show schematic energy level alignments of pn junctions, pin homojunctions, more complex pin heterojunctions with Zener-diode characteristics, as well as a complete OLED stack. The results allow a deeper insight in the working principle of such devices. / Mit Hilfe der Photoelektronenspektroskopie werden in der vorliegenden Arbeit Energieniveaus an Grenzflächen von organischen Halbleitern untersucht, wobei ein Hauptaugenmerk auf dem Einfluss und den Eigenschaften dotierter Schichten liegt. Bei der Untersuchung grundlegender Eigenschaften eines p-dotierten organischen Halbleiters können Ladungsträgerkonzentrationen bis zu 2*10E20 cm-3 nachgewiesen werden. Eine Variation der Dotierkonzentration zeigt einen überproportionalen Einfluss der Ladungsträger auf die Position des Ferminiveaus verglichen mit Experimenten an anorganischen Schichten. Durch den Vergleich mit der Anzahl Dotanden in der Schicht kann gezeigt werden, dass dabei nur etwa 5% der Dotanden einen vollständigen Ladungstransfer eingehen. Eine detaillierte Untersuchungen der Zustandsdichte jenseits des HOMOs (Highest Occupied Molecular Orbital) zeigt, dass die exponentiell abfallende Flanke der Zustandsdichte weiter in die Bandlücke hineinreicht als üblicherweise angenommen. Das Ferminiveau erfährt bei steigender Dotierung ein Pinning an diesen Zuständen, was für eine signifikante Ladungsträgerkonzentration spricht. Weiterhin wurden Untersuchungen zu Metal Top- und Grundkontakten durchgeführt. Es kann gezeigt werden, dass die Ursache für die Entstehung unsymmetrischer Strom-Spannungskurven, trotz eines symmetrischen Probenaufbaus, an einer Reaktion zwischen dem Molekül und den Metallatomen liegt. Dadurch entsteht eine um 1.16eV reduzierte Injektionsbarriere für Elektronen am Topkontakt. Weitere detaillierte Untersuchungen an diesen Topkontakten zeigen, dass im Falle von Silber als Metall diese Grenzfläche von Diffusionsprozessen dominiert ist. Im Gegensatz dazu zeigt das unedle Metall Aluminium keine Diffusion und führt zu abrupten Grenzflächen. Im ersten Fall kann zudem eine Monolage vom Molekül auf dem Metallkontakt nachgewiesen werden, die unabhängig von der Metalldicke aufschwimmt. Zuletzt werden Bauelemente oder Teile solcher mit Photoelektronenspektroskopie vermessen. Hierbei werden die Grenzflächendipole, die Ausbildung von Verarmungszonen, die Energieangleichung in Mischschichten und der Einfluss der Eingebauten Spannung untersucht. Es können die Banddiagramme von pn-Übergängen, einfachen pin Homoübergängen, komplexeren pin Heteroübergänge mit Zener-Dioden Verhalten sowie eine gesamte OLED gezeigt werden. Die Ergebnisse erlauben einen tieferen Einblick in die Arbeitsweise solcher Bauelemente.

In this report, the operation of a normally-off vertical gallium nitride (GaN) metal-oxide field effect transistor with a threshold voltage of 5 V is demonstrated. A crucial step during device fabrication is the formation of the highly n-doped source layer. The authors infer that the use of molecular beam epitaxy (MBE) is highly beneficial for suppressing diffusion of the magnesium (Mg) p-type dopants from the body layer grown by metal-organic vapor phase epitaxy into the source cap. Repassivation of the previously activated Mg acceptors by a hydrogen out-diffusion treatment is suppressed in the ultrahigh vacuum growth environment. Structural and electrical data indicate that the defect density of the GaN substrate is currently limiting device performance much more compared to other effects like varying surface morphology resulting from fluctuations in III/N stoichiometry during the MBE growth.

Amorphous Silicon(a-Si) films have attracted the attention of several investigators as it is an economical material for devices. One of the problems that is addressed is the doping of these films after they are prepared. In this thesis, we investigated the effects of doping amorphous Sil­icon films(prepared by r.f. sputtering) with Aluminium(Al) by ther­mal diffusion. Amorphous Silicon films have been prepared on glass substrates at optimal process parameters. Then, the a-Si films are coated with Al by vacuum evaporation and subjected to heating in N2 atmosphere in the temperature range 300°C to 600°C for different durations. After etching Al layer, it has been found that some of the films which are heated around 550°C contain filament like polycrystalline regions surrounding islands of a-Si. This structure has been confirmed through Scanning Electron Mi-croscope(SEM) photographs and electrical conductivity measurements. SEM photographs indicate that, bright regions of amorphous mate­rial are surrounded by dark regions of relatively higher conducting boundaries. The electrical conductivity study shows that there is sharp increase in conductivity of Al doped films, which is attributed to the conduct­ing polycrystalUne filament. A simple model has been proposed to explain the variation of con­ductivity of these transformed films, with process parameters and with temperature. Schottky barrier diodes have been fabricated using these trans­formed materials and their characteristics explained.