Universiẗat, Diss., 2005--Freiburg (Breisgau). / Erscheinungsjahr an der Haupttitelstelle: 2004.

We report on the characteristics of a ZnO based metal insulator semiconductor (MIS) diode comprised of a heterostructure of n-ZnO nanorods/n-GaN. The MIS structure consisted of unintentional - doped n type ZnO nanorods grown on n-GaN sample using hydrothermal synthesis at low temperature (100°). The ZnO nanorod layer was vertically grown from the GaN sample, having the diameter 100nm and length 2µm. Then, an insulator layer for electrical isolation was deposited on the top of ZnO nanorod layer by using spin coating method. A metal layer (gold) was finally deposited on the top. The I-V dependences show a rectifying diode like behavior with a leakage current of 2.10⁻⁵ A and a threshold voltage of about 3V. Depend on the thickness of the insulator, the I-V dependences of the n-ZnO/n-GaN heterostructure was varied from rectifying behavior to Ohmic and nearly linear. / Singapore-MIT Alliance (SMA)

La thèse porte sur cette couche de nitrure ou d’oxynitrure de silicium. Le déroulé prévoit l’élaboration des deux types de dépôts par voie gazeuse (par Chemical Vapor Infiltration CVI), la caractérisation de ces dépôts (par tous les moyens scientifiques à disposition et jugés utiles), ainsi que des essais en conditions proches de l’application visée (haute température, présence de phases liquides) pour juger de l’efficacité de ces dépôts et notamment effectuer une comparaison entre le nitrure et l’oxynitrure. Une étude complète des paramètres modifiables lors de l’élaboration et de leur effet sur la chimie (et par conséquent l’influence sur le comportement du matériau en conditions d’utilisation) représente le cœur du travail considéré. / Ceramics are usually used at high temperature because of their refractory nature. However, they are too brittle to be submitted to high stresses, such as in the rotating parts of aircraft engines. One way to reduce the brittleness of ceramics is to design them as composites. The fiber/matrix architecture displays a damageable character thanks to a suitable interfacial layer, which is deposited on the fiber cloths before the infiltration with molten silicon. The aim of the thesis is to propose and evaluate a solution to protect the fiber reinforcement during the impregnation step with liquid silicon. This solution involves the deposition of a protective layer made of silicon nitride or oxynitride.

Filmes finos de nitreto de silício com excesso de nitrogênio foram depositados sobre silício por sputtering reativo para obter estruturas emissoras de luz. As amostras foram modificadas por implantação iônica para verificar a influência dos dopantes arsênio (As) e boro (B) nos espectros de fotoluminescência (PL). As medidas de PL foram realizadas na faixa de temperatura entre 15-300 K e apresentaram uma emissão entre os comprimentos de onda 370-870 nm. Os dopantes introduziram uma emissão em 725 nm na banda de emissão, principalmente as dopadas com As. Foram realizadas medidas de microscopias para verificar a presença de nanoestruturas assim como a distribuição dos dopantes no material. As imagens de microscopias confirmaram a presença de nanocristais de nitreto de silício nas fases α, β e γ e identificaram a presença do dopante B nas fases cristalinas. O modelo de condução de Pool-Frenkel domina o transporte de portadores, indicando que a condução ocorre pelos níveis intrabandas, característica que definiu o modo que as recombinações radiativas ocorreram. As medidas de eletroluminescência (EL) apresentaram uma emissão centrada nos comprimentos de onda 760 e 880 nm (polarização negativa) e 1010 nm (polarização positiva) revelando diferenças significativas quando comparadas com as medidas de PL. Essa diferença esta associada à maneira como os elétrons populam os níveis intrabanda (excitação óptica para PL e elétrica para EL) que resulta em recombinações radiativas em diferentes comprimentos de ondas. A intensidade dos espectros de EL manifestou uma dependência quase linear com a densidade de corrente para ambas as polarizações. As medidas de EL em campos alternados exibiram um espectro de emissão composto pela soma das bandas obtidas separadamente em cada uma das polarizações. Medidas de EL em diferentes temperaturas (50-300 K) foram realizadas para investigar a influência da temperatura nos processos de recombinação radiativa. A intensidade exibiu uma redução com o aumento da temperatura, devido ao aumento do acoplamento elétron-fônon. / Silicon nitride with excess of nitrogen thin films were deposited on silicon substrate by reactive sputtering in order to obtain light emitting structures. Samples were modified by ion implantation of arsenic (As) and boron (B) to ascertain dopant leverage at photoluminescence (PL) spectra. PL measurements were performed at temperature ranging from 15 K up to 300 K and showed a band emission between wavelength 370 and 870 nm. An emission centered at 725 nm was observed in doped samples; especially in the presence of As. Microscope images showed crystalline structures of α-Si3N4, β-Si3N4 and γ-Si3N4 and confirmed boron dopant in nanocrystalline structures. Pool-Frenkel conduction model dominates electron transport in non-stoichiometric silicon nitride films due to intraband levels, phenomenon that has a huge contribution to electroluminescence (EL) emission. EL signals were composed by two peaks centered at 760 and 880 nm (negative bias – EL-N) and one peak at 1010 nm (positive bias – EL-P). Diffences between PL and EL spectra exhibit a clear dependence on the mode of excitation (photo and current source) on radiative recombination process. EL intensity had almost a linear increase with current density for both polarizations. EL measurements under AC voltage were composed by a superposition of the signals from EL-N and EL-P signals. Photo and electroluminescence measurements were collected at different temperatures (50 to 300 K) in order to investigate the temperature influence on the radiative recombination. The EL intensity was decreasing with temperature increasing, due to electron-phonon interactions.

Filmes finos de nitreto de silício com excesso de nitrogênio foram depositados sobre silício por sputtering reativo para obter estruturas emissoras de luz. As amostras foram modificadas por implantação iônica para verificar a influência dos dopantes arsênio (As) e boro (B) nos espectros de fotoluminescência (PL). As medidas de PL foram realizadas na faixa de temperatura entre 15-300 K e apresentaram uma emissão entre os comprimentos de onda 370-870 nm. Os dopantes introduziram uma emissão em 725 nm na banda de emissão, principalmente as dopadas com As. Foram realizadas medidas de microscopias para verificar a presença de nanoestruturas assim como a distribuição dos dopantes no material. As imagens de microscopias confirmaram a presença de nanocristais de nitreto de silício nas fases α, β e γ e identificaram a presença do dopante B nas fases cristalinas. O modelo de condução de Pool-Frenkel domina o transporte de portadores, indicando que a condução ocorre pelos níveis intrabandas, característica que definiu o modo que as recombinações radiativas ocorreram. As medidas de eletroluminescência (EL) apresentaram uma emissão centrada nos comprimentos de onda 760 e 880 nm (polarização negativa) e 1010 nm (polarização positiva) revelando diferenças significativas quando comparadas com as medidas de PL. Essa diferença esta associada à maneira como os elétrons populam os níveis intrabanda (excitação óptica para PL e elétrica para EL) que resulta em recombinações radiativas em diferentes comprimentos de ondas. A intensidade dos espectros de EL manifestou uma dependência quase linear com a densidade de corrente para ambas as polarizações. As medidas de EL em campos alternados exibiram um espectro de emissão composto pela soma das bandas obtidas separadamente em cada uma das polarizações. Medidas de EL em diferentes temperaturas (50-300 K) foram realizadas para investigar a influência da temperatura nos processos de recombinação radiativa. A intensidade exibiu uma redução com o aumento da temperatura, devido ao aumento do acoplamento elétron-fônon. / Silicon nitride with excess of nitrogen thin films were deposited on silicon substrate by reactive sputtering in order to obtain light emitting structures. Samples were modified by ion implantation of arsenic (As) and boron (B) to ascertain dopant leverage at photoluminescence (PL) spectra. PL measurements were performed at temperature ranging from 15 K up to 300 K and showed a band emission between wavelength 370 and 870 nm. An emission centered at 725 nm was observed in doped samples; especially in the presence of As. Microscope images showed crystalline structures of α-Si3N4, β-Si3N4 and γ-Si3N4 and confirmed boron dopant in nanocrystalline structures. Pool-Frenkel conduction model dominates electron transport in non-stoichiometric silicon nitride films due to intraband levels, phenomenon that has a huge contribution to electroluminescence (EL) emission. EL signals were composed by two peaks centered at 760 and 880 nm (negative bias – EL-N) and one peak at 1010 nm (positive bias – EL-P). Diffences between PL and EL spectra exhibit a clear dependence on the mode of excitation (photo and current source) on radiative recombination process. EL intensity had almost a linear increase with current density for both polarizations. EL measurements under AC voltage were composed by a superposition of the signals from EL-N and EL-P signals. Photo and electroluminescence measurements were collected at different temperatures (50 to 300 K) in order to investigate the temperature influence on the radiative recombination. The EL intensity was decreasing with temperature increasing, due to electron-phonon interactions.

Four silicon nitride powder blends vary in starting powder characteristics, glass chemistry, and phase composition. This work focuses on how these properties influence densification behavior, microstructural development, and the resulting mechanical performance of dense ceramics. Previous work completed on alpha-rich, low oxide containing (8 wt%), and fine silicon nitride powder (GS-44) showed high hardness equiaxed with grained ceramic. GS-44 served as an excellent precursor for the matrix phase material in graphene reinforced composites, which resulted in 235% increase in toughness and high hardness retention [1] with the addition of 1.5 vol% graphene. As the GS-44 powder is no longer in production, investigative work into other commercial powders and customization of powder blends was initiated. Commercial blends were selected based on availability, high alpha content, fine particle size, and additive chemistry (Al2O3, MgO, and Y2O3). The objective was to understand which powder characteristics led to a ceramic design that contained high hardness, strength, and toughness properties in order to increase the use of silicon nitride in extreme temperature environments. One such example is aerospace and structural applications that require a high-performance material that is lightweight and good thermal stability. Strong covalent bonding in silicon nitride make densification of powders extremely difficult; thereby, sintering additives are necessary to promote liquid phase sintering processes. Compaction of ceramic powders was carried out using a spark plasma sintering (SPS) furnace by utilizing a pulsed direct current through a conductive graphite die that encapsulates the sample powder. SPS was preferred over other conventional sintering methods owing to its high heating rate and short dwell times at the sintering target temperature. Thus, SPS provides superior control for tailoring the final silicon nitride properties by producing a hard alpha-phase and tough beta-phase microstructures. The custom blend developed had an appreciable amount of media wear included during the milling process that increased the additive content. Development of the custom blend was used to understand the effect of a larger additive content. Commercial GS-44 blend was used as the control to track the effect of adjusting specific surface area and oxide content in silicon nitride powder systems (HCS-M, C-R3, and UA-SN). The mechanical results for the four matrix systems, showed that toughness increased with grain coarsening and minimization of alumina content in beta silicon nitride. Based on these findings it is important to determine tradeoffs (i.e. balance of high hardness, toughness, and strength) to engineer an optimal ceramic that can be used for structural and aerospace applications.

Doctor of Philosophy / Department of Chemistry / Eric A. Maatta / Polyoxometalates have attracted much attention over the last few decades and have been studied in a wide variety of fields such as catalysis, medicine, imaging, photochromism, and magnetic materials. While many of these systems are easy to prepare, the ability to functionalize polyoxometalates is an ongoing challenge. Two approaches used to functionalize polyoxometalates involve insertion of metal fragments into a lacunary polyoxometalate or the direct replacement of terminal oxo ligands with the isoelectronic organoimido ligand. This process has been proven successful in many cases and with a wide variety of organoimido compounds. One of our group’s goals has been to synthesize a functionalized hexamolybdate species that is capable of metal coordination. However, previous results have been hindered because the electron withdrawing effect of the cluster is transmitted to the metal binding sites. In order to combat the electron withdrawing effect of the cluster, 4-amino piperidine dithiocarbamate ligands, which have no conjugation in the ring and are capable of metal binding, have been synthesized and characterized. A series of transition metal complexes have been made and a single crystal has been grown of a nickel(II) complex. Attempts to attach these species to clusters are described. Imido hexamolybdate clusters have been functionalized with styryl and iodophenyl groups. The styrene functionalized hexamolybdate was copolymerized with 4- chloromethylstyrene in moderate yields. This copolymer has the capabilities for further substitution at the chloromethyl group. The iodophenyl functionalized hexamolybdate was fully characterized including a single-crystal X-ray structural determination. This functionalized hexamolybdate can be used in carbon-carbon bond formation through coupling reactions. A chromium(V) nitrido polyoxometalate has been synthesized from a lacunary Keggin precursor and characterized. This nitrido species shows promise as a nitrogen transfer agent. Likewise, this nitrido species could be an entry point to other derivatives through reactions with various nucleophiles and electrophiles.

Optoelectronic devices fabricated from nitride semiconductors include blue and green light emitting diodes (LEDs) and laser diodes (LDs). To design efficient devices, the structure and composition of the constituent materials must be well-characterised. Traditional microscopy techniques used to examine nitride semiconductors include transmission electron microscopy (TEM), and atomic force microscopy (AFM). This thesis describes the study of nitride semiconductor materials using these traditional methods, as well as atom probe tomography (APT), a technique more usually applied to metals that provides three-dimensional (3D) compositional information at the atomic scale. By using both APT and correlative microscopy techniques, a more complete understanding of the material can be gained, which can potentially lead to higher-efficiency, longer-lasting devices. Defects, such as threading dislocations (TDs), can harm device performance. An AFM-based technique was used to show that TDs affect the local electrical properties of nitride materials. To investigate any compositional changes around the TD, APT studies of TDs were attempted, and evidence for oxygen enrichment near the TD was observed. The dopant level in nitride devices also affects their optoelectronic properties, and the combination of APT and TEM was used to show that Mg dopants were preferentially incorporated into pyramidal inversion domains, with a Mg content two orders of magnitude above the background level. Much debate has been focused on the microstructural origin of charge carrier localisation in InGaN. Alloy inhomogeneities have often been suggested to provide this localisation, yet APT has revealed InGaN quantum wells to be a statistically random alloy. Electron beam irradiation in the TEM caused damage to the InGaN, however, and a statistically significant deviation from a random alloy distribution was then observed by APT. The alloy homogeneity of InAlN was also studied, and this alloy system provided a unique opportunity to study gallium implantation damage to the APT sample caused during sample preparation by the focused ion beam (FIB). The combination of APT with traditional microscopy techniques made it possible to achieve a thorough understanding of a wide variety of nitride semiconductor materials.

The present work focuses on the growth and characterizations of GaN and InN layers and nanostructures on p-Si(100) and p-Si(111) substrates by plasma-assisted molecular beam epitaxy and the studies of GaN/p-Si and InN/p-Si heterojunctions properties. The thesis is divided in to seven different chapters. Chapter 1 gives a brief introduction on III-nitride materials, growth systems, substrates, possible device applications and technical background. Chapter 2 deals with experimental techniques including the details of PAMBE system used in the present work and characterization tools for III-nitride epitaxial layers as well as nanostructures. Chapter 3 involves the growth of GaN films on p-Si(100) and p-Si(111) substrates. Phase pure wurtzite GaN films are grown on Si (100) substrates by introducing a silicon nitride layer followed by low temperature GaN growth as buffer layers. GaN films grown directly on Si (100) are found to be phase mixtured, containing both cubic and hexagonal modifications. The x-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy studies reveal that the significant enhancement in the structural and optical properties of GaN films grown with silicon nitride buffer layer grown at 800 oC, when compared to the samples grown in the absence of silicon nitride buffer layer and with silicon nitride buffer layer grown at 600 oC. Core-level photoelectron spectroscopy of SixNy layers reveals the sources for superior qualities of GaN epilayers grown with the high temperature substrate nitridation process. The discussion has been carried out on the typical inverted rectification behavior exhibited by n-GaN/p-Si heterojunctions. Considerable modulation in the transport mechanism is observed with the nitridation conditions. The heterojunction fabricated with the sample of substrate nitridation at high temperature exhibites superior rectifying nature with reduced trap concentrations. Lowest ideality factors (~1.5) are observed in the heterojunctions grown with high temperature substrate nitridation which is attributed to the recombination tunneling at the space charge region transport mechanism at lower voltages and at higher voltages space charge limited current conduction is the dominating transport mechanism. Whereas, thermally generated carrier tunneling and recombination tunneling are the dominating transport mechanisms in the heterojunctions grown without substrate nitridation and low temperature substrate nitridation, respectively. A brief comparison of the structural, optical and heterojunction properties of GaN grown on Si(100) and Si(111) has been carried out. Chapter 4 involves the growth and characterizations of InN nanostructures and thinfilms on p-Si(100) and p-Si(111) substrates. InN QDs are grown on Si(100) at different densities. The PL characteristics of InN QDs are studied. A deterioration process of InN QDs, caused by the oxygen incorporation into the InN lattice and formation of In2O3/InN composite structures was established from the results of TEM, XPS and PL studies. The results confirm the partial oxidation of the outer shell of the InN QDs, while the inner core of the QDs remains unoxidized. InN nanorods are grown on p-Si(100), structural characterizations are carried out by SEM, and TEM. InN nanodots are grown on p-Si(100), structural characterizations are performed. InN films were grown on Si(100) and Si(111) substrates and structural characterizations are carried out. Chapter 5 deals with the the heterojunction properties of InN/p-Si(100) and InN/p-Si(111).The transport behavior of the InN NDs/p-Si(100) diodes is studied at various bias voltages and temperatures. The temperature dependent ZB BH and ideality factors of the forward I-V data are observed, while it is governed through the modified Richardson’s plot. The difference in FB BH and C-V BH and the deviation of ideality factor from unity indicate the presence of inhomogeneities at the interface. The band offsets derived from C-V measurements are found to be Δ EC=1.8 eV and Δ EV =1.3 eV, which are in close agreement with Anderson’s model. The band offsets of InN/p-Si heterojunctions are estimated using XPS data. A type-III band alignment with a valence band offset of Δ EV =1.39 eV and conduction band offset of ΔEC=1.81 eV is identified. The charge neutrality level model provides a reasonable description of the band alignment of the InN/p-Si interface. The interface dipole deduced by comparison with the electron affinity model is 0.06 eV. The transport studies of InN NR/p-Si(100) heterojunctions have been carried out by conductive atomic force microscopy (CAFM) as well as conventional large area contacts. Discussion of the electrical properties has been carried out based on local current-voltage (I-V) curves, as well as on the 2D conductance maps. The comparative studies on transport properties of diodes fabricated with InN NRs and NDs grown on p-Si(100) substrates and InN thin films grown on p-Si(111) substrates have also been carried out. Chapter 6 deals with the growth and characterizations of InN/GaN heterostructures on p-Si(100) and p-Si(111) substarets and also on the InN/GaN/p-Si heterojunction properties. The X-ray diffraction (XRD), scanning electron microscopy (SEM) studies reveal a considerable variation in crystalline quality of InN with grown parameters. Deterioration in the rectifying nature is observed in the case of InN/GaN/p-Si(100) heterojunction substrate when compared to InN/GaN/p-Si (111) due to the defect mediated tunneling effect, caused by the high defect concentration in the GaN and InN films grown on Si(100) and also due to the trap centers exist in the interfaces. Reduction in ideality factor is also observed in the case of n-InN/n-GaN/p–Si(111) when compared to n-InN/n-GaN/p–Si(100) heterojunction. The sum of the ideality factors of individual diodes is consistent with experimentally observed high ideality factors of n-InN/n-GaN/p–Si double heterojunctions due to double rectifying heterojunctions and metal semiconductor junctions. Variation of effective barrier heights and ideality factors with temperature are confirmed, which indicate the inhomogeneity in barrier height, might be due to various types of defects present at the GaN/Si and InN/GaN interfaces. The dependence of forward currents on both the voltage and temperatures are explained by multi step tunneling model and the activation energis were estimated to be 25meV and 100meV for n-InN/n-GaN/p–Si(100) and n-InN/n-GaN/p–Si(111) heterojunctions, respectively. Chapter 7 gives the summary of the present study and also discusses about future research directions in this area.

abstract: This dissertation covers my doctoral research on the cathodoluminescence (CL) study of the optical properties of III-niride semiconductors. The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high frequency applications. Compared to traditional Si-based devices, GaN-based devices offer superior breakdown properties, faster switching speed, and reduced system size. Some of the current device designs involve lateral p-n junctions which require selective-area doping. Dopant distribution in the selectively-doped regions is a critical issue that can impact the device performance. While most studies on Mg doping in GaN have been reported for epitaxial grown on flat c-plane substrates, questions arise regarding the Mg doping efficiency and uniformity in selectively-doped regions, where growth on surfaces etched away from the exact c-plane orientation is involved. Characterization of doping concentration distribution in lateral structures using secondary ion mass spectroscopy lacks the required spatial resolution. In this work, visualization of acceptor distribution in GaN:Mg epilayers grown by metalorganic chemical vapor deposition (MOCVD) was achieved at sub-micron scale using CL imaging. This was enabled by establishing a correlation among the luminescence characteristics, acceptor concentration, and electrical conductivity of GaN:Mg epilayers. Non-uniformity in acceptor distribution has been observed in epilayers grown on mesa structures and on miscut substrates. It is shown that non-basal-plane surfaces, such as mesa sidewalls and surface step clusters, promotes lateral growth along the GaN basal planes with a reduced Mg doping efficiency. The influence of surface morphology on the Mg doping efficiency in GaN has been studied. The second part of this thesis focuses on the optical properties of InGaN for photovoltaic applications. The effects of thermal annealing and low energy electron beam irradiation (LEEBI) on the optical properties of MOCVD-grown In0.14Ga0.86N films were studied. A multi-fold increase in luminescence intensity was observed after 800 °C thermal annealing or LEEBI treatment. The mechanism leading to the luminescence intensity increase has been discussed. This study shows procedures that significantly improve the luminescence efficiency of InGaN, which is important for InGaN-based optoelectronic devices. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020