<p>Light emitting diode (LED)-based solid state displays (SSD) have attracted growing interest due to their advantages in terms of contrast ratio, brightness, viewing angle, and response time compared to liquid crystal displays. GaN based III-nitride thin film materials are suitable materials for SSD due to their wide and tunable bandgaps. However, the large size and costly manufacturing process of commercially available GaN-based LED chips limit the potential uses of LEDs as the pixels of SSD.</p> <p>In this work, tiny single crystal beta-phase (111) oriented SiC whiskers 2 microns in diameter and 18 microns in length are proposed as the substrates for GaN growth due to their small lattice constant mismatch (3%) with GaN, their conductive nature and their small size for potential use in SSD pixels. Aligned SiC whiskers with (111) planes exposed in an alumina matrix prepared by a precise manipulation and alignment method of SiC whiskers including a series of steps was developed in this work. The alignment degree of whiskers achieved in this work is higher than conventional extrusion methods, and a sintering approach capable of forming an aligned alumina/SiC composite was developed and understood using a self-limiting oxidation reaction mechanism.</p> <p>To take advantage of the potential versatility, scalability and cost effectiveness of sputtering for SSD manufacturing, a reactive sputtering system was built for a detailed investigation of GaN thin film growth nucleation and subsequent growth behavior on SiC. 6H-SiC single crystal substrates were chosen as a reference substrate for SiC whiskers. An XRRC indicates that a high quality single crystalline GaN thin film was successfully grown epitaxially on 6H-SiC by sputtering. Two-dimensional X-ray diffraction and scanning transmission electron microscopy results demonstrated that the epitaxial growth of GaN thin films relies on the short range order and/or crystalline area of the native oxide layer in GaN/SiC interface for the first time.</p> / Doctor of Philosophy (PhD)

Gallium nitride (GaN) and its related semiconductor alloys are attracting tremendous interest for their wide range of applications in blue and green LEDs, diode lasers, high-temperature and high-power electronics. Nanomaterials such as InGaN/GaN core-shell three-dimensional nanostructures are seen as a breakthrough technology for future solid-state lighting and nano-electronics devices. In a core-shell LED, the active semiconductor layers grown around a GaN core enable control over a wide range of wavelengths and applications. In this thesis the capability for the heteroepitaxial growth of a proof-of-principle core-shell LED is advanced. A design that can be applied at the wafer scale using metalorganic vapor phase epitaxy (MOVPE) crystal growth on highly uniform GaN nanorod (NR) structures is proposed. This project demonstrates understanding over the growth constraints of active layers and dopant layers. The impact of reactor pressure and temperature on the morphology and on the incorporated InN mole fraction was studied for thick InGaN shells on the different GaN crystal facets. Mg doping and effectiveness of the p-n junction for a core-shell structure was studied by extensive growth experiments and characterization. Sapphire and Si substrates were used, and at all the stages of growth and fabrication. The structures were optimized to achieve geometry homogeneity, high-aspect-ratio, incorporation homogeneity for InN and Mg dopant. The three-dimensional nature of NRs and their light emission provided ample challenges which required adaptation of characterization and fabrication techniques for a core-shell device. Finally, an electrically contacted core-shell LED is demonstrated and characterized. Achieving a proof-of-principle core-shell device could be the starting point in the development of nanostructure-based devices and new physics, or in solving technical problems in planar LEDs, such as the polarization of emitted light, the quantum-confined Stark effect, efficiency droop, or the green gap.

This dissertation describes the development of III-nitride (III-N) bipolar devices for optoelectronic and electronic applications. Research mainly involves device design, fabrication process development, and device characterization for Geiger-mode gallium nitride (GaN) deep-UV (DUV) p-i-n avalanche photodiodes (APDs), indium gallium nitride (InGaN)/GaN-based violet/blue laser diodes (LDs), and GaN/InGaN-based npn radio-frequency (RF) double-heterojunction bipolar transistors (DHBTs). All the epitaxial materials of these devices were grown in the Advanced Materials and Devices Group (AMDG) led by Prof. Russell D. Dupuis at the Georgia Institute of Technology using the metalorganic chemical vapor deposition (MOCVD) technique. Geiger-mode GaN p-i-n APDs have important applications in DUV and UV single-photon detections. In the fabrication of GaN p-i-n APDs, the major technical challenge is the sidewall leakage current. To address this issue, two surface leakage reduction schemes have been developed: a wet-etching surface treatment technique to recover the dry-etching-induced surface damage, and a ledged structure to form a surface depletion layer to partially passivate the sidewall. The first Geiger-mode DUV GaN p-i-n APD on a free-standing (FS) c-plane GaN substrate has been demonstrated. InGaN/GaN-based violet/blue/green LDs are the coherent light sources for high-density optical storage systems and the next-generation full-color LD display systems. The design of InGaN/GaN LDs has several challenges, such as the quantum-confined stark effect (QCSE), the efficiency droop issue, and the optical confinement design optimization. In this dissertation, a step-graded electron-blocking layer (EBL) is studied to address the efficiency droop issue. Enhanced internal quantum efficiency (ɳi) has been observed on 420-nm InGaN/GaN-based LDs. Moreover, an InGaN waveguide design is implemented, and the continuous-wave (CW)-mode operation on 460-nm InGaN/GaN-based LDs is achieved at room temperature (RT). III-N HBTs are promising devices for the next-generation RF and power electronics because of their advantages of high breakdown voltages, high power handling capability, and high-temperature and harsh-environment operation stability. One of the major technical challenges to fabricate high-performance RF III-N HBTs is to suppress the base surface recombination current on the extrinsic base region. The wet-etching surface treatment has also been employed to lower the surface recombination current. As a result, a record small-signal current gain (hfe) > 100 is achieved on GaN/InGaN-based npn DHBTs on sapphire substrates. A cut-off frequency (fT) > 5.3 GHz and a maximum oscillation frequency (fmax) > 1.3 GHz are also demonstrated for the first time. Furthermore, A FS c-plane GaN substrate with low epitaxial defect density and good thermal dissipation ability is used for reduced base bulk recombination current. The hfe > 115, collector current density (JC) > 141 kA/cm², and power density > 3.05 MW/cm² are achieved at RT, which are all the highest values reported ever on III-N HBTs.

abstract: This dissertation presents research findings regarding the exploitation of localized surface plasmon (LSP) of epitaxial Ag islands as a means to enhance the photoluminescence (PL) of Germanium (Ge) quantum dots (QDs). The first step of this project was to investigate the growth of Ag islands on Si(100). Two distinct families of Ag islands have been observed. “Big islands” are clearly faceted and have basal dimensions in the few hundred nm to μm range with a variety of basal shapes. “Small islands” are not clearly faceted and have basal diameters in the 10s of nm range. Big islands form via a nucleation and growth mechanism, and small islands form via precipitation of Ag contained in a planar layer between the big islands that is thicker than the Stranski-Krastanov layer existing at room-temperature. The pseudodielectric functions of epitaxial Ag islands on Si(100) substrates were investigated with spectroscopic ellipsometry. Comparing the experimental pseudodielectric functions obtained for Si with and without Ag islands clearly identifies a plasmon mode with its dipole moment perpendicular to the surface. This observation is confirmed using a simulation based on the thin island film (TIF) theory. Another mode parallel to the surface may be identified by comparing the experimental pseudodielectric functions with the simulated ones from TIF theory. Additional results suggest that the LSP energy of Ag islands can be tuned from the ultra-violet to the infrared range by an amorphous Si (α-Si) cap layer. Heterostructures were grown that incorporated Ge QDs, an epitaxial Si cap layer and Ag islands grown atop the Si cap layer. Optimum growth conditions for distinct Ge dot ensembles and Si cap layers were obtained. The density of Ag islands grown on the Si cap layer depends on its thickness. Factors contributing to this effect may include the average strain and Ge concentration on the surface of the Si cap layer. The effects of the Ag LSP on the PL of Ge coherent domes were investigated for both α-Si capped and bare Ag islands. For samples with low-doped substrates, the LSPs reduce the Ge dot-related PL when the Si cap layer is below some critical thickness and have no effect on the PL when the Si cap layer is above the critical thickness. For samples grown on highly-doped wafers, the LSP of bare Ag islands enhanced the PL of Ge QDs by ~ 40%. / Dissertation/Thesis / Doctoral Dissertation Physics 2015

Die vorliegende Arbeit befasst sich mit dem Einfluss einer hyperthermischen Stickstoffionenbestrahlung (Ekin < 25 eV) auf das Galliumnitrid-Schichtwachstum. Dabei wird insbesondere der Einfluss einer Oberflächenrekonstruktion, einer Strukturierung der Oberfläche, einer Zwischenschicht (Pufferschicht) und der Einfluss verschiedener Siliziumsubstratorientierungen auf das epitaktische Wachstum von dünnen Galliumnitrid-Schichten nach einer hyperthermischen Stickstoffionenbestrahlung diskutiert. Ziel war es, möglichst dünne, epitaktische und defektarme Galliumnitrid-Schichten zu erhalten. Für die Charakterisierung der Galliumnitrid-Schichten und der Siliziumsubstrate standen diverse Analysemethoden zur Verfügung. Die kristalline Oberflächenstruktur konnte während des Wachstums mittels Reflexionsbeugung hochenergetischer Elektronen beobachtet werden. Nachfolgend wurde die Oberflächentopografie, die kristalline Struktur und Textur, sowie die optischen Eigenschaften der Galliumnitrid-Schichten mittels Rasterkraftmikroskopie, Röntgenstrahl-Diffraktometrie, hochauflösender Transmissionselektronenmikroskopie und Photolumineszenzspektroskopie untersucht.

A novel process for low-temperature (LT) epitaxial growth of silicon carbide (SiC) by replacing the growth precursor propane with chloro-methane was recently developed at Mississippi State University. However, only limited information was available about the defects and impurity incorporation in the various types of epitaxial layers produced by this new method like blanket epitaxial layers, selectively grown epitaxial mesas, and highly doped epitaxial layers, prior to their comprehensive characterization in this work. Molten potassium hydroxide (KOH) etching, mechanical polishing and a variety of other characterizing techniques were used to delineate and identify the defects both in the epilayer and substrates. Under optimum growth conditions, the concentration of defects in the epitaxial layers was found to be less than that in the substrate, which established the good quality of the LT growth process. Defect concentrations, on selectively grown epitaxial layers, strongly depended on the crystallographic orientation of the mesa sidewall. The addition of HCl to the growth process, aimed at increasing the growth rate, caused a significant concentration of triangular defects (TDs) to be formed in the epitaxial layers. The TDs were traced down to the substrate by a combination of repeated polishing and molten KOH etching steps. The TDs were found not to originate from any substrate defects. Their origin was traced to polycrystalline silicon islands which form on the surface during growth and subsequently get evaporated away, which had made it impossible to detect them and suspect their influence on the TD generation prior to this work. The TDs were found to include single or multiple stacking faults bound by partial dislocations and, in some cases, inclusions of other SiC polytypes. Gradual degradation of the epitaxial morphology was found in heavily aluminum doped p+ layers, with an increase in the level of doping, followed by much steeper degradation when approaching the solubility limit of Al in 4H-SiC. Precipitates were the dominating defect at the highest levels of doping and were observed beyond a doping of 3.5x1020 cm-3. A dislocation generation model for heavily doped epitaxial layers was developed accounting for the stress in the lattice caused by Al doping.

No description available.

[pt] Se faz um estudo detalhado sobre o crescimento de GaInNAs pela técnica de metalorganic vapor phase epitaxy (MOVPE) com o objetivo de diminuir a contaminação residual do carbono no material. Para isso se pesquisa a influência dos precursores assim como dos diferentes parâmetros de crescimento na morfologia, na contaminação residual e na incorporação de diferentes elementos nos materiais. A temperatura, a taxa de crescimento, a razão As/III, o conteúdo do nitrogênio e os diferentes precursores são os parâmetros alterados de uma amostra para a outra. Como alguns precursores não foram usados antes para este material semicondutor, inicialmente apenas o GaAs foi examinado, passando posteriormente aos materiais ternários GaInAs e GaNAs, para finalmente obter o GaInNAs. Através da caracterização das amostras obtém-se a qualidade dos materiais assim como a contaminação residual do carbono. São utilizadas técnicas de difração de raios-X de alta resolução (HR-XRD), microscopia de força atômica (AFM), perfil eletroquímico de capacitância-tensão (ECV), espectrometria de massa de íons secundários (SIMS), fotoluminescência (PL) e técnicas in-situ próprias do reator MOVPE para avaliar o efeito dos parâmetros de crescimento epitaxial sobre a qualidade das estruturas obtidas, assim como sobre a incorporação dos diferentes elementos nos materiais. O crescimento do GaInNAs no reator CRIUS foi bem sucedido com algumas combinações dos precursores. Se confirmou que o alto nível do carbono está relacionado à quantidade do nitrogênio no cristal e que o carbono é fornecido pelos grupos metil dos precursores, principalmente pelo TMGa, seguido do TMIn. Todas as medidas para reduzir a incorporação residual do carbono foram bem sucedidas. O uso de precursores do grupo III sem grupo metil reduz significativamente a dopagem residual do carbono. Finalmente foram crescidas células solares para avaliar o material no dispositivo. / [en] A detailed investigation the GaInNAs growth by metalorganc vapor phase epitaxy (MOVPE) is performed in order to reduce the carbon background in the material. For this, the precursors , as well as the different growth parameters influence on crystal morphology, carbon background and the incorporation of different elements on the semiconductor material, are investigated. The temperature, the growth rate, the As/III ratio, the nitrogen content, and the different precursors were varied from one sample to another. Particularly, since some precursors were never used for this semiconductor material, initially, only GaAs was examined, later the GaInAs and the GaNAs ternary materials were grown, to finally obtain the GaInNAs. The samples characterization was used to assess materials quality, as well as the carbon background incorporation. Different characterization techniques such as High-Resolution X-Ray Diffraction (HR-XRD), Atomic Force Microscopy (AFM), Electrochemical Capacitance-Voltage (ECV) and In-Situ measurements were used to evaluate the effect of the epitaxial growth parameters on the quality of the obtained structures, as well as on the different elements incorporation in the semiconductor material. GaInNAs was successfully grown in the MOVPE reactor with particular precursors combinations. The relation between the high carbon level and the nitrogen amount in the crystal was confirmed, as well as the fact that carbon is supplied by methyl-groups of the precursors, especially TMGa, followed by TMIn. All measures to reduce carbon background incorporation were successful. The use of methyl group free III precursors significantly reduces the carbon background incorporation. Finally, solar cells were grown to evaluate the material in the device.

L’intégration de matériaux ferromagnétiques dans des hétérostructures semi-conductrices offre aujourd'hui de nouvelles perspectives dans le domaine de l’électronique de spin. Dans ce manuscrit sont présentés les résultats de la croissance par épitaxie par jets moléculaires d’hétérostructures de Mn5Ge3 sur Ge(111). Le Mn5Ge3 est un composé ferromagnétique jusqu'à température ambiante qui a l’avantage de pouvoir s’intégrer directement au Ge, semiconducteur du groupe IV. S'agissant d'un matériau relativement nouveau, un des efforts majeurs a porté sur la maîtrise de la croissance des couches minces de Mn5Ge3 par la technique d'épitaxie en phase solide (SPE). Un fort accent a été mis sur les caractérisations structurales, la détermination des relations d'épitaxie avec le Ge(111), afin de les relier aux propriétés magnétiques des films. La seconde partie de ce travail a été consacrée à l'étude des processus cinétiques d'incorporation de carbone dans les couches minces de Mn5Ge3. La combinaison des différents moyens de caractérisations structurales et magnétiques a permis d'aboutir à une augmentation notable de la température de Curie tout en conservant une excellente qualité structurale de la couche et de l'interface avec le Ge et une stabilité thermique jusqu’à 850°C. Tous ces résultats indiquent que les couches minces de Mn5Ge3épitaxiées sur Ge(111) apparaissent comme des candidats à fort potentiel pour l'injection de spin dans les semi-conducteurs du groupe IV / Spin-electronics based on ferromagnetic metal/semiconductor systems offer a pathway toward integration of information storage and processing in a single material. This emerging fieldaims to create a new generation of electronic devices where two degrees of freedom will be associated: spin and charge of carriers. In this context, the outcome of this thesis is toelaborate a novel ferromagnetic compound, namely Mn5Ge3, on Ge using molecular beamepitaxy method. The interests in this compound are manyfold: it can be stabilized as a uniquephase on Ge(111) in the form of epitaxial thin films, it is ferromagnetic until room temperature and it is compatible with Si-based conventional microelectronics. In this work,one major effort was devoted to the epitaxial growth of Mn5Ge3 on Ge using Solid PhaseEpitaxy method. By combining structural and magnetic characterizations, we demonstrated high quality epitaxial thin Mn5Ge3 films with good magnetic properties. We also studied theeffect carbon incorporation on the structural and magnetic properties of epitaxial Mn5Ge3films. The carbon-doped films exhibit a high Curie temperature with an atomically smoothinterface and a high thermal stability. All these results show that Mn5Ge3 is a promisingcandidate opening up the ways for spin injection via tunnel effect through the Schottky barrierinto Ge

Le développement des dispositifs issus de l'électronique de spin nécessite de nouveaux matériaux qui permettent d'injecter de manière efficace le courant polarisé en spin dans des semiconducteurs. Parmi de nombreux matériaux utilisés comme injecteurs de spin, les semiconducteurs ferromagnétiques dilués (DMS), obtenus en dopant des semiconducteurs avec des impuretés magnétiques tels que Mn ou Co, sont considérés comme des candidats potentiels pour l'injection de spin. Ces matériaux dopés deviennent ferromagnétiques tout en conservant leurs propriétés semiconductrices. Par conséquent, ils présentent une similarité d'impédance électrique par rapport aux substrats semiconducteurs, ce qui rend efficace l'injection de courant polarisé en spin dans ces derniers. Dans ce contexte, l'objectif principal de cette thèse consiste à étudier la cinétique de croissance des semiconducteurs ferromagnétiques dilués GeMn. Nous cherchons à déterminer les paramètres clés de la croissance des couches de GeMn, à savoir la température du substrat, et la concentration en Mn. Pour la fabrication de dispositifs électroniques fonctionnels, le challenge crucial est d'obtenir des DMS ayant une température de Curie (TC) bien supérieure à la température ambiante. Nous nous sommes donc concentrés sur la cinétique de formation de la phase nanocolonnaire GeMn possédant une TC au-delà de 400 K. / The development of active spintronic devices requires new materials, which enable to efficiently inject spin-polarized currents into non-magnetic semiconductors. Among numerous materials that can be used as spin injectors, diluted magnetic semiconductors (DMS), obtained by doping standard semiconductors with magnetic impurities, such as Mn or Co, have emerged as potential candidates for spin injection. The materials become ferromagnetic while conserving their semiconducting properties. They exhibit therefore natural impedance match to host semiconductors and are expected to efficiently inject spin-polarized currents into semiconductors. In this context, the main objectives of this thesis work consist in studying the growth kinetics of GeMn-based diluted magnetic semiconductors. We aim at determining the main growth parameters, such as the substrate temperature and the Mn concentration, that govern the growth process of GeMn layers. Since for device applications it is crucial to obtain DMS exhibiting a Curie temperature (TC) well above room temperature, we have focused our attention to the kinetic formation of the GeMn nanocolumn phase, which exhibits a Curie temperature higher than 400 K.