Dissertations / Theses on the topic 'III-Nitride Materials'
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Kumaresan, Vishnuvarthan. "Novel substrates for growth of III-Nitride materials." Thesis, Paris 6, 2016. http://www.theses.fr/2016PA066538/document.
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Un des avantages majeurs des nanofils (NFs) semi-conducteurs est la possibilité d'intégrer ces nano-matériaux sur divers substrats. Cette perspective est particulièrement intéressante pour les nitrures d'éléments III qui manquent d'un substrat idéal. Nous avons étudié l'utilisation de nouveaux supports pour la croissance de NFs de GaN en épitaxie par jets moléculaires assistée par plasma. Nous avons exploré trois approches avec une caractéristique commune : le support de base est un substrat amorphe bas-coût. Pour deux d'entre elles, une fine couche d'un matériau cristallin est déposée sur ce support pour promouvoir la croissance épitaxiale des NFs. Dans la première approche, nous avons formé un film mince de Si poly-cristallin par "cristallisation induite par l'aluminium (AIC-Si)". Les conditions ont été optimisées pour obtenir une forte texture de fibre orientée [111] du film de Si qui nous a permis de faire croitre des NFs de GaN verticaux. La même idée a été mise en ¿uvre avec le graphène transféré sur SiOx. Nous avons montré pour la première fois dans la littérature que les NFs de GaN adoptent une orientation basale bien définie par rapport au graphène. La troisième approche consiste à faire croitre des NFs directement sur les substrats amorphes. Nous avons utilisé la silice thermique et la silice fondue. Nous avons examiné le temps de latence avant la formation des premiers germes et obtenu des NFs de GaN de bonne verticalité sur les deux types de silice. Sur la base de nos observations, nous concluons que la croissance épitaxiale de NFs de GaN sur graphène est particulièrement prometteuse pour le développement de dispositifs flexiblesA major advantage of semiconductor nanowires (NWs) is the possibility to integrate these nano-materials on various substrates. This perspective is particularly attractive for III-nitrides, for which there is a lack of an ideal substrate. We examined the use of novel templates for growing GaN NWs by plasma assisted molecular beam epitaxy. We explored three approaches with a common feature: the base support is a cost-efficient amorphous substrate and a thin crystalline material is deposited on the support to promote epitaxial growth of GaN NWs.In the first approach, we formed polycrystalline Si thin films on amorphous support by a process called aluminum-induced crystallization (AIC-Si). The conditions of this process were optimized to get a strong [111] fiber-texture of the Si film which enabled us to grow vertically oriented GaN NWs. The same idea was implemented with graphene as an ultimately thin crystalline material transferred on SiOx. We illustrated for the first time in literature that GaN NWs and the graphene layer have a single relative in-plane orientation. We propose a plausible epitaxial relationship and demonstrate that the number of graphene layers has a strong impact on GaN nucleation. Proof-of-concept for selective area growth of NWs is provided for these two approaches. As a simple approach, the possibility of growing NWs directly on amorphous substrates was explored. We use thermal silica and fused silica. Self-induced GaN NWs were formed with a good verticality on both substrates. Based on our observations, we conclude that the epitaxial growth of GaN NWs on graphene looks particularly promising for the development of flexible devices
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Kim, Kyounghoon. "Growth and characterization of III-nitride photonic materials /." Search for this dissertation online, 2004. http://wwwlib.umi.com/cr/ksu/main.
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Ren, Christopher Xiang. "Multi-microscopy characterisation of III-nitride devices and materials." Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/264158.
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III-nitride optoelectronic devices have become ubiquitous due to their ability to emit light efficiently in the blue and green spectral ranges. Specifically, III-nitride light emitting diodes (LEDs) have become widespread due to their high brightness and efficiency. However, III-nitride devices such as single photon sources are also the subject of research and are promising for various applications. In order to improve design efficient devices and improve current ones, the relationship between the structure of the constituent materials and their optical properties must be studied. The optical properties of materials are often examined by photoluminescence or cathodoluminescence, whilst traditional microscopy techniques such a transmission electron microscopy and scanning electron microscopy are used to elucidate their structure and composition. This thesis describes the use of a dual-beam focussed ion beam/scanning electron microscope (FIB/SEM) in bridging the gap between these two types of techniques and providing a platform on which to perform correlative studies between the optical and structural properties of III-nitride materials. The heteroepitaxial growth of III-nitrides has been known to produce high defect densities, which can harm device performance. We used this correlative approach to identify hexagonal defects as the source of inhomogeneous electroluminescence (EL) in LEDs. Hyperspectral EL mapping was used to show the local changes in the emission induced by the defects. Following this the FIB/SEM was used to prepare TEM samples from the apex of the defects, revealing the presence of p-doped material in the active region caused by the defect. APSYS simulations confirmed that the presence of p-doped material can enhance local EL. The deleterious effects of defects on the photoelectrochemical etching of cavities were also studied. We performed TEM analysis of an edge-defect contained in unetched material on the underside of a microdisk using FIB/SEM sample preparation methods. The roughness and morphology of microdisk and nanobeam cavities was studied using FIB-tomography (FIBT), demonstrating how the dual-beam instrument may be used to access the 3D morphology of cavities down to the resolution of the SEM and the slicing thickness of the FIB. This tomography approach was further extended with electron tomography studies of the nanobeam cavities, a technique which provided fewer issues in terms of image series alignment but also the presence of reconstruction artefacts which must be taken into account when quantitatively analysing the data. The use of correlative techniques was also used to establish the link between high Si content in an interlayer running along the length of microrods with changes in the optical emission of these rods. The combination of CL, FIB/SEM and TEM-based techniques has made it possible to gain a thorough understanding of the link between the structural and optical properties in a wide variety of III-nitride materials and devices.
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Zhang, Hengfang. "Hot-wall MOCVD of N-polar group-III nitride materials." Licentiate thesis, Linköpings universitet, Halvledarmaterial, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-175502.
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Group III-Nitride semiconductors: indium nitride (InN), gallium nitride (GaN), aluminum nitride (AlN) and their alloys continue to attract significant scientific interest due to their unique properties and diverse applications in photonic and electronic applications. Group-III nitrides have direct bandgaps which cover the entire spectral range from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN). This makes III-nitride materials suitable for high-efficient and energy-saving optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs). The Nobel Prize in Physics 2014 was awarded for the invention of efficient GaN blue LEDs, which further accelerated the research in the field of group III-nitride materials. GaN and related alloys are also suitable for high-temperature, high-power and high-frequency electronic devices with performance that cannot be delivered by other semiconductor technologies such as silicon (Si) and gallium arsenide (GaAs). For example, GaN-based high electron mobility transistors (HEMTs) have been widely adopted for radio frequency (RF) communication and power amplifiers, high-voltage power switches in radars, satellites, and wireless base stations for 5G. Recently, nitrogen (N)-polar group-III nitrides have drawn much attention due to their advantages over their metal-polar counterparts in e.g. HEMTs. These include feasibility to fabricate ohmic contacts with low resistance, an enhanced carrier confinement with a natural back barrier, and improved device scalability. Despite intensive research, the growth of micrometer-thick high-quality N-polar GaN based materials remains challenging. One of the major problems to develop device-quality N-polar nitrides is the high surface roughness, which results from the formation of hexagonal hillocks or step-bunching. Another significant hurdle is the unintentional polarity inversion, which reduces the crystalline quality and prohibits device fabrication. This licentiate thesis focuses on the development of N-polar AlN and GaN heterostructures on SiC substrates for HEMT RF applications. The overall aim is to exploit the advantages of the hot-wall MOCVD concept to grow high-quality N-polar HEMT structures for higher operational frequencies and improved device performance. In order to achieve this goal, special effort is dedicated to understanding the effects of growth conditions and substrate orientation on the structural properties and polarity of AlN, GaN and AlGaN grown by hot-wall MOCVD. N-polar AlN nucleation layers (NLs) with layer by layer growth mode and step-flow growth mode can be achieved on on-axis and 4_ offaxis SiC (000¯1), respectively, by carefully controlling V/III ratio and growth temperature. Utilizing scanning transmission electron microscopy (STEM) we have established a comprehensive picture of the atomic arrangements, local polarity and polarity evolution in AlN, GaN/AlN and AlGaN/GaN/AlN in the cases of low-temperature and high-temperature AlN NLs both for on-axis and off-axis substrates. We have shown that typically employed methods for polarity determination using potassium hydroxide wet etching could not provide conclusive results in the case of mixed-polar AlN as Al-polar domains may be easily over-etched and remain undetected. Atomic scale electron microscopy is therefore needed to accurately determine the polarity. We further have developed growth strategy and have optimized the epitaxial process for N-polar GaN, and have demonstrated high quality N-polar AlGaN/GaN/AlN heterostructures.Additional funding agencies: Chalmers University of technology; ABB; Ericsson; Epiluvac; FMV; Gotmic; Saab; SweGaN; UMS; Swedish Foundation for Strategic Research under Grants No. FL12-0181, No. RIF14-055, and No. EM16-0024; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No.2009- 00971.
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West, Allen M. "Effects of dislocations on electronic properties of III-nitride materials." [Gainesville, Fla.] : University of Florida, 2005. http://purl.fcla.edu/fcla/etd/UFE0009281.
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Bao, An. "Investigation on the properties of nanowire structures and hillocks of Group-III nitride materials." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/276187.
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Group-III nitride materials are increasingly important, because of their semiconducting properties and bandgaps tuneable across a wide range from the infrared to ultraviolet. They are of particular interest for optoelectronic and power electronic applications. The studies on nitride materials are comprehensive, and one way to categorise them is based on the scale of the material, namely: (a) 3D bulk materials, for example the development of 3D bulk nitride substrate; (b) epitaxial layers, for example GaN/InGaN 2D quantum well based light emitting diodes (LEDs); (c) 1D nitride nanowires and (d) 0D quantum dots, for example InGaN quantum dot based single photon sources. This thesis uses a multimicroscopy concept to investigate various group-III nitride nanowires and hillocks. Multiple different microscopy techniques were applied to the same specific nanostructure or defect. This allows the properties of the materials of interest to be linked directly to the nanostructures or defects, providing a more complete picture of the samples that have been studied. The multiple microscopy techniques used to conduct the work in this thesis include (scanning) transmission electron microscopy ((S)TEM), cathodoluminescence (CL), focused ion beam (FIB) and atomic force microscopy (AFM). Specifically, AFM was used to characterise the morphology of the sample on a sub-nanometer scale. The crystalline structures were characterised using (S)TEM, and the in-situ energy dispersive X-ray spectroscopy (EDS) was used to conduct compositional analysis of the selected sites. CL was used to reveal the optoelectronic properties by analysing the emission wavelengths of the materials, excited by the electron beam. FIB was the technique used to prepare site-specific samples to be measured in (S)TEM. A detailed explanation of these characterisation techniques was also included. In the context of the studies on nitride materials, nitride nanowires and their heterostructures are a particular research focus. They combine the unique properties of III-nitride materials together with the advantages induced by the nanowire geometry. This thesis explores three different nanowire systems: a GaN nanowire structure incorporating a GaN/Sc$_x$Ga$_{1-x}$N axial heterostructure grown by molecular beam epitaxy (MBE); GaN/InGaN core-shell nanowires fabricated by a hybrid approach combining metalorganic vapour phase epitaxy (MOVPE) and dry etching techniques; and AlGaN nanowires on free standing AlGaN substrates fabricated by MBE and inductively coupled plasma (ICP) etching. The optoelectronic properties, compositions and structures of these nanowires were studied in detail. Moreover, a comprehensive review on the properties, growth methods and applications of group-III nitride nanowires is also included in this thesis. Apart from nanowires, a lot of effort has been focusing on the improvement of the quality of epitaxial layers of GaN and its alloys, and they currently have an even wider perspective than nitride nanowires. The understanding of defects within the epitaxial layers is crucial in order to mitigate the their adverse effects, leading to the increased emphasis on defect analysis. Hillocks are a type of defects found on GaN epilayers, which are less well studied than other defects such as dislocations and stacking faults. As a consequence, the formation mechanisms of hillocks remain controversial. In this context, after a review on the past studies on GaN hillocks, this thesis also investigates two types of hillocks, i.e. hillocks on GaN p-i-n diodes and hillocks on GaN grown on patterned sapphire substrates (PSS). Their nanoscale structures, properties and formation mechanisms are studied.
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Eiting, Christopher James. "Growth of III-V nitride materials by MOCVD for device applications /." Digital version accessible at:, 1999. http://wwwlib.umi.com/cr/utexas/main.
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Crawford, Samuel Curtis. "Synthesis of III-V nitride nanowires with controlled structure, morphology, and composition." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/88370.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 173-182).
The III-V nitride materials system offers tunable electronic and optical properties that can be tailored for specific electronic and optoelectronic applications by varying the (In,Ga,Al)N alloy composition. While nitride thin films tend to suffer from high dislocation densities due to the lattice mismatch with growth substrates, nanowires can be grown dislocation-free on highly mismatched substrates including silicon. Furthermore, axial and radial junction configurations offer unique nanoscale device architectures that enable more optimal device design. In order to realize the potential benefits of III-V nitride nanowires, precise control of nanowire synthesis is required. This thesis describes the development of experimental techniques and theoretical models that guide the synthesis of Ill-V nitride and other compound semiconductor nanowires with control over material structure, morphology, and composition. First, GaN nanowires were synthesized with control over nanowire orientation, morphology, and defect density. Substrate orientation was used to control whether nanowires grew preferentially in the polar [0001] direction or the nonpolar [1-100] direction. Film deposition on the nanowire sidewalls was effectively minimized by reducing the Ga precursor flux and internanowire spacing. Using nonpolar-oriented GaN nanowires with uniform diameter, the diameter-dependent growth rate was modeled to demonstrate that growth is limited by nucleation at the perimeter of the seed/nanowire interface. Finally, Ni- and Au-seeded GaN nanowires were directly compared, and the higher growth rate and reduced defect density in Ni-seeded nanowires were consistent with a reduced seed/nanowire interfacial energy. Next, nonpolar-oriented InN/InGaN axial heterostructure nanowires were grown by introducing Ga precursors during InN nanowire growth. The formation of GaN shells placed an upper limit on the allowable Ga precursor flux. Shell deposition was minimized by operating at higher temperature and pressure. However, a reduction in the local supply of Ga to the seed particle also limited InGaN formation. Therefore, brief high-flux pulses were used at lower pressure to form InN/InGaN axial heterostructures with minimal shell formation. Electron tomography and energy dispersive X-ray spectroscopy were used to analyze the Ga-driven driven changes in nanowire morphology and composition, respectively. The reduction in nanowire diameter upon the introduction of Ga was found to be driven by changes in seed particle composition. A flow-controlled approach was developed to modulate the diameter along individual nanowires, which can enable unique properties including enhanced light trapping in nanowire arrays and increased phonon scattering in thermoelectrics. In InN nanowires, a reduction in V flow produced segments with larger diameters and slower growth rates. A reduction in III flow in GaN nanowires also produced segments with slower growth rates, but thinner diameters. These trends are a consequence of the separate pathways traveled by the III and V sources to the site of reaction, enabling control over the incorporation rate of III source into the seed particle and the extraction rate of III source out of the seed particle, respectively. Based on these promising results, models were developed to explore the potential for template-free nanowire diameter modulation via particle-mediated growth. The results from diameter-modulated InN and GaN nanowires were evaluated considering contributions of seed particle volume, wetting angle, and three-dimensional morphology to the observed diameter changes. To achieve large diameter ratios using liquid seed particles, significant changes in both seed particle volume and wetting angle are necessary. Furthermore, the model was used to evaluate the surface energy and morphology of the liquid/solid interface. The interface was found to not be flat, contrary to common assumptions, which has significant implications for nanowire growth models. Finally, we extended the flow-controlled diameter modulation approach to GaAs nanowires, demonstrating that the technique is generally applicable to particle-mediated compound semiconductor nanowires. Both the III and V sources were varied during growth, producing similar trends in diameter and growth rate as with III-V nitride nanowires. Notably, three different types of [111]B-oriented nanowires were observed and had discrete differences in diameter modulation, growth rate, and cross-sectional shape, which were attributed to differences in seed particle phase. By controlling growth conditions during nanowire nucleation, each of the three types of nanowires were preferentially produced, indicating that the seed particle phase can be controllably varied in compound-forming seed alloys. Together, these results provide a foundation for fabricating III-V nitride and other nanowires with control over material structure, morphology, and composition. Experimental techniques and theoretical models were developed that enable control over growth direction, tapering, growth rate, defect density, composition, and diameter. These tools are helpful in achieving nanowires with rationally tailored properties for functional nanowire-based devices.
by Samuel Curtis Crawford.
Ph. D.
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Nguyen, Hieu. "Molecular beam epitaxial growth, characterization and device applications of III-Nitride nanowire heterostructures." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=107905.
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Recently, group III-nitride nanowire heterostructures have been extensively investigated. Due to the effective lateral stress relaxation, such nanoscale heterostructures can be epitaxially grown on silicon or other foreign substrates and can exhibit drastically reduced dislocations and polarization fields, compared to their planar counterparts. This dissertation reports on the achievement of a new class of III-nitride nanoscale heterostructures, including InGaN/GaN dot-in-a-wires and nearly defect-free InN nanowires on a silicon platform. We have further developed a new generation of nanowire devices, including ultrahigh-efficiency full-color light emitting diodes (LEDs) and solar cells on a silicon platform.We have identified two major mechanisms, including poor hole transport and electron overflow, that severely limit the performance of GaN-based nanowire LEDs. With the incorporation of the special techniques of p-type modulation doping and AlGaN electron blocking layer in the dot-in-a-wire LED active region, we have demonstrated phosphor-free white-light LEDs that can exhibit, for the first time, internal quantum efficiency of > 50%, negligible efficiency droop up to ~ 2,000A/cm2, and extremely high stable emission characteristics at room temperature, which are ideally suited for future smart lighting and full-color display applications.We have also studied the epitaxial growth, fabrication and characterization of InN:Mg/i-InN/InN:Si nanowire axial structures on n-type Si(111) substrates and demonstrated the first InN nanowire solar cells. Under one-sun (AM 1.5G) illumination, the devices exhibit a short-circuit current density of ~ 14.4 mA/cm2, open circuit voltage of 0.14 V , fill factor of 34.0%, and energy conversion efficiency of 0.68%. This work opens up exciting possibilities for InGaN nanowire-based, full solar-spectrum third-generation solar cells.Récemment, les hétérostructures à base de nitride et de groupe III ont fait l'objet de recherches intensives. Grâce à la relaxation latérale effective du stress, de telles hétérostructures d'échelle nanométrique peuvent être déposés sur du Silicium ou d'autres substrats. Celles-ci démontrent une réduction dramatique des dislocations et des champs de polarisations comparativement à leurs contreparties planes. Cette dissertation rapporte l'accomplissement d'une nouvelle classe de matériau nanométrique, soit des hétérostructures III-nitride incluant InGaN/GaN point dans fils ainsi que des nanofils d'InN presque sans défauts sur du Silicium. De plus, nous avons développé une nouvelle génération de dispositifs à base de nanofils, incluant des diodes émettrices de lumière (LEDs) à efficacité ultra haute et spectre visible complet ainsi que des cellules solaires sur une gaufre de Silicium. Nous avons identifié 2 mécanismes majeurs, incluant le faible transport des trous et le surplus d'électrons, qui limitent sérieusement la performance des LEDs à base de nanofils de GaN. Avec l'ajout de certaines techniques spéciales de modulation de type p, et une couche bloquante d'électrons faite de AlGaN dans la région active de la LED point dans fil. Par ailleurs, nous avons démontré des LEDs blanche sans phosphore qui démontrent, pour la première fois, une efficacité quantique supérieure à 50% ainsi qu'une baisse d'efficacité négligeable jusqu'à ~ 2,000A/cm2 et des caractéristiques d'émissions très hautes et stables à température pièce. Celles-ci sont donc toutes désignées pour des applications d'illumination intelligentes et des écrans pleines couleurs. La croissance par épitaxie, la fabrication et la caractérisation des nanofils d'InN:Mg/i-InN/InN:Si axiaux sur des substrats de Si(111) de type n et démontré la première cellule solaire à base d'InN. Sous l'illumination d'un soleil (AM 1.5G), les dispositifs démontrent une densité de courant de ~ 14.4 mA/cm2 en court-circuit, un voltage de circuit ouvert de 0.14V, un facteur de remplissage de 34.0% et une efficacité de conversion d'énergie de 0.68%. Ce travail ouvre des portes excitantes pour des cellules solaires plein spectre de troisième génération à base de nanofils d'InGaN.
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Eriksson, Martin. "Photoluminescence Characteristics of III-Nitride Quantum Dots and Films." Doctoral thesis, Linköpings universitet, Halvledarmaterial, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-139766.
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III-Nitride semiconductors are very promising in both electronics and optical devices. The ability of the III-Nitride semiconductors as light emitters to span the electromagnetic spectrum from deep ultraviolet light, through the entire visible region, and into the infrared part of the spectrum, is a very important feature, making this material very important in the field of light emitting devices. In fact, the blue emission from Indium Gallium Nitride (InGaN), which was awarded the 2014 Nobel Prize in Physics, is the basis of the common and important white light emitting diode (LED). Quantum dots (QDs) have properties that make them very interesting for light emitting devices for a range of different applications, such as the possibility of increasing device efficiency. The spectrally well-defined emission from QDs also allows accurate color reproduction and high-performance communication devices. The small size of QDs, combined with selective area growth allows for an improved display resolution. By control of the polarization direction of QDs, they can be used in more efficient displays as well as in traditional communication devices. The possibility of sending out entangled photon pairs is another QD property of importance for quantum key distribution used for secure communication. QDs can hold different exciton complexes, such as the neutral single exciton, consisting of one electron and one hole, and the biexciton, consisting of two excitons. The integrated PL intensity of the biexciton exhibits a quadratic dependence with respect to the excitation power, as compared to the linear power dependence of the neutral single exciton. The lifetime of the neutral exciton is 880 ps, whereas the biexciton, consisting of twice the number of charge carriers and lacks a dark state, has a considerably shorter lifetime of only 500 ps. The ratio of the lifetimes is an indication that the size of the QD is in the order of the exciton Bohr radius of the InGaN crystal making up these QDs in the InGaN QW. A large part of the studies of this thesis has been focused on InGaN QDs on top of hexagonal Gallium Nitride (GaN) pyramids, selectively grown by Metal Organic Chemical Vapor Deposition (MOCVD). On top of the GaN pyramids, an InGaN layer and a GaN capping layer were grown. From structural and optical investigations, InGaN QDs have been characterized as growing on (0001) facets on truncated GaN pyramids. These QDs exhibit both narrow photoluminescence linewidths and are linearly polarized in directions following the symmetry of the pyramids. In this work, the neutral single exciton, and the more rare negatively charged exciton, have been investigated. At low excitation power, the integrated intensity of the PL peak of the neutral exciton increases linearly with the excitation power. The negatively charged exciton, on the other hand, exhibits a quadratic power dependence, just like that of the biexciton. Upon increasing the temperature, the power dependence of the negatively charged exciton changes to linear, just like the neutral exciton. This change in power dependence is explained in terms of electrons in potential traps close to the QD escaping by thermal excitation, leading to a surplus of electrons in the vicinity of the QD. Consequently, only a single exciton needs to be created by photoexcitation in order to form a negatively charged exciton, while the extra electron is supplied to the QD by thermal excitation. Upon a close inspection of the PL of the neutral exciton, a splitting of the peak of just below 0.4 meV is revealed. There is an observed competition in the integrated intensity between these two peaks, similar to that between an exciton and a biexciton. The high energy peak of this split exciton emission is explained in terms of a remotely charged exciton. This exciton state consists of a neutral single exciton in the QD with an extra electron or hole in close vicinity of the QD, which screens the built-in field in the QD. The InGaN QDs are very small; estimated to be on the order of the exciton Bohr radius of the InGaN crystal, or even smaller. The lifetimes of the neutral exciton and the negatively charged exciton are approximately 320 ps and 130 ps, respectively. The ratio of the lifetimes supports the claim of the QD size being on the order of the exciton Bohr radius or smaller, as is further supported by power dependence results. Under the assumption of a spherical QD, theoretical calculations predict an emission energy shift of 0.7 meV, for a peak at 3.09 eV, due to the built-in field for a QD with a diameter of 1.3 nm, in agreement with the experimental observations. Studying the InGaN QD PL from neutral and charged excitons at elevated temperatures (4 K to 166 K) has revealed that the QDs are surrounded by potential fluctuations that trap charge carriers with an energy of around 20 meV, to be compared with the exciton trapping energy in the QDs of approximately 50 meV. The confinement of electrons close to the QD is predicted to be smaller than for holes, which accounts for the negative charge of the charged exciton, and for the higher probability of capturing free electrons. We have estimated the lifetimes of free electrons and holes in the GaN barrier to be 45 ps and 60 ps, in consistence with excitons forming quickly in the barrier upon photoexcitation and that free electrons and holes get trapped quickly in local potential traps close to the QDs. This analysis also indicates that there is a probability of 35 % to have an electron in the QD between the photoexcitation pulses, in agreement with a lower than quadratic power dependence of the negatively charged exciton. InN is an attractive material due to its infrared emission, for applications such as light emitters for communication purposes, but it is more difficult to grow with high quality and low doping concentration as compared to GaN. QDs with a higher In-composition or even pure InN is an interesting prospect as being a route towards increased quantum confinement and room temperature device operation. For all optical devices, p-type doping is needed. Even nominally undoped InN samples tend to be heavily n-type doped, causing problems to make pn-junctions as needed for LEDs. In our work, we present Mg-doped p-type InN films, which when further increasing the Mg-concentration revert to n-type conductivity. We have focused on the effect of the Mg-doping on the light emission properties of these films. The low Mg doped InN film is inhomogeneous and is observed to contain areas with n-type conductivity, so called n-type pockets in the otherwise p-type InN film. A higher concentration of Mg results in a higher crystalline quality and the disappearance of the n-type pockets. The high crystalline quality has enabled us to determine the binding energy of the Mg dopants to 64 meV. Upon further increase of the Mg concentration, the film reverts to ntype conductivity. The highly Mg doped sample also exhibits a red-shifted emission with features that are interpreted as originating from Zinc-Blende inclusions in the Wurtzite InN crystal, acting as quantum wells. The Mg doping is an important factor in controlling the conductivity of InN, as well as its light emission properties, and ultimately construct InN-based devices. In summary, in this thesis, both pyramidal InGaN QDs and InGaN QDs in a QW have been investigated. Novel discoveries of exciton complexes in these QD systems have been reported. Knowledge has also been gained about the challenging material InN, including a study of the effect of the Mg-doping concentration on the semiconductor crystalline quality and its light emission properties. The outcome of this thesis enriches the knowledge of the III-Nitride semiconductor community, with the long-term objective to improve the device performance of III-Nitride based light emitting devices.
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Wang, Jingzhou. "Optical and Electrical Study of the Rare Earth Doped III-nitride Semiconductor Materials." Ohio University / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1478177382556951.
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Kent, Thomas Frederick. "III-Nitride Nanostructures for Optoelectronic and Magnetic Functionalities: Growth, Characterization and Engineering." The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1408564155.
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Sarwar, ATM Golam. "Extreme Band Engineering of III-Nitride Nanowire Heterostructures for Electronic and Photonic Application." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1451358029.
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Gong, Yipin. "MOCVD growth and optical investigation of III-nitride materials including non-polar and semi-polar GaN." Thesis, University of Sheffield, 2014. http://etheses.whiterose.ac.uk/4991/.
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This thesis focuses on the growth of high quality III-nitride materials by MOCVD along both c-direction and non-polar a-direction. High temperature AlN buffer layer is introduced, which can effectively reduce the dislocation density of the overlying layer. Stimulated emission at 340nm has been obtained by such AlN buffer techniques, which mechanism is understood by studying the strain relaxation in the QW structure with RSM measurements. A non-polar overgrowth approach has also been established for the growth of non-polar GaN, leading to an impressive improvement in the crystalline quality. Meanwhile, optical investigation on c-plane InGaN/GaN MQW nanorod structures has been performed, demonstrating an significantly enhanced photoluminescence emission. It has been concluded that the enhancement is due to the reduction on quantum confined Stark effect caused by strain relaxation in the MQW structure.
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Domènech, i. Amador Núria. "Phonons in III-nitride thinfilms, bulk and nanowires: a closer look into InN vibrational properties." Doctoral thesis, Universitat de Barcelona, 2015. http://hdl.handle.net/10803/348867.
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This thesis is devoted to the study of the interactions of phonons in indium nitride (InN) and materials of the (In,Ga)N system with wurtzite structure. For this purpose, we present Raman spectroscopy on nanowires (NWs), thin films and bulk samples, in order to adress phonon interactions in these materials. We also present Brillouin spectrsocopy measurements of InN thin films, from which a reliable set of elastic constants is proposed.
We have studied the phonon anharmonic interactions and phonon decay channels of InN, both in thin films and NWs. The temperature dependence of Raman peak width of all the phonon modes has been studied using a model that considers the contribution of three- and four-phonon processes, taking into account the phonon density of states obtained by ab-initio calculations.
In InN thin films, we find that the E2h phonon mode mainly decays through 4-phonon processes, whereas the extremely narrow E2l mode can decay only through up-conversion processes. The LO and the TO modes are found to decay through 3-phonon and 4-phonon interactions. In InN NWs we found the same phonon decay channels but phonon linewidths are significantly reduced, indicating a higher crystalline quality. The lifetimes of the phonon modes are derived from the measured phonon linewidths. The long-lived E2l phonon exhibits the largest lifetime, which is mainly limited by impurity scattering. We also study the anharmonic decay of high-frequency LVMs H complexes in Mg-doped InN, which can be explained by considering dephasing due to quasi-elastic acoustic phonon scattering.
We have discussed the relevant electronic resonances that affect Raman scattering in the (In,Ga)N system. We show that the optical excitation of the longitudinal optical modes in InN occurs via the Martin's double resonance both in InN layers and nanostructures, even though the defect density of the latter is significantly lower. By performing wavelength-dependent measurements on InN thin films and NWs, the A1(LO) and the E1(LO) wave-vector dispersion close to zone-center have been obtained. We have also studied the impurity-mediated cascade mechanism of multiphonons in InGaN layers. To ascertain the role of the impurities we have studied as-grown samples in comparison with He+-implanted InGaN layers. UV Raman-scattering measurements allow us to measure up to fifth order multiphonon scattering due to cascade mechanism. Relative multiphonon intensities depend on the indium concentration and implantation dose.
Finally, we have studied the LO-Phonon-Plasmon Coupled Modes (LOPCMs) in InN and GaN using the Lindhard-Mermin model. We have determined the electron density in undoped, Si-doped and Mg-doped NWs. We have also studied a bulk, ammonothermally-grown Si-doped GaN sample. No evidence of LOPCMs was detected in the Ga-polar face, probably due to the higher defect density existing in this sample sector. We have detected both branches of the LOPCMs in the N-polar face, and we have made a study of the distribution of the free charge density by means of confocal micro-Raman measurements.Aquesta tesi està dedicada a l’estudi de les interaccions dels fonons en nitrur d'indi (InN) i en semiconductors del sistema (In.Ga)N amb estructura wurtzita. Amb aquest objectiu es presenten estudis d'espectroscòpia Raman en capes primes, nanofils (NWs), i mostres bulk, que han permès abordar de manera global les interaccions dels fonons en aquests materials. Hem estudiat les interaccions anharmòniques i els canals de decaïment dels fonons de InN, tant en capes primes com en NWs. La dependència de l’amplada del pic Raman amb la temperatura de tots els modes fonònics s’ha estudiat utilitzant un model anharmònic que considera la contribució dels processos de tres i quatre fonons, i tenint en compte la densitat d'estats de fonons obtinguda mitjançant càlculs ab-initio. L'anàlisi dels temps de vida fonònics i de la dependència amb temperatura de les freqüències permet afirmar que els NWs tenen una estructura més relaxada que les capes primes. També hem estudiat el decaïment anharmònic de modes locals de vibració corresponents a complexos d'H en InN fortament dopat amb Mg. Hem estudiat les ressonàncies en el sistema (In,Ga)N i la influència de la densitat d’impureses en l’eficiència dels mecanismes ressonants. Hem demostrat que la dispersió Raman de modes òptics longitudinals en el InN es produeix a través de la doble ressonància del Martin tant en capes primer com en nanoestructures, tot i que la densitat de defectes d'aquestes últimes és significativament menor. Hem estudiat també el mecanisme de cascada mediat per impureses, a través del qual es produeix la dispersió de multifonons, en capes primes de InGaN amb diferent composició i diferent grau d’implantació d'ions d'He, i hem comprovat que les intensitats relatives dels multifonons depenen de la concentració d’indi i de la dosi de la implantació. Finalment, hem estudiat l’acoblament de fonons polars amb els plasmons mitjançant el model dielèctric de Lindhard-Mermin, amb la finalitat d’investigar la densitat d’electrons lliures utilitzant espectroscòpia Raman. Hem determinat la concentració d'electrons en NWs de InN sense dopar, dopats amb Si i dopats amb Mg. També hem fet un estudi de la distribució de la densitat de càrrega en una mostra de GaN ammonotermal mitjançant mesures de micro-Raman confocal.
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Choi, Suk. "Growth and characterization of III-nitride materials for high efficiency optoelectronic devices by metalorganic chemical vapor deposition." Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/45823.
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Efficiency droop is a critical issue for the Group III-nitride based light-emitting diodes (LEDs) to be competitive in the general lighting application. Carrier spill-over have been suggested as an origin of the efficiency droop, and an InAlN electron-blocking layer (EBL) is suggested as a replacement of the conventional AlGaN EBL for improved performance of LED. Optimum growth condition of InAlN layer was developed, and high quality InAlN layer was grown by using metalorganic chemical vapor deposition (MOCVD). A LED structure employing an InAlN EBL was grown and its efficiency droop performance was compared with a LED with an AlGaN EBL. Characterization results suggested that the InAlN EBL delivers more effective electron blocking over AlGaN EBL. Hole-injection performance of the InAlN EBL was examined by growing and testing a series of LEDs with different InAlN EBL thickness. Analysis results by using extended quantum efficiency model shows that further improvement in the performance of LED requires better hole-injection performance of the InAlN EBL. Advanced EBL structures such as strain-engineered InAlN EBL and compositionally-graded InAlN EBLs for the delivery of higher hole-injection efficiency were also grown and tested.
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Namkoong, Gon. "Molecular beam epitaxy grown III-nitride materials for high-power and high-temperture applications : impact of nucleation kinetics on material and device structure quality." Diss., Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/16426.
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Jackson, Christine M. "Correlations of Electronic Interface States and Interface Chemistry on Dielectric/III Nitride Heterostructures for Device Applications." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu15257361319909.
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Carnevale, Santino D. "Catalyst-free III-nitride Nanowires by Plasma-assisted Molecular Beam Epitaxy: Growth, Characterization, and Applications." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1374066626.
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Antunez, de Mayolo Eduardo. "Study of the Optical Properties of sp2-Hybridized Boron Nitride." Thesis, Linköpings universitet, Tillämpad optik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-125738.
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Nitride-based semiconductor materials make it possible to fabricate optoelectronic devices that operate in the whole electromagnetic range, since the band gaps of these compounds can be modified by doping. Among these materials, the sp2-hybridized boron nitride has properties that make it a potential candidate for integration in devices operating in the short-wavelength limit, under harsh environment conditions, due to the strength of the B-N bond. Nevertheless, this binary compound has been the less studied material among the nitrides, due to the lack of complete control on the growth process. This thesis is focused on the study of the optical properties of sp2-hybridized boron nitride grown by hot-wall chemical vapor deposition (CVD) method, at the Department of Physics, Chemistry and Biology, at Linköping University, Sweden. The samples received for this study were grown on c-plane aluminum nitride as the buffer layer, which in turn was grown by nitridation on c- plane oriented sapphire, as the substrate material. The first objective of the research presented in this thesis was the development of a suitable ellipsometry model in a spectral region ranging from the infrared to the ultraviolet zones of the electromagnetic spectrum, with the aim of obtaining in the process optical properties such as the index of refraction, the energy of the fundamental electronic interband transition, the frequencies for the optical vibrational modes of the crystal lattice, as well as their broadenings, and the numerical values of the dielectric constants; and on the other hand, structural parameters such as the layers thicknesses, and examine the possibility of the presence of roughness or porosity on the boron nitride layer, which may affect the optical properties, by incorporating their effects into the model. The determination of these parameters, and their relation with the growth process, is important for the future adequate design of heterostructure-based devices that incorporate this material. In particular, emphasis has been put on the modeling of the polar lattice resonance contributions, with the TO- LO model, by using infrared spectroscopic ellipsometry as the characterization technique to study the phonon behavior, in the aforementioned spectral region, of the boron nitride. On the other hand, spectroscopic ellipsometry in the visible-ultraviolet spectral range was used to study the behavior of the material, by combining a Cauchy model, including an Urbach tail for the absorption edge, and a Lorentz oscillator in order to account for the absorption in the material in the UV zone. This first step on the research project was carried out at Linköping University. The second objective in the research project was to carry out additional studies on the samples received, in order to complement the information provided by the ellipsometry model and to improve the model itself, provided that it was possible. The characterization techniques used were X-ray diffraction, which made it possible to confirm that in fact boron nitride was present in the samples studied, and made it possible to verify the crystalline quality of the aforementioned samples, and in turn relate it to the quality of the ellipsometry spectra previously obtained; the Raman spectroscopy made it possible to further verify and compare the crystalline qualities of the samples received, as well as to obtain the frequency for the Raman active B-N stretching vibration in the basal plane, and to compare this value with that corresponding to the bulk sp2-boron nitride; scanning electron microscopy made it possible to observe the rough surface morphologies of the samples and thus relate them to some of the conclusions derived from the ellipsometry model; and finally cathodoluminescence measurements carried out at low temperature (4 K) allowed to obtain a broad band emission, on all the samples studied, which could be related to native defects inside the boron nitride layers, i.e., boron vacancies. Nevertheless, no trace of a free carrier recombination was observed. Considering that the hexagonal-boron nitride is nowadays considered to be a direct band gap semiconductor, it may be indirectly concluded, in principle, that the dominant phase present in the samples studied was the rhombohedral polytype. Moreover, it can be tentatively concluded that the lack of an observable interband recombination may be due to the indirect band gap nature of the rhombohedral phase of the boron nitride. Spectroscopic ellipsometry does not give a definite answer regarding this issue either, because the samples analyzed were crystalline by nature, thus not being possible to use mathematical expressions for the dielectric function models that incorporate the band gap value as a fitting parameter. Therefore, the nature of the band gap emission in the rhombohedral phase of the boron nitride is still an open research question. On the other hand, luminescent emissions originating from radiative excitonic recombinations were not observed in the cathodoluminescence spectra. This second step of the project was carried out at the Leroy Eyring Center for Solid State Science at Arizona State University.
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May, Brelon J. "Investigation and Engineering of the Homogeneity and Current Injection of Molecular Beam Epitaxy Grown III-Nitride Nanowire Ultraviolet Light Emitting Diodes." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1546385850422501.
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Venkatachalam, Anusha. "Investigation of self-heating and macroscopic built-in polarization effects on the performance of III-V nitride devices." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/29669.
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Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2010.Committee Chair: Yoder, Douglas; Committee Member: Graham, Samuel; Committee Member: Allen, Janet; Committee Member: Klein, Benjamin; Committee Member: Voss, Paul. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Reitmeier, Zachary J. "The Chemistry and Surface Microstructure of Si-Based Substrates and their Effect on the Evolution of the Microstructures of III-Nitride Films Grown via Metalorganic Vapor Phase Epitaxy." NCSU, 2005. http://www.lib.ncsu.edu/theses/available/etd-03202005-194018/.
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The present research was undertaken with the goals of understanding the evolution of defects and strain in heteroepitaxial AlN and GaN films deposited via metalorganic vapor phase epitaxy and minimizing those defects through manipulation of the substrate. As observed with atomic force microscopy (AFM), AlN initially grew in the form of flat-topped islands on as-received SiC substrates. Threading dislocations (TDs) observed in transmission electron microscopy (TEM) images initiated at the AlN/SiC interface as the result of defects at the surface of the mechanically polished substrate and/or condensation of point defects. GaN initially grew in the Stranski-Krastanov mode on AlN/SiC before transitioning to the dislocation-mediated step flow mode. The TDs in GaN resulted from the propagation of the TDs present in the AlN layer. The biaxial strain in the GaN layers varied with buffer layer material and layer thickness yet all samples investigated remained in residual compression due to incomplete relaxation of the coherent strain. The presence of strain during the initial growth of AlxGa1-xN layers directly on as-received SiC also resulted in phase-separated regions of Al-rich and Al-poor film. A high temperature hydrogen etch was then used to remove mechanical polishing scratches from the SiC substrates. Subsequently deposited AlN layers featured reduced pit density and the elimination of scratch-induced undulations. GaN layers deposited with AlN buffer layers on these substrates resulted in slightly reduced TD densities as observed by AFM, TEM, and high resolution X-ray diffraction (HRXRD). Regions of dramatically reduced dislocation densities were observed by HRXRD, TEM, and cathodoluminescence for GaN layers on stripe-patterned Si substrates. However, long growth times resulted in outdiffusion of Si from the substrate and subsequent film roughening. Finally, it was demonstrated that the presence of ammonia during heating of GaN templates to the growth temperature for homoepitaxy resulted in removal of carbon- and oxygen-based contaminants from the template surface.
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Olsson, Kevin. "Optimization of gas flow uniformity in enhancement of Metal Organic Chemical Vapor Deposition growth for III-nitrides." Thesis, Linköpings universitet, Halvledarmaterial, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-157378.
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The thesis focuses on the gas flow profile optimization of a non-conventional injector in a hot-wall MOCVD system. The injector’s gas flow profile is simulated with CFD and demonstrates awell-behaved laminar flow with a parabolic profile. To ensure the theory is in coherence with the reality, a qualitative study with five thermocouples in a test graphite piece of the was performed. First the thesis will take you through an introduction of the semiconductor field to arrive in a problem formulation. Then you will read about the principles of MOCVD systems, fluid dynamics principles and thermocouple theory. The experiment’s way of approach is thendescribed through all steps from blue print to results. A discussion about the result and the conclusion will be read before the proposals of future work based on the thesis work. The laminar flow is confirmed according to the resulting data and the limitations of the system is set to two different cases depending on background temperature. At 1000 °C a laminar flow is strongly indicated to be obtained at position 3A, closest to the growth area, within the gas flow range of 25 SLM regardless of background pressure, except for 700 mBar indicating turbulent flow for 15 SLM an up. At 20 and 200 mBar the laminar flow limit is suggested by data to be even higher and reaching a value of 35 SLM. At 450 °C the data indicate a laminar flow up to 20 SLM at position 3A regardless of background pressure condition, except for 700 mBar where the data indicate a laminar flow at 35 and 40 SLM. 50 mBar strongly indicates a laminar flow profile up to a gas flow of 35 SLM. With a background pressure of 20 mBar, the data suggests a laminar flow profile up to at least 25 SLM. At 100 mBar the data indicates a laminar flow within the range of 30 SLM.
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Young, Craig Alexander. "Fabrication of photonic microstructures in group III nitride material." Thesis, University of Glasgow, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.249984.
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Serban, Alexandra. "Magnetron Sputter Epitaxy of Group III-Nitride Semiconductor Nanorods." Licentiate thesis, Linköpings universitet, Tunnfilmsfysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-141595.
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The III-nitride semiconductors family includes gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and related ternary and quaternary alloys. The research interest on this group of materials is sparked by the direct bandgaps, and excellent physical and chemical properties. Moreover, the ternary alloys (InGaN, InAlN and AlGaN) present the advantage of bandgap tuning, giving access to the whole visible spectrum, from near infrared into deep ultraviolet wavelengths. The intrinsic properties of III-nitride materials can be combined with characteristical features of nanodimension and geometry in nanorod structures. Moreover, nanorods offer the advantage of avoiding problems arising from the lack of native substrates, like lattice and thermal expansion, film – substrate mismatch. The growth and characterization of group III-nitride semiconductos nanorods, namely InAlN and GaN nanorods, is presented in this thesis. All the nanostructures were grown by employing direct-current reactive magnetron sputter epitaxy. InxAl1−xN self-assembled, core-shell nanorods on Si(111) substrates were demonstrated. A comprehensive study of temperature effect upon the morphology and composition of the nanorods was realized. The radial nanorod heterostructure consists of In-rich cores surrounded by Al-rich shells with different thicknesses. The spontaneous formation of core-shell nanorods is suggested to originate from phase separation due to spinodal decomposition. As the growth temperature increase, In desorption is favored, resulting in thicker Al-rich shells and larger nanorod diameters. Both self-assembled and selective-area grown GaN nanorods are presented. Self-assembled growth of GaN nanorods on cost-effective substrates offers a cheaper alternative and simplifies device processing. Successful growth of high- quality GaN (exhibiting strong bandedge emission and high crystalline quality) on conductive templates/substrates such as Si, SiC, TiN/Si, ZrB2/Si, ZrB2/SiC, Mo, and Ti is supported by the possibility to be used as electrodes when integrated in optoelectronic devices. The self-assembled growth leads to mainly random nucleation, resulting in nanorods with large varieties of diameters, heights and densities within a single growth run. This translates into non-uniform properties and complicates device processing. These problems can be circumvented by employing selective-area growth. Pre-patterned substrates by nano-sphere lithography resulted in GaN nanorods with controlled length, diameter, shape, and density. Well-faceted c-axis oriented GaN nanorods were grown directly onto the native SiOx layer inside nano-opening areas, exhibiting strong bandedge emission at room- temperature and single-mode lasing. Our studies on the growth mechanism revealed a different growth behavior when compared with selective-area grown GaN nanorods by MBE and MOCVD. The time-dependent growth series helped define a comprehensive growth mechanism from the initial thin wetting layer formed inside the openings, to the well-defined, uniform, hexagonal NRs resulted from the coalescence of multiple initial nuclei.
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Trybus, Elaissa Lee. "Molecular beam epitaxy growth of indium nitride and indium gallium nitride materials for photovoltaic applications." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/28108.
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Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009.Committee Chair: Doolittle, W. Alan; Committee Member: Ferguson, Ian; Committee Member: Graham, Samuel; Committee Member: Rohatgi, Ajeet; Committee Member: Shen, Shyh-Chiang.
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Miller, Eric Justin. "Influence of material properties on device design and performance in III-V nitride alloys /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2003. http://wwwlib.umi.com/cr/ucsd/fullcit?p3091322.
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Yoo, Dongwon. "Growth and Characterization of III-Nitrides Materials System for Photonic and Electronic Devices by Metalorganic Chemical Vapor Deposition." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/16220.
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A wide variety of group III-Nitride-based photonic and electronic devices have opened a new era in the field of semiconductor research in the past ten years. The direct and large bandgap nature, intrinsic high carrier mobility, and the capability of forming heterostructures allow them to dominate photonic and electronic device market such as light emitters, photodiodes, or high-speed/high-power electronic devices. Avalanche photodiodes (APDs) based on group III-Nitrides materials are of interest due to potential capabilities for low dark current densities, high sensitivities and high optical gains in the ultraviolet (UV) spectral region. Wide-bandgap GaN-based APDs are excellent candidates for short-wavelength photodetectors because they have the capability for cut-off wavelengths in the UV spectral region (λ < 290 nm). These intrinsically solar-blind UV APDs will not require filters to operate in the solar-blind spectral regime of λ < 290 nm. For the growth of GaN-based heteroepitaxial layers on lattice-mismatched substrates, a high density of defects is usually introduced during the growth; thereby, causing a device failure by premature microplasma, which has been a major issue for GaN-based APDs. The extensive research on epitaxial growth and optimization of Alx Ga 1-x N (0 ≤ x ≤ 1) grown on low dislocation density native bulk III-N substrates have brought UV APDs into realization. GaN and AlGaN UV p-i-n APDs demonstrated first and record-high true avalanche gain of > 10,000 and 50, respectively. The large stable optical gains are attributed to the improved crystalline quality of epitaxial layers grown on low dislocation density bulk substrates. GaN p-i-n rectifiers have brought much research interest due to its superior physical properties. The AIN-free full-vertical GaN p-i-n rectifiers on n - type 6H-SiC substrates by employing a conducting AIGaN:Si buffer layer provides the advantages of the reduction of sidewall damage from plasma etching and lower forward resistance due to the reduction of current crowding at the bottom n -type layer. The AlGaN:Si nucleation layer was proven to provide excellent electrical properties while also acting as a good buffer role for subsequent GaN growth. The reverse breakdown voltage for a relatively thin 2.5 μm-thick i -region was found to be over -400V.
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Rennesson, Stéphanie. "Développement de nouvelles hétérostructures HEMTs à base de nitrure de gallium pour des applications de puissance en gamme d'ondes millimétriques." Phd thesis, Université Nice Sophia Antipolis, 2013. http://tel.archives-ouvertes.fr/tel-00943619.
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Les matériaux III-N sont présents dans la vie quotidienne pour des applications optoélectroniques (diodes électroluminescentes, lasers). Les propriétés remarquables du GaN (grand gap, grand champ de claquage, champ de polarisation élevé, vitesse de saturation des électrons importante...) en font un candidat de choix pour des applications en électronique de puissance à basse fréquence, mais aussi à haute fréquence, par exemple en gamme d'ondes millimétriques. L'enjeu de ce travail de thèse consiste à augmenter la fréquence de travail des transistors tout en maintenant une puissance élevée. Pour cela, des hétérostructures HEMTs (High Electron Mobility Transistors) sont développées et les épaisseurs de cap et de barrière doivent être réduites, bien que ceci soit au détriment de la puissance délivrée. Une étude sera donc menée sur l'influence des épaisseurs de cap et de barrière ainsi que le type de barrière (AlGaN, AlN et InAlN) de manière à isoler les hétérostructures offrant le meilleur compromis en termes de fréquence et de puissance. De plus, les moyens mis en œuvre pour augmenter la fréquence de travail entrainent une dégradation du confinement des électrons du canal. De manière à limiter cet effet, une back-barrière est insérée sous le canal. Ceci fera l'objet d'une deuxième étude. Enfin, une étude de la passivation de surface des transistors sera menée. La combinaison des ces trois études permettra d'identifier la structure optimale pour délivrer le plus de puissance à haute fréquence (ici à 40 GHz).
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Hoffman, Timothy B. "Optimization and characterization of bulk hexagonal boron nitride single crystals grown by the nickel-chromium flux method." Diss., Kansas State University, 2016. http://hdl.handle.net/2097/32797.
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Doctor of PhilosophyDepartment of Chemical Engineering
James H. Edgar
Hexagonal boron nitride (hBN) is a wide bandgap III-V semiconductor that has seen new interest due to the development of other III-V LED devices and the advent of graphene and other 2-D materials. For device applications, high quality, low defect density materials are needed. Several applications for hBN crystals are being investigated, including as a neutron detector and interference-less infrared-absorbing material. Isotopically enriched crystals were utilized for enhanced propagation of phonon modes. These applications exploit the unique physical, electronic and nanophotonics applications for bulk hBN crystals. In this study, bulk hBN crystals were grown by the flux method using a molten Ni-Cr solvent at high temperatures (1500°C) and atmospheric pressures. The effects of growth parameters, source materials, and gas environment on the crystals size, morphology and purity were established and controlled, and the reliability of the process was greatly improved. Single-crystal domains exceeding 1mm in width and 200μm in thickness were produced and transferred to handle substrates for analysis. Grain size dependence with respect to dwell temperature, cooling rate and cooling temperature were analyzed and modeled using response surface morphology. Most significantly, crystal grain width was predicted to increase linearly with dwell temperature, with single-crystal domains exceeding 2mm in at 1700°C. Isotopically enriched ¹⁰B and ¹¹B hBN crystal were produced using a Ni-Cr-B flux method, and their properties investigated. ¹⁰B concentration was evaluated using SIMS and correlated to the shift in the Raman peak of the E[subscript 2g] mode. Crystals with enrichment of 99% ¹⁰B and >99% ¹¹B were achieved, with corresponding Raman shift peaks at 1392.0 cm⁻¹ and 1356.6 cm⁻¹, respectively. Peak FWHM also decreased as isotopic enrichment approached 100%, with widths as low as 3.5 cm⁻¹ achieved, compared to 8.0 cm⁻¹ for natural abundance samples. Defect selective etching was performed using a molten NaOH-KOH etchant at 425°C-525°C, to quantify the quality of the crystals. Three etch pit shapes were identified and etch pit width was investigated as a function of temperature. Etch pit density and etch pit activation energy was estimated at 5×10⁷ cm⁻² and 60 kJ/mol, respectively. Screw and mixed-type dislocations were identified using diffraction-contrast TEM imaging.
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Lochner, Zachary M. "Heterojunction bipolar transistors and ultraviolet-light-emitting diodes based in the III-nitride material system grown by metalorganic chemical vapor deposition." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/49032.
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The material and device characteristics of InGaN/GaN heterojunction bipolar transistors (HBTs) grown by metalorganic chemical vapor deposition are examined. Two structures grown on sapphire with different p-InxGa1-xN base-region compositions, xIn = 0.03 and 0.05, are presented in a comparative study. In a second experiment, NpN-GaN/InGaN/GaN HBTs are grown and fabricated on free-standing GaN (FS-GaN) and sapphire substrates to investigate the effect of dislocations on III-nitride HBT epitaxial structures. The performance characteristics of HBTs on FS-GaN with a 20×20 m2 emitter area exhibit a maximum collector-current density of ~12.3 kA/cm2, a D.C. current gain of ~90, and a maximum differential gain of ~120 without surface passivation. For the development of deep-ultraviolet optoelectronics, several various structures of optically-pumped lasers at 257, 246, and 243 nm are demonstrated on (0001) AlN substrates. The threshold-power density at room temperature was reduced to as low as 297 kW/cm2. The dominating polarization was measured to be transverse electric in all cases. InAlN material was developed to provide lattice matched, high-bandgap energy cladding layers for a III-N UV laser structure. This would alleviate strain and dislocation formation in the structure, and also mitigate the polarization charge. However, a gallium auto-doping mechanism was encountered which prevents the growth of pure ternary InAlN, resulting instead in quaternary InAlGaN. This phenomenon is quantitatively examined and its source is explored.
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Jessen, Gregg Huascar. "Investigation and Characterization of AlGaN/GaN Device Structures and the Effects of Material Defects and Processing on Device Performance." Connect to this title online, 2002. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1038605384.
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Thesis (Ph. D)--Ohio State University, 2002.Title from first page of PDF file. Document formatted into pages; contains xxx,198 p.: ill. (some col.). Includes abstract and vita. Advisor: Leonard J. Brillson, Dept. of Electrical Engineering. Includes bibliographical references (p. 188-198).
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Briere, Gauthier. "Réalisation de méta-optiques à base de matériaux semi-conducteurs III-V pour des applications dans le visible." Thesis, Université Côte d'Azur (ComUE), 2019. http://www.theses.fr/2019AZUR4075.
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Depuis de récentes années de nouveaux composants optiques ont fait leur apparition. Ces composants connus sous les noms de « Méta-optiques » ou encore « Métasurfaces », rendent possible le contrôle et la mise en forme du front d’onde de la lumière permettant alors de mettre en forme n’importe quel faisceau incident et ainsi créer des fonctionnalités optiques classiques telles que focaliser ou dévier la lumière, ou alors des fonctionnalités aux propriétés surprenantes telle que la réalisation de métahologramme dépendant en polarisation. En effet, grâce à l’arrangement périodique de résonateurs de dimensions géométriques sous-longueur d’onde, il est alors possible d’obtenir un contrôle local arbitraire du faisceau incident. Néanmoins, même si de nombreuses applications ont pu être démontré dans la communauté, seuls quelques matériaux se retrouvent être compatibles pour le développement industriel de ces composants. De plus, afin de passer de composant passif à actif, pour la réalisation de composant dynamique, il est nécessaire de passer de matériau diélectrique à matériau semi-conducteur. C’est pourquoi dans ces travaux, nous nous sommes intéressés à l’utilisation d’un matériau semi-conducteur qui est le Nitrure de Gallium pour la réalisation de composants métasurfaciques. Nous présentons alors dans un premier temps une étude numérique des nanostructures utilisées lors de ces travaux. Puis nous montrons comment est réalisée la conception de nos méta-optiques en présentant la méthode de design et les procédés de nanofabrications employés, notamment une nouvelle technique de gravure, compatible uniquement avec les matériaux cristallins et préservant leurs propriétés optiques. Nous exposons ensuite différentes applications où nos composants sont utilisés telles que : la réalisation de métalentilles de large ouverture numérique et de large surface, l’optimisation de réseaux métasurfaciques permettant d’atteindre des efficacités de diffraction supérieur à 80% ou encore la réalisation expérimentale de méta-hologramme permettant de conserver l’information du moment angulaire orbitale du faisceau incidentIn the past years, new optical components have appeared. These components, known as "meta-optics" or "metasurfaces", made it possible to control and to shape the wavefront of the light. This allows the control of any incident beam and the creation of conventional optical functionalities, such as focusing or deflecting the light, or functionalities with additional features such as the possibility of creating polarization-dependent meta-holograms. Indeed, thanks to the periodic arrangement of resonators with sub-wavelength geometric dimensions, it is possible to obtain an arbitrary local control of the incident beam. Nevertheless, even though many applications have been demonstrated in the community, only a few materials are found to be compatible for the industrial development of these components. In addition, in order to pass from passive to active components for the fabrication of dynamic devices, it is necessary to switch from dielectric materials to semiconductor materials. For these reasons, we are interested in the use of a semiconductor material, Gallium Nitride, for the development of metasurface components. We first present a numerical study of the nanostructures used during this work. Then, we show how the design of our meta-optics is done by presenting the numerical conception method and nanofabrication processes used, which includes a new etching technique compatible only with crystalline materials while preserving their optical properties. Finally, we suggest different applications where our components can be used, such as: the development of metalenses with high numerical aperture and large surface; the optimization of metasurface high contrast gratings allowing to reach diffraction efficiencies higher than 80%; or the fabrication of meta-holograms preserving the information of the orbital angular momentum of the incident beam
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"Polarization Effects in Group III-Nitride Materials and Devices." Doctoral diss., 2012. http://hdl.handle.net/2286/R.I.14643.
Full textAbstract:
abstract: Group III-nitride semiconductors have wide application in optoelectronic devices. Spontaneous and piezoelectric polarization effects have been found to be critical for electric and optical properties of group III-nitrides. In this dissertation, firstly, the crystal orientation dependence of the polarization is calculated and in-plane polarization is revealed. The in-plane polarization is sensitive to the lateral characteristic dimension determined by the microstructure. Specific semi-polar plane growth is suggested for reducing quantum-confined Stark effect. The macroscopic electrostatic field from the polarization discontinuity in the heterostructures is discussed, b ased on that, the band diagram of InGaN/GaN quantum well/barrier and AlGaN/GaN heterojunction is obtained from the self-consistent solution of Schrodinger and Poisson equations. New device design such as triangular quantum well with the quenched polarization field is proposed. Electron holography in the transmission electron microscopy is used to examine the electrostatic potential under polarization effects. The measured potential energy profiles of heterostructure are compared with the band simulation, and evidences of two-dimensional hole gas (2DHG) in a wurtzite AlGaN/ AlN/ GaN superlattice, as well as quasi two-dimensional electron gas (2DEG) in a zinc-blende AlGaN/GaN are found. The large polarization discontinuity of AlN/GaN is the main source of the 2DHG of wurtzite nitrides, while the impurity introduced during the growth of AlGaN layer provides the donor states that to a great extent balance the free electrons in zinc-blende nitrides. It is also found that the quasi-2DEG concentration in zinc-blende AlGaN/GaN is about one order of magnitude lower than the wurtzite AlGaN/GaN, due to the absence of polarization. Finally, the InAlN/GaN lattice-matched epitaxy, which ideally has a zero piezoelectric polarization and strong spontaneous polarization, is experimentally studied. The breakdown in compositional homogeneity is triggered by threading dislocations with a screw component propagating from the GaN underlayer, which tend to open up into V-grooves at a certain thickness of the InxAl1-xN layer. The V-grooves coalesce at 200 nm and are filled with material that exhibits a significant drop in indium content and a broad luminescence peak. The structural breakdown is due to heterogeneous nucleation and growth at the facets of the V-grooves.Dissertation/Thesis
Ph.D. Physics 2012
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Solanke, Swanand Vishnu. "Integration of Layered Materials with Group-III Nitride Semiconductors for Dual Band Photodetection." Thesis, 2019. https://etd.iisc.ac.in/handle/2005/4441.
Full textAbstract:
In many applications, simultaneous detection in two distinct bands, UV and IR regime is required. An instrumentation in which detection in both the bands achievable using single device would be highly desirable owing to its cost effectiveness, ease in fabrication and ease in operation. Many attempts were made to integrate two materials to make device capable in dual band detection. One of such attempts was epitaxial stacking of two different members of Group-III Nitride family members, possessing different bandgap, to form single device. However, such epi-stacks suffer from lattice mismatch, difficulty in growth and fabrication, traps and dislocations generating because of lattice mismatch, thus affecting electrical and optical performance of the device. In such difficulties, layered materials (or commonly referred as 2D semiconductors) have gained big interest thanks to their exciting properties. Layered materials (LMs) can be transferred virtually on any substrate using very simple methods without worrying about lattice mismatch issues. They have excellent electrical and optical properties which make them attractive candidates for optoelectronic applications.
Through this work, we attempt to integrate layered materials with GaN, a wide bandgap Group-III Nitride semiconductor and show that, by using simple integration techniques and extreme bandgap engineering by exploiting band alignments in heterojunction, it is possible to achieve dual band photo detection.
We started with Integration of MoS2 with GaN. By fabricating MSM device such that metal formed contact only on MoS2 and not on GaN, we showed that it is possible to extract photocarriers generated in GaN layer underneath through MoS2 layer. The detection spectra showed two sharp edges in spectral responsivity (SR) graph, one at 365 nm (UV) and another at 685 nm (Visible). Normalised SR was found out to be 127 A/W and 33 A/W at 365 nm and 685 nm respectively. Device showed persistent photoconductivity (PCC) making it a slow device. Detectivity at ~3.3 x 1011 Jones at 532 nm laser excitation. Next, in similar approach, devices were fabricated by integrating β-In2Se3 with GaN. SR graph showed two distinct band edges, one at 365 nm (UV) and another at 850 nm (NIR) making it 1st demonstration of simultaneous dual band detection. Normalised SR were calculated to be 1.75 A/W and 32.7 mA/W at 365 nm and 850 nm respectively. Device showed very less PCC compared to MoS2/GaN device making it little faster. A detectivity of ~1.6x109 Jones was found at 805 nm which is 1st report by such device at 805 nm laser excitation. We then attempted with vertical photodetector (PD) by integrating α-In2Se3 with GaN. This time device showed two sharp band steps in SR graph, one at 365 nm and another at 850 nm with considerable rejection in visible band. Device showed dual band detection in both positive and negative bias. Normalised SR was recorded to be 1.9 A/W for 3V bias and ~0.088 A/W for -3V bias at 365 nm. While at 850 nm, NSR was found out to be ~0.07 A/W (-3V) and 0.05 A/W (3V). detectivity was found out to be 3.6 x 1010 Jones at -3V and 1.2 x 1010 Jones at 3V for the excitation wavelength of 850 nm. Device showed faster response compared to previously mentioned lateral devices making it suitable for communication applications. Finally, we attempted to show suitability of N-polar GaN MSM PD for the photodetection applications. Device exhibited excellent spectral responsivity with sharp step at 365 nm with UV-to-Vis ratio of ~250. Finally, it was concluded that, improvement in the device performance can be achieved by improvement in the growth quality of N-polar GaN, dark current reduction and device architecture.
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"Growth, Characterization, and Thermodynamics of III-Nitride Semiconductors." Doctoral diss., 2011. http://hdl.handle.net/2286/R.I.9035.
Full textAbstract:
abstract: III-nitride alloys are wide band gap semiconductors with a broad range of applications in optoelectronic devices such as light emitting diodes and laser diodes. Indium gallium nitride light emitting diodes have been successfully produced over the past decade. But the progress of green emission light emitting devices has been limited by the incorporation of indium in the alloy, mainly due to phase separation. This difficulty could be addressed by studying the growth and thermodynamics of these alloys. Knowledge of thermodynamic phase stabilities and of pressure - temperature - composition phase diagrams is important for an understanding of the boundary conditions of a variety of growth techniques. In this dissertation a study of the phase separation of indium gallium nitride is conducted using a regular solution model of the ternary alloy system. Graphs of Gibbs free energy of mixing were produced for a range of temperatures. Binodal and spinodal decomposition curves show the stable and unstable regions of the alloy in equilibrium. The growth of gallium nitride and indium gallium nitride was attempted by the reaction of molten gallium - indium alloy with ammonia at atmospheric pressure. Characterization by X-ray diffraction, photoluminescence, and secondary electron microscopy show that the samples produced by this method contain only gallium nitride in the hexagonal phase. The instability of indium nitride at the temperatures required for activation of ammonia accounts for these results. The photoluminescence spectra show a correlation between the intensity of a broad green emission, related to native defects, and indium composition used in the molten alloy. A different growth method was used to grow two columnar-structured gallium nitride films using ammonium chloride and gallium as reactants and nitrogen and ammonia as carrier gasses. Investigation by X-ray diffraction and spatially-resolved cathodoluminescence shows the film grown at higher temperature to be primarily hexagonal with small quantities of cubic crystallites, while the one grown at lower temperature to be pure hexagonal. This was also confirmed by low temperature photoluminescence measurements. The results presented here show that cubic and hexagonal crystallites can coexist, with the cubic phase having a much sharper and stronger luminescence. Controlled growth of the cubic phase GaN crystallites can be of use for high efficiency light detecting and emitting devices. The ammonolysis of a precursor was used to grow InGaN powders with different indium composition. High purity hexagonal GaN and InN were obtained. XRD spectra showed complete phase separation for samples with x < 30%, with ~ 9% indium incorporation in the 30% sample. The presence of InGaN in this sample was confirmed by PL measurements, where luminescence from both GaN and InGaN band edge are observed. The growth of higher indium compositions samples proved to be difficult, with only the presence of InN in the sample. Nonetheless, by controlling parameters like temperature and time may lead to successful growth of this III-nitride alloy by this method.Dissertation/Thesis
Ph.D. Physics 2011
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"Growth of III-nitride nano-materials by chemical vapor deposition." 2006. http://library.cuhk.edu.hk/record=b5892788.
Full textAbstract:
Hong Liang = 用化学气相淀积方法生长氮化物纳米材料 / 洪亮.Thesis (M.Phil.)--Chinese University of Hong Kong, 2006.
Includes bibliographical references.
Text in English; abstracts in English and Chinese.
Hong Liang = Yong hua xue qi xiang dian ji fang fa sheng chang dan hua wu na mi cai liao / Hong Liang.
Acknowledgements --- p.ii
Abstract --- p.iii
Contents --- p.v
Chapter Chapter 1 --- Introduction
Chapter 1.1 --- Background --- p.1
Chapter 1.2 --- Motivation --- p.2
Chapter 1.2.1 --- A1N and AlGaN nanowires --- p.2
Chapter 1.2.2 --- CVD --- p.3
Chapter 1.3 --- Our work --- p.3
Chapter Chapter 2 --- Experiment --- p.7
Chapter 2.1 --- CVD system --- p.7
Chapter 2.2 --- Sources and Substrates --- p.7
Chapter 2.3 --- Growth of A1N nanowires --- p.8
Chapter 2.4 --- Growth of AlGaN nanowires --- p.9
Chapter Chapter 3 --- Characterization --- p.11
Chapter 3.1 --- Scanning Electron Microscopy --- p.11
Chapter 3.1.1 --- Topographic images by secondary electrons --- p.11
Chapter 3.1.2 --- Elemental Analysis by Energy Dispersive X-ray --- p.12
Chapter 3.2 --- Transmission Electron Microscopy --- p.12
Chapter 3.3 --- X-Ray Diffraction --- p.14
Chapter 3.4 --- Micro-Raman --- p.15
Chapter Chapter 4 --- Results and Discussion --- p.18
Chapter 4.1 --- A1N nano-structures --- p.18
Chapter 4.1.1 --- A1N nano-leaves grown on silicon substrates --- p.18
Chapter 4.1.2 --- A1N nanowires grown on silicon substrates --- p.19
Chapter 4.1.3 --- SiNx nanowires grown on silicon substrates --- p.22
Chapter 4.1.4 --- A1N nanowires grown on sapphire substrates --- p.26
Chapter 4.1.5 --- Comparison with the results of other research groups --- p.31
Chapter 4.2 --- AlGaN nano-structures --- p.33
Chapter 4.2.1 --- AlGaN nanowires grown on silicon substrates --- p.33
Chapter 4.2.2 --- Temperature dependence --- p.38
Chapter 4.2.3 --- The influence of the mass ratio (Ga/Al) in the precursor metal sources --- p.43
Chapter 4.2.4 --- Substrate effect --- p.46
Chapter Chapter 5 --- Suggestion of the growth mechanism --- p.51
Chapter 5.1 --- Growth mechanisms: an introduction --- p.51
Chapter 5.2 --- The growth mechanisms for our produced samples --- p.57
Chapter 5.2.1 --- Growth mechanism for A1N nanowires --- p.58
Chapter 5.2.2 --- Growth mechanism for AlGaN nanowires --- p.61
Chapter 5.2.3 --- Substrate effect --- p.65
Chapter Chapter 6 --- Conclusions --- p.71
Appendix --- p.73
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Sarigiannidou, Eirini. "Electron Microscopy and III-Nitride Nanostructured." Phd thesis, 2004. http://tel.archives-ouvertes.fr/tel-00937274.
Full textAbstract:
Ce mémoire de thèse présente une étude structurale de puits et de boîtes quantiques GaN/AIN élaborés par épitaxie par jets moléculaires. La technique d'investigation est la microscopie électronique à transmission, utilisée en modes (i) haute résolution, (ii) imagerie filtrée,(iii) conventionnel et (iv) faisceau convergent. Un chapitre est consacré à l'analyse quantitative des images haute résolution par la méthode de projection et l'analyse de la phase géométrique. Ces méthodes sont analysées et optimisées (par exemple utilisation d'images "off-axis"). Dans les super-réseaux (SL) de puits quantiques GaN/AIN les polarités Ga et N sont analysées. Nous démontrons la supériorité de la qualité structurale des faces Ga: interfaces plus abruptes et uniformes, absence de domaines d'inversion et contraintes moins importantes. Nous analysons aussi l'évolution des nanostructures (puits ou boîtes) durant le processus d'encapsulation et nous prouvons que la croissance de l'AIN induit un amincissement des puits quantiques et une réduction isotrope de la taille des boîtes quantiques. Ce phénomène est attribué à un mécanisme d'échange entre les deux métaux et dépend de la relaxation des couches de GaN. Dans un SL de boîtes quantiques GaN/AIN nous examinons la distribution des contraintes et nous démontrons en combinant des analyses haute résolution, des calculs théoriques et de la diffraction X que l'alignement vertical des boîtes est du à une différence de l'état de contrainte de la couche d'AIN. Enfin, nous prouvons que le dopage au Mg à fortes concentrations d'une couche de GaN face N favorise la conversion de la structure de wurtzite à zinc-blende.
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Loeber, David A. S. "Stimulated emission and lasing in III-V nitride heterostructures." 1997. https://scholarworks.umass.edu/dissertations/AAI9737554.
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Stimulated emission, lasing, and related properties of III-V nitride heterostructures are studied. A strain-dependent semi-empirical tight-binding model is developed, using the valence force-field model of Keating, to predict the atomic positions in the strained wurtzite crystal lattice. Predicted deformation potentials and strain-induced exciton splitting are shown to closely match data in the literature. The spectral properties of the edge luminescence from GaN-AlGaN heterostructures is investigated. The existence of stimulated emission is demonstrated and a measurement of the optical gain spectra is reported. In addition, the light emission properties of GaN-AlGaN separate confinement heterostructures is studied. The measured luminescence properties are improved for active region designs with fewer, thicker wells. An analysis of the trend is presented demonstrating recombination at the well-barrier interface as a significant factor. The results also indicate that the quantum wells experience compressive strain from the lattice mismatch with the AlGaN cladding layers. Further experimental results demonstrate that the commonly observed surface stimulated emission is related to in-plane optical gain, and is observed most commonly in samples with rough surface morphology. Photopumping results from GaN-AlGaN laser platelets are presented and discussed. Laser oscillation in GaN-AlGaN separate confinement heterostructures is demonstrated in which the optical cavity is formed unintentionally by parallel cracks in the epilayer. The observed laser modes are broad and shift to shorter wavelengths with increasing pump intensity. An analysis is presented revealing that mode shifting resulting from carrier-induced refractive index changes restricts the observation of laser modes to short optical cavities. GaN-AlGaN Bragg reflectors are investigated through reflectivity modeling and characterization. A transfer-matrix model is developed with an empirical relation for the refractive indices and predictions of the model are compared with data in the literature. Experimental results are then presented and compared with the predictions of the model. The design and characterization of GaN-AlGaN vertical cavity surface emitting lasers is studied. Luminescence spectra are presented from two devices which demonstrate sharp, highly-polarized, regularly spaced modes for pump intensities above a threshold. The spacing of the laser modes is shown to match the mode spacing predicted by the reflectivity model.
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"Optical Properties of III-Nitride Semiconductors for Power Electronics and Photovoltaics." Doctoral diss., 2020. http://hdl.handle.net/2286/R.I.62699.
Full textAbstract:
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
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Kong, Wei. "Lattice Engineering of III-Nitride Heterostructures and Their Applications." Diss., 2016. http://hdl.handle.net/10161/12195.
Full textAbstract:
III-Nitride materials have recently become a promising candidate for superior applications over the current technologies. However, certain issues such as lack of native substrates, and high defect density have to be overcome for further development of III-Nitride technology. This work presents research on lattice engineering of III-Nitride materials, and the structural, optical, and electrical properties of its alloys, in order to approach the ideal material for various applications. We demonstrated the non-destructive and quantitative characterization of composition modulated nanostructure in InAlN thin films with X-ray diffraction. We found the development of the nanostructure depends on growth temperature, and the composition modulation has impacts on carrier recombination dynamics. We also showed that the controlled relaxation of a very thin AlN buffer (20 ~ 30 nm) or a graded composition InGaN buffer can significantly reduce the defect density of a subsequent epitaxial layer. Finally, we synthesized an InAlGaN thin films and a multi-quantum-well structure. Significant emission enhancement in the UVB range (280 – 320 nm) was observed compared to AlGaN thin films. The nature of the enhancement was investigated experimentally and numerically, suggesting carrier confinement in the In localization centers.
Dissertation
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"Design, Growth, and Characterization of III-Sb and III-N Materials for Photovoltaic Applications." Doctoral diss., 2019. http://hdl.handle.net/2286/R.I.53926.
Full textAbstract:
abstract: Photovoltaic (PV) energy has shown tremendous improvements in the past few decades showing great promises for future sustainable energy sources. Among all PV energy sources, III-V-based solar cells have demonstrated the highest efficiencies. This dissertation investigates the two different III-V solar cells with low (III-antimonide) and high (III-nitride) bandgaps.
III-antimonide semiconductors, particularly aluminum (indium) gallium antimonide alloys, with relatively low bandgaps, are promising candidates for the absorption of long wavelength photons and thermophotovoltaic applications. GaSb and its alloys can be grown metamorphically on non-native substrates such as GaAs allowing for the understanding of different multijunction solar cell designs. The work in this dissertation presents the molecular beam epitaxy growth, crystal quality, and device performance of AlxGa1−xSb solar cells grown on GaAs substrates. The motivation is on the optimization of the growth of AlxGa1−xSb on GaAs (001) substrates to decrease the threading dislocation density resulting from the significant lattice mismatch between GaSb and GaAs. GaSb, Al0.15Ga0.85Sb, and Al0.5Ga0.5Sb cells grown on GaAs substrates demonstrate open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. In addition, a detailed study is presented to demonstrate the temperature dependence of (Al)GaSb PV cells.
III-nitride semiconductors are promising candidates for high-efficiency solar cells due to their inherent properties and pre-existing infrastructures that can be used as a leverage to improve future nitride-based solar cells. However, to unleash the full potential of III-nitride alloys for PV and PV-thermal (PVT) applications, significant progress in growth, design, and device fabrication are required. In this dissertation, first, the performance of
ii
InGaN solar cells designed for high temperature application (such as PVT) are presented showing robust cell performance up to 600 ⁰C with no significant degradation.
In the final section, extremely low-resistance GaN-based tunnel junctions with different structures are demonstrated showing highly efficient tunneling characteristics with negative differential resistance (NDR). To improve the efficiency of optoelectronic devices such as UV emitters the first AlGaN tunnel diode with Zener characteristic is presented. Finally, enabled by GaN tunnel junction, the first tunnel contacted InGaN solar cell with a high VOC value of 2.22 V is demonstrated.Dissertation/Thesis
Doctoral Dissertation Electrical Engineering 2019
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Reed, Mason Jacob. "Light emitting diodes and dilute magnetic semiconductors in the III-nitride materials system." 2005. http://www.lib.ncsu.edu/theses/available/etd-07052005-162129/unrestricted/etd.pdf.
Full text
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"Modeling, Growth and Characterization of III-V and Dilute Nitride Antimonide Materials and Solar Cells." Doctoral diss., 2017. http://hdl.handle.net/2286/R.I.44054.
Full textAbstract:
abstract: III-V multijunction solar cells have demonstrated record efficiencies with the best device currently at 46 % under concentration. Dilute nitride materials such as GaInNAsSb have been identified as a prime choice for the development of high efficiency, monolithic and lattice-matched multijunction solar cells as they can be lattice-matched to both GaAs and Ge substrates. These types of cells have demonstrated efficiencies of 44% for terrestrial concentrators, and with their upright configuration, they are a direct drop-in product for today’s space and concentrator solar panels. The work presented in this dissertation has focused on the development of relatively novel dilute nitride antimonide (GaNAsSb) materials and solar cells using plasma-assisted molecular beam epitaxy, along with the modeling and characterization of single- and multijunction solar cells.
Nitrogen-free ternary compounds such as GaInAs and GaAsSb were investigated first in order to understand their structural and optical properties prior to introducing nitrogen. The formation of extended defects and the resulting strain relaxation in these lattice-mismatched materials is investigated through extensive structural characterization. Temperature- and power-dependent photoluminescence revealed an inhomogeneous distribution of Sb in GaAsSb films, leading to carrier localization effects at low temperatures. Tuning of the growth parameters was shown to suppress these Sb-induced localized states.
The introduction of nitrogen was then considered and the growth process was optimized to obtain high quality GaNAsSb films lattice-matched to GaAs. Near 1-eV single-junction GaNAsSb solar cells were produced. The best devices used a p-n heterojunction configuration and demonstrated a current density of 20.8 mA/cm2, a fill factor of 64 % and an open-circuit voltage of 0.39 V, corresponding to a bandgap-voltage offset of 0.57 V, comparable with the state-of-the-art for this type of solar cells. Post-growth annealing was found to be essential to improve Voc but was also found to degrade the material quality of the top layers. Alternatives are discussed to improve this process. Unintentional high background doping was identified as the main factor limiting the device performance. The use of Bi-surfactant mediated growth is proposed for the first time for this material system to reduce this background doping and preliminary results are presented.Dissertation/Thesis
Doctoral Dissertation Electrical Engineering 2017
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Rajpalke, Mohana K. "Semipolar And Nonpolar Group III-Nitride Heterostructures By Plasma-Assisted Molecular Beam Epitaxy." Thesis, 2012. https://etd.iisc.ac.in/handle/2005/2483.
Full textAbstract:
Group III-nitride semiconductors are well suited for the fabrication of devices including visible-ultraviolet light emitting diodes, high-temperature and high-frequency devices. The wurtzite III-nitride based heterostructures grown along polar c-direction have large internal electric fields due to discontinuities in spontaneous and piezoelectric polarizations. For optoelectronic devices, such as light-emitting diodes and laser diodes, the internal electric field is deleterious as it causes a spatial separation of electron and hole wave functions in the quantum wells, which decreases emission efficiency. Growth of GaN-based heterostructures in alternative orientations, which have reduced (semipolar) or no polarization (nonpolar) in the growth direction, has been a major area of research in the last few years. The correlation between structural, optical and transport properties of semipolar and nonpolar III-nitride would be extremely useful. The thesis focuses on the growth and characterizations of semipolar and nonpolar III-nitride heterostructures by plasma-assisted molecular beam epitaxy.
Chapter 1 provides a brief introduction to the III-nitride semiconductors. The importance of semipolar and nonpolar III-nitride heterostructures over conventional polar heterostructures has been discussed.
Chapter 2 deals with the descriptions of molecular beam epitaxy system and working principles of different characterization tools used in the present work.
Chapter 3 addresses the molecular beam epitaxial growth of nonpolar (1 1 -2 0) and semipolar (1 1 -2 2) GaN on sapphire substrates. An in-plane orientation relationship is found to be [0 0 0 1] GaN || [-1 1 0 1] sapphire and [-1 1 0 0] GaN || [1 1 -2 0] sapphire for nonpolar GaN on r-sapphire substrates. Effect of growth temperature on structural, morphological and optical properties of nonpolar GaN has been studied. The growth temperature plays a major role in controlling crystal quality, morphology and emission properties of nonpolar a-plane GaN. The a-plane GaN shows crystalline anisotropy nature and it has reduced with increase in the growth temperature. The surface roughness was found to decrease with increase in growth temperature and film grown at 760°C shows reasonably smooth surface with roughness 3.05 nm. Room temperature photoluminescence spectra show near band emission peak at 3.434 -3.442 eV. The film grown at 800 ºC shows broad yellow luminescence peak at
2.2 eV. Low temperature photoluminescence spectra show near band emission at 3.483 eV along with defect related emissions. Raman spectra exhibit blue shift due to compressive strain in the film. An in-plane orientation relationship is found to be [1 -1 00] GaN || [1 2-1 0] sapphire and [-1 -1 2 3] GaN || [0 0 0 1] sapphire for semipolar GaN on m-plane sapphire substrates. The surface morphology of semipolar GaN film is found to be reasonably smooth with pits on the surface. Room temperature photoluminescence shows the near band emission (NBE) at 3.432 eV, which is slightly blue shifted compared to the bulk GaN. The Raman E2 (high) peak position observed at 569.1 cm1.
Chapter 4 deals with the fabrication and characterizations of Au/nonpolar and Au/semipolar GaN schottky diodes. The temperature-dependent current–voltage measurements have been used to determine the current mechanisms in Schottky diodes fabricated on nonpolar a-plane GaN and semipolar GaN epilayers. The barrier height (φb) and ideally factor (η) estimated from the thermionic emission model are found to be temperature dependent in nature indicate the deviations from the thermionic emission (TE) transport mechanism. Low temperature I-V characteristics of Au/ GaN Schottky diode show temperature independent tunnelling parameter. Barrier heights calculated from XPS are found to be 0.96 eV and 1.13 eV for Au/nonpolar GaN and Au/semipolar GaN respectively.
Chapter 5 demonstrates the growth of InN on r-sapphire substrates with and without GaN buffer layer. InN film and nanostructures are grown on r-sapphire without GaN buffer layer and they are highly oriented along (0002) direction. The electron microscopy study confirms the nanostructures are vertically aligned and highly oriented along the (0001) direction. The Raman studies of InN nanostructures show the SO modes along with the other possible Raman modes. The band gap of InN nanostructures is found to be 0.82 eV. InN grown with a-plane GaN buffer shows nonpolar orientated growth. Growth temperature dependent studies of nonpolar a-plane InN epilayers are carried out. The valence band offset value is calculated to be 1.31 eV for nonpolar a-plane InN/GaN heterojunctions. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 1.41 eV.
Chapter 6 deals with the temperature dependent I-V characteristics of the nonpolar a-plane (1 1 -2 0) InN/GaN heterostructures. The measured values of barrier height and ideality factor from the TE model show the temperature dependent variation. The double Gaussian distribution has mean barrier height values ( ϕb ) of 1.17 and 0.69 eV with standard deviation (σs ) of 0.17 and 0.098 V, respectively. The modified Richardson plot ln (Is/T2)-q2σ2/2k2T2 ) versus q/kT in the temperature range of 350 – 500 K, yielded the Richardson constant of 19.5 A/cm2 K2 which is very close to the theoretical value of 24 A/cm2 K2 for n-type GaN. The tunneling parameters E0 found to be temperature independent at low temperature range (150 –300 K).
Chapter 7 concludes with the summary of present investigations and the scope for future work.
47
Rajpalke, Mohana K. "Semipolar And Nonpolar Group III-Nitride Heterostructures By Plasma-Assisted Molecular Beam Epitaxy." Thesis, 2012. http://hdl.handle.net/2005/2483.
Full textAbstract:
Group III-nitride semiconductors are well suited for the fabrication of devices including visible-ultraviolet light emitting diodes, high-temperature and high-frequency devices. The wurtzite III-nitride based heterostructures grown along polar c-direction have large internal electric fields due to discontinuities in spontaneous and piezoelectric polarizations. For optoelectronic devices, such as light-emitting diodes and laser diodes, the internal electric field is deleterious as it causes a spatial separation of electron and hole wave functions in the quantum wells, which decreases emission efficiency. Growth of GaN-based heterostructures in alternative orientations, which have reduced (semipolar) or no polarization (nonpolar) in the growth direction, has been a major area of research in the last few years. The correlation between structural, optical and transport properties of semipolar and nonpolar III-nitride would be extremely useful. The thesis focuses on the growth and characterizations of semipolar and nonpolar III-nitride heterostructures by plasma-assisted molecular beam epitaxy.
Chapter 1 provides a brief introduction to the III-nitride semiconductors. The importance of semipolar and nonpolar III-nitride heterostructures over conventional polar heterostructures has been discussed.
Chapter 2 deals with the descriptions of molecular beam epitaxy system and working principles of different characterization tools used in the present work.
Chapter 3 addresses the molecular beam epitaxial growth of nonpolar (1 1 -2 0) and semipolar (1 1 -2 2) GaN on sapphire substrates. An in-plane orientation relationship is found to be [0 0 0 1] GaN || [-1 1 0 1] sapphire and [-1 1 0 0] GaN || [1 1 -2 0] sapphire for nonpolar GaN on r-sapphire substrates. Effect of growth temperature on structural, morphological and optical properties of nonpolar GaN has been studied. The growth temperature plays a major role in controlling crystal quality, morphology and emission properties of nonpolar a-plane GaN. The a-plane GaN shows crystalline anisotropy nature and it has reduced with increase in the growth temperature. The surface roughness was found to decrease with increase in growth temperature and film grown at 760°C shows reasonably smooth surface with roughness 3.05 nm. Room temperature photoluminescence spectra show near band emission peak at 3.434 -3.442 eV. The film grown at 800 ºC shows broad yellow luminescence peak at
2.2 eV. Low temperature photoluminescence spectra show near band emission at 3.483 eV along with defect related emissions. Raman spectra exhibit blue shift due to compressive strain in the film. An in-plane orientation relationship is found to be [1 -1 00] GaN || [1 2-1 0] sapphire and [-1 -1 2 3] GaN || [0 0 0 1] sapphire for semipolar GaN on m-plane sapphire substrates. The surface morphology of semipolar GaN film is found to be reasonably smooth with pits on the surface. Room temperature photoluminescence shows the near band emission (NBE) at 3.432 eV, which is slightly blue shifted compared to the bulk GaN. The Raman E2 (high) peak position observed at 569.1 cm1.
Chapter 4 deals with the fabrication and characterizations of Au/nonpolar and Au/semipolar GaN schottky diodes. The temperature-dependent current–voltage measurements have been used to determine the current mechanisms in Schottky diodes fabricated on nonpolar a-plane GaN and semipolar GaN epilayers. The barrier height (φb) and ideally factor (η) estimated from the thermionic emission model are found to be temperature dependent in nature indicate the deviations from the thermionic emission (TE) transport mechanism. Low temperature I-V characteristics of Au/ GaN Schottky diode show temperature independent tunnelling parameter. Barrier heights calculated from XPS are found to be 0.96 eV and 1.13 eV for Au/nonpolar GaN and Au/semipolar GaN respectively.
Chapter 5 demonstrates the growth of InN on r-sapphire substrates with and without GaN buffer layer. InN film and nanostructures are grown on r-sapphire without GaN buffer layer and they are highly oriented along (0002) direction. The electron microscopy study confirms the nanostructures are vertically aligned and highly oriented along the (0001) direction. The Raman studies of InN nanostructures show the SO modes along with the other possible Raman modes. The band gap of InN nanostructures is found to be 0.82 eV. InN grown with a-plane GaN buffer shows nonpolar orientated growth. Growth temperature dependent studies of nonpolar a-plane InN epilayers are carried out. The valence band offset value is calculated to be 1.31 eV for nonpolar a-plane InN/GaN heterojunctions. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 1.41 eV.
Chapter 6 deals with the temperature dependent I-V characteristics of the nonpolar a-plane (1 1 -2 0) InN/GaN heterostructures. The measured values of barrier height and ideality factor from the TE model show the temperature dependent variation. The double Gaussian distribution has mean barrier height values ( ϕb ) of 1.17 and 0.69 eV with standard deviation (σs ) of 0.17 and 0.098 V, respectively. The modified Richardson plot ln (Is/T2)-q2σ2/2k2T2 ) versus q/kT in the temperature range of 350 – 500 K, yielded the Richardson constant of 19.5 A/cm2 K2 which is very close to the theoretical value of 24 A/cm2 K2 for n-type GaN. The tunneling parameters E0 found to be temperature independent at low temperature range (150 –300 K).
Chapter 7 concludes with the summary of present investigations and the scope for future work.
48
Chandrasekar, Hareesh. "Dissimilar Hetero-Interfaces with Group III-A Nitrides : Material And Device Perspectives." Thesis, 2016. http://etd.iisc.ac.in/handle/2005/2740.
Full textAbstract:
Group III-A nitrides (GaN, AlN, InN and alloys) are materials of considerable contemporary interest and currently enable a wide variety of optoelectronic and high-power, high-frequency electronic applications. All of these applications utilize device structures that employ a single or multiple hetero-junctions, with material compositions varying across the interface. For example, the workhorse of GaN based electronic devices is the high electron mobility transistor (HEMT) which is usually composed of an AlGaN/GaN hetero-junction, where a two-dimensional electron gas (2DEG) is formed due to differences in polarization between the two layers. In addition to such hetero-junctions in the same material family, formation of hetero-interfaces in nitrides begins right from the epitaxy of the very first layer due to the lack of native substrates for their growth. The consequences of such "dissimilar" hetero-junctions typically manifest as large defect densities at this interface which in turn gives rise to defective films. Additionally, if the substrate is also a semiconductor, the electrical properties at such dissimilar semiconductor-nitride hetero-junctions are particularly important in terms of their influence on the performance of nitride devices. Nevertheless, the large defect densities at such dissimilar 3D-3D semiconductor interfaces, which translate into more trap states, also prevents them from being used as active device layers to say nothing of reliability considerations arising because of these defects. Recently, the advent of 2D materials such as graphene and MoS2 has opened up avenues for Van der Waal’s epitaxy of these layered films with practically any other material. Such defect-free integration enables dissimilar semiconductor hetero-junctions to be used as active device layers with carrier transport across the 2D-3D hetero-interface. This thesis deals with hetero-epitaxial growth platforms for reducing defect densities, and the material and electrical properties of dissimilar hetero-junctions with the group III-A nitride material system.
49
Chandrasekar, Hareesh. "Dissimilar Hetero-Interfaces with Group III-A Nitrides : Material And Device Perspectives." Thesis, 2016. http://hdl.handle.net/2005/2740.
Full textAbstract:
Group III-A nitrides (GaN, AlN, InN and alloys) are materials of considerable contemporary interest and currently enable a wide variety of optoelectronic and high-power, high-frequency electronic applications. All of these applications utilize device structures that employ a single or multiple hetero-junctions, with material compositions varying across the interface. For example, the workhorse of GaN based electronic devices is the high electron mobility transistor (HEMT) which is usually composed of an AlGaN/GaN hetero-junction, where a two-dimensional electron gas (2DEG) is formed due to differences in polarization between the two layers. In addition to such hetero-junctions in the same material family, formation of hetero-interfaces in nitrides begins right from the epitaxy of the very first layer due to the lack of native substrates for their growth. The consequences of such "dissimilar" hetero-junctions typically manifest as large defect densities at this interface which in turn gives rise to defective films. Additionally, if the substrate is also a semiconductor, the electrical properties at such dissimilar semiconductor-nitride hetero-junctions are particularly important in terms of their influence on the performance of nitride devices. Nevertheless, the large defect densities at such dissimilar 3D-3D semiconductor interfaces, which translate into more trap states, also prevents them from being used as active device layers to say nothing of reliability considerations arising because of these defects. Recently, the advent of 2D materials such as graphene and MoS2 has opened up avenues for Van der Waal’s epitaxy of these layered films with practically any other material. Such defect-free integration enables dissimilar semiconductor hetero-junctions to be used as active device layers with carrier transport across the 2D-3D hetero-interface. This thesis deals with hetero-epitaxial growth platforms for reducing defect densities, and the material and electrical properties of dissimilar hetero-junctions with the group III-A nitride material system.
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Bhat, Thirumaleshwara N. "Group III Nitride/p-Silicon Heterojunctions By Plasma Assisted Molecular Beam Epitaxy." Thesis, 2012. https://etd.iisc.ac.in/handle/2005/2454.
Full textAbstract:
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.