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AlGaN-based ultraviolet LEDs attract significant interests recently for their versatile applications. Germicidal irradiation in the UV-C wavelengths can combat future pandemic outbreaks while wavelengths in the longer UV-B/UV-A range have wideapplications in biochemical sensing and medical treatment. However, commercial AlGaN-based ultraviolet LEDs have much lower power efficiency (< 5%) than the traditional incumbent technologies mercury lamps due to many challenges this ultra-wide bandgap material poses in the epitaxial MOCVD growth, device design and fabrication technologies. In this dissertation, I will first discuss AlN epitaxy technologies on SiC and sapphire substrates by MOCVD. With optimized growth condition, we have achieved highly conductive n-type Al0.6Ga0.4N with around 5 mΩ-cm and demonstrated low contact resistance Vanadium based ohmic contact on n-type Al0.6Ga0.4N. I will then present how doping efficiency in p-type AlGaN can be massively improved with polarization engineering to achieve efficient carrier injection for UV LEDs. Next, I will discuss some aspects of the active region device design and MOCVD growth optimization that can further enhance the device performance. Combining the developments in each of these areas with improved fabrication and packagingtechnologies, we have demonstrated UV LEDs with world class efficiencies from deep UV (below 250 nm) to UV-A wavelength (320+ nm). Lastly, I will present some recent developments of UV-A LEDs results on GaN templates and discuss how some of these technologies can be utilized for future UV laser diodes developments.

Global population increase has highlighted the scarcity of resources, specifically energy and potable water, and emphasized the need to develop new technologies to help meet these growing needs. Additionally, the impact of the historic use of carbon-intensive fuel sources to meet energy demand has become increasingly apparent through the environmental repercussions of anthropogenic climate change. Since energy and water security intrinsically consider cost and access to resources, disruptive technologies to improve energy and water security must exhibit translatability, scalability, and low-cost. Harnessing abundant renewable energy sources such as solar, hydro, and wind is a promising solution to carbon-intensive energy production, while also providing low-cost energy. The coupling of renewable energy resources with energy-intensive and expensive water remediation processes such as reverse osmosis, can lower their costs. However, the efficient storage of intermittent renewable resources during peaks hours of production for delivery of energy during times of peak demand has proven a challenge for decades. Furthermore, the development of robust membrane materials that drastically impact the cost of water remediation, especially considering increasing water pollutants from anthropogenic activity, have yet to be realized in recent years. As a result, research and development of more efficient energy storage and robust membrane technologies is vital for meeting society’s growing demand for energy and water remediation without increasing anthropogenic greenhouse gas production. A thorough understanding of both fundamental cationic-host relationships and gaseous precursor-substrate interactions are necessary for the intelligent design of next generation energy storage and membrane materials. This dissertation work focuses on the design of synthetic parameters for controlled material property engineering, the development of electrochemical methods elucidating cationic behavior within host materials, structural and electronic changes to host material as a function of cationic identity and amount, as well as the synthesis of a new compositional space.In this work, emphasis was placed upon identifying reaction conditions in a metal organic chemical vapor deposition system (MOCVD) that allows control over layer stacking and defect population in 2D metal chalcogenides. The MOCVD is a unique chemical vapor deposition system in which the concentrations of metal organic precursors in the gaseous phase can be finitely controlled to deliver highly specific precursor ratios for thin film synthesis. Additionally, the aqueous intercalation of cation(s) within 3D metal chalcogenides to observe cationic behavior as well as structurally and electronically induced changes to the host was investigated. These structural and electronic changes were investigated through a suite of electrochemical experiments to observe cationic behavior, X-ray absorption spectroscopy (XAS) for electronic changes, X-ray diffraction (XRD) for structural changes, and theoretical computational modeling to elucidate energetics of various scenarios. All these components aided in the development of an expanded fundamental understanding of material synthesis design and cationic behavior for remediation and energy storage applications, respectively.Chapter 1 presents an overview of the state-of-the-art in water desalination membrane and energy storage technology. There is also an introduction to the 2D and 3D molybdenum chalcogenide materials focused on in this work, inclusive of their structure, properties and historical applications. Chapter 1 also presents the current shortcomings of current state-of-the-art technologies and how the molybdenum chalcogenides presented have potential to overcome those barriers.Chapter 2 describes the development of experimental parameters for 2D transition metal dichalcogenides (e.g., MoS2 and MoSe2) in a MOCVD system. Major principles affecting crystal growth in the MOCVD system (thermodynamics, kinetics, and mass transport) and the need to deconvolute the “black box” of synthesis is also discussed. Focus is placed on using our understanding of the major principles affecting crystal growth to develop methodology in which wide-area coverage, layer-by-layer growth, and defect population are controlled. Lastly, we discuss major findings and current issues in the field, mainly the reproducibility of studies, and uniform reporting and open-access databases can mitigate that.Chapter 3 details the structural and electronic changes of the 3D molybdenum chalcogenide, Mo6S8, as a function of stoichiometric amount of Cu content. Electrochemical responses were correlated to structural changes and stoichiometry via ex-situ XRD analysis. Electronic changes to the host Mo6S8 material as a function of Cu content was observed via changes in character of the S K-pre-edge feature which was analyzed using X-ray Absorption Near-Edge Structure (XANES). We observed a depression of the S K-pre-edge feature as a function of increasing Cu content and posed using XANES pre-edge features as method to monitor metal content in other materials. Theory Density Functional Theory (DFT) and density of states (DOS) calculations were also employed by collaborators at CU Boulder to discuss the metastability of Cu4Mo6S8¬ at open-circuit voltage and metal-to-semiconductor bandgap transitions as a function of Cu content in the Mo6S8 system (i.e., CuxMo6S8; x=1-4).Chapter 4 outlines the dual intercalation of both Cu and Zn within the binary Mo6S8 system via electrochemical methods in aqueous electrolytes. Unique structural changes were identified via ex-situ XRD where apparently Cu and Zn had a similar structural effect on the host material due to their similarity in ionic radius, despite their difference in charge (Cu+1 vs Zn2+). Despite both Cu and Zn being present in the host material, confirmed via Energy-Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS), the resulting XRD pattern resembled that of pure-phase Cu2Mo6S8. Lastly, this work reports the synthesis of the CuxZnyMo6S8 composition for the first time.

The InGaN material system spans from the ultraviolet (363nm) to the near infrared (1.8µm). It has found tremendous success in solid state lighting, leading to huge improvements in both lifetime and efficiencies of white light sources. InGaN-based light sources can be found in most households, automobiles and displays today. However, these successes are mostly limited to low In-fraction devices emitting in the violet and blue wavelengths. This dissertation focuses on long wavelength InGaN devices emitting in the red, far-red and near infrared.Because of the large lattice mismatch between InN and GaN, high composition InGaN necessary for long wavelength devices is highly strained when grown on a GaN template. A novel method of biaxially strain-relaxing an InGaN buffer across a planar substrate is demonstrated by leveraging the temperature sensitivity of high composition InGaN layers. The thermal decomposition of an InGaN underlayer to form voids is shown to relax an InGaN buffer across the entire substrate. Quantum wells grown on these buffers show red-shifted emission and a decrease in the strain-state of the active region. We then optimize the growth conditions of the relaxed InGaN buffers to demonstrate smooth, highly relaxed InGaN in a single MOCVD growth.Next, we demonstrate devices on the strain-relaxed buffers that compare favorably with those using other relaxation methods. The devices show remarkably low forward voltage and high growth temperature compared to conventionally grown counterparts. Using this relaxation method, far-red LEDs emitting over 750nm are achievable. We show red 5 × 5 µm^2 micro-LEDs emitting at 640nm with > 0.45% EQE measured on-wafer.Finally, we explore growth of InN quantum dots on semipolar (2021) and (2021) substrates, two orientations previously unexplored in InN or In-rich InGaN. We map out growth parameters and characterize the crystal growth on the two planes, demonstrating room temperature photoluminescence in the 1300nm range. An unconventional p-down device structure is proposed and progress is shown towards a first electrically-injected InN LED with photoluminescence shown from a full device. Key challenges are identified, namely the temperature sensitivity of InN and poor crystal quality from the resulting low temperature growth.

Germicidal ultraviolet (GUV) light with wavelengths between 250 nm and 300 nm was first discovered to have a strong disinfectant effect in the 19th century. GUV irradiation is a chemical-free, species-agnostic disinfection technology: some example applications include disinfection of medical equipment and buildings; municipal water treatment, residential water purification, and consumer devices like water bottles; air disinfection in offices, schools, planes, cruise ships, and retail spaces, and countless others. Most importantly, a low-cost, durable, portable, and efficient solid-state GUV light source could bring these disinfection applications to low-income and developing regions where most sewage enters waterways untreated, and the lack of sterile environments and equipment worsens healthcare outcomes. The current incumbent technology for GUV light emission is the mercury lamp. This technology has remained largely unchanged for the past century—it is efficient and inexpensive, and has a large market due to its use for phosphor-converted white light (“fluorescent lighting”). On the other hand, this technology relies on toxic mercury vapor contained within a hot quartz bulb, requires complex and inefficient driving electronics, and cannot be reliably dimmed or rapidly switched on/off. To meet growing demand for disinfection technologies, and for GUV to expand into residential, consumer, and smart applications, a solid-state GUV light emitter is needed. A UV LED would have many benefits, much like the now-mature solid-state blue and white LEDs which have revolutionized the lighting and display industries. Semiconductor LEDs can be produced at smaller sizes, lower costs, higher efficiencies, and longer lifetimes than their 19th-century vapor lamp counterparts; these semiconductor devices can be dimmed or switched rapidly with no warm-up or flicker, and are readily produced in a variety of target wavelengths. Currently available UV LED technologies based on the AlGaN materials system are much less efficient (3-5%) than Hg lamps (20-30%), due to a number of challenges related to AlGaN material quality, chemical purity, process technologies, and optical properties. Therefore, there is a strong desire to improve the efficiency, cost, and lifetime of UV emitters to match their visible LED counterparts, which have efficiencies well over 50% and lifetimes over 10,000 h. In this dissertation, I discuss the ways in which AlN and AlGaN defect density on foreign substrates has been greatly reduced using improved MOCVD growth methods, achieving a dislocation density of 2x108 cm-2 on SiC. Next, I will explain why n-type and p-type doping (both of which are needed for LED operation) of AlGaN are challenging, and how both have been greatly improved using growth condition modification and polarization engineering, respectively. Using the methods outlined in this dissertation, we achieved n-type AlxGa1-xN (x>0.65) resistivities below 10 m-cm in material grown on three different substrate platforms (4H-SiC, 6H-SiC, and sapphire), and on two different MOCVD reactors. I will discuss active region engineering for increased light emission power, as well as an analysis of the various LED processing methods and packaging architectures used in the devices shown. Combining improvements in material quality, electrical conductivity, and optical quality, we have increased GUV LED external quantum efficiency from 2% to 10%.

With the advent of things like autonomous vehicles, augmented reality, high-capacity wireless networks, and high performance computing, the ways in which we sense, process and transfer information are evolving, and this is driving innovation in semiconductor electronics and photonics. At the forefront of this evolution III-V semiconductors like indium phosphide (InP) and gallium arsenide (GaAs) present themselves as high performance materials both in electronics (high mobility, heterojunctions) and photonic (direct bandgap, light emission) devices. Growth of these materials is possible via metal organic chemical vapor deposition (MOCVD), an ideal tool because highly scalable and industry standard for high volume production and throughput.In this work we explore several MOCVD selective area growth (SAG) approaches for the growth of III-V based technologies.First, we further develop a type of SAG called template assisted selective epitaxy (TASE) as a way to integrate horizontal heterojunctions (HJs) in a laterally grown structure while also allowing for planar gating. We show successful growth of horizontal HJs of InAs, GaAs and InGaAs, included in InP structures and characterized to show abrupt interfaces and crystalline material. The orientation of the templates and the substrate is chosen so that a flat vertical facet appears at the growth front allowing for the HJs to be horizontal, unlike typical planar epitaxy, enabling the design of novel electronic HJ devices like a low energy triple-HJ tunnel field effect transistor.We then explore how III-V materials can be integrated onto CMOS compatible silicon substrates via SAG to create a low defect density pseudo substrate for the subsequent regrowth of active gain materials and the integration of photonic devices. This is attempted by leveraging aspect ratio trapping effects of TASE on Silicon or SOI (silicon on insulator) while scaling its size to a large enough area to accommodate photonic structures like micro disk lasers.Finally, we demonstrate a different SAG approach where deep (>5 micron) recesses are etched into (001) silicon. The recesses are deep enough to allow application of mature defect engineering techniques (low temperature buffers, defect filtering layers and superlattices) and obtaining an antiphase boundary (APB)-free GaAs micro ridge, while remaining underneath surface, facilitating coupling with adjacent waveguide structures.

AlGaN is among the ultra-wide band gap semiconductors that are projected to power a wide range of the next-generation electronic and photonic applications. It has been applied primarily for the development of germicidal UV LEDs that are crucial for virus disinfection and gained an extra importance in the era of COVID-19 pandemic, with proven efficacy against wide range of viruses - including SARS-CoV-2 - that rivals the most mature disinfection technologies. However, state-of-the-art deep UV LEDs are still lagging behind visible LEDs in terms of output power, lifetime and cost. Our approach to tackle this issue relies on silicon carbide as a growth substrate in contrast to sapphire. Firstly, The advantages gained by employing SiC as a substrate will be highlighted. Secondly, growth of Al-rich AlGaN on SiC by Metalorganics chemical vapor deposition (MOCVD) will be the main focus of this thesis, with emphasis on the AlGaN growth kinetics, doping and active region, Detailing simple, yet robust, solutions for AlGaN cracking, doping inefficiency and multiquantum wells (MQW) growth—solutions that enabled the growth of up to 5 micron-thick crack-free AlGaN films on SiC (among the highest reported), and the growth of highly conductive Al-rich n-AlGaN films up to 83% Al molar fraction with conductivities that match and exceed the best reported in the literature, as well as the growth of MQW with tunable and monochromatic emission in the UV range of maximal disinfection efficacy (250 nm to 280 nm).. In Al-rich AlGaN, the increased ionic character of Al-N bond leads to dissimilar kinetical behavior between Al and Ga adatoms, as well as precipitates several key challenging issues such as greater deviation from ideal unit cell structure, anisotropic light emission and inefficient doping. This work tackles these limits and explores growth and processing techniques to address these challenges, specifically; optimization of n-doped AlGaN will be illustrated with focus on the effect of the different growth parameters on the conductivity and ohmic contact formation. Studies on the impact of MOCVD growth conditions on the UV LEDs performance will be presented, emphasizing how the growth outcome can be influenced by other external parameters; starting from chamber geometry and conditioning to substrate polishing, preparation and pretreatment. Then, the device performance of UVC led will be summarized along with new solutions to further improve the overall performance.

Synthetic two-dimensional (2D) mono-elemental crystals, namely X-enes, have recently emerged as a new frontier for atomically thin nanomaterials with on-demand properties. Among X-enes, antimonene, the β-phase allotrope of antimony, is formed by atoms arranged in buckled hexagonal rings bearing a comparatively higher environmental stability with respect to other players of this kind. However, the exploitation of monolayer or few-layer antimonene and other 2D materials in novel opto-electronic devices is still hurdled by the lack of scalable processes. Here, we demonstrated the viability of a bottom-up process for the epitaxial growth of antimonene-like nanocrystals (ANCs), based on a Metal-Organic Chemical Vapor Deposition (MOCVD) process, assisted by gold nanoparticles (Au NPs) on commensurate (1 1 1)-terminated Ge surfaces. The growth mechanism was investigated by large- and local-area microstructural analysis, revealing that the etching of germanium, catalyzed by the Au NPs, led to the ANCs growth on the exposed Ge (1 1 1) planes. As a supportive picture, ab-initio calculations rationalized this epitaxial relationship in terms of compressively strained β-phase ANCs. Our process could pave the way to the realization of large-area antimonene layers by a deposition process compatible with the current semiconductor manufacturing technology.

Gallium nitride (GaN) is now widely used in commercial white Light Emitting Diodes (LEDs) thanks to the emergence of high-brightness GaN blue LEDs in 1990s. In addition to its application in solid-state lighting, GaN has been also vowed as a strong contender for next-generation high power and frequency devices due to its high critical electric field (3.3 MV/cm) and high mobility of the 2-dimensional electron gas (2DEG) at the aluminum gallium nitride (AlGaN)/GaN interface. Lateral AlGaN/GaN high-electron-mobility-transistors (HEMTs) have been available as commercial off-the-shelf devices since 2005. However, with the demand for even higher power at reduced chip area and cost and with better thermal management at high currents, vertical device architectures have emerged as the chosen structure to meet these demands. But vertical devices that can hold high power require thick and high quality GaN layers. Recent developments of bulk GaN substrate growth technologies allowed vertical GaN device with thick drift layer to be more feasible. However, GaN substrate technology is challenged with cost, reliability and uniformity issues even at the currently commercially available 2” (diameter) substrates. Therefore, GaN vertical power devices on cheap substrates without compromising the GaN material quality remains to be of great interest. Si substrates with their fab-scale integrated circuit technology can propel the development of commercial vertical high power GaN devices. The biggest challenge for realizing thick GaN layers on Si to hold high voltage in the vertical direction is the large thermal and lattice mismatch between GaN and Si that leads to cracking of the GaN layers beyond only a few micrometers.In major part of this dissertation, we will focus on the epitaxy techniques of thick crack-free GaN layers on Si by selective area growth (SAG) and the fabrication of vertical GaN switches. The epitaxy technique developed in this work resulted in crack-free thick GaN layers on Si that are of high quality with low dislocation densities and low background doping in order to sustain high breakdown voltages. The developed processes hold the potential to significantly advance the fundamental electronic materials research in power devices and their efficient system level integration.

With the increased demand for controlled deterministic growth of III–V semiconductors at the nanoscale, the impact and interest of understanding defect formation and crystal structure switching becomes increasingly important. Vapor–liquid–solid (VLS) growth of semiconductor nanocrystals is an important mechanism for controlling and studying the formation of individual crystal layers and stacking defects. Using in situ studies, combining atomic resolution of transmission electron microscopy and controlled VLS crystal growth using metal organic chemical vapor deposition, we investigate the simplest achievable change in atomic layer stacking–single twinned layers formed in GaAs. Using Au-assisted GaAs nanowires of various diameters, we study the formation of individual layers with atomic resolution to reveal the growth difference in forming a twin defect. We determine that the formation of a twinned layer occurs significantly more slowly than that of a normal crystal layer. To understand this, we conduct thermodynamic modeling and determine that the propagation of a twin is limited by the energy cost of forming the twin interface. Finally, we determine that the slower propagation of twinned layers increases the probability of additional layers nucleating, such that multiple layers grow simultaneously. This observation challenges the current understanding that continuous uniform epitaxial growth, especially in the case of liquid-metal assisted nanowires, proceeds one single layer at a time and that its progression is limited by the nucleation rate.

A new principle for supplying volatile precursors to MOCVD gas-phase chemical deposition systems is proposed, based on a two-stage evaporation of an organic solution of precursors from a soaked cotton thread, which passes sequentially through the zones of evaporation of the solvent and precursors. The technological capabilities of TSF-MOCVD (Thread-Solution Feed MOCVD) are demonstrated based on examples of obtaining thin epitaxial films of СеО2, h-LuFeO3 and thin-film heterostructures β-Fe2O3/h-LuFeO3. The results of studying the obtained films by X-ray diffraction, energy dispersive X-rayanalysis, and high- and low-resolution transmission microscopy are presented. Using the TSF module, one can finely vary the crystallisation conditions, obtaining coatings of the required degree of crystallinity, as evidenced by the obtained dependences of the integral width of the h-LuFeO3 reflection on the film growth rate. Based on the TEM and XRD data, it was concluded that β-Fe2O3 grows epitaxially over the h-LuFeO3 layer. Thus, using TSF-MOCVD, one can flexibly change the composition of layered heterostructures and obtain highly crystalline epitaxial films with a clear interface in a continuous deposition process

Gallium Nitride has gained prominence in the field of power electronics due to its high bandgap, high critical electric field, high mobility and high saturation-drift velocity. This means that GaN can be used to make devices that have a low on-resistance along with a high breakdown voltage. High frequency GaN HEMTs have been commercially available since 2006 and still being improved. The high power market is just starting to tap into GaN devices. Vertical transistors are especially attractive for high power applications, as they provide the possibility of a high breakdown voltage at a low chip size, thus, a low cost price. In addition to this, GaN devices can also use the two-dimensional electron gas formed at the GaN/AlGaN heterojunction to obtain a higher current.Several GaN based power devices are being investigated - both vertical and lateral. Our group has been focusing on the Oxide GaN-interlayer Field Effect Transistor (OGFET) and the Current Aperture Vertical Electron Transistor (CAVET). On the lateral side, power HEMTs are being developed. This work focuses on materials development for the various types of power devices. A primary motive for this thesis is for it to serve as a reference manual for researchers working on GaN power devices. Vertical power devices are united by a common feature – a thick drift region with a low n-type carrier concentration. Through simulations, it has been shown that the optimal carrier concentration to simultaneously achieve a low Ron and a high breakdown voltage is about 1 x 1016 cm-3. Through our experiments, it was demonstrated that it is also the carrier concentration range where the electron mobility peaks. Achieving such a low doping can be problematic as it is in the range of impurity dopants such as Carbon and Oxygen. A low doping and record mobility was achieved for vertical devices by MOCVD and applied to the OGFET and the CAVET. Devices were grown both, on sapphire and on bulk GaN and high breakdown voltages were achieved.Power devices require an elaborate growth and fabrication process, including regrowth on p-GaN doped by Magnesium and regrowth within trenches. Magnesium is known to diffuse into subsequent layers during regrowth, lowering device performance. A novel approach to circumvent this issue is presented, which can eliminate the need to remove the sample from the MOCVD chamber altogether. A low temperature GaN layer grown via flow modulation epitaxy successfully suppressed the Mg penetration at a rate of 5 nm/dec, the lowest ever by MOCVD, leading to the formation of an abrupt p-GaN:Mg/GaN junction.Regrowth within trenches was optimized for different applications. GaN was grown conformally within narrow trenches by exploring various growth conditions. A complete filling of trench was also achieved for a different set of applications. Finally, conformal regrowth of high composition, low temperature AlGaN was optimized for deep-recessed gate HEMTs to lower gate leakage. All of these were planar regrowths and no masks were used.

High-quality gate dielectrics are key to securing the performance and reliability needed in GaN-based electronics to drive widespread adoption. GaN has emerged as a strong contender to serve a range of applications in the rapidly growing field of power electronics due to its attractive wide-bandgap and high mobility. As power devices regularly experience high levels of current and high fields, a robust gate dielectric offers the only gating solution that can permit long-term, stable switching operations under these strenuous electrical conditions. However, GaN does not possess a high-quality native oxide, and despite more than a decade of research efforts, a definitive agreement on a preferred gate dielectric material and its deposition technique have yet to be reached. As it currently stands, the long-term performance of GaN transistors is limited by the gate dielectric and its interface with GaN. Hence, the technical development of high-quality gate dielectrics, such as the work illustrated here, will be a prerequisite to the success of GaN for energy-efficient power electronics. This dissertation documents the material selection, growth, and characterization of gate dielectrics, as well as the development of the gate integration process into GaN power transistors. This work describes the first uses of metalorganic chemical vapor deposition (MOCVD) to explore the growth design space of in situ gate oxides on III-nitride epitaxy. In addition, MOCVD growth was used to tune the crystallinity and interfacial/bulk properties of the gate oxides. Metal-oxide-semiconductor (MOS) capacitors served as test structures for electrically characterizing various MOCVD gate oxides for the following features: oxide leakage current, breakdown field, fixed charge, interface and border trap density, charge-to-breakdown, and time-to-breakdown capabilities. MOCVD growth studies commenced with Al2O3 gate dielectrics on GaN and tuning of their electrical properties. A turning point came in the technical development of Al2O3 when the growth space of amorphous Al2O3 was found to be limited. Investigations progressed to AlSiO dielectrics, which was formed by supplying Si into the Al2O3 growth process. This alloying growth strategy extended the amorphous stability as well as improved the electrical properties. Amorphous Al2O3 and AlSiO exhibited superior characteristics over crystalline Al2O3 dielectrics, with lower leakage, higher breakdown, and reduced interface-state and border trap densities. Moreover, AlSiO excelled with a predicted lifetime of 20 years for high electric fields >3 MV/cm based on time-dependent dielectric breakdown measurements. A gate regrowth process was developed to integrate MOCVD dielectrics into vertical GaN trench MOSFETs. The process consisted of an annealing treatment to repair processing damage, followed by a GaN regrowth prior to dielectric deposition to recreate the in situ, high-quality dielectric/GaN interface. This work reports the first growth developments of AlSiO gate dielectrics for GaN-based devices and concludes with the first demonstration of a high-performance AlSiO-gated GaN power FET.

While most commercial gallium nitride (GaN) devices are grown in the (0001) Ga-polar orientation, the N-polar (0001 ̅) orientation is advantageous for heterostructures which can benefit from reversed polarization fields including transistors, photodetectors, solar cells, and optoelectronic devices. One particularly attractive application is tunnel junctions, since GaN/(In,Ga)N/GaN tunnel junction devices rely on the piezoelectric polarization in the N-polar orientation. In these devices the tunneling probability is proportional to the indium composition in the InGaN layer, motivating the investigation of the upper limit of indium incorporation in N-polar InGaN layers embedded into GaN. Devices with high indium mole fraction active regions (>0.25) typically suffer from high defect densities and low quantum efficiency, primarily due to 1) the large 10% lattice mismatch between InN and GaN, and 2) the low thermal stability of InN, requiring significantly lower deposition temperatures compared to GaN layers. The work presented in this thesis addresses the above challenges in multiple ways.First, the deposition of N-polar InGaN layers for tunnel devices will be detailed including the optical, structural, and electrical properties of layers with indium compositions up to 0.46. The electrical performance of N-polar GaN/In0.35Ga0.65N/GaN tunnel diodes will be discussed, including temperature-dependent measurements which confirmed tunneling behavior under reverse bias. Second, the growth of N-polar InN quantum dots and thin films on vicinal GaN base layers will be presented. For thin layers, quantum dot-like features were spontaneously formed to relieve the strain between the InN and GaN layers. For thicker layers above 10 nm, high electron mobilities up to 706 cm2/Vs were measured using Hall effect measurements indicating high quality layers. The properties of GaN/InN/GaN double heterostructures will be presented as well.Next, the use of lattice-engineered InGaN pseudo-substrates (PSs) as base layers for InGaN deposition will be discussed as a strategy to mitigate the lattice mismatch when growing layers with high indium content. Relaxed N-polar InGaN films were grown by MOCVD on N-polar InGaN PSs using a novel digital approach which enabled the deposition of thick layers while maintaining smooth surfaces, and InGaN/GaN multiple quantum wells were deposited on both N-polar and Ga-polar InGaN PSs. The use of the InGaN PSs resulted in InGaN layers with about 50% higher In compositions and enhanced optical properties compared to those grown on traditional GaN templates. Finally, the impurity incorporation behavior of N-polar layers grown at reduced temperatures in the InGaN growth regime will be presented along with the growth and process optimization for the fabrication of reduced temperature p-GaN layers. Although the majority of this thesis focuses on the growth of indium-containing layers, these studies are useful for the fabrication of N-polar devices grown at low temperatures. By minimizing the impurity incorporation and optimizing the p-GaN growth process, p-contact resistances as low as 2.77 mΩcm2 were achieved.

The aim of the study is to understand the effects of NH3 flow rate in the initial part of high temperature (HT) GaN growth on structural and optical characteristics of the HT-GaN layer grown on dome shaped sapphire susbtrate by Metal Organic Chemical Vapor Deposition (MOCVD) system. High resolution x-ray diffraction (HRXRD) and photoluminescence (PL) measurements were performed to characterization the growing GaN epilayer. It is observed that the using of different NH3 flow rate in the initial part of HT-GaN growth has an effect on both full-width at half-maximum (FWHM) values obtained from HRXRD results and intensities of yellow luminescence peaks. It is seen that the FWHM values obtained from the symmetric (00.2) omega scan increased as the NH3 flow rates in the initial part of HT-GaN growth increased. It is demonstrated that the intensities of yellow luminescence peaks are very sensitive to NH3 flow rates in the initial part of HT-GaN growth.

Iron oxide thin films were grown directly on three‐dimensional nickel foam via metalorganic chemical vapor deposition (MOCVD) in the temperature range of 250–450 °C using Fe(CO)5 as precursor. Iron oxide (α‐Fe2O3) films were formed at low substrate temperatures (250–350 °C), whereas the additional growth of an underlying NiO film occurred at substrate temperatures above 350 °C. The electrochemical activities of the as‐formed binder‐free and noble metal‐free electrodes were tested for the oxygen evolution reaction (OER) in alkaline media. An overpotential reduced by 250 mV at a current density of 50 mA cm−2 and a lower Tafel slope of 55 mV dec−1 compared to bare nickel foam were found for the best‐performing electrocatalyst, while the long‐term stability of the as‐formed electrodes was proven by chronopotentiometry. The surface morphology of the iron oxide films was characterized by scanning electron microscopy, whereas the crystallographic phase as well as the elemental composition were determined by X‐ray diffraction, energy‐dispersive X‐ray spectroscopy, X‐ray photoelectron spectroscopy, and time‐of‐flight secondary ion mass spectrometry in the pre‐ and the post‐catalytic state., Electrocatalysts from the gas phase: α‐Fe2O3 thin films are grown via metalorganic chemical vapor deposition on three‐dimensional Ni foam as sustainable and earth‐abundant electrocatalyst for the oxygen evolution reaction in alkaline media.

International audience; Novel adducts of lithium hexafluoroacetylacetonato {Li(hfa)} with polyethers (monoglyme = {CH3OCH2CH2OCH3}, diglyme = {CH3O(CH2CH2O)2CH3}, triglyme = {CH3O(CH2CH2O)3CH3} and tetraglyme {CH3O(CH2CH2O)4CH3} have been synthesized through a single step reaction and characterized by FT-IR spectroscopy, 1H, and 13C NMR, single crystal X-ray diffraction studies along with thermal analysis.

International audience; Peculiar and unique growth mechanisms involved in semiconductor nanowires (NWs) pave the way to the achievement of new crystallographic phases and remarkable material properties, and hence, studying polytypism in semiconductor NWs arouses a strong interest for the next generation of electronic and photonic applications. In this context, the growth of ZnS nanowires has been investigated, as bulk ZnS compound exhibits numerous unstable polytypes at high temperatures, but their stable occurrence is highly anticipated in a nanowire due to its special quasi-dimensional shape and growth modes. In this work, the idea is to provide a change in the growth mechanism via the physical state of catalyst droplet (liquid or solid) and hence, study the induced structural modifications in ZnS nanowires. The HRTEM images of VLS (via liquid alloyed catalyst) grown ZnS NWs show periodic stacking faults, which is precisely identified as a stacking sequence of cubic or hexagonal individual planes leading to an astonishing 15R crystal polymorph. This crystallographic phase is observed for the first time in nanowires. Contrastingly, NWs grown with VSS (via solid catalyst) show crystal polytypes of zinc blende and wurtzite. We calculate and discuss the role of cohesive energies in the formation of such ZnS polytypes. Further, we present the selection rules for the crystallization of such 15R structure in NWs and discuss the involved VLS and VSS growth mechanisms leading to the formation of different crystal phases.

We report optical phonon vibration modes in ensembles of self-catalyzed InP/InAs/InP multi core-shell one-dimensional nanostructures (nanopillars and nanocones) grown on InP(111)B substrates using liquid indium droplets as a catalyst via metal-organic chemical vapor deposition. We characterized the Raman vibration modes of InAs E1(TO), InAs A1(TO), InAs E1(LO), InP E1(TO), InP A1(LO), and InP E1(LO) from the ensemble of as-grown nanostructures. We also identified second-order Raman vibration modes, associated with InP E1(2TO), E1(LO+TO), and E1(2LO), in the InP/InAs/InP core-shell nanopillars and nanocones. Raman spectra of InP/InAs/InP nanopillars showed redshift and broadening of LO modes at low-frequency branches of InAs and InP. Due to the polar nature in groups III–V nanowires, we observed strong frequency splitting between InAs E1(TO) and InAs A1(LO) in InP/InAs/InP nanocones. The Raman resonance intensities of InP and InAs LO modes are found to be changed linearly with an excitation power. By tilting the substrate relative to the incoming laser beam, we observed strong suppression of low-frequency branch of InP and InAs LO phonon vibrations from InP/InAs/InP nanocones. The integrated intensity ratio of InP E1(TO)/E1(LO) for both nanostructures is almost constant at 0-degree tilt, but the ratio of the nanocones is dramatically increased at 30-degree tilt. Our results suggest that Raman spectroscopy characterization with a simple substrate tilting method can provide new insights into non-destructive characterization of the shape, structure, and composition of the as-grown nanostructures for the wafer-scale growth and integration processing of groups III–V semiconducting hetero-nanostructures into nanoelectronics and photonics applications.

Chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells prepared via metal-organic chemical vapor deposition (MOCVD) are one of the candidates for highly advanced photovoltaic devices. This is because of their effectiveness and potential for reducing production costs through large-scale production. However, research on MOCVD-prepared solar cells is progressing slower than that on other types of solar cells, primarily because the preparation of CuInSe2 (CIS)-based films via MOCVD is relatively more sophisticated. In this study, we analyzed CIS solar cells prepared via three-stage MOCVD and processed with relatively simple precursors and techniques. We achieved an energy-conversion efficiency of 7.39% without applying a buffer layer. Instead, we applied a Cu-deficient layer to create a buried pn junction. Ultimately, we demonstrated that the fabrication of fully-MOCVD-processed CIS photovoltaic devices is feasible.

The integration of different electronic materials systems together has gained increasing interest in recent years, with the III-nitrides being a favorable choice for a variety of electronic applications. To increase flexibility in integration options, growing nitrides material directly on semi-processed wafers would be advantageous, necessitating low temperature (LT) growth schemes. In this work, the growth of AlN and GaN was conducted via metalorganic chemical vapor deposition (MOCVD) using both NH3 and DMHy as N-precursors. The relationships between growth rate versus temperature were determined within the range of 300 to 550 °C. The growth of AlN/GaN heterostructures was also investigated herein, employing flow modulation epitaxy MOCVD at 550 °C. Subsequent samples were studied via atomic force microscopy, X-ray diffraction, TEM, and Hall measurements. Two-dimensional electron gases were found in samples where the LT AlN layer was grown with NH3, with one sample showing high electron mobility and sheet charge of 540 cm2/V∙s and 3.76 × 1013 cm−2, respectively. Inserting a LT GaN layer under the LT AlN layer caused the mobility and charge to marginally decrease while still maintaining sufficiently high values. This sets the groundwork towards use of LT nitrides MOCVD in future electronic devices integrating III-nitrides with other materials.

In this study, we suppressed the parasitic emission caused by electron overflow found in typical ultraviolet B (UVB) and ultraviolet C (UVC) light-emitting diodes (LEDs). The modulation of the p-layer structure and aluminum composition as well as a trade-off in the structure to ensure strain compensation allowed us to increase the p-AlGaN doping efficiency and hole numbers in the p-neutral region. This approach led to greater matching of the electron and hole numbers in the UVB and UVC emission quantum wells. Our UVB LED (sample A) exhibited clear exciton emission, with its peak near 306 nm, and a band-to-band emission at 303 nm. The relative intensity of the exciton emission of sample A decreased as a result of the thermal energy effect of the temperature increase. Nevertheless, sample A displayed its exciton emission at temperatures of up to 368 K. In contrast, our corresponding UVC LED (sample B) only exhibited a Gaussian peak emission at a wavelength of approximately 272 nm.

In this paper, we report the successful demonstration of bright InGaN-based microLED devices emitting in the red spectral regime grown by metal organic chemical vapor deposition (MOCVD) on c-plane semi-relaxed InGaN substrates on sapphire. Through application of an InGaN/GaN base layer scheme to ameliorate high defect density and maintain appropriate lattice constant throughout the growth, high-In quantum wells (QWs) can be grown with improved crystal quality. Improvement to the design of the growth scheme also yields higher power output resulting in an increase to the external quantum efficiency (EQE). Combined, these two improvements allow for an 80 × 80 μm2 microLED device emitting at 609 nm to achieve 0.83% EQE. Furthermore, the true In content of the QW is measured using atomic probe tomography (APT) to confirm the improved In incorporation during high temperature active region growth. These developments represent advancement toward the realization of bright, highly efficient red III-nitride LEDs to be used in RGB applications under one material system.

We examine full InGaN-based microLEDs on c-plane semi-relaxed InGaN substrates grown by metal organic chemical vapor deposition (MOCVD) that operate across a wide range of emission wavelengths covering nearly the entire visible spectrum. By employing a periodic InGaN base layer structure with high temperature (HT) GaN interlayers on these semi-relaxed substrates, we demonstrate robust μLED devices. A broad range of emission wavelengths ranging from cyan to deep red are realized, leveraging the indium incorporation benefit of the relaxed InGaN substrate with an enlarged lattice parameter. Since a broad range of emission wavelengths can be realized, this base layer scheme allows the tailoring of the emission wavelength to a particular application, including the possibility for nitride LEDs to emit over the entire visible light spectrum. The range of emission possibilities from blue to red makes the relaxed substrate and periodic base layer scheme an attractive platform to unify the visible emission spectra under one singular material system using III-Nitride MOCVD.

International audience; Layered semiconductor gallium selenide (GaSe) is considered a potential candidate for optoelectronic applications because of its direct band gap. Monocrystalline material is, however, a prerequisite to fully exploit these properties in devices, where one-dimensional nano-objects could be considered as a model system. As a consequence of their large surface-to-volume ratio, nano-objects such as nanoribbons are interesting for photodetection applications. Here, we report the vapor–liquid–solid growth of GaSe nanoribbons by MOCVD on 300 mm silicon substrates. A growth model is proposed on the basis of a comprehensive study of the impact of the growth parameters on the nanoribbon morphology. The nanoribbon microstructure is investigated by HR-STEM and Raman spectroscopy characterizations. HR-STEM and TEM cross-sectional observations coupled with EDX analyses reveal a monocrystalline nanoribbon core covered with a native gallium-oxide shell. Test devices are made by contacting individual nanoribbon. The current versus voltage (I–V) characteristic obtained over a range of temperature (−50 to 100 °C) in the dark and under white light illumination is fitted on the basis of a back-to-back Schottky diode model. A stable and repeatable dynamic photoresponse is measured from the GaSe nanoribbons, with an ION/IOFF ratio of 17 at room temperature.

Highly conductive silicon-doped AlGaN and ohmic contacts are needed for deep-UV LEDs and ultrawide bandgap electronics. We demonstrate improved n-Al0.65Ga0.35N films grown by metal–organic chemical vapor deposition (MOCVD) on sapphire substrates using a low V/III ratio (V/III = 10). A reduced V/III ratio improves repeatability and uniformity by allowing a wider range of silicon precursor flow conditions. AlxGa1−xN:Si with x &gt, 0.5 typically has an electron concentration vs. silicon concentration trend that peaks at a particular “knee” value before dropping sharply as [Si] continues to increase (self-compensation). The Al0.65Ga0.35N:Si grown under the lowest V/III conditions in this study does not show the typical knee behavior, and instead, it has a flat electron concentration trend for [Si] &gt, 3 × 1019 cm−3. Resistivities as low as 4 mΩ-cm were achieved, with corresponding electron mobility of 40 cm2/Vs. AFM and TEM confirm that surface morphology and dislocation density are not degraded by these growth conditions. Furthermore, we report vanadium-based ohmic contacts with a resistivity of 7 × 10−5 Ω-cm2 to AlGaN films grown using a low V/III ratio. Lastly, we use these highly conductive silicon-doped layers to demonstrate a 284 nm UV LED with an operating voltage of 7.99 V at 20 A/cm2, with peak EQE and WPE of 3.5% and 2.7%, respectively.

III-N-based electronics and optoelectronics are reaching great levels of sophistication in the areas of power electronics, RF amplifiers, lighting, and display technologies. Much of the success of these technologies can be traced to superior or unique material properties that make III-N solid state devices the ideal choices for their applications. Consequently, state of the art devices are being pushed to the limit of what may be fabricated due to strain considerations in the AlGaN and InGaN systems. In order to continue the advancement of III-N based technologies toward greater performance, into new niches, and open up new markets, it is necessary to exploit the entire (Al,In,Ga)N system to its fullest potential.The utility of AlInGaN is multifaceted. These materials can be used for strain management, fabrication of lattice-matched devices, and polarization engineering to manipulate electric fields within device active regions, or even create high-conductivity charge slabs. Unlike ternary alloys, there is no single unique combination of band gap, polarization charge, and lattice constant, which results in greater device design freedom. However, to effectively utilize these materials, reliable growth processes must be established, and the material parameters critical to device design must be characterized.This thesis describes the progress in AlInGaN development at UCSB beginning with identification and exploration of the AlInGaN growth parameter space, using understanding from ternary alloys as a springboard into quaternary growth. From there, the thesis progresses to the establishment of a design toolbox for AlInGaN based devices via electrical characterization of these materials. Challenges associated with the AlInGaN system, coupled with sparse literature on the topic, necessitated the design of experiments to isolate and characterize the material parameters from measurements of solid-state devices. Electrical characterization focused on the net polarization charge at heterojunction interfaces, as well as the effects of Schottky barrier height inhomogeneity on both electrostatics and transport in diodes. The quantum mechanical scattering at the metal-semiconductor junction will be discussed, as will its physical origin and impact on diode current. A major goal of this thesis was to establish a device design toolbox populated with information of experimentally calculated net polarization charge at AlInGaN/GaN interfaces and Schottky barrier heights. This goal was accomplished and the information was established for future device designers in the field.The thesis concludes with a discussion of the application and exploitation of the unique effects observed in AlInGaN materials to device design. Future outlook will be given on avenues for research in AlInGaN materials and AlInGaN-based devices, and direction will be provided to finish populating the (electrical) device design toolbox with conduction band offset measurements.

The III-Nitride materials system provides a fascinating platform for developing optoelectronic devices, such as solar cells and LEDs, which have the power to dramatically improve the efficiency of our power consumption and reduce our environmental footprint. Finding ways to make these devices more efficient is key to driving their widespread adoption. This dissertation focuses on the intersection of challenges in physics and metalorganic chemical vapor deposition (MOCVD) growth at the nanoscale when designing for device efficiency.In order to create the best possible InGaN solar cell, a multiple quantum well (MQW) active region design had to be employed to prevent strain relaxation related degradation. There were two competing challenges for MQW active region design and growth. First, it was observed current collection efficiency improved with thinner quantum barriers, which promoted efficient tunneling transport instead of inefficiency thermally activated escape. Second, GaN barriers could planarize surface defects in the MQW region under the right conditions and when grown thick enough. A two-step growth method for thinner quantum barriers was developed that simultaneously allowed for tunneling transport and planarized V-defects. Barriers as thin as 4 nm were employed in MQW active regions with up to 30 periods without structural or electrical degradation, leading to record performance. Application of dielectric optical coatings greatly reduced surface reflections and allowed a second pass of light through the device. This both demonstrated the feasibility of multijunction solar integration and boosted conversion efficiency to record levels for an InGaN solar cell.III-N LEDs have achieved state-of-the-art performance for decades, but still suffer from the phenomena of efficiency droop, where device efficiency drops dramatically at high power operation. Droop is exacerbated by the polarization-induced electric fields in InGaN quantum wells, which originate from a lack of inversion symmetry in GaN’s wurtzite crystal structure. These fields can be screened by using highly doped layers, but the extreme dopant densities predicted by simulation for complete screening may require using Ge as an alternative n-type dopant to Si. GaN:Ge layers with excellent electrical characteristics were grown by MOCVD with doping densities exceeding 1020 cm-3. However, their surface morphologies were very poor and they proved a poor screening dopant in LED structures. Using Si as the n-type screening dopant, LEDs with single QW active regions were grown, packaged, and tested. Biased photoluminescence showed strong evidence of complete polarization screening. The LEDs had low droop, but also low peak efficiencies. Possible explanations for trends in efficiency with varying QW width and field screening will be discussed.

The effect of Mg doping on the electrical and optical properties of the p-GaN/AlGaN structures on a Si substrate grown by metal organic chemical vapor deposition was investigated. The Hall measurement showed that the activation efficiency of the sample with a 450 sccm Cp2Mg flow rate reached a maximum value of 2.22%. No reversion of the hole concentration was observed due to the existence of stress in the designed sample structures. This is attributed to the higher Mg-to-Ga incorporation rate resulting from the restriction of self-compensation under compressive strain. In addition, by using an AlN interlayer (IL) at the interface of p-GaN/AlGaN, the activation rate can be further improved after the doping concentration reaches saturation, and the diffusion of Mg atoms can also be effectively suppressed. A high hole concentration of about 1.3 × 1018 cm−3 can be achieved in the p-GaN/AlN-IL/AlGaN structure.

This paper presents the current status of medium-wave infrared (MWIR) detectors at the Military University of Technology’s Institute of Applied Physics and VIGO System S.A. The metal–organic chemical vapor deposition (MOCVD) technique is a very convenient tool for the deposition of HgCdTe epilayers, with a wide range of compositions, used for uncooled infrared detectors. Good compositional and thickness uniformity was achieved on epilayers grown on 2-in-diameter, low-cost (100) GaAs wafers. Most growth was performed on substrates, which were misoriented from (100) by between 2° and 4° in order to minimize growth defects. The large lattice mismatch between GaAs and HgCdTe required the usage of a CdTe buffer layer. The CdTe (111) B buffer layer growth was enforced by suitable nucleation procedure, based on (100) GaAs substrate annealing in a Te-rich atmosphere prior to the buffer deposition. Secondary-ion mass spectrometry (SIMS) showed that ethyl iodide (EI) and tris(dimethylamino)arsenic (TDMAAs) were stable donor and acceptor dopants, respectively. Fully doped (111) HgCdTe heterostructures were grown in order to investigate the devices’ performance in the 3–5 µm infrared band. The uniqueness of the presented technology manifests in a lack of the necessity of time-consuming and troublesome ex situ annealing.

Purpose The purpose of this paper is to investigate the effect of growth temperature on the evolution of indium incorporation and the growth process of InGaN/GaN heterostructures. Design/methodology/approach To examine this effect, the InGaN/GaN heterostructures were grown using Taiyo Nippon Sanso Corporation metal-organic chemical vapor deposition (MOCVD) SR4000-HT system. The InGaN/GaN heterostructures were epitaxially grown on 3.4 µm undoped-GaN (ud-GaN) and GaN nucleation layer, respectively, over a commercial 2” c-plane flat sapphire substrate. The InGaN layers were grown at different temperature settings ranging from 860°C to 820°C in a step of 20°C. The details of structural, surface morphology and optical properties were investigated using X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), atomic force microscopy and ultraviolet-visible (UV-Vis) spectrophotometer, respectively. Findings InGaN/GaN heterostructure with indium composition up to 10.9% has been successfully grown using the MOCVD technique without any phase separation detected within the sensitivity of the instrument. Indium compositions were estimated through simulation fitting of the XRD curve and calculation of Vegard’s law from UV-Vis measurement. The thickness of the structures was determined using the Swanepoel method and the FE-SEM cross-section image. Originality/value This paper report on the effect of MOCVD growth temperature on the growth process of InGaN/GaN heterostructure, which is of interest in solid-state lighting technology, especially in light-emitting diodes and solar cell application.

To obtain high-quality n-type doped β-Ga2O3 films, silane was used as an n-type dopant to grow Si-doped β-Ga2O3 films on (100) β-Ga2O3 substrates by metal-organic chemical vapor deposition (MOCVD). The electron concentrations of the Si-doped β-Ga2O3 films obtained through experiments can be stably controlled in the range of 6.5 × 1016 cm−3 to 2.6 × 1019 cm−3, and the ionization energy of Si donors is about 30 meV, as determined by analysis and calculation. The full width at half maxima of the rocking curves of the (400) crystal plane of all doped films was less than 500 arcsec, thus showing high crystal quality, while the increase of the doping concentration increased the defect density in the β-Ga2O3 films, which had an adverse effect on the crystal quality and surface morphology of the films. Compared with heteroepitaxial Si-doped β-Ga2O3 films, homoepitaxial Si-doped β-Ga2O3 films exhibited higher quality, lower defect density, and more stable electron concentration, which make them more conductive for preparing Ga2O3-based power devices.

Metal organic chemical vapour deposition (MOCVD) was used with a mixed single solid source precursors of copper dithiocarbamate (CuC10H16N2O2S4) and zinc dithiocarbamate (ZnC10H16N2O2S4) to prepare pyrolysed copper zinc sulphide thin films. The precursors were analysed by Fourier Transform Infrared Spectroscopy (FTIR), to determine the structures present. The films were deposited on glass substrates at a temperature of 420 0C. Experimental results showed that the compositional studies were found to be complementary. The structural analyses revealed that the film is crystalline with an average grain size of approximately 4 μm. An optical band gap energy of 2.50 eV was obtained with an absorbance that decreases with wavelength. The electrical characterization gave values of sheet resistance, resistivity and conductivity of the film as 5.13 x10-6 Ω/Sq, 2.27 x 10-1 Ω.cm and 3.61 x 10-2 (Ω.cm)-1 respectively. The findings confirmed that the synthesized precursors are potential materials for depositing high quality copper zinc sulphide thin films.

We report a high thermal stability of kappa gallium oxide grown on c-plane sapphire substrate by metal organic chemical vapor deposition. Kappa gallium oxide is widely known as a metastable polymorph transitioning its phase when subjected to a high temperature. Here, we show the kappa gallium oxide whose phase is stable in a high temperature annealing process at 1000 °C. These oxide films were grown at 690 °C under nitrogen carrier gas. The materials showed high electrical resistivity when doped with silicon, whereas the film conductivity was significantly improved when doped with both indium and silicon. This work provides a pathway to overcoming limitations for the advance in utilizing kappa gallium oxide possessing superior electrical characteristics.

2G HTS wires are capable of transferring huge amounts of electrical energy without loss. An increase in the current-carrying capacity in these materials is possible due to an increase in the thickness of the superconducting layer; however, there is a problem with the appearance of impurity orientations and other defects with increasing thickness. We have proposed a solution of this problem by increasing the thickness of the superconducting layer by the MOCVD method using interlayers of yttrium oxide.The aim of this study was the production of thick composite films with yttrium oxide interlayers and high critical current density. In addition, we want to show the effectiveness of the approach of introducing yttrium oxide interlayers for the reduction of the number of parasitic orientations and defects with an increase in HTS film thickness.The deposition of YBа2Cu3O7–dand Y2O3 films was carried out layer by layer using reel-to-reel MOCVD equipment. 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International audience; Due to their polymorphism, TiO 2 films are quintessential components of state-ofthe-art functional materials and devices for various applications from dynamic random access memory to solar water splitting. However, contrary to other semiconductors/dielectric materials, the relationship between structural/morphological and electrical properties at the nano and microscales remains unclear. In this context, the morphological characteristics of TiO 2 films obtained by metalorganic chemical vapor deposition (MOCVD) and plasma-enhanced chemical vapor deposition (PECVD), the latter including nitrogen doping, are investigated and they are linked to their in-and out-plane electrical properties. A transition from dense to tree-like columnar morphology is observed for the MOCVD films with increasing deposition temperature. It results in the decrease in grain size and the increase in porosity and disorder, and subsequently, it leads to the decrease in lateral carrier mobility. The increase in nitrogen amount in the PECVD films enhances the disorder in their pillar-like columnar morphology along with a slight increase in density. A similar behavior is observed for the out-plane current between the low temperature MOCVD films and the undoped PECVD ones. The pillar-like structure of the latter presents a lower in-plane resistivity than the low temperature MOCVD films, whereas the out-plane resistivity is lower. The treelike columnar structure exhibits poor in-and out-plane conductivity properties, whereas pillar-like and dense TiO 2 exhibits similar in-and out-plane conductivities even if their morphologies are noticeably different.

γ-phase aluminum oxide (γ-Al2O3) films are grown on MgO (100) wafers by metal organic chemical vapor deposition (MOCVD). Post-annealing process is conducted to study the influence of annealing temperature on the properties of the films. Structural analyses indicate that all the deposited and annealed films present a preferred growth orientation of γ-Al2O3 (220) along the MgO (200) direction. And the film annealed at 1100 °C exhibits the best film quality compared with those of the films grown and annealed at other temperatures. Scanning electron microscopy measurements also imply the best surface morphology for the γ-Al2O3 film annealed at 1100 °C, which is in good accordance with the structural analyses. Optical transmittance spectra show good transparency for all the deposited and annealed films in the visible wavelength region with an average transmittance value of 83.5%. The optical bandgaps are estimated to be in the range of 5.56–5.79 eV for the deposited and annealed films. Semiconductor films with high optical transmittance in the visible region as well as wide bandgaps are appropriate for the manufacture of transparent optoelectronic devices and ultraviolet optoelectronic devices.

We report a highly conductive gallium oxide doped with both silicon and indium grown on c-plane sapphire substrate by MOCVD. From a superlattice structure of indium oxide and gallium oxide doped with silicon, we obtained a highly conductive material with an electron hall mobility up to 150 cm2/V·s with the carrier concentration near 2 × 1017 cm−3. However, if not doped with silicon, both Ga2O3:In and Ga2O3 are highly resistive. Optical and structural characterization techniques such as X-ray, transmission electron microscope, and photoluminescence, reveal no significant incorporation of indium into the superlattice materials, which suggests the indium plays a role of a surfactant passivating electron trapping defect levels.

We report the post-growth thermal annealing and the subsequent phase transition of Ga2O3 grown on c-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). We demonstrated the post-growth thermal annealing at temperatures higher than 900 &deg, C under N2 ambience, by either in situ or ex situ thermal annealing, can induce phase transition from nominally metastable &kappa, to thermodynamically stable &beta, phase. This was analyzed by structural characterizations such as high-resolution scanning transmission electron microscopy and x-ray diffraction. The highly resistive as-grown Ga2O3 epitaxial layer becomes conductive after annealing at 1000 &deg, C. Furthermore, we demonstrate that in situ annealing can lead to a crack-free &beta, Ga2O3.

Bi2Te3 is attracting a renewed interest, due to its topological insulator properties; however, even using advanced physical and chemical deposition techniques, the growth of high-crystal quality layers on substrates allowing its technological employment, such as Si, is very challenging, due to the structural complexity of Bi2Te3. In this work, we present the optimized large area growth of topological insulator Bi2Te3 epitaxial layers on un-buffered i-Si (111) substrates via Metal-Organic Vapor Phase Epitaxy (MOVPE), which is of crucial importance for future integration into CMOS compatible spintronic devices. We found that the key to maximizing the layer quality requires a balanced control of the reactor pressure (P), growth temperature (T), and growth time (t). Within a proper parameter window, the grown Bi2Te3 thin layers are crystalline, stoichiometric, and highly uniform, also at the local scale. They exhibit a rhombohedral crystalline structure, and they are [001] out-of-plane oriented on the i-Si (111) substrate. Low temperature magnetoresistance measurements revealed clear Weak Antilocalization (WAL) effects, demonstrating that the optimized MOVPE - grown Bi2Te3 is a topological insulator, hence opening further possibilities for its technology-transfer to innovative devices.

Interest in iridium and platinum has been steadily encouraged due to such unique properties as exceptional chemical inertia and corrosion resistance, high biological compatibility, and mechanical strength, which are the basis for their application in medical practice. Metal-organic chemical vapor deposition (MOCVD) is a promising method to fabricate Ir and Pt nanomaterials, multilayers, and heterostructures. Its advantages include precise control of the material composition and microstructure in deposition processes at relatively low temperatures onto non-planar substrates. The development of MOCVD processes is inextricably linked with the development of the chemistry of volatile precursors, viz., specially designed coordination and organometallic compounds. This review describes the synthesis methods of various iridium and platinum precursors, their thermal properties, and examples of the use of MOCVD, including formation of films for medical application and bimetallics. Although metal acetylacetonates are currently the most widely used precursors, the recently developed heteroligand Ir(I) and Pt(IV) complexes appear to be more promising in both synthetic and thermochemical aspects. Their main advantage is their ability to control thermal properties by modifying several types of ligands, making them tunable to deposit films onto different types of materials and to select a combination of compatible compounds for obtaining the bimetallic materials.

Tungsten fiber-reinforced tungsten (Wf/W) composites are being developed to improve the intrinsic brittleness of tungsten. In these composites, engineered fiber/matrix interfaces are crucial in order to realize toughening mechanisms. For such a purpose, yttria (Y2O3), being one of the suitable interface materials, could be realized through different coating techniques. In this study, the deposition of thin films of yttria on a 150 µm tungsten wire by physical and chemical vapor deposition (PVD and CVD) techniques is comparatively investigated. Although fabrication of yttria is feasible through both CVD and PVD routes, certain coating conditions such as temperature, growth rate, oxidation of Wf, etc., decide the qualitative nature of a coating to a particular extent. In the case of PVD, the oxidation of Wf is highly reduced compared to the WO3 formation in high-temperature CVD coating processes. Yttria-coated tungsten fibers are examined comprehensively to characterize their microstructure, phase, and chemical composition using SEM, XRD, and Raman spectroscopy techniques, respectively.

This work presents the results of structure, microstructure, and chemical composition investigations performed on ZrO2-Sm2O3 layers synthesized by MOCVD (metal-organic chemical vapor deposition) using Zr(tmhd)4 and Sm(tmhd)3 as reactants on quartz glass substrate. The molar percentage of Sm(tmhd)3 used to obtain the layers at both 500 and 550 &deg, C was 14% and 22.75%, respectively. Synthesis parameters were selected so that the value of the extended criterion Grx/Rex2 (Gr&mdash, Grashof number, Re&mdash, Reynolds number, x&mdash, the distance from the gas inflow point to the CVD (MOCVD) reactor) could be maintained below 0.01. It was determined from XRD (X-ray diffraction) analyses that the layers deposited at 500 &deg, C contained small amounts of a crystalline phase and the layers obtained at 550 &deg, C contained greater amounts of the crystalline phase (solid solution). SEM (scanning electron microscope) observations have also shown that the crystalline phase is present in the layers synthesized at 500 &deg, C, as well as at 550 &deg, C. EDS (energy dispersive spectroscopy) studies have indicated that molar content of Sm2O3 in the crystalline phase is lower in comparison to the amount present in the respective amorphous phase. The larger the Sm2O3 content in the layer, the higher the growth rate.

Currently, X-ray photoelectron spectroscopy (XPS) is widely used to characterize the nanostructured material surface. The ability to determine the atom distribution and chemical state with depth without the sample destruction is important for studying the internal structure of the coating layer several nanometers thick, and makes XPS the preferable tool for the non-destructive testing of nanostructured systems. In this work, ultra-soft X-ray spectroscopy methods are used to study hidden layers and interfaces of pyrolytic tungsten carbide nanoscale coatings on the multi-walled carbon nanotube (MWCNT) surfaces. XPS measurements were performed using laboratory spectrometers with sample charge compensation, and Near Edge X-ray Absorption Fine Structure (NEXAFS) studies using the Russian&ndash, German dipole beamline (RGBL) synchrotron radiation at BESSY-II. The studied samples were tested by scanning and transmission electron microscopy, X-ray diffractometry, Raman scattering and NEXAFS spectroscopy. It was shown that the interface between MWCNT and the pyrolytic coating of tungsten carbide has a three-layer structure: (i) an interface layer consisting of the outer graphene layer carbon atoms, forming bonds with oxygen atoms from the oxides adsorbed on the MWCNT surface, and tungsten atoms from the coating layer, (ii) a non-stoichiometric tungsten carbide WC1-x nanoscale particles layer, (iii) a 3.3 nm thick non-stoichiometric tungsten oxide WO3-x layer on the WC1-x/MWCNT nanocomposite outer surface, formed in air. The tungsten carbide nanosized particle&rsquo, s adhesion to the nanotube outer surface is ensured by the formation of a chemical bond between the carbon atoms from the MWCNT upper layer and the tungsten atoms from the coating layer.

The present study evaluated the main factors that influence the photocatalytic activity of titanium dioxide (TiO2) films grown by metalorganic chemical vapor deposition (MOCVD) at 400 and 500 °C, in different growth times. The photocatalytic behavior was analyzed by measuring the methyl orange dye degradation at different pH values. Structural and morphological characteristics, and the recyclability of the catalysts for several cycles were also investigated. Anatase phase was identified in all films. The higher photodegradation performances were obtained at acidic pH. The results demonstrated that the photocatalyst thickness is an important parameter in heterogenous photocatalysis. The best photocatalytic result occurred for the 395 nm-thick TiO2 film grown at 400 °C, which presented 65.3% of the dye degradation under UV light. The recyclability experiments demonstrated that the TiO2 films grown by MOCVD present a great stability after several photocatalytic cycles, which allows their practical application for water treatment with high efficiency.

In this work, we have achieved synthesizing large-area high-density &beta, Ga2O3 nanowires on c-plane sapphire substrate by metal&ndash, organic chemical vapor deposition assisted with Au nanocrystal seeds as catalysts. These nanowires exhibit one-dimensional structures with Au nanoparticles on the top of the nanowires with lengths exceeding 6 &mu, m and diameters ranging from ~50 to ~200 nm. The &beta, Ga2O3 nanowires consist of a single-crystal monoclinic structure, which exhibits strong (201) orientation, confirmed by transmission electronic microscopy and X-ray diffraction analysis. The PL spectrum obtained from these &beta, Ga2O3 nanowires exhibits strong emissions centered at ~360 and ~410 nm, respectively. The energy band gap of the &beta, Ga2O3 nanowires is estimated to be ~4.7 eV based on an optical transmission test. A possible mechanism for the growth of &beta, Ga2O3 nanowires is also presented.

The paper is devoted to the structure and properties of the composite material based on multi-walled carbon nanotubes (MWCNTs) covered with pyrolytic iron and chromium. Fe/MWCNTs and Cr/MWCNTs nanocomposites have been prepared by the metal organic chemical vapor deposition (MOCVD) growth technique using iron pentacarbonyl and bis(arene)chromium compounds, respectively. Composites structures and morphologies preliminary study were performed using X-ray diffraction, scanning and transmission electron microscopy and Raman scattering. The atomic and chemical composition of the MWCNTs&rsquo, surface, Fe-coating and Cr-coating and interface&mdash, (MWCNTs surface)/(metal coating) were studied by total electron yield method in the region of near-edge X-ray absorption fine structure (NEXAFS) C1s, Fe2p and Cr2p absorption edges using synchrotron radiation of the Russian-German dipole beamline (RGBL) at BESSY-II and the X-ray photoelectron spectroscopy (XPS) method using the ESCALAB 250 Xi spectrometer and charge compensation system. The absorption cross sections in the NEXAFS C1s edge of the nanocomposites and MWCNTs were measured using the developed approach of suppressing and estimating the contributions of the non-monochromatic background and multiple reflection orders radiation from the diffraction grating. The efficiency of the method was demonstrated by the example of the Cr/MWCNT nanocomposite, since its Cr2p NEXAFS spectra contain additional C1s NEXAFS in the second diffraction order. The study has shown that the MWCNTs&rsquo, top layers in composite have no significant destruction, the MWCNTs&rsquo, metal coatings are continuous and consist of Fe3O4 and Cr2O3. It is shown that the interface between the MWCNTs and pyrolytic Fe and Cr coatings has a multilayer structure: a layer in which carbon atoms along with epoxy &ndash, C&ndash, O&ndash, bonds form bonds with oxygen and metal atoms from the coating layer is formed on the outer surface of the MWCNT, a monolayer of metal carbide above it and an oxide layer on top. The iron oxide and chromium oxide adhesion is provided by single, double and epoxy chemical binding formation between carbon atoms of the MWCNT top layer and the oxygen atoms of the coating, as well as the formation of bonds with metal atoms.

In this study, we investigated the effects of InGaN/GaN-based interlayer (IL) and electron emitting layer (EEL) consisting of a GaN barrier layer grown with different metal-organic (MO) precursors of gallium (Ga), which were grown underneath the active layer. The growth behavior of GaN with triethyl Ga (TEGa) showed an increasing growth time due to a lower growth rate compared with GaN grown with trimethyl Ga (TMGa), resulting in the formation of columnar domains and grain boundary with reduced defect. UV-A light emitting diode (LED) chips with three types of ILs and EELs, grown with different MO sources, were fabricated and evaluated by light output power (LOP) measurements. The LOP intensity of UVLED-III with the GaN barrier layer-based IL and EEL grown by TEGa was enhanced by 1.5 times compared to that of the IL and EEL grown with TMGa at 300 mA current injection. Use of the GaN barrier layer in ILs and EELs grown by TEGa improved the crystal quality of the post grown InGaN/GaN multiple quantum well, which reduces leakage current. Therefore, for the UV-A LED with ILs and EELs grown with TEGa MO precursors, electrical and optical properties were improved significantly.