1,720,998 research outputs found
Magnesium Gallate Thin Films for Application in High Power Electronics
No full textThis thesis aims to investigate the crystalline growth of magnesium gallate (MGO) thin films using pulsed laser deposition (PLD) and the impact of thermal annealing on film properties for advancing its application in power electronics. For that, MGO films were deposited on the c-plane sapphire substrates with modified PLD parameters suitable for crystalline growth over a broad range of temperatures and oxygen pressure. The preferential growth of MGO films along the [111] crystal orientation has been observed, accompanied by an increase in crystallinity with rising temperatures and oxygen pressures. Moreover, the as-grown MGO films have the six-fold rotational symmetry and an epitaxial relationship of 30° with the sapphire substrate. Within the films, the constituent magnesium (Mg) has an oxidation state of +2, while gallium (Ga) exists predominantly in the +3 chemical state. Additionally, oxygen vacancies in the film increase with decreasing partial pressure during the PLD-growth. The direct bandgap of MgGa2O4 films is determined to be ~5.27±0.03 electron-volts (eV). The MGO film surface has exhibited a granular morphology with an increasing trend in grain size as the temperatures and oxygen pressure increased. The refractive index and thickness of these films is obtained to be in the range of ~1.90±0.02 and ~70±2.0 nm, respectively. Post-growth oxygen-annealed MGO samples show an increasing trend in position-shift of the diffraction peaks with temperature suggesting an alloy or change in composition. The presence of aluminum (Al) within the film layer after the anneal indicates the Al-diffusion from substrate. This also compliments the previous assumption of compositional change in the MGO films which possibly caused the position-change of diffraction peaks. Furthermore, the individual photoemission lines reveal an oxidation state of +2, +3, and +3, for Mg, Ga, and Al, respectively for the annealed MGO film. In addition, transmission electron micrographs exhibit the preferential orientation of magnesium gallate films with an individually uniform distribution of the compositional elements and diffused aluminum in the film layer. The annealed MGO films show an increase in the bandgap with temperatures probably due to the aluminum incorporation for forming an alloy. The complex refractive index is estimated to be ~1.863 at 632nm for annealing at 700℃ and later, a decreasing trend is observed with temperature. The rms roughness of annealed samples follows a decreasing trend with temperatures indicating an improved surface quality upon the thermal treatment.Engineerin
Secured and Versatile IoT Connectivity: A Novel LoRa-Based Approach with Enhanced Encryption, Key Exchange, Real-Time Data Visualization, and Dynamic Storage Integration
No full textThis research ventures into uncharted territory considering the growing need for secure and effective long-range communication resulting from the rise of Internet-of-Things (IoT) applications. By incorporating the recently approved National Institute of Standards and Technology (NIST) ASCON cryptographic standard and the Advanced Encryption Standard (AES) 256, this project enhances the security of IoT networks within a cost-effective framework. To this end, Long Range (LoRa) technology, a low-power wide-area network protocol, serves as the primary communication technology. The experiment involves two transmitters, each equipped with different environmental sensors, communicating via LoRa technology. The security model employed is two-fold: It relies on the ASCON and AES-256 standards to ensure robust data encryption. Alongside these, the Elliptic Curve Diffie-Hellman (ECDH) protocol is implemented for secure key exchange. A unique three-fold data visualization strategy is introduced, encompassing real-time updates on a locally hosted webpage, local storage in the ESP32 in a CSV file format, and secure cloud storage on the Adafruit IO platform. Using parameters like signal-to-noise ratio (SNR), received signal strength indicator (RSSI), latency, power consumption, and memory consumptions, performance comparisons with networks that only use AES encryption show that the system that uses the ASCON standards performs better than AES-256. This groundbreaking application of ASCON and AES-256 in LoRa communication establishes a remarkable precedent. Not only does it emphasize affordability, but it also enhances security, a vital factor in developing reliable future IoT applications. In summary, this thesis contributes a practical approach to integrating cryptographic standards within IoT networks, offering a novel perspective for ensuring secure, long-range communication within IoT system.Engineerin
Optimization of Seeding Parameters for Growing Large-Scale Polycrystalline Diamond on β-Ga2O3
No full textβ-Ga2O3 is a promising semiconductor, with an ultra-wide bandgap of ~4.8 eV and a critical electric breakdown field of 8 MVcm-1, which has the potential to outperform the GaN and SiC based high power devices available in the market. However, the thermal conductivity of βGa2O3 is significantly low, which can drastically reduce the lifespan of devices made from this propitious material. The problem of poor thermal management in β-Ga2O3 based devices can be solved by growing a large area thick diamond layer on the single crystal β-Ga2O3. However, it is challenging to grow diamonds on β-Ga2O3 due to the large surface energy difference and thermal mismatch between the β-Ga2O3 and the diamond films. Another key factor for the difficulties in growing diamonds via the chemical vapor deposition (CVD) process is poor seeding density. In this study, the polymer-assisted electrostatic dip seeding technique is optimized by tuning different parameters such as spinning speed, spinning time, baking periods, and ultrasonication periods. The optimized seeding technique achieves a high seeding density of ~1.82 ×1011 cm-2. The diamond films are grown at different growth periods to investigate the nucleation mechanism. Different characterization techniques, such as atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD) diffractometer, are used to analyze the surface morphology and quality of the diamond crystals. SEM images support the large-scale deposition of diamond crystals on β-Ga2O3. It is found that longer deposition periods lead to larger diamond crystals, as well as an increase in film thickness. Furthermore, the analysis of full width half maximum (FWHM) and phase purity of the diamond film through Raman spectra and X-ray spectroscopy (XRD) implies that the crystallinity and quality of the diamond film improves with longer growth period. The Raman spectra of the diamond films show increasing blue shift from its equilibrium position with increased thickness, indicating that longer growth periods result in higher compressive stress in the diamond films. The increased compressive stress causes the diamond films to peel off, which has been overcome by increasing the cooling period. In a nutshell, our findings pave the way for growing large-area diamond films on β-Ga2O3 to address the thermal management issue in β-Ga2O3-based power and communication devices.Engineerin
Pulsed Laser Annealing to Optimize Structural Properties of GaN Buffer Layer in GaN/A1GaN HEMTS
No full textIn recent years, Gallium Nitride (GaN) has gathered significant attention within the semiconductor industry for its versatile applications in optoelectronics, high-power semiconductor devices, and as a compelling alternative to traditional silicon-based technologies. Known for its exceptional electrical properties, GaN boasts a wide bandgap, high electron mobility, and superior thermal conductivity, making it a preferred choice for power electronics, RF components, and optoelectronics. Its ability to operate at higher voltages and frequencies than traditional siliconbased semiconductors has led to significant advancements in efficiency and performance across various industries, including telecommunications and renewable energy. Despite these favorable attributes, GaN-based Aluminum Gallium Nitride/Gallium Nitride (AlGaN/GaN) High Electron Mobility Transistors (HEMTs) face challenges related to high dislocation density during multilayer fabrication processes. Dislocations in the GaN layer can detrimentally impact the mobility of the two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface, thus affecting device performance. While extensive research has focused on optimizing layer composition, thickness, and growth conditions, the correlation between thermal treatment and dislocations, and its implications for device performance, remains underexplored. This study employs a distinct approach to enhance the structural and electrical properties of GaN in AlGaN/GaN HEMTs through pulsed PLA. PLA offers a rapid and localized heating method that mitigates the drawbacks associated with conventional thermal annealing techniques, such as prolonged heating times and high thermal budgets. By meticulously adjusting PLA conditions, this study aims to minimize dislocation density in the GaN layer, thereby improving overall device performance. Using advanced characterization techniques, including Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD), we monitored the layer-by-layer growth of HEMTs and quantified the dislocation density. The results revealed a 3.75-fold improvement in the GaN buffer layer's dislocation density under specific PLA conditions. This reduction in dislocation density is attributed to the enhanced atomic mobility induced by the controlled rapid and localized heating of PLA, which facilitates the annihilation of dislocations. Furthermore, electrical characterization through Hall measurements demonstrated significant enhancements in the 2DEG channel's properties. The mobility showed a 30% improvement, under optimal PLA conditions. These improvements are crucial for high-power and high-frequency device applications, as they directly impact the efficiency and reliability of AlGaN/GaN HEMTs. This study highlights the potential of PLA as an effective technique for improving the structural and electrical properties of GaN in AlGaN/GaN HEMTs. The findings emphasize PLA's ability to overcome the limitations of conventional annealing methods, offering a lower thermal budget and enhanced control over dislocation density. This study lays the groundwork for future research on PLA, suggesting its applicability to other semiconductor materials and paving the way for advancements in ultrafast device technology. The successful implementation of PLA in GaN-based HEMTs can lead to significant improvements in device performance, making it a promising technique for the next generation of high-power and high-frequency electronic devices.Engineerin
Investigation of the Growth Parameters and Electronic Structure of Q-Carbon, and Its Integration with GaN
No full textQ-carbon, a quenched form of carbon, is a recently discovered carbon structure that has tremendous properties, compatibility, and potential for use in device fabrication and other electronic applications. However, the non-equilibrium nature of the synthesis process and a very small window of parameters have limited the growth of large area Q-carbon to very low thermally conductive substrates. This thesis aims to overcome these limitations through extensive tuning of the parameters of diamond-like-carbon (DLC), the base structure required for the formation of Qcarbon. The as-deposited DLC films on sapphire substrates contain 15% to 82% sp3 content which is obtained through the optimization of laser energy density, frequency, and deposition temperature during pulsed laser deposition (PLD). These films follow some noticeable trends, such as the sp3 content increasing with temperature and beyond a maximum value, it decreases with frequency and fluence. Simulation of laser interaction with materials (SLIM) is used to find the optimum pulsed laser annealing (PLA) energy required for the formation of large-area Q-carbon, and the results are reproduced experimentally to obtain large-area Q-carbon on sapphire substrates using different DLC films while varying the PLA energy density ranging from 0.3 J/cm2 to 1.2 J/cm2. Thereby, this study expands the potential for highly thermally conductive substrates and offers the baseline parameters for further studies of Q-carbon. Later, this thesis also addresses the heat dissipation issue in GaN high-power semiconductor devices arising from the insufficient thermal conductivity for efficient heat dissipation during device operation and increased power density requirement in modern microelectronic systems. In this research, a thin Q-carbon interlayer was deposited on GaN in order to facilitate diamond-GaN integration. The Q-carbon on GaN provides the nucleation base for consistent, high-quality diamond development, shields the GaN layer from degradation, and eliminates the thermal mismatch between GaN and diamond. Thus, this unique combination holds the potential to solve GaN devices' heat management problem and get us closer to GaN's theoretical potential for wireless/5-g communications, high-power devices, and other sophisticated microelectronic systems. Moreover, the characteristics of both forms of amorphous carbon, i.e., DLC and Q-carbon, depend on the proportion of the sp2-sp3 hybridization. Thus, it is crucial to examine the effects of the composition on the optical properties, work function, and valence band structure of these thin films due to the applicability and possibilities of these materials in electronic applications and device construction. The chemical composition, valence band, and work function of diamond, nitrogen-doped, and undoped DLC and Q-carbon films are examined using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). According to the findings, the ratio of sp2-sp3 bonding and the nitrogen partial pressure both have a significant impact on the valence band structure. The work function of the DLC films was measured to be 3.82-4.91 eV, and for Q-carbon, it ranged from 3.17 to 4.05 eV depending on the etching level, doping, and sp3 percentage. Furthermore, the in-depth XPS analysis of the Q-carbon films reveals some new insights into the characteristics of PLA growth, such as the diffusion of substrate material into the film. The findings of this research will serve as a foundation for the development of a thorough understanding of the electronic band structures and work functions of amorphous carbon films, particularly Q-carbon, which has great potential for a variety of electronic applications, such as electron field emission devices, high-power electronics, superconductivity, and sensing devices.Engineerin
Growth and Characterization of CuGa2O4 Thin Film Using Pulsed Laser
No full textCopper gallate (CuGa2O4) thin films hold significant potential for various applications in the field of semiconductors and optoelectronics. In this study, high-quality crystalline CuGa2O4 thin films were grown on c-plane sapphire substrates using the pulsed laser deposition (PLD) technique by optimizing the growth parameters (i.e., temperature, oxygen chamber pressure, and laser energy density). The obtained XRD patterns validated that the preferred orientation of the crystalline CuGa2O4 thin film is along the [111] direction. The crystallinity of the films improved with increasing substrate temperature while maintaining a constant O2 pressure, as assessed by measuring the full width at half maximum (FWHM) of the prominent (222) plane. Reducing the oxygen pressure leads to the emergence of β-Ga2O3 peaks rather than the CuGa2O4 peaks due to the incomplete oxidation of copper (Cu). XRD phi (φ) scans revealed a sixfold rotational symmetry of CuGa2O4 and a 30º epitaxial relationship between the film and the sapphire substrate. X-ray photoelectron spectroscopy (XPS) spectra confirmed the presence of Cu1+, Cu2+, and Ga3+ oxidation states in the films. Again, under optimized conditions, increasing laser energy density resulted in complete oxidation of Cu and increased film thickness from 51.8 nm to 183.4 nm. The direct bandgap of CuGa2O4 films was determined to be 4.51±0.01 eV by analyzing UV-Vis absorbance spectra using the Tauc equation. The refractive index was found to be 2.05±0.01 based on the spectroscopic ellipsometry data. While the impact of increasing laser energy density was negligible on parameters such as crystal structure, rotational symmetry, oxidation states of gallium (Ga), oxygen (O), bandgap, and refractive index of CuGa2O4 thin film, notable alterations in the oxidation state of Cu and film thickness were observed.
While the film grown at 550°C was amorphous, annealing it at 600°C exhibited the preferred orientation of CuGa2O4, denoting the role of annealing temperature in crystallization. As the annealing temperature increased, the crystallinity of the films decreased, as indicated by the increasing FWHM of the diffraction peaks. Furthermore, all the samples annealed at temperatures ranging from 600°C to 900°C exhibited six-fold rotational symmetry in the phi scan. However, the peak intensity for the sample annealed at 900°C was lower due to decreased crystallinity. XPS survey scans of the samples annealed at (600-900) °C revealed that aluminum diffusion from the sapphire substrate occurs with annealing temperatures of 700°C and beyond, as evidenced by the XPS data. Additionally, a decrease in the absorption spectra and an increase in the bandgaps from 4.5 to 5.45 eV with annealing temperature further validated the diffusion of Al from the substrate. This investigation into the epitaxial growth of ultrawide bandgap CuGa2O4 thin films has the potential to enhance its applications in the semiconductor industry, particularly for optoelectronics and photonics in the UV region.Engineerin
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Investigation of the Electrical Properties of (InxGa1-x)2O3 Alloys Grown using Pulsed Laser Deposition
No full textWide bandgap semiconductor materials are gaining popularity because of their favorable features for increased power and high breakdown performance. Beta-gallium oxide (β-Ga2O3) is one such material with direct bandgap of 4.9eV having exceptional thermal and chemical stability. Materials that can preserve favorable energy alignment when manufacturing heterojunctions and alloys are needed to improve device performance. Thus, studying the bandgap engineering susceptibility and surface electrical characteristics of In2O3 and β-Ga2O3 alloys can lead to the discovery of a plethora of new potential applications. (InxGa1-x)2O3 alloys have highly tunable electrical and optical properties that can be used in transparent conductor technologies and solar-blind photodetectors. As a result, it is critical to develop an understanding of the structural and electrical behavior of these metal oxide alloys. In this research, alloys of Ga2O3 with In2O3, ternary (InxGa1–x)2O3 were grown for x =25% as a function of growth parameters such as substrate temperature (Ts), and oxygen partial pressure (Po2) using Pulsed Laser Deposition (PLD) technique. Amorphous to crystalline phase transformation was observed with increasing substrate temperature. Crystalline phases were detected representing both the monoclinic and cubic bixbyite phases. Ellipsometry study showed a slight thickness variation due to evaporation of volatile Ga2O and In2O suboxides. Temperature dependent Hall measurement analysis showed mobility and resistivity of the alloys are of the order ~ 14-30 cm2 /Vs and ~7×10-2 -9×10-2 Ω.cm respectively. Higher mobility was observed for the amorphous alloys compared to the crystalline samples. X-ray photoelectron spectroscopy (XPS) analyses indicated that the (InxGa1–x)2O3 alloy contains a mixture of Ga and In cation valence states and oxygen vacancies were reduced when the oxygen partial pressure during deposition was increased. The reduction of the oxygen vacancies was attributed to the suppression of internal defects due to the oxygen vacancies. Thus, reduction of the oxygen vacancies was thought to be a possible reason of increasing mobility with increasing partial pressure for the crystalline alloys. To further investigate the role of the reduction of internal defects, (InxGa1–x)2O3 alloys were deposited on sapphire substrate with a Ga2O3 buffer layer. The homoepitaxial buffer layer successfully increased the mobility of the crystalline alloys consistent with reducing the internal defects.Engineerin
Investigations into (InxGa1-x)2O3 structures grown on c-plane sapphire using pulsed laser deposition
No full textAlloys of Ga2O¬3 with In2O3, ternary (InxGa1–x)2O3 were grown for x ≤ 15% as a function of growth parameters such as substrate temperature (Ts), oxygen partial pressure (Po2), and laser power using Pulsed Laser Deposition (PLD) technique. The structural property of the as-grown layers was studied based on the growth parameters employing X-ray diffraction (XRD) analysis. Besides the monoclinic phase reflections of Ga2O¬3 and cubic bixbyite phase reflections of In2O3, an extra peak was observed for indium concentration, x ≥ 8% at specific growth conditions. The indium incorporation was reflected in a band gap reduction between 4.98 and 4.64 eV, as determined using UV-Vis analysis. Ellipsometry study showed a slight thickness variation due to evaporation of volatile Ga2O and In2O suboxides. X-ray photoelectron spectroscopy (XPS) analyses indicated that the (InxGa1–x)2O3 alloy contains a mixture of Ga and In cation valence states and oxygen vacancies that were reduced when the oxygen partial pressure during deposition was increased. During the analysis of the XRD spectra of the (In0.10Ga0.90)2O3 alloy, the observation of an extra peak led to the investigation of metastable polymorphs of (InxGa1–x)2O3. This polymorph, κ-(InxGa1–x)2O3 was confirmed through X-ray diffraction phi-scan of the {122} reflection representing the orthorhombic phase. The spontaneous polarization of κ-(InxGa1–x)2O3 makes it a promising candidate for 2DEG HEMT and ferroelectric devices. Detailed investigation of the impact of growth conditions in the κ-phase indicated that the appearance of this phase is due to specific growth conditions and does not depend on the nucleation process. The impact of PLD growth parameters such as laser power, substrate temperature, and oxygen partial pressure was investigated using XRD to establish a phase diagram for the κ-phase and β-phase of (InxGa1–x)2O3.Engineerin
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