Wide-Bandgap Semiconductors: A Critical Analysis of GaN, SiC, AlGaN, Diamond, and Ga₂O₃ Synthesis Methods, Challenges, and Prospective Technological Innovations

3.1. Molecular Beam Epitaxy (MBE) Molecular Beam Epitaxy (MBE) is a sophisticated physical vapor deposition technique that enables the fabrication of semiconductor materials with atomic precision, allowing meticulous control over layer thickness and composition. MBE operates in ultra-high vacuum (UHV) conditions, typically between 8−10 and 10−12 Torr, which are essential for precisely regulating the composition and morphology of deposited layers, thus optimizing the electronic and optoelectronic properties of the materials. This process involves the deposition of atoms or molecules from a vapor phase onto a heated crystalline substrate, creating high-quality crystalline structures with minimal defects [81,82,83]. MBE allows for the growth of layers with atomic-layer control, enabling the creation of structures with abrupt interfaces and tailored doping profiles [83]. This method is particularly advantageous for fabricating complex semiconductor heterostructures and engineering band gaps essential for wide bandgap (WBG) semiconductors. Materials like Gallium Nitride (GaN), Silicon Carbide (SiC), and Aluminum Gallium Nitride (AlGaN) are commonly grown using MBE due to their wide bandgaps, high breakdown fields, and high electron saturation velocities, making them ideal for high-power, high-frequency, and high-temperature electronic devices [8,84]. A key feature of MBE is the use of Reflection High-Energy Electron Diffraction (RHEED) for real-time growth monitoring, enabling precise adjustments during deposition to maintain optimal conditions for layer formation [84]. MBE’s growth mechanisms support the formation of two-dimensional thin films, one-dimensional nanorods, and zero-dimensional quantum dots, allowing complete carrier confinement on both lattice-matched and lattice-mismatched substrates [85]. 3.1.1. MBE Technique Process In MBE, high-purity materials like gallium and arsenic are heated in effusion cells or electron-beam evaporators until they sublime, creating atomic or molecular beams. These beams travel through the vacuum chamber and condense onto the heated substrate, forming epitaxial layers one atomic layer at a time [83]. This precise control over atomic-layer growth ensures high-quality, clean interfaces and enables the fabrication of complex semiconductor heterostructures with minimal defects. The MBE process is governed by several kinetic processes, including adsorption, desorption, surface diffusion, migration, incorporation, and decomposition of reactive atoms or molecules on the heated substrate [85]. Real-time monitoring techniques like RHEED provide feedback on surface structure and growth rates, allowing adjustments to beam intensities and substrate temperature to optimize layer formation [84]. The schematic diagram of MBE is shown in Figure 2. The growth mechanisms in MBE enable the formation of two-dimensional thin films, one-dimensional nanorods, and zero-dimensional quantum dots, which are capable of complete carrier confinement on both lattice-matched and lattice-mismatched substrates [85]. 3.1.2. MBE Advantages, Applications, and Challenges MBE offers several advantages, including its ability to create high-quality epitaxial films with atomic precision, defect-free layers, and tailored doping profiles [86]. Its low-temperature processes mitigate autodoping issues and support the fabrication of complex structures like quantum wells and superlattices essential for III–V semiconductors and wide-bandgap materials like GaN [85,87]. However, MBE also faces challenges. Its slow growth rate, typically between 0.01 and 0.3 μm/min, limits scalability for mass production. The requirement for costly UHV systems adds to its limitations, and surface contamination or structural defects can compromise epitaxial layer quality [87]. Despite these limitations, MBE remains crucial in semiconductor fabrication and research. It plays a key role in developing devices like light-emitting diodes (LEDs), lasers, solar cells, and high-electron-mobility transistors (HEMTs). Its ability to create high-purity heterostructures makes it indispensable in advanced research areas, including nanotechnology and quantum computing [88,89]. As the demand for high-performance electronic devices grows, MBE continues to drive industrial and academic advancements in semiconductor technology and material science. 3.2. Chemical Vapor Deposition (CVD) or Epitaxial Chemical Vapor Deposition (E-CVD) Chemical Vapor Deposition (CVD) is a versatile technique employed for the deposition of thin films and nanostructures, crucial in various technological applications such as electronics and photovoltaics. CVD operates through chemical reactions in the vapor phase, allowing precise control over film properties, which is essential for enhancing the quality and uniformity of deposited materials [90,91]. Variants like Plasma-Enhanced CVD (PECVD) and Hot-Filament CVD (HFCVD) are notable for their effectiveness in growing materials like diamond and silicon carbide (SiC). Microwave plasma CVD, for instance, achieves high nucleation densities for diamond films on SiC substrates by optimizing parameters like methane concentration and substrate temperature [91]. PECVD enhances the traditional CVD process by utilizing plasma energy to drive chemical reactions at lower temperatures compared to thermal CVD methods, making it advantageous for temperature-sensitive substrates. HFCVD employs a heated filament to thermally decompose precursor gases, facilitating efficient and cost-effective deposition of materials like diamonds. Advancements in CVD technology continue to enable the synthesis of high-purity thin films, including 2D materials, while addressing challenges related to process control and scalability [92]. 3.2.1. Chemical Vapor Deposition Process CVD is a sophisticated process where vapor-phase precursors react chemically on or near a heated substrate, leading to the deposition of solid films. The process typically involves three stages: heating, reaction, and cooling. The reaction chamber is heated to a predetermined temperature, vaporizing the precursors. During the reaction stage, these precursors undergo surface reactions, diffusion, and desorption, with temperature control being crucial for uniform film thickness and quality [89]. The deposition occurs at high temperatures (900–1400 °C) under low pressure, optimizing the Gibbs free energy for solid formation [90,93]. PECVD uses radio frequency (RF) energy to ionize precursor gases, creating reactive species that facilitate film deposition at lower temperatures. HFCVD, on the other hand, uses a heated filament to decompose precursor gases, promoting efficient material breakdown and uniform deposition. A schematic diagram of PECVD and HFCVD is shown in Figure 3. 3.2.2. CVD Advantages, Applications, and Challenges CVD techniques offer significant advantages in semiconductor manufacturing, particularly in the growth and fabrication of high-quality thin films. CVD allows precise control over film composition and thickness, enabling the production of materials with tailored properties essential for advanced applications such as 2D materials and polymeric films [92]. The method’s scalability and repeatability make it suitable for industrial applications, including solar cells and water filtration systems [94]. Specific CVD variants like PECVD enhance deposition rates and support lower temperature processing, which is crucial for temperature-sensitive substrates. HFCVD provides improved film adhesion and uniformity, making it advantageous for complex geometries in semiconductor devices [95]. CVD techniques are integral to advancing semiconductor technology through their versatility and efficiency. A comparison of the variant CVD is shown in Table 2. Plasma-enhanced CVD and hot-filament CVD are pivotal for growing semiconductor materials like diamond and SiC, which are essential for high-performance power electronics. Diamond’s ultra-wide bandgap (5.47 eV) and exceptional thermal conductivity make it promising for high-power applications, including Schottky barrier diodes and field-effect transistors (FETs) that operate at elevated temperatures [56,96]. HFCVD has advanced single-crystal diamond production, achieving growth rates over 10 µm/h while addressing challenges like filament contamination and doping for improved crystallinity [96]. SiC, known for its high breakdown voltage and thermal stability, is suitable for high-voltage applications [97]. Despite its advantages, CVD faces challenges impacting its efficacy and cost-effectiveness. High precursor costs limit process scalability, especially in integrated circuit manufacturing, where cost efficiency is crucial [98]. Achieving uniform film deposition remains complex due to the interactions between gas-phase and surface reactions, leading to potential variations in film quality [99,100]. Precise temperature control is essential, as deviations can trigger unwanted chemical reactions, affecting film characteristics. The opaque nature of CVD reactors further complicates optimization for consistent results [99]. Addressing these challenges is vital for advancing CVD technologies. 3.3. Metal-Organic Chemical Vapor Deposition (MOCVD) Metal-Organic Chemical Vapor Deposition (MOCVD) is a critical chemical process utilized for the deposition of thin layers of semiconductor materials, particularly in the mass production of compound semiconductors like Gallium Nitride (GaN) and transition metal dichalcogenides (TMDs). This technique allows precise control over growth conditions, including precursor flux and reactor geometry, essential for achieving high-quality thin films with uniformity and desired properties [100]. MOCVD is advantageous because it operates at high temperatures and under chalcogen overpressures, facilitating the epitaxial growth of monolayers and heterostructures. It is widely adopted in the semiconductor industry, especially for optoelectronic devices where thin film quality directly impacts device performance and reliability [101,102]. MOCVD remains one of the most prevalent techniques for the scalable production of advanced semiconductor materials [102]. 3.3.1. MOCVD Technique Process The MOCVD process for depositing GaN involves critical gas-phase chemical reactions between metal-organic precursors, such as trimethylgallium (TMG), and hydrides like ammonia (NH₃). In the reactor, TMG decomposes to release gallium, while NH₃ provides nitrogen, forming GaN on a heated substrate. The efficiency of these reactions is influenced by the flow rates of TMG and NH₃, as well as the carrier gas composition, typically a mixture of hydrogen and nitrogen, which enhances deposition rates and film uniformity [101,102]. Reactor design and operating conditions, including pressure and temperature gradients, significantly optimise growth processes and minimise impurities [103]. A typical MOCVD reactor includes a gas inlet for precursors, a substrate holder for the heated substrate, and an exhaust system for byproducts, ensuring effective material deposition and removal of excess gases. The schematic diagram of MOCVD is shown in Figure 4. 3.3.2. MOCVD Advantages, Applications and Challenges MOCVD offers significant advantages in producing materials like GaN, AlGaN, and InGaN, particularly for LEDs and solar cells. Advanced reactor designs, such as multi-wafer systems, enable simultaneous processing of multiple substrates, enhancing uniformity and deposition rates while minimizing impurities [101,105]. The scalability of MOCVD supports effective transitions from laboratory research to large-scale production, ensuring consistent device performance across manufacturing stages [105]. Its versatility allows deposition on different substrates, crucial for complex structures like multi-junction solar cells, while in situ monitoring ensures precise control over growth conditions [101]. MOCVD plays a crucial role in growing semiconductors for high-power electronics, optoelectronics, and laser diodes. It enables epitaxial growth of high-quality materials like AlGaN and Ga₂O₃, which are essential for devices such as power diodes and LEDs. MOCVD has been successfully used to fabricate AlGaN-based P-i-N and Schottky diodes, achieving impressive breakdown voltages and high on/off ratios, critical for high-power applications [106]. It also facilitates Ga₂O₃ growth, demonstrating the high throughput necessary for commercial power and photodetector application [107]. Optimizing growth parameters like temperature and gas composition significantly enhances laser diode efficiency and output power [108]. Despite its advantages, MOCVD faces challenges, including high defect densities and difficulties achieving uniform layer growth. Defect densities can reach levels as high as 108 cm⁻² (Table 3), considerably higher than those achieved through Molecular Beam Epitaxy (MBE) [109,110]. This can limit device performance, especially in LEDs and high-power electronics where material quality is critical. Achieving uniformity is complicated by the large lattice mismatch and thermal expansion coefficient differences between GaN and silicon substrates, leading to cracking and dislocation formation. Variations in temperature, precursor flow rates, and reactor conditions can result in non-uniform thickness and composition across the wafer, affecting device yield and reliability. Safety concerns arise from the use of toxic precursors like trimethyl gallium (TMGa) and ammonia (NH₃), necessitating stringent handling protocols [110]. Advancements in buffer layer technology and growth rate optimization continue to mitigate these issues [109]. 3.4. Hydride Vapor Phase Epitaxy (HVPE) Hydride Vapor Phase Epitaxy (HVPE) is a prominent technique for growing AlGaN and thick gallium nitride (GaN) layers, essential for producing bulk substrates used in GaN-based devices. This method is known for its cost-effectiveness and efficiency in mass production, enabling high-quality GaN crystals with minimal non-uniformity, achieving rates as low as 1% [111]. HVPE employs advanced reactor designs, including vertical gas injectors and high-capacity gallium precursor sources, facilitating continuous growth and enhancing reproducibility. The technique supports various doping methods, which are crucial for tailoring the electrical properties of GaN to meet specific device requirements [112]. HVPE remains a viable solution for addressing the demand for high-quality GaN substrates, advancing nitride technology development [113]. 3.4.1. Hydride Vapor Phase Epitaxy Process The hydride vapor phase epitaxy (HVPE) process utilizes hydrogen chloride (HCl) and metal sources, such as gallium, to synthesize gallium nitride (GaN) through a vapor-phase reaction with ammonia (NH₃). In this process, liquid gallium is introduced as a precursor and reacts with HCl gas to form gallium chloride (GaCl), which reacts with NH₃ in a high-temperature growth zone to form GaN on a substrate within a specialized growth chamber. The HVPE apparatus typically consists of a source chamber for gas mixing and a growth chamber where the deposition occurs, allowing for efficient gas flow and uniform temperature distribution, which is crucial for high-quality GaN production [114]. The schematic diagram of HVPE is shown in Figure 5. Advanced reactor designs, such as axisymmetric vertical gas injectors, enhance growth rate uniformity and scalability, enabling the production of large-diameter GaN crystals [111]. This method is recognized for its high growth rates compared to other techniques, making it the preferred choice for mass-producing GaN substrates [114]. 3.4.2. HVPE Advantages, Applications, and Challenges Hydride Vapor Phase Epitaxy (HVPE) offers significant advantages for producing thick epitaxial layers, particularly in the growth of bulk GaN substrates essential for high-performance devices like LEDs and high-electron-mobility transistors (HEMTs). HVPE is characterized by high growth rates, which surpass those of Metal-Organic Chemical Vapor Deposition (MOCVD) and other methods, making it ideal for applications requiring substantial layer thickness [116,117]. The technique enables the production of large-area substrates with reduced defect concentrations, enhancing the quality of electronic-grade epitaxial films [117]. HVPE-grown bulk GaN exhibits superior electrical properties, such as high carrier mobility and low specific contact resistivity, which are critical for the efficiency of vertical power devices [112,116]. HVPE’s ability to produce high-quality, thick GaN layers positions it as a pivotal method in advancing semiconductor technology. Hydride vapor phase epitaxy (HVPE) grown GaN layers are pivotal for various applications, particularly in power devices, RF transistors, and optoelectronics. The uniformity of GaN crystal growth, influenced by the flow field in the HVPE reactor, is crucial for achieving high-quality layers essential for device performance [118]. GaN-based high electron mobility transistors (HEMTs) exhibit superior characteristics such as low leakage current and high thermal conductivity, making them ideal for high-power and RF applications, including military and telecommunications systems [47]. The integration of GaN with other wide-bandgap semiconductors like β-Ga₂O₃ enhances the performance of optoelectronic devices, leveraging their complementary properties for improved efficiency [119]. Innovative substrate solutions, such as GaN-on-porous silicon, have been developed to reduce RF losses while maintaining high performance, thus broadening the applicability of GaN in advanced electronic systems [120]. Hydride Vapor Phase Epitaxy (HVPE) faces several challenges, including high defect density, substrate bowing, and lattice mismatch during growth. High defect densities, particularly in GaAs and AlN layers, can arise from factors such as inappropriate growth conditions and substrate misorientation, leading to issues like microcrystal inclusions and spiral hillocks, which degrade surface quality [121]. Substrate bowing is often exacerbated by thermal stresses during growth, particularly in thick layers, which can result in non-uniformities and further defects [121]. Lattice mismatch between the substrate and the epitaxial layer can lead to dislocation formation, impacting the overall performance of devices like solar cells [122]. Optimizing growth parameters, such as V/III ratios and growth rates, is crucial to mitigate these issues and enhance the quality of HVPE-grown materials [122,123]. 3.5. Liquid Phase Epitaxy (LPE) Liquid Phase Epitaxy (LPE) is a crystal growth technique where semiconductor layers are deposited from a molten solution onto a substrate, primarily used for thick layers of materials like GaAs, GaP, InP, and III-V compounds. This method enables the production of high-quality single-crystal films essential for applications such as light-emitting diodes (LEDs), laser diodes, and photovoltaic cells [123]. LPE operates under conditions far from thermodynamic equilibrium, allowing the growth of lattice-mismatched layers with controlled composition gradients [124]. Despite its historical prominence, LPE remains valuable for producing thick films and high-quality materials for magneto-optical devices [125] and has recently shown potential for growing thin films at lower temperatures, enhancing its versatility. 3.5.1. LPE Technique Process The LPE process involves heating a solution containing the desired material, such as gallium for GaAs, until it becomes molten, typically at temperatures ranging from several hundred to over 1200 °C [123]. The substrate is dipped into the molten solution, and as the temperature decreases, a thin epitaxial layer solidifies on the substrate from the liquid phase [124]. Precise control over temperature and melt composition ensures high-quality growth, as variations can lead to defects or non-uniform layers [126]. Techniques like in situ monitoring and equilibrium phase diagrams help maintain optimal growth conditions [126], enabling the production of high-purity, low-defect epitaxial layers essential for optoelectronics and semiconductor technologies [127]. A schematic diagram of LPE is shown in Figure 6. 3.5.2. LPE Advantages, Applications and Challenges LPE is a cost-effective and straightforward method for growing thick semiconductor layers, making it attractive for optoelectronics applications such as LEDs and photodetectors. Its simplicity allows the use of relatively inexpensive equipment and low operational costs, facilitating adoption in both academic and industrial settings [126]. LPE is well-suited for heterostructure growth, particularly III-V compounds, due to its ability to produce high-purity materials with low defect densities [124,127]. Its selective and equilibrium growth capability enhances its utility in creating complex multilayer structures necessary for high-efficiency devices. LPE has been successfully applied in developing solar cells (Table 4), particularly through the integration of Ge quantum dots within GaP matrices, demonstrating improved efficiency under concentrated sunlight [128]. The method’s ability to grow complex structures at lower temperatures makes it a cost-effective choice for multilayer semiconductor applications. Despite its advantages, LPE faces challenges in achieving uniformity over large areas, especially for ultrathin layers, due to its limitations in controlling growth parameters compared to techniques like Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) [123]. Maintaining sharp interfaces between layers is difficult due to the complex dynamics of the liquid phase, which can lead to diffusion and intermixing at boundaries, compromising structural integrity [125]. Non-uniform thickness and composition across the substrate are common issues due to LPE’s growth rates and temperature conditions, making it less suitable for applications requiring precise layer control [123]. In contrast, MBE and MOCVD offer superior deposition rate control and environmental conditions, enabling high-quality, ultrathin films with well-defined interfaces [124,129]. 3.6. Atomic Layer Epitaxy (ALE) Atomic Layer Epitaxy (ALE), a variant of Atomic Layer Deposition (ALD), is an advanced epitaxial technique that enables the deposition of materials one atomic layer at a time, achieving atomic-level precision. This method is particularly advantageous for creating ultrathin layers and nanostructures, employing self-limiting surface reactions that ensure uniformity and control over film thickness, often down to angstrom levels [130,131]. Initially developed for thin-film electroluminescence, ALE now encompasses a wide range of materials, including high-k dielectrics essential for modern electronics. Its ability to finely tune microstructures and chemical compositions makes it invaluable in optoelectronics, quantum information technology, and renewable energy devices [131,132]. ALE continues to demonstrate potential for enhancing device performance through precise control of interfaces and material properties [132]. 3.6.1. ALE Technique Process The ALE process involves a cyclic sequence of precursor introductions, depositing one atomic layer per cycle and providing exceptional film uniformity and quality. The process starts with the preparation of a clean, smooth substrate. A first gaseous precursor (Precursor A) in each cycle adsorbs onto the substrate’s surface, forming a monolayer through self-limiting adsorption. A purge step follows to remove unreacted Precursor A and excess gases. Then, a second precursor (Precursor B) reacts with the adsorbed layer to form a new atomic layer, releasing byproducts. Another purge step removes these byproducts, completing one deposition cycle. Repeating this cycle enables precise layer-by-layer growth to achieve the desired film thickness [133]. A schematic diagram of ALE is shown in Figure 7. 3.6.2. ALE Advantages, Applications and Challenges ALE offers atomic-scale precision in film growth, enabling exceptional control over thickness and composition, which is crucial for nanostructures and superlattices. It allows for the deposition of materials with atomic-level accuracy, producing high-quality films with sharp interfaces essential for semiconductor device heterostructures [135,136]. ALE’s ability to create pinhole-free films across complex surfaces enhances the performance of electronic components like transistors and sensors by ensuring uniformity and conformality [136]. Integrating ALE with other processes supports the development of advanced materials with tailored properties, advancing nanoscale engineering in applications like gas sensors and photovoltaics [137,138]. ALE is also critical in growing ultrathin films for high-performance transistors, quantum wells, and memory devices. It enables precise control over thickness and uniformity, which is crucial for fabricating two-dimensional materials like transition metal dichalcogenides (TMDs) and graphene, which offer superior electrical properties and scalability for commercial applications [139,140]. For instance, TMDs such as MoS₂ demonstrate high carrier mobility and tunable bandgaps, making them ideal for flexible electronics and non-volatile memory devices [141,142]. ALE’s ability to grow continuous multilayer films at low temperatures enhances the integration of these materials into existing semiconductor processes, addressing uniformity and performance challenges across large substrates [142]. Despite its advantages, ALE faces challenges that affect its scalability for thick-layer deposition and large-scale production. A key issue is its inherently low growth rates, depositing films one atomic layer at a time, making it inefficient for applications requiring substantial material thickness [143,144]. Maintaining precise control over precursor gases and reactor conditions poses another challenge; variations in these parameters can lead to inconsistencies in film quality and uniformity [144]. Meticulous gas flow and pressure management within the reaction chamber is essential, as any disturbances can introduce impurities and affect film compactness. Addressing these challenges is crucial for enhancing ALE’s viability in industrial applications [144,145]. 3.7. Pulsed Laser Deposition (PLD) Pulsed Laser Deposition (PLD) is an advanced thin-film deposition technique that utilizes high-energy pulsed lasers to ablate target materials, enabling the deposition of thin films onto various substrates. It is particularly beneficial for producing complex oxides, metallic films, and high-temperature superconductors due to its ability to create highly crystalline films with tailored properties. The process involves the ejection of material from a target in plasma form, which then condenses on a substrate, often a single-crystal oxide, facilitating thin-film growth [146,147]. PLD’s flexibility allows for the deposition of films with varying compositions and structures, making it suitable for optoelectronics, solar cells, and emerging advanced materials [148,149]. Recent advancements in hardware and simulation methods have enhanced its scalability and precision, improving film quality on larger substrates [148,150]. 3.7.1. Pulsed Laser Deposition Process PLD is a physical vapor deposition technique that creates high-quality thin films by directing high-powered laser pulses onto a solid target. This process vaporizes the target material, forming a plasma plume of ions, atoms, and molecules condensing on a substrate within a vacuum chamber. The deposition process consists of four main stages: laser ablation, plasma expansion, material deposition, and nucleation and growth, all of which determine the film’s crystallinity and uniformity. Precise control of laser pulse energy, fluence, and target-substrate distance is essential for optimizing deposition rates and ensuring uniform film layers. A schematic diagram of PLD is shown in Figure 8. Higher laser fluence increases the ablation rate but, if excessive, can cause non-uniform deposition or substrate damage [147,151]. The distance between the target and substrate influences plume dynamics, with an optimal distance ensuring uniform condensation, which is critical for microelectronics and nanophotonics applications (Table 5) [151]. Proper parameter adjustments are vital to achieving the desired film properties and performance [149]. 3.7.2. PLD Advantages, Applications and Challenges PLD offers precise stoichiometric control of deposited films, which is essential for complex materials such as high-temperature superconductors and multilayered oxides. This technique enables accurate transfer of target material composition to the substrate, ensuring films with tailored properties that align with simulation results [149]. It is particularly effective in growing epitaxial films on substrates with mismatched lattice parameters, which is crucial for high-quality crystalline structures in advanced applications [147]. PLD’s versatility allows for the deposition of various materials—including metals, semiconductors, and insulators—within a single chamber, eliminating the need for extensive chemical processing and streamlining fabrication [153]. It is widely used in growing complex oxide films such as YBCO, LaAlO₃, and SrTiO₃, which are essential for superconducting devices, photovoltaics, and thin-film transistors. The technique enables precise control over film stoichiometry and morphology, facilitating the fabrication of high-quality multilayered films and heterostructures, which are crucial for advanced microelectronics and optoelectronic devices [152,154]. Phase-field modeling of SrTiO₃ growth under PLD further demonstrates its ability to produce epitaxial films with desired properties, while the deposition of AZO thin films highlights the potential for crack-free structures with superior electrical and optical characteristics [154,155]. PLD enables the formation of nanostructures, such as quantum dots, enhancing the functionality of laser diodes and microelectronic components [151]. However, PLD faces challenges in achieving uniform, high-quality epitaxy over large areas, particularly for complex materials. Maintaining consistent laser fluence is critical, as fluctuations can affect ablation rates, leading to target contamination and substrate damage [154]. Key parameters such as substrate temperature and laser energy density must be optimized to mitigate these issues [155]. Scaling up PLD for large-area deposition or high-throughput manufacturing is challenging due to the high costs of equipment and the need for precise control over deposition parameters [149,153]. Although innovative hardware designs and simulation methods have improved PLD’s reliability and efficiency, the inherent complexities of the process remain a barrier to widespread industrial adoption [152,153]. 3.8. Ammonothermal Growth Techniques Ammonothermal growth is a technique used to produce high-quality gallium nitride (GaN) crystals, essential for advanced semiconductor applications. This method utilizes supercritical ammonia as a solvent to dissolve and recrystallize GaN on a seed crystal, facilitating the formation of bulk GaN substrates with low defect densities and high structural quality. Unlike other growth techniques, ammonothermal growth enables the fabrication of native GaN substrates, which minimize lattice mismatches and defects, improving device performance [156,157]. 3.8.1. Ammonothermal Growth Process The process occurs in a high-pressure autoclave (Figure 9), where the GaN seed crystal is positioned to optimize growth quality. Supercritical ammonia is introduced to dissolve GaN nutrients, typically metallic Ga or polycrystalline GaN. The temperature and pressure are precisely controlled to achieve the desired solubility, with mineralizers enhancing dissolution rates [156]. Once the GaN is dissolved, the material is transported to the lower-temperature growth region, where it recrystallizes onto the seed crystal. This temperature gradient-driven process ensures uniform deposition and high structural integrity [158,159]. After sufficient crystal growth, the autoclave is cooled gradually, and the resulting GaN crystal is harvested and processed into wafers for device fabrication. 3.8.2. Ammonothermal Growth Advantages, Applications and Challenges Ammonothermal GaN substrates are integral to optoelectronics, power electronics, and high-frequency applications. They are widely used in LEDs, laser diodes, and photodetectors, where their superior crystal quality enhances efficiency [159]. In power electronics, GaN-on-GaN devices benefit from high breakdown voltage and thermal stability, making them ideal for high-power transistors and converters [161,162]. Their low defect density and high electron mobility also make them suitable for radio-frequency (RF) and microwave applications [163,164]. The ammonothermal method produces low-defect, high-purity GaN crystals, ensuring superior material properties for advanced semiconductor devices. Unlike other techniques, this method can be scaled up for producing large-diameter GaN wafers, reducing manufacturing costs [158,159,164]. It allows for growth in multiple orientations, including polar, non-polar, and semi-polar, enabling customized device designs [165]. Despite its advantages, ammonothermal growth has limitations. Low growth rates compared to hydride vapor phase epitaxy (HVPE) result in longer production times and increased costs. The high-pressure and high-temperature conditions require specialized autoclaves, making the process expensive and technically demanding [166,167]. While defect density is lower, point defects and native defects can still occur, requiring strict process control to minimize their impact [168,169]. 3.9. EFG Growth Techniques Edge-Defined Film-Fed Growth (EFG), also known as the Stepanov method, is a crystal growth technique widely used for producing high-quality semiconductors. This method enables the fabrication of large-area, defect-minimized crystals suitable for power electronics, optoelectronics, and photovoltaics. Unlike conventional techniques, EFG allows direct control over crystal shape and structure using a specialized die, reducing material waste and enhancing production efficiency [170,171]. 3.9.1. EFG Growth Process The EFG method begins with crucible preparation and material melting, where high-purity raw materials, such as gallium oxide (Ga₂O₃) or silicon, are melted at high temperatures to create a uniform molten pool [167]. A high-temperature-resistant die, typically made of iridium, shapes the emerging crystal as molten material is fed through the die to form a thin film, which solidifies upon controlled pulling (Figure 10) [172,173]. The growth rate is precisely controlled to ensure structural uniformity and minimize defects, with typical pulling rates ranging from 2–10 mm/h for Ga₂O₃ crystals [170,173]. After crystallization, the material undergoes cooling and annealing to relieve internal stress and improve crystalline quality [170,171]. The final processing steps involve cutting and polishing the crystals into wafers for semiconductor applications [174,175]. 3.9.2. EFG Growth Advantages, Applications and Challenges EFG-grown crystals are extensively used in power electronics, where ultra-wide bandgap materials like Ga₂O₃ enable high-power and high-temperature operation. In optoelectronics, EFG-grown Ga₂O₃ supports the fabrication of deep-ultraviolet (DUV) detectors and LEDs due to its high crystalline quality. The method is also applied in photovoltaics, producing low-cost silicon ribbons for solar cells with efficiencies exceeding 14%. Ga₂O₃ crystals are used in sensor applications, offering high sensitivity and stability under extreme conditions [174,175]. The EFG method is cost-effective compared to other crystal growth techniques like Czochralski (CZ), as it minimizes material waste and enables high-yield production [174]. The use of a die allows precise control over the solid-liquid interface, leading to high-quality crystals with low defect densities [171,172]. EFG is also highly scalable, making it suitable for large-area crystal growth in industrial applications. Furthermore, it offers versatility, allowing the growth of Ga₂O₃, silicon, and other semiconductor materials for diverse applications [170,171]. Despite its advantages, the EFG method presents several challenges. Defect control remains a significant concern, as oxygen vacancies and stacking faults can form during the growth process, requiring optimization of pulling rates and die design to minimize defects [170,176]. Impurity incorporation can also degrade the crystal’s optical and electrical properties, necessitating strict growth environment control and high-purity raw materials [170,171]. The high temperatures used in EFG introduce thermal stress, which can cause crystal cracking or warping, requiring careful annealing and cooling strategies [170,171]. Although EFG is scalable, further advancements are needed to increase throughput and reduce production costs for large-scale applications. 3.10. Comparative Analysis of Epitaxial Growth Techniques Epitaxial growth techniques play a vital role in fabricating high-performance semiconductor materials, each offering unique advantages and facing distinct challenges. These methods differ in terms of growth rates, material quality, and environmental impact, making them suitable for specific applications in electronic and optoelectronic devices. The following table (Table 6) provides a comparative overview of the most prominent epitaxial growth techniques, highlighting their growth rates, the quality of materials they produce, and their respective environmental implications.