Search Results (558)

Graphene’s exceptional carrier mobility and broadband absorption make it promising for ultrafast photodetection. However, its low optical absorption limits responsivity, while the absence of a bandgap results in high dark current, constraining the signal-to-noise ratio and efficiency. Although silicon (Si) photodetectors normally offer fabrication compatibility, their performance is severely hindered by interface trap states and optical shading. To overcome these limitations, we demonstrate an epitaxial graphene/n-Si heterojunction photodiode. This device utilizes graphene epitaxially grown on germanium integrated with a transferred Si thin film, eliminating polymer residues and interface defects common in transferred graphene. As a result, the fabricated photodetector achieves an ultralow dark current of 1.2 × 10⁻⁹ A, a high responsivity of 1430 A/W, and self-powered operation at room temperature. This work provides a strategy for high-sensitivity and low-power photodetection and demonstrates the practical integration potential of graphene/Si heterostructures for advanced optoelectronics. Full article

This work presents a comprehensive study of the structural, luminescent, and photoconversion properties of epitaxial composite phosphor converters based on single crystalline films of Ce³⁺-activated Ca_2−xY_1+xMg_1+xSc_1−xSi₃O₁₂:Ce (x = 0–0.25) (CYMSSG:Ce) garnet, grown using the liquid phase epitaxy (LPE) method on single-crystal Y₃Al₅O₁₂ (YAG) and YAG:Ce substrates. The main goal of this study is to elucidate the structure–composition–property relationships that influence the photoluminescence and photoconversion efficiency of these film–substrate composite converters, aiming to optimize their performance in high-power white light-emitting diode (WLED) applications. Systematic variation in the Y³⁺/Sc³⁺/Mg²⁺ cationic ratios within the garnet structure, combined with the controlled tuning of film thickness (ranging from 19 to 67 µm for CYMSSG:Ce/YAG and 10–22 µm for CYMSSG:Ce/YAG:Ce structures), enabled the precise modulation of their photoconversion properties. Prototypes of phosphor-converted WLEDs (pc-WLEDs) were developed based on these epitaxial structures to assess their performance and investigate how the content and thickness of SCFs affect the colorimetric properties of SCFs and composite converters. Clear trends were observed in the Ce³⁺ emission peak position, intensity, and color rendering, induced by the Y³⁺/Sc³⁺/Mg²⁺ cation substitution in the film converter, film thickness, and activator concentrations in the substrate and film. These results may be useful for the design of epitaxial phosphor converters with tunable emission spectra based on the epitaxially grown structures of garnet compounds. Full article

In this paper, the first demonstration of a highly strained In_0.8Ga_0.2As/In_0.2Ga_0.8Sb type-II superlattice structure grown on InAs substrates by molecular beam epitaxy (MBE) for long-wavelength infrared detection was reported. Novel methodologies were developed to optimize the As and Sb flux growth conditions. The quality of the epitaxial layer was characterized using multiple analytical techniques, including differential interference contrast microscopy, atomic force microscopy, high-resolution X-ray diffraction, and high-resolution transmission electron microscopy. The high-quality superlattice structure, with a total thickness of 1.5 μm, exhibited exceptional surface morphology with a root-mean-square roughness of 0.141 nm over a 5 × 5 μm² area. Single-element devices with PIN architecture were fabricated and characterized. At 77 K, these devices demonstrated a 50% cutoff wavelength of approximately 12.1 μm. The long-wavelength infrared PIN devices exhibited promising performance metrics, including a dark current density of 7.96 × 10⁻² A/cm² at −50 mV bias and a high peak responsivity of 4.90 A/W under zero bias conditions, both measured at 77 K. Furthermore, the devices achieved a high peak quantum efficiency of 65% and a specific detectivity (D*) of 2.74 × 10¹⁰ cm·Hz^1/2/W at the peak responsivity wavelength of 10.7 µm. These results demonstrate the viability of this material system for long-wavelength infrared detection applications. Full article

Epitaxial growth of β-Ga₂O₃ nanowires on silicon substrates was realized by the low-pressure chemical vapor deposition (LPCVD) method. The as-grown Si/Ga₂O₃ heterojunctions were employed in the Underwater DUV detection. It is found that the carrier type as well as the carrier concentration of the silicon substrate significantly affect the performance of the Si/Ga₂O₃ heterojunction. The p-Si/β-Ga₂O₃ (2.68 × 10¹⁵ cm⁻³) devices exhibit a responsivity of up to 205.1 mA/W, which is twice the performance of the devices on the n-type substrate (responsivity of 93.69 mA/W). Moreover, the devices’ performance is enhanced with the increase in the carrier concentration of the p-type silicon substrates; the corresponding device on the high carrier concentration substrate (6.48 × 10¹⁷ cm⁻³) achieves a superior responsivity of 845.3 mA/W. The performance enhancement is mainly attributed to the built-in electric field at the p-Si/n-Ga₂O₃ heterojunction and the reduction in the Schottky barrier under high carrier concentration. These findings would provide a strategy for optimizing carrier transport and interface engineering in solar-blind UV photodetectors, advancing the practical use of high-performance solar-blind photodetectors for underwater application. Full article

This study provides new insight into the mechanisms governing solid state dewetting (SSD) in SiGe alloys and underscores the potential of this bottom-up technique for fabricating self-organized defect-free nanostructures for CMOS-compatible photonic and nanoimprint applications. In particular, we investigate the SSD of Si_1−xGe_x thin films grown by molecular beam epitaxy on silicon-on-insulator (SOI) substrates, focusing on and clarifying the interplay of dewetting dynamics, strain elastic relaxation, and SiGe/SOI interdiffusion. Samples were annealed at 820 °C, and their morphological and compositional evolution was tracked using atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Raman spectroscopy, considering different annealing time steps. A sequential process typical of the SiGe alloy has been identified, involving void nucleation, short finger formation, and ruptures of the fingers to form nanoislands. XRD and Raman data reveal strain relaxation and significant Si-Ge interdiffusion over time, with the Ge content decreasing from 29% to 20% due to mixing with the underlying SOI layer. EDX mapping confirms a Ge concentration gradient within the islands, with higher Ge content near the top. Full article

The integration of thick homoepitaxial layers on silicon carbide (SiC) substrates is critical for enabling high-voltage power devices, yet it remains challenged by substrate surface quality and wafer geometry evolution. This study investigates the relationship between substrate preparation—particularly chemical mechanical planarization (CMP)—and the impact on wafer bow, total thickness variation (TTV), local thickness variation (LTV), and defect propagation during epitaxial growth. Seven 150 mm, 4° off-axis, prime-grade 4H-SiC substrates from a single ingot were processed under high-volume manufacturing (HVM) conditions and grown with epitaxial layers ranging from 12 μm to 100 μm. Metrology revealed a strong correlation between increasing epitaxial thickness and geometric deformation, especially beyond 31 μm. Despite initial surface scratches from CMP, hydrogen etching and buffer layer deposition significantly mitigated scratch propagation, as confirmed through defect mapping and SEM/FIB analysis. These findings provide a deeper understanding of the substrate-to-epitaxy integration process and offer pathways to improve manufacturability and yield in thick-epilayer SiC device fabrication. Full article

BaBiO₃ has recently gained significant research attention as a parent material for an interesting family of alloyed compositions with multiple technological applications. In order to grow a variety of structures, a versatile deposition tool such as molecular beam epitaxy must be employed. In this work, the molecular beam epitaxy growth of BaBiO₃ on SrTiO₃(001) and MgO(001) substrates is studied. When grown by molecular beam epitaxy on SrTiO₃(001) or MgO(001) substrates, BaBiO₃ is known to have two competing orientations, namely (001) and (011). Characterization of the thin film is carried out by X-ray diffraction, X-ray reflectivity, atomic force microscopy, Rutherford backscattering, and transmission electron microscopy. Pathways to block the growth of BaBiO₃(011) and to grow only the technologically relevant BaBiO₃(001) are described for both substrates. An understanding of the enabling mechanism of the co-growth is established from an epitaxial point of view. This can be beneficially utilized for the growth of different compositions in the BaBiO₃ material family in a more controlled manner. Full article

SnSe₂, as a prominent member of the post-transition metal dichalcogenides, exhibits many intriguing physical phenomena and excellent thermoelectric properties, calling for both fundamental study and potential application in two-dimensional (2D) devices. In this article, we realized the molecular beam epitaxial growth of SnSe₂ films on a $\sqrt{3}\times \sqrt{3}$-Sn reconstructed Si(111) surface. The analysis of reflection high-energy electron diffraction reveals the in-plane lattice orientation as SnSe₂[110]//$\sqrt{3}$-Sn [112]//Si [110]. In addition, the flat morphology of SnSe₂ film was identified by scanning tunneling microscopy (STM), implying the relatively strong adsorption effect of $\sqrt{3}$-Sn/Si(111) substrate to the SnSe₂ adsorbates. Subsequently, the interfacial charge transfer was observed by X-ray photoemission spectroscopy. Afterwards, the direct characterization of electronic structures was obtained via angle-resolved photoemission spectroscopy. In addition to proving the presence of interfacial charge transfer again, a new relatively flat in-gap band was found in monolayer and few-layer SnSe₂, which disappeared in multi-layer SnSe₂. The interface strain-induced partial structural phase transition of thin SnSe₂ films is presumed to be the reason. Our results provide important information on the characterization and effective modulation of electronic structures of SnSe₂ grown on $\sqrt{3}$-Sn/Si(111), paving the way for the further study and application of SnSe₂ in 2D electronic devices. Full article

In the present study, a thermal–electromagnetic hydrodynamics model has been used to study welding temperature and melt flow characteristics during the laser welding of 316L steel. This welding was performed using an assisted electromagnetic field. In addition, a Monte Carlo model was used to study grain growth during solidification with the purpose of achieving a better understanding of the control of the microstructure. Based on the numerical model, which has been validated by experimental data, the effects of the current intensity of the electromagnetic field on the temperature distribution, melt flow characteristics, and grain growth are discussed here in detail. The simulation results showed that both Marangoni convection and welding temperature could be controlled by the magnetic damping effect, and that they increased due to the electromagnetic heating effect when using an electromagnetic field. Furthermore, when controlling the temperature distribution and melt flow velocity in the laminar flow of the melt pool, which was assisted by a 30 A current intensity of the electromagnetic field, the temperature gradient decreased by 13.5%. This decrease could be even larger than 50% when a turbulent flow was formed in the melt pool, which has here been demonstrated for a current intensity of 100 A. In addition, undercooling was found to decrease because of the increase in the melt flow velocity when using an assistive electromagnetic field. This led to a longer nucleation time in the melt pool. Furthermore, more and larger directional columnar grains, grown by the driving force of the temperature gradient, could be formed after the consumption of the small, nucleated grains near the solid–liquid interface. In short, by controlling the temperature distribution and melt flow velocity, the required grain morphology (equiaxed or columnar) and dimension (radius, length, or width) can be controlled by coarsening and epitaxial growth. Full article

We report the growth and optical characterization of single-crystal BiFe_1−xMn_xO₃ thin films directly on SrTiO₃/Si(001) substrates using molecular beam epitaxy. X-ray diffraction confirmed epitaxial growth, film crystallinity, and sharp interface quality. Scanning electron microscopy and energy dispersive X-ray spectroscopy verified uniform film morphology and successful Mn incorporation. Spectroscopic ellipsometry revealed a systematic bandgap reduction with increasing Mn concentration, from 2.7 eV in BiFeO₃ to 2.58 eV in BiFe_0.74Mn_0.26O₃, consistent with previous reports on Mn-doped BiFeO₃. These findings highlight the potential of BiFe₁₋_xMn_xO₃ films for bandgap engineering, advancing their integration into silicon-compatible multifunctional optoelectronic and photovoltaic applications. Full article

Monitoring trace toluene exposure is critical for early-stage lung cancer screening via breath analysis, yet conventional chemiresistive sensors face fundamental limitations, including compromised selectivity in complex VOC matrices and humidity-induced signal drift, with prevailing p–n heterojunction architectures suffering from inherent charge recombination and environmental instability. Herein, we pioneer a 2D core–shell n–n heterojunction strategy through rational design of TiO₂@MoS₂ heterostructures, where vertically aligned MoS₂ nanosheets are epitaxially grown on 2D TiO₂ derived from graphene-templated synthesis, creating built-in electric fields at the heterojunction interface that dramatically enhance charge carrier separation efficiency. At 240 °C, the TiO₂@MoS₂ sensor exhibits a superior response (R_a/R_g = 9.8 to 10 ppm toluene), outperforming MoS₂ (R_a/R_g = 2.8). Additionally, the sensor demonstrates rapid response/recovery kinetics (9 s/16 s), a low detection limit (50 ppb), and excellent selectivity against interfering gases and moisture. The enhanced performance is attributed to unidirectional electron transfer (TiO₂ → MoS₂) without hole recombination losses, methyl-specific adsorption through TiO₂ oxygen vacancy alignment, and steric exclusion of non-target VOCs via size-selective MoS₂ interlayers. This work establishes a transformative paradigm in gas sensor design by leveraging n–n heterojunction physics and 2D core–shell synergy, overcoming long-standing limitations of conventional architectures. Full article

Gallium nitride (GaN) is commonly used in various semiconductor systems owing to its high mobility and thermal stability; however, the production of GaN thin films using the currently employed methods requires improvement. To facilitate the growth of high-quality GaN epitaxial thin films, this study explored the crystallographic structures, properties, and influences of chromium nitride (CrN) buffer layers sputtered under various conditions. The crystallographic orientation of CrN played a crucial role in determining the GaN film quality. For example, even when the crystallinity of the CrN (111) plane was relatively low, a single-phase CrN (111) buffer layer could provide a more favorable template for GaN epitaxy compared to cases where both the CrN (111) and Cr₂N (110) phases coexisted. The significance of a low-temperature (LT) GaN nucleation layer deposited onto the CrN buffer layers was assessed using atomic force microscopy and contact angle measurements. The X-ray phi scan results confirmed the six-fold symmetry of the grown GaN, further emphasizing the contribution of an LT-GaN nucleation layer. These findings offer insights into the underlying mechanisms governing GaN thin film growth and provide guidance for the optimization of the buffer layer conditions to achieve high-quality GaN epitaxial films. Full article

The transition of SiC MOSFET structure from planar to trench-based architectures requires the optimization of gate dielectric layers to improve device performance. This study utilizes a range of characterization techniques to explore the interfacial properties of ZrO₂ and SiO₂/ZrO₂ gate dielectric films, grown via atomic layer deposition (ALD) in SiC epitaxial trench structures to assess their performance and suitability for device applications. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements showed the deposition of smooth film morphologies with roughness below 1 nm for both ZrO₂ and SiO₂/ZrO₂ gate dielectrics, while SE measurements revealed comparable physical thicknesses of 40.73 nm for ZrO₂ and 41.55 nm for SiO₂/ZrO₂. X-ray photoelectron spectroscopy (XPS) shows that in SiO₂/ZrO₂ thin films, the binding energies of Zr 3d_5/2 and Zr 3d_3/2 peaks shift upward compared to pure ZrO₂. Electrical characterization showed an enhancement of E_BR (3.76 to 5.78 MV·cm⁻¹) and a decrease of I_{ON_EBR} (1.94 to 2.09 × 10⁻³ A·cm⁻²) for the SiO₂/ZrO₂ stacks. Conduction mechanism analysis identified suppressed Schottky emission in the stacked film. This indicates that the incorporation of a thin SiO₂ layer effectively mitigates the small bandgap offset, enhances the breakdown electric field, reduces leakage current, and improves device performance. Full article

The anisotropic magnetoresistance (AMR) effect is widely used in microscale and nanoscale magnetic sensors. In this study, we investigate the correlation between AMR and the crystal structure, epitaxial relationship, and magnetic properties of Co₅₀Fe₅₀ thin films deposited on rigid MgO and flexible mica substrates. The AMR ratio is approximately 1.6% for CoFe films on mica, lower than the 2.5% observed in epitaxially grown films on MgO substrates. The difference is likely due to the well-defined easy axis in the single domain epitaxial thin films on MgO, which enhances the AMR ratio. Microscopic strain induced by lattice mismatch and bending on flexible substrates were determined using grazing incidence X-ray diffraction and extended X-ray absorption fine structure techniques. These results showed that neither microscopic nor macroscopic strain (below 0.5%) affects the AMR ratio on mica, suggesting its suitability for magnetic sensors in flexible and wearable devices. Additionally, investigating M-H loops under various growth temperatures, lattice mismatch conditions, and bending strains could further benefit the fabrication and integration of the micro-scale magnetic sensors in the microelectronic industry. Full article

Ultra-high-temperature (>1500 °C) sensors play vital roles in ensuring operational excellence in variety of energy-related applications, such as power plant boilers and gas turbine engines. Crystalline sapphire fibers have enormous potential to replace conventional expensive precious metal (e.g., Pt/Rh)-based high-temperature (>1500 °C) sensors by offering higher environmental robustness and distributed sensing capabilities. However, a lack of proper cladding substantially compromises the performance of the sensor. To overcome this fundamental limitation, we develop a hybrid growing method to fabricate low-loss clad crystalline sapphire fibers. We grow a higher-refractive-index doped crystalline sapphire fiber core using the laser-heated pedestal growth (LHPG) method and lower-refractive-index undoped crystalline sapphire fiber cladding using the liquid-phase epitaxy (LPE) method. Furthermore, due to the existence of this cladding layer, a single mode of operation can be achieved at a core diameter size of 30 μm. The experimental results confirm that the grown clad crystalline sapphire fiber can survive in extremely high-temperature (>1500 °C) harsh environments due to the matched coefficient of thermal expansion (CTE) between the fiber core and the cladding. The numerical results also indicate a temperature sensing accuracy of 3.5 °C. This opens the door for developing point and distributed fiber sensor networks capable of enduring extremely harsh environments at extremely high temperatures. Full article