Epitaxial Growth Research Articles

Controlled epitaxial growth of strain-induced large-area bilayer MoS2 by chemical vapor deposition based on two-stage strategy

Two-dimensional (2D) bilayer transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their promising applications in the fields of electronics, optoelectronics, valleytronics and nonlinear optics. However, the precise synthesis of large-area, high-yield and uniform bilayer MoS2 semiconductors remains a significant challenge. Herein, we have developed one two-stage chemical vapor deposition strategy based on strain engineering, enabling the controlled preparation of large-area (4 × 6 cm2) bilayer MoS2 nanostructures. Systematic characterizations indicate that compressive strain was introduced during the growth of first layer MoS2, which effectively induces the synthesis of the second layer MoS2. First-principles calculations based on density functional theory further reveal the mechanism of strain induced controllable growth of bilayer MoS2. Field-effect transistors based on AA and AB stacking bilayer MoS2 have been fabricated exhibiting excellent electronic properties. Our work provides a new pathway for the precise preparation of bilayer TMDCs nanostructures, offering experimental support for their application in the field of electronic and optoelectronic devices.

Recent Trends in Tailoring External Acidity in Zeolites for Catalysis.

Zeolites are crystalline aluminosilicates with well-defined microporous structures that have found several applications in catalysis. In recent years, great effort has been devoted to defining strategies aimed at tuning structural and acidity properties to improve the catalytic performance of zeolites. Depending on the zeolitic structure, the acid sites located inside the crystals catalyze reactions by exploiting the internal channel shape-selectivity. In contrast, strong acid sites located on the external surface do not offer the possibility to control the size of molecules involved in the reactions. This aspect generally leads to a loss of selectivity toward desired products and to the uncontrolled production of coke. Passivating surface acidity is a promising way to overcome deactivation issues and to enhance the catalytic performance of zeolites. This Mini-Review aims to provide, for the first time, a complete overview of the techniques employed in recent years to neutralize strong external acid sites. Both chemical and liquid vapor deposition of silicates have been widely employed to passivate the external surface acidity of zeolites. In recent years, the epitaxial growth of layers of aluminum-free zeolite, e.g., silicalite-1, over the surface of the acidic zeolite has been proposed as a new approach to neutralize strong external acid sites controlling diffusional phenomena. NH3-TPD, FT-IR, SEM-EDX, and other techniques have been used to provide information about the level of control of the external strong acidity of passivated zeolites. In this Mini-Review, both passivation treatments and characterization techniques are compared and advantages and disadvantages deeply discussed to elucidate the effect of passivation procedures on physical features and especially the catalytic behavior.

Cubic Mn3Ge thin films stabilized through epitaxial growth as a candidate noncollinear antiferromagnet

Metallic antiferromagnets with chiral spin textures induce Berry curvature-driven anomalous and spin Hall effects that arise from the topological structure of their electronic bands. Here, we use epitaxial engineering to stabilize (111)-oriented thin films of Mn3Ge with a cubic phase. This cubic phase is distinct from tetragonal ferrimagnetic and hexagonal noncollinear antiferromagnetic structures with the same chemical composition. First-principles calculations indicate that cubic Mn3Ge will preferentially form an all-in/all-out triangular spin texture. We present evidence for this noncollinear antiferromagnetism through magnetization measurements with a Néel temperature of 490 K. First-principles calculations of the corresponding band structure indicate the presence of Weyl points. These highlight cubic Mn3Ge as a candidate material for topological antiferromagnetic spintronics.

Topological hydrogen-bonded organic frameworks (HOFs) and their electronic applications in sensor, memristor, and neuromorphic computing

Recently, an emerging class of hydrogen-bonded organic frameworks (HOFs) has become an appealing member of organic material family, attributed to their layered self-assembly structures, high-crystalline, and environmentally friendly characteristics, which have rapidly propelled their development in the field of electronic devices. In this context, we focus on the latest category of topological HOFs, with particular attention given to cutting-edge experimental demonstrations, exceptional electrical performances, and promising applications. First, a concise concept and fundamental mechanism of HOFs are provided, elucidating the potential correlation between structural designs and material properties. Subsequently, a comprehensive summary is presented on the preparation and synthesis methods, such as hydrothermal techniques, epitaxial growth, electro-deposition, among others. Notably, the latest advancements in HOFs-based electronics are thoroughly introduced and discussed, along with their applications in sensors, memristors, artificial synapses, neuromorphic computing, and human perception systems. Finally, the future challenges and prospects of topological HOFs are elaborated upon with the aim of providing valuable guidance for high-performance HOF-based electronics.

Boosting the efficiency of InGaN-based green LEDs grown on Si through buffer strain engineering

InGaN-based micro-light-emitting diodes (LEDs) grown on Si have gained tremendous interest for full-color displays. Strain management is a key challenge for the epitaxial growth of InGaN-based long-wavelength LEDs on Si because the accumulated compressive strain can severely limit In incorporation and degrade the quality of InGaN multi-quantum wells (MQWs) when the conventional Al-composition step-graded AlN/AlGaN buffer is used for strain control. In this work, we demonstrate a promising approach to effectively reduce the in-plane residual compressive stress of GaN by using an AlN single-layer buffer. The in-plane lattice parameter of the GaN underlayer was increased from 3.183 to 3.189 Å with the residual compressive stress at room temperature reduced from 0.37 to ∼0 GPa, which significantly improved the In incorporation of InGaN MQWs and extended the photoluminescence wavelength from 510 to 550 nm. A remarkably high internal quantum efficiency of 78% was thus achieved for the as-grown InGaN-based green LEDs on Si. This work paves the way for the growth of high-efficiency InGaN-based long-wavelength micro-LEDs.

Epitaxial Growth of Nano‐Sized Pb Islands on SrTiO3 Substrate: Competition Between Electron Correlation and Enhanced Superconductivity

AbstractThe discovery of the interface‐enhanced superconductivity in the single‐layer film of FeSe epitaxially grown on SrTiO3 substrates has triggered a flurry of activity in the field of superconductivity. It raised the hope to find more conventional high transition temperature (Tc) superconductors that are purely driven by the electron‐phonon interaction at ambient pressure. Here the epitaxial growth of the Pb nano‐sized islands on SrTiO3 (001) substrates with the island volumes ranging from 286 to 4945 nm3 is reported by molecular beam epitaxy, followed by systematic scanning tunneling microscopic/spectroscopic (STM/S) investigation. The observed STS gap for the nanoscale islands highly dependent on the volumes of nano‐sized islands, can be divided into three regions. By performing a detailed spectroscopic investigation, it is founded that superconductivity in the volume above 3700 nm3 (Region I) has a zero temperature energy gap (Δ(0)) and Tc of 6.8 meV and 9.8 K obtained by BCS fitting, showing the largely‐enhanced Δ and slightly‐increased Tc by comparing to the bulk Pb (1.4 meV and 7.2 K). As the volume in the range from 1300 to 3700 nm3 (Region II), a large Coulomb gap induced by electron correlation emerged and shows a volume‐dependent behavior, suggesting the reduced size can enhance electron correlation in Pb islands. As the volume decreases down to Region III, enhanced electron correlation and Coulomb gap become more dominant and superconductivity is totally suppressed. The experiment reveals that an electron‐electron interaction in nano‐sized Pb islands can be significantly enhanced by reducing the island sizes and suppresses the superconductivity, thus demonstrates a competition between superconductivity and electron correlation as the volume varies.

Investigation and Comparison of the Performance for β-Ga2O3 Solar-Blind Photodetectors Grown on Patterned and Flat Sapphire Substrate

Ga2O3, with its large band gap, is a promising material suitable for utilization in solar-blind photodetection. Sapphire with a higher lattice match with Ga2O3 was used as the substrate for epitaxial growth of Ga2O3. Here, the epitaxial layers of Ga2O3 were deposited by MOCVD on patterned sapphire substrates. The structure of epitaxial Ga2O3 layers on patterned substrates has been identified by X-ray diffractometry. To investigate the influence of the patterned substrates on the formation of epitaxial layers, thin Ga2O3 layers were grown on a flat sapphire substrate under the same conditions. Both types of samples were β-phase. However, no improvement in the layers’ crystalline quality was discovered when utilizing patterned sapphire substrates. In addition, the performance of the obtained two types of Ga2O3 photodetectors was compared. The photoelectric properties, such as responsivity, response speed, and detection capability, were different in the case of flat samples.

Over 500°C stable transparent conductive oxide for optoelectronics

AbstractHigh‐temperature stable transparent conductive oxides (TCOs) are highly desirable in optoelectronics but are rarely achieved due to the defect generation that is inevitable during high‐temperature air annealing. This work reports unprecedented stability in aluminum and fluorine co‐doped ZnO (AFZO) films prepared by pulse laser deposition. The AFZO can retain a mobility of 60 cm2 V−1 s−1, an electron concentration of 4.5 × 1020 cm−3, and a visible transmittance of 91% after air‐annealing at 600°C. Comprehensive defect characterization and first principles calculations have revealed that the offset of substitutional aluminum by zinc vacancy is responsible for the instability observed in aluminum‐doped ZnO, and the pairing between fluorine substitution and zinc vacancy ensures the high‐temperature stability of AFZO. The utility of AFZO in enabling the epitaxial growth of (AlxGa1−x)2O3 film within a high‐temperature, oxygen‐rich environment is demonstrated, facilitating the development of a self‐powered solar‐blind ultraviolet Schottky photodiode. Furthermore, the high‐mobility AFZO transparent electrode enables perovskite solar cells to achieve improved power conversion efficiency by balancing the electron concentration‐dependent conductivity and transmittance. These findings settle the long‐standing controversy surrounding the instability in TCOs and open up exciting prospects for the advancement of optoelectronics.image

Novel edge-epitaxial MOF on MOF for efficient nizatidine removal

BackgroundMetal organic frameworks (MOFs) have shown potential for photocatalytic of pollutants, via generation of reactive oxygen species like singlet oxygen (1O2). However, enhancing the photocatalytic performance of MOFs remains a challenge. Addressing this challenge is particularly important because removing pharmaceuticals like nizatidine (NZT) is crucial to prevent the formation of carcinogenic N-nitrosodimethylamine during water treatment processes. MethodsA novel MOF on MOF photocatalyst (PCN-134/PCN-134) was synthesized through an epitaxial edge growth strategy. Firstly, PCN-134 was utilized as the seed MOF, upon which the secondary MOF Zr-BTB was grown epitaxially on its edges. Subsequently, the photosensitizer Tetrakis(4-carboxyphenyl) porphyrin was integrated onto the surface of Zr-BTB via a microwave-assisted solvothermal process. Significant findingsCompared to the pristine PCN-134, the PCN-134/PCN-134 exhibited significantly enhanced photodegradation efficiency towards NZT under optimal conditions (85.07% vs. 47.5 %). It had been demonstrated that 1O2 was the key reactive oxygen species responsible for the degradation of NZT. Notably, the photodegradation system had excellent immunity to interference, with a drop in removal of no more than 5 % and 15 % in the presence of common ions and soluble organics, respectively. Additionally, it showed excellent removal for NZT in various water matrices. This study highlights the potential of the MOF on MOF epitaxial growth strategy in enhancing the photocatalytic performance of MOFs for environmental remediation applications.

Controllable chemical synthesis of core-shell NaGdF4:Yb3+/Er3+@Bi4O5I2/Bi5O7I Z-scheme heterojunction for efficient tetracycline hydrochloride photodegradation

Constructing heterojunction photocatalysts that can make additional utilization of near-infrared (NIR) light has been a global scientific problem and has gained widespread research interest. Upconversion nanoparticles (UCNPs) collaborate with semiconductors to construct UCNPs-based photocatalysts that are used to improve solar energy utilization. In this work, the as-prepared UCNPs-based photocatalysts consisting of NaGdF4:Yb3+/Er3+ (NGFYE) and bismuth oxyiodide (BOI) are synthesized by chemical epitaxial growth method and in-situ calcination process. The crystalline phase, morphology, optical and electrochemical properties of NGFYE@BOI samples are investigated in detail. Besides, Bi4O5I2/Bi5O7I Z-type heterojunction is confirmed by EPR test. Moreover, the photocatalytic elimination of tetracycline hydrochloride (TCH) and its photocatalytic mechanism by NGFYE@BOI samples are explored. This work may provide a promising idea for the efficient and controlled synthesis of UCNPs-based photocatalysts, which will contribute to the future utilization of solar energy.

Interfacial epitaxy of multilayer rhombohedral transition-metal dichalcogenide single crystals.

Rhombohedral-stacked transition-metal dichalcogenides (3R-TMDs), which are distinct from their hexagonal counterparts, exhibit higher carrier mobility, sliding ferroelectricity, and coherently enhanced nonlinear optical responses. However, surface epitaxial growth of large multilayer 3R-TMD single crystals is difficult. We report an interfacial epitaxy methodology for their growth of several compositions, including molybdenum disulfide (MoS2), molybdenum diselenide, tungsten disulfide, tungsten diselenide, niobium disulfide, niobium diselenide, and molybdenum sulfoselenide. Feeding of metals and chalcogens continuously to the interface between a single-crystal Ni substrate and grown layers ensured consistent 3R stacking sequence and controlled thickness from a few to 15,000 layers. Comprehensive characterizations confirmed the large-scale uniformity, high crystallinity, and phase purity of these films. The as-grown 3R-MoS2 exhibited room-temperature mobilities up to 155 and 190 square centimeters per volt second for bi- and trilayers, respectively. Optical difference frequency generation with thick 3R-MoS2 showed markedly enhanced nonlinear response under a quasi-phase matching condition (five orders of magnitude greater than monolayers).

Investigation into epitaxial growth optimization of a novel AlGaN/GaN HEMT structure for application in UV photodetectors

Investigation into epitaxial growth optimization of a novel AlGaN/GaN HEMT structure for application in UV photodetectors

Bifractality in the one-dimensional Wolf-Villain model.

We introduce a multifractal optimal detrended fluctuation analysis to study the scaling properties of the one-dimensional Wolf-Villain (WV) model for surface growth. This model produces coarsened surface morphologies for long timescales (up to 10^{9} monolayers) and its universality class remains an open problem. Our results for the multifractal exponent τ(q) reveal an effective local roughness exponent consistent with a transient given by the molecular beam epitaxy (MBE) growth regime and Edwards-Wilkinson (EW) universality class for negative and positive q values, respectively. Therefore, although the results corroborate that long-wavelength fluctuations belong to the EW class in the hydrodynamic limit, as conjectured in the recent literature, a bifractal signature of the WV model with an MBE regime at short wavelengths was observed.

Experimental demonstration of tunable hybrid improper ferroelectricity in double-perovskite superlattice films

Hybrid improper ferroelectricity can effectively avoid the intrinsic chemical incompatibility of electronic mechanism for multiferroics. Perovskite superlattices, as theoretically proposed hybrid improper ferroelectrics with simple structure and high technological compatibility, are conducive to device integration and miniaturization, but the experimental realization remains elusive. Here, we report a strain-driven oxygen octahedral distortion strategy for hybrid improper ferroelectricity in La2NiMnO6/La2CoMnO6 double-perovskite superlattices. The epitaxial growth mode with mixed crystalline orientations maintains a large strain transfer distance more than 90 nm in the superlattice films with lattice mismatch less than 1%. Such epitaxial strain permits sustainable long-range modulation of oxygen octahedral rotation and tilting, thereby inducing and regulating hybrid improper ferroelectricity. A robust room-temperature ferroelectricity with remnant polarization of ~ 0.16 μC cm−2 and piezoelectric coefficient of 2.0 pm V−1 is obtained, and the density functional theory calculations and Landau-Ginsburg-Devonshire theory reveal the constitutive correlations between ferroelectricity, octahedral distortions, and strain. This work addresses the gap in experimental studies of hybrid improper ferroelectricity for perovskite superlattices and provides a promising research platform and idea for designing and exploring hybrid improper ferroelectricity.

Microalloying design in directed energy deposition of complex-concentrated alloy: Pre-nitriding to inhibit hot cracking and promote mechanical properties

With the development of ultra-supercritical (USC) power generation technology, the increase of steam temperature puts higher requirements for high-temperature performance and processing reliability of superalloys. Here, a new complex-concentrated alloy (CCA) is developed for manufacturing the devices in USC circumstances. For the laser additive manufacturing of CCA, avoiding the hot cracking is crucial to guarantee reliable printability and favorable service performance. In this work, the nitrogen (N) is introduced innovatively by pre-nitriding powder to ensure sufficient N to combine with the matrix during the directed energy deposition process. The solidification temperature range can be narrowed to inhibit the hot cracking with the N-doping effectively. As a result, the sample with N-doping exhibited an extraordinary performance of strength (1083.51 MPa) and plasticity (20.83 %). The strengthening mechanisms mainly include solid solution strengthening, grain refinement strengthening, and dislocation strengthening. The elimination of defects leads to an increase in ductility in the meantime. In addition, the dynamic oscillation of the molten pool caused by pre-nitriding powder also has a nonnegligible impact on the breakdown of grain epitaxy growth and the initiation of grain refinement. These findings shed new light on the alloy design concept to inhibit hot cracking and promote mechanical properties.

Interband cascade light-emitting diodes grown on silicon substrates using GaSb buffer layer

Interband cascade light-emitting diodes (ICLEDs) offer attractive advantages for infrared applications, which would greatly expand if high-quality growth on silicon substrates could be achieved. This work describes the formation of threading dislocations in ICLEDs grown monolithically on GaSb-on-Silicon wafers. The epitaxial growth is done in two stages: the GaSb-on-Silicon buffer is grown first, followed by the ICLED growth. The buffer growth involves the nucleation of a 10-nm-thick AlSb buffer layer on the silicon surface, followed by the GaSb growth. The AlSb nucleation layer promotes the formation of 90° and 60° interfacial misfit dislocations, resulting in a highly planar morphology for subsequent GaSb growth that is almost 100% relaxed. The resulting GaSb buffer for growth of the ICLED has a threading dislocation density of ∼107/cm2 after ∼3 μm of growth. The fabricated LEDs showed variations in device performance, with some devices demonstrating comparable light–current–voltage curves to those for devices grown on GaSb substrates, while other devices showed somewhat reduced relative performance. Cross-sectional transmission electron microscopy observations of the inferior diodes indicated that the multiplication of threading dislocations in the active region had most likely caused the increased leakage current and lower output power. Enhanced defect filter layers on the GaSb/Si substrates should provide more consistent diode performance and a viable future growth approach for antimonide-based ICLEDs and other infrared devices.

Vertical Interconnections by Electroless Pd Atoms on Immersion Au Surface for Heterogeneous Integration

Electroless Pd atoms were employed to bond vertical interconnection for chip-stacking applications without pressure and at a temperature below 100 °C. Vertical interconnections were created by channeling an electroless Pd plating solution through microchannels, enabling the reduced Pd atoms to assemble naturally within the gaps between two opposing Cu pillars. Notably, the Pd deposition resulted in epitaxial growth along the orientation of the Cu pillar surface, suggesting improved electrical properties for future high-frequency applications. In this study, highly uniform electroless Pd-bonded Cu pillar joints were fabricated using a patterned microfluidic system. The required bonding time was significantly correlated with the plating temperature. Furthermore, the electroless Pd-bonded joints exhibit a robust strength of approximately 55 MPa. Pd exhibits greater skin depth than other plating materials, which helps reduce signal loss and latency during high-frequency signal transmission. This innovative approach offers promise as a potential candidate for future 5G applications requiring low bonding stress and minimal thermal budget.

In-situ TEM investigation of dislocation healing and recrystallization in nanoscratched silicon at elevated temperatures up to 800 °C

Nanoscratching introduces detrimental surface and subsurface defects like amorphous regions, dislocations, and stacking faults in monocrystalline silicon, hindering its application in microelectronics and high-performance optics. This study leverages in-situ transmission electron microscopy to unveil the thermal evolution of these defects in atomic scale. A key finding is the amorphous phase recrystallization starting at ∼500 °C. Epitaxial growth from the crystalline-amorphous boundary, guided by adjacent crystal planes, restores the original diamond structure phase. By 700–800 °C, almost complete recrystallization occurs, maintaining similar interplanar spacing despite residual crystal distortions and dislocations. Notably, heating above 600 °C results in the gradual vanishing of stacking faults, suggesting a dynamic thermal evolution of the crystal defects induced by surface nanoscratching. This work demonstrates thermal annealing as a promising strategy to mitigate nanoscratch-induced defects, paving the way for defect-free-surface of silicon components in ultra-precision machining processes. It offers valuable insights into the interplay between nanoscratching, temperature, and defect evolution, laying the groundwork for surface and subsurface defects elimination in silicon under thermal fields.

Uncovering grain and subgrain microstructure at the scale of additive manufacturing melt tracks with a scalable cellular automaton solidification model

Metal additive manufacturing, characterized by rapid solidification, yields refined grains with a distinctive cellular subgrain microstructure that plays a pivotal role in determining material properties. Due to the significant computational expense demanded to simulate the required physics with submicron spatial resolution, their numerical simulations have been limited to proof-of-concept studies to either 2D or small subregions of a melt pool. In this study, an open-source, scalable, solidification code, muMatScale, based on the cellular automaton method, has been developed to predict the grain and the underlying subgrain microstructure over an entire melt pool. The model incorporates flexible parallelization schemes, utilizing MPI and OpenMP GPU Offloading, in addition to appropriate multi-physics specific to non-equilibrium rapid solidification in AM. The impact of nucleation parameters on grain microstructures was investigated with a focus on grain size variations and morphology transitions. With selected nucleation parameters, the simulation predicted the grain size, subgrain morphology, crystallographic orientation, and microsegregation aligned with experimental measurements. The model demonstrates that epitaxial grain growth is a dominant factor at the melt pool boundary, influencing grain size variation under different grain sizes in the build plate while maintaining consistent primary dendrite arm spacing under identical thermal conditions. The highly efficient numerical model enables large-scale simulations with a spatial resolution of 100 nm or less, unveiling unprecedented insights into thermal and solutal diffusion driven grain growth, and the subgrains with microsegregation within grains in 3D across scales. muMatScale will enable the linking of submicron length-scale microstructure to part-level material behavior by investigating fundamental solidification problems at the intercellular scale in many-track and many-layer builds.

Epitaxial Growth of the Large-Scale, Highly-Ordered 3D GaN-Truncated Pyramid Array Toward an Ultrahigh Rejection Ratio and Responsivity Visible-Blind Ultraviolet Photodetection.

The micro- and nanostructures of III-nitride semiconductors captivate strong interest owing to their distinctive properties and myriad potential applications. Nevertheless, challenges endure in managing the damage inflicted on crystals through top-down processes or achieving extensive control over the large-area growth of these microstructures via bottom-up methods, thereby impacting their optical and electronic properties. Here, we present novel epitaxially grown 3D GaN truncated pyramid arrays (TPAs) on patterned Si substrates, devoid of any catalyst. These GaN TPAs feature highly ordered, large-scale structures, attributed to the utilization of 3D Si substrates and thin AlN interlayers to alleviate epitaxial strains and limit dislocation formation. Comprehensive characterization via scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and cathodoluminescence attests to the superior structural and optical attributes of these crystals. Furthermore, photoluminescence and ultraviolet (UV)-visible diffuse reflectance spectroscopy reveal sharp band-edge emission and significant light trapping in the UV bands. Employing these GaN TPAs, we constructed metal-semiconductor-metal visible-blind UV photodetectors (PDs) incorporating Ti3C2 MXene as Schottky electrodes. These PDs display exceptional responsivity, achieving 5.32 × 103 mA/W at 255 nm and an ultrahigh UV/visible rejection ratio (R255nm/R450nm) approaching 106, which are 1-2 orders of magnitude higher than most recently reported works. This exploration showcases novel GaN-based microstructures characterized by uniformity, ordered geometry, and exemplary crystalline integrity, paving the way for developing optoelectronic applications.