Epitaxial Growth Research Articles

Epitaxial growth in one dimension.

. The final structure and properties of layers grown by epitaxy techniques are determined in the very early stage of the process. This review describes one-dimensional models for epitaxial growth, emphasizing the basic theoretical concepts employed to analyze nucleation and aggregation phenomena in the submonolayer regime. The main findings regarding the evolution of quantities defining the properties of the system, such as monomer and island densities, and the associated island size, gap length, and capture zone distributions are discussed, as well as the analytical tools used to evaluate them. A concise overview of the most extended algorithms used to simulate the growth processes is provided. Some relevant experimental results are discussed, establishing connections with existing theoretical studies.

Preparation of high purity phase (Bi, Pb)-2223 superconducting films by sol–gel method

In this work, a (Bi, Pb)2Sr2Ca2Cu3O10+δ ((Bi, Pb)-2223) solution with good film-forming capability and chemical stability was synthesized through using metal acetates as raw materials, acrylic acid as an additive, and anhydrous methanol as a solvent to adjust the sol formula. Subsequently, (Bi, Pb)-2223 thin films were fabricated on LaAlO3 (LAO) single-crystal substrates via dip-coating technology at different heat treatment temperatures, and the structures and properties of (Bi, Pb)-2223 films were characterized. It was found that when the sol stoichiometric ratio of Bi: Pb: Sr: Ca: Cu was controlled at 1.9: 0.35: 2: 2: 3 and the gel film was heat-treated at 860 °C and dwelled for 120 min in an mixed atmosphere of N2: O2 = 96: 4, the prepared thin film with a pure (Bi, Pb)-2223 superconducting phase exhibited excellent biaxial texture, a dense surface structure, and had a T c of 106 K and a ΔT c of 6 K. The microstructure analysis revealed a layered epitaxial growth of the film along the LAO substrate with a regular arrangement of atoms and a consistent interlayer spacing.

Electric-Field-Driven Localization of Molecular Nanowires in Wafer-Scale Nanogap Electrodes.

As integrated circuits continue to scale toward the atomic limit, bottom-up processes, such as epitaxial growth, have come to feature prominently in their fabrication. At the same time, chemistry has developed highly tunable molecular semiconductors that can perform the functions of ultimately scaled circuit components. Hybrid techniques that integrate programmable structures comprising molecular components into devices however are sorely lacking. Here we demonstrate a wafer-scale process that directs the localization of a conductive polymer, Mw = 20 kg mol-1 polyaniline, from dilute solutions into 50 nm vertical nanogap device architectures using electric-field-driven self-assembly. The resulting metal-polymer-metal junctions were characterized by electron microscopy, Raman spectroscopy and transport measurements demonstrating that our technique is highly selective, assembling conductive polymers only in electrically activated nanogaps. Our results represent a step toward scalable hybrid nanoelectronics that seamlessly integrate established lithographic top-down fabrication with bottom-up synthesized molecular functional circuit components.

New Insights into MAI Additives in 2D-Assisted 3D Controlled Crystallization Toward High-Quality α-Phase FAPbI3 Perovskites.

The highly oriented 2D perovskite templates of n = 1 have typically been created to attain controllable and oriented crystallization of 3D α-phase formamidinium lead triiodide (α-FAPbI3) perovskites. However, the role of methylammonium iodide (MAI), a widely used α-FAPbI3 phase stabilizer, in regulating the growth dynamics of 2D/3D perovskites is generally ignored. Herein, Ruddlesden-Popper type n = 1 2D octylammonium lead iodide(OA2PbI4) perovskites are added into FAPbI3 precursor solution. The template of n = 2 (OA2MAPb2I7), which is spontaneously constructed by the mixture of n = 1 2D and methylammonium chloride (MACl), acts as a skeleton to template the epitaxial growth of α-FAPbI3. However, the volatilization of MACl inevitably causes damage to the 2D structure during thermal annealing. This study reveals that small amounts of less volatile MAI additive enables the creation of stable 2D template, leading to more controlled vertical orientation crystallization. Consequently, the high-quality mixed-dimensional perovskite film delivers a high efficiency of 24.19% together with improved intrinsic stability. This work provides an in-depth understanding of 2D-assisted controlled epitaxial growth of α-FAPbI3.

Epitaxial growth of excitonic single crystals and heterostructures: Oxides and nitrides

Epitaxial growth of excitonic single crystals and heterostructures: Oxides and nitrides

Prospects for β-Ga2O3: now and into the future

Abstract This review describes the progress of research on gallium oxide as a material for power devices, covering the development of bulk crystal growth through to epitaxial growth, defect evaluations, device processes and development, all based on the author's research experiences. During the last decade or so, the epi-wafer size has been expanded to 4–6 inches, and Schottky barrier diodes and field-effect transistors capable of ampere-class operation and with breakdown voltages of several kV have been demonstrated. On the other hand, challenges to the practical application of gallium oxide power devices, such as the cost of epi-wafers, killer defects, purity of epitaxial layer, etc., have also become apparent. This paper provides a comprehensive summary of the history of these developments, including not only papers but also patents and conference presentations, and gives my personal views on the prospects for this material’s continued development.

Hierarchically Micro-, Meso-, and Macro-Porous MOF Nanosystems for Localized Cross-Scale Dual-Biomolecule Loading and Guest-Carrier Cooperative Anticancer Therapy.

Mass transfer of bulky molecules, e.g., bioenzymes, particularly for cross-scale multibiomolecules, imposes serious challenges for microporous metal-organic frameworks (MOFs). Here, we create a hierarchically porous MOF heterostructure featuring highly region-ordered micro-, meso-, and macro-pores by growing a microporous ZIF-8 shell onto a hollow Prussian blue core through an epitaxial growth strategy. This allows for localized loading of large bioenzyme glucose oxidase (GOx) and small drug 5-fluorouracil (5-FU) within specific pores simultaneously and triggers unique guest-carrier cooperative anticancer capabilities. The stable ZIF-8 outer layer effectively blocks the core pores, preventing the undesired leakage of GOx into normal tissues. The acidity-induced ZIF-8 degradation gradually releases Zn2+ and loaded 5-FU for chemotherapy under acidic tumor microenvironments. With the loss of the shielding effect of the ZIF-8 coating, the released GOx depletes intratumoral glucose (Glu) for starvation therapy. Notably, an accelerated cascade reaction occurs between ZIF-8 decomposition and GOx release, facilitated by the modulator factor of Glu. This culminates in the realization of synergistic cancer therapy, as comprehensively demonstrated by in vitro and in vivo experiments, as well as transcriptome sequencing analyses. Our work not only introduces a hierarchically porous MOF heterostructure with highly region-ordered pores but also provides a perspective for guest-carrier cooperative anticancer therapy.

High-performance GaN ultraviolet polarization-sensitive photodetector based on ferroelectric polarization LiNbO3

High responsivity ultraviolet (UV) photodetectors (PDs) are essential for abundant civilian and military applications. Gallium nitride (GaN) has emerged as an ideal material for UV PD fabrication due to its favorable properties. However, the quality of GaN epitaxial layers significantly impacts device performance and reliability. Sapphire-based GaN epitaxial growth technology enables the realization of high-quality GaN epitaxial layers, making it the preferred choice for GaN substrates. Nonetheless, the thermal expansion coefficient mismatch between sapphire and GaN can lead to crystal mismatch and stress accumulation at high temperatures, affecting device performance and reliability. In contrast, lithium niobate (LiNbO3) exhibits similar coefficients of thermal expansion to GaN, mitigating crystal mismatch and stress accumulation issues. Here, we report the realization of a GaN UV PD by laminating GaN membrane onto ferroelectric LiNbO3 through selective electrochemical etching of the sapphire-based GaN epitaxial film. The LiNbO3-based GaN PD achieves a specific high on/off ratio of 107. At a 5 V bias voltage, the device exhibits a high peak responsivity of 1.712 × 103 A/W under 325 nm laser illumination. Furthermore, the device demonstrates excellent performance for polarization light detection, with a polarization ratio of approximately 54.95. Exploiting the local ferroelectric polarization of x-cut LiNbO3, the photogenerated electron–hole pairs in GaN are efficiently separated by the electrostatic field from the polarization of ferroelectric LiNbO3, resulting in enhanced light-to-electric conversion efficiency. Our work presents a method for fabricating high responsivity GaN-based UV PD, showcasing the potential of integrating ferroelectric LiNbO3 to enhance device performance.

Advanced Mechanical Transfer of Micro-LEDs Enabled by Structurally Modified Wide Sapphire Nanomembranes through Thermal Reflow of Photoresist.

Micro light-emitting diodes (micro-LEDs) are pivotal in next-generation display technologies, driven by the need for high pixel density. This study introduces a novel methodology utilizing wide sapphire nanomembranes (W-SNM) as a dual-purpose template for high-quality epitaxial growth and the mechanical lift-off of individual micro-LEDs. Micro-LEDs grow individually on W-SNM, obviating the chip singulation process. By employing mechanical fracturing of the thin W-SNM, our method facilitates the transfer of micro-LEDs without the conventional laser lift-off (LLO) process. Previously introduced sapphire nanomembranes (SNM) have shown promise in enhancing epitaxial layer quality; however, they encountered challenges in managing micro-LED size variation and achieving efficient mechanical transfer. Here, we apply simple yet effective adjustments to the SNM structure, specifically, its elevation and widening. This strategic modification allows micro-LEDs to endure applied forces without incurring cracks or defects, ensuring that only the targeted W-SNM are selectively fractured. The mechanically transferred vertical 15 × 15 μm2 micro-LED device operates at an optimal turn-on voltage of 3.3 V. Finite element simulations validate the mechanical strain distribution between the W-SNM and GaN when pressure is applied, confirming the efficacy of our design approach. This pioneering methodology offers a streamlined, efficient pathway for the production and mechanical transfer of micro-LEDs, presenting new avenues for their integration into next-generation, high-performance displays.

Influencing the surface quality of free-standing wurtzite gallium nitride in ultra-high vacuum: Stoichiometry control by ammonia and bromine adsorption

Gallium nitride (GaN), a wide bandgap semiconductor with absorption and emission in the ultraviolet/visible range, is proposed as an alternative to metallic surfaces for assembling organic molecular structures aiming at optoelectronic applications. However, the formation of a persistent surface oxide layer in air considerably limits the use of GaN for well-defined interfaces. In this work, we have investigated, characterized and processed n-type free-standing c-plane hexagonal wurtzite GaN crystals grown by hydride vapor phase epitaxy and ammonothermal growth methods. Surface cleaning and full removal of the oxide layer on GaN surfaces could be reproducibly achieved via sputtering and annealing cycles, as evidenced by X-ray photoelectron spectroscopy and low-energy electron diffraction. Scanning tunneling microscopy, however, indicated substantial roughening of the GaN surface and the formation of unwanted Ga-rich islands and clusters. Although ammonia (NH3) and bromine (Br) treatments compensated the N/Ga atoms ratio reduced by sputtering, the surface morphology remained rough, exhibiting randomly shaped and distributed hillocks. In addition, we studied the effect of electron bombardment on the surface quality of GaN during NH3 annealing, on-surface debromination and polymerization of 1,3,5-Tris(4-bromophenyl) benzene on GaN and removal of Ga atoms by Br atoms during the desorption.

Construction of PCN-222@PCN-134 as sorbent of dispersive solid-phase extraction for extraction and enrichment of sulphonamides

A MOF-on-MOF composite (PCN-222@PCN-134) was synthesized by seed epitaxial growth method and applied as sorbent to extract sulfonamides (SAs) in different sample matrices. Using PCN-222@PCN-134 as sorbent, a dispersive solid-phase extraction was developed to clean-up and preconcentration of SAs prior to HPLC-DAD analysis. The linear range of the method are achieved between 0.1 and 200 ng/mL, and the limits of detection are in the range of 0.03–0.27 ng/mL. Comparison with pristine MOFs, the performance of PCN-222@PCN-134 composite is significantly improved. The enrichment factors are obtained in the range of 254–333. The adsorption kinetics and adsorption isotherms are in accordance with pseudo-second-order and Langmuir models, respectively. The spiked recovery of real samples including egg and urine are between 76.4 and 120.1 % with relative standard deviation not higher than 12.1 %. After 14 times of use, the recovery rate of sorbent to SAs is still above 80 %. This work demonstrates that PCN-222@PCN-134 possesses excellent extraction performance to SAs, which can be used to clean-up and preconcentration of trace SAs in different sample matrices, such as egg and urine.

Assessing the Potential and Limitations of PbS and HgTe Colloidal Quantum Dot Infrared Detectors for Free Space Optical Communication

AbstractFree space optical communication with infrared light is a promising secure wireless optical communication technology, where infrared photodetectors are the core component. To date, such applications are based on epitaxial growth narrow‐band semiconductors facing the challenge of large‐area fabrication. Infrared colloidal quantum dots (CQDs) are of interest because of the high‐throughput solution processing. Here, large‐area CQD photodetectors with adjustable wavelengths covering 1.3–2.0 µm. For 1 × 1 mm2 CQD photodetector, the response time achieved within 1 µs at room temperature, responded sharply to the 25 kbps PRBS‐7 communication code is demonstrated. For 1.5 × 1.5 cm2 large‐area CQD photodetectors, a communication rate of 2 kbps is achieved. This work is a step toward the CQD application in the field of optical communications.

Microstructure and mechanical properties of large Ti6Al4V components by electron beam powder bed fusion

Electron beam powder bed fusion (EB-PBF)-built Ti–6Al–4V(Ti6Al4V) has increasingly shown great potential for orthopedic implant and aerospace applications in recent years. Large components prepared by additive manufacturing (AM) have heterogeneous microstructures, defect size, and residual stress in the building direction due to the high temperature gradient. The samples with a height of 170 mm ware prepare via EB-PBF. The microstructure, mechanical properties, defect size, and residual stress along the building direction have been systematically study in this work. The grains epitaxial growth along the build direction as coarse prior β columnar grains. The residual compressive stress increases from the sample bottom to top. The max void size increases from 23.8 μm at the bottom to 108 μm at the top, and the mechanical properties gradually deteriorate along the building direction, which is related to the temperature gradient on the building direction of the sample. It is found that the ultimate tensile strength decreases from 916 ± 9 MPa at the bottom to 876 ± 7 MPa at the top, and the elongation to failure decreases from 9.13 ± 0.61 % to 6.49 ± 0.34 %. Meanwhile, the hardening ability is also weakened along the building direction. This study reveals the evolution process of the sample's microstructure, mechanical properties, and defects, providing data support for further control of material defects.

Exploring the effects of molecular beam epitaxy growth characteristics on the temperature performance of state-of-the-art terahertz quantum cascade lasers

This study conducts a comparative analysis, using non-equilibrium Green’s functions (NEGF), of two state-of-the-art two-well (TW) Terahertz Quantum Cascade Lasers (THz QCLs) supporting clean 3-level systems. The devices have nearly identical parameters and the NEGF calculations with an abrupt-interface roughness height of 0.12 nm predict a maximum operating temperature (Tmax) of ~ 250 K for both devices. However, experimentally, one device reaches a Tmax of ~ 250 K and the other a Tmax of only ~ 134 K. Both devices were fabricated and measured under identical conditions in the same laboratory, with high quality processes as verified by reference devices. The main difference between the two devices is that they were grown in different MBE reactors. Our NEGF-based analysis considered all parameters related to MBE growth, including the maximum estimated variation in aluminum content, growth rate, doping density, background doping, and abrupt-interface roughness height. From our NEGF calculations it is evident that the sole parameter to which a drastic drop in Tmax could be attributed is the abrupt-interface roughness height. We can also learn from the simulations that both devices exhibit high-quality interfaces, with one having an abrupt-interface roughness height of approximately an atomic layer and the other approximately a monolayer. However, these small differences in interface sharpness are the cause of the large performance discrepancy. This underscores the sensitivity of device performance to interface roughness and emphasizes its strategic role in achieving higher operating temperatures for THz QCLs. We suggest Atom Probe Tomography (APT) as a path to analyze and measure the (graded)-interfaces roughness (IFR) parameters for THz QCLs, and subsequently as a design tool for higher performance THz QCLs, as was done for mid-IR QCLs. Our study not only addresses challenges faced by other groups in reproducing the record Tmax of ~ 250 K and ~ 261 K but also proposes a systematic pathway for further improving the temperature performance of THz QCLs beyond the state-of-the-art.

Molecular beam epitaxy growth and doping modulation of topological semimetal NiTe2

In this study, high-quality thin films of the topological semimetal phase NiTe2 were prepared using the molecular beam epitaxy technique, confirmed through x-ray diffraction with pronounced Laue oscillations. Electrical transport experiments reveal that thick films have properties similar to bulk materials. By employing co-deposition, we introduced either magnetic or non-magnetic elements during the growth of thinner films, significantly altering their electrical properties. Notably, the magnetic element Cr induces long-range ferromagnetic ordering, leading to the observation of a significant anomalous Hall effect in NiTe2 thin films. The Hall conductivity remains nearly constant well below the Curie temperature, indicating the correlation with the intrinsic topological nature of the band structure. Theoretical first principles band calculations support the generation of the Weyl semimetal state in the material through magnetic doping. These findings pave the way for exploring more magnetic Weyl semimetals and related low-dimensional quantum devices based on topological semimetals.

Heteroepitaxial MOF-on-MOF Photocatalyst for Solar-Driven Water Splitting.

Assembly of different metal-organic frameworks (MOFs) into hybrid MOF-on-MOF heterostructures has been established as a promising approach to develop synergistic performances for a variety of applications. Here, we explore the performance of a MOF-on-MOF heterostructure by epitaxial growth of MIL-88B(Fe) onto UiO-66(Zr)-NH2 nanoparticles. The face-selective design and appropriate energy band structure alignment of the selected MOF constituents have permitted its application as an active heterogeneous photocatalyst for solar-driven water splitting. The composite achieves apparent quantum yields for photocatalytic overall water splitting at 400 and 450 nm of about 0.9%, values that compare much favorably with previous analogous reports. Understanding of this high activity has been gained by spectroscopic and electrochemical characterization together with scanning transmission and transmission electron microscopy (STEM, TEM) measurements. This study exemplifies the possibility of developing a MOF-on-MOF heterostructure that operates under a Z-scheme mechanism and exhibits outstanding activity toward photocatalytic water splitting under solar light.

Confinement of excited states in two-dimensional, in-plane, quantum heterostructures

Two-dimensional (2D) semiconductors are promising candidates for optoelectronic application and quantum information processes due to their inherent out-of-plane 2D confinement. In addition, they offer the possibility of achieving low-dimensional in-plane exciton confinement, similar to zero-dimensional quantum dots, with intriguing optical and electronic properties via strain or composition engineering. However, realizing such laterally confined 2D monolayers and systematically controlling size-dependent optical properties remain significant challenges. Here, we report the observation of lateral confinement of excitons in epitaxially grown in-plane MoSe2 quantum dots (~15-60 nm wide) inside a continuous matrix of WSe2 monolayer film via a sequential epitaxial growth process. Various optical spectroscopy techniques reveal the size-dependent exciton confinement in the MoSe2 monolayer quantum dots with exciton blue shift (12-40 meV) at a low temperature as compared to continuous monolayer MoSe2. Finally, single-photon emission (g2(0) ~ 0.4) was also observed from the smallest dots at 1.6 K. Our study opens the door to compositionally engineered, tunable, in-plane quantum light sources in 2D semiconductors.

Leakage mechanisms of sub-pA InGaAs/GaAs nano-ridge waveguide photodetectors monolithically integrated on a 300-mm Si wafer

We report on a comprehensive temperature dependent dark current study of high-quality InGaAs/GaAs multi quantum well waveguide photodetectors monolithically integrated on silicon. They are integrated through metalorganic vapor-phase selective-area epitaxial growth in a 300 mm CMOS pilot line. Defects resulting from the metamorphic growth of III-V devices on Si make these devices susceptible to different leakage mechanisms at higher operating temperatures. For the high-temperature operation of complex photonics-electronics integrations, understanding the leakage mechanisms of the devices has critical significance. This will help to optimize designs promptly and ensure the reliability and longevity of such devices under extreme operating conditions. The photodetector devices exhibit dark currents below 1 pA, at room temperature and −1 V bias voltage, limited by the noise floor of the measurement setup. To resolve the different leakage mechanisms contributing to the dark current, the devices were measured at elevated temperatures and the results were cross-validated with device simulations. The devices exhibited very low dark currents, with a median below 0.1 nA at 195 °C, suggesting very high-quality material growth. Through device models, leakage mechanisms related to Shockley-Read-Hall (SRH) recombination at bulk volume defects are found to be the main factor contributing to the dark current. The surface SRH recombination is found to be limited, yet affecting the forward bias dark current due to the shortening of the diffusion paths of the majority carriers. Also, the device model shows that the actual dark currents at room temperature can be as low as 0.01 pA, more than 1-order lower than the measured levels. This study emphasizes the high quality of the III-V nano-ridge waveguide devices grown on Si, which can potentially expand the capabilities of silicon photonics platforms further.

Construction of electron-interactive CoMoO4 @ CoP core–shell structure on boron-doped graphene aerogel as strongly interface coupled hybrid electrodes for high flexible supercapacitor

Exploring high energy density, lightweight and self-supporting flexible electrodes is essentially significant to flexible energy storage equipment. Herein, boron-doped three-dimensional porous graphene aerogel (BGA) is engineered and novel flake-like CoP encapsulating flower-like CoMoO4 core–shell structure is arranged on it (CoMoO4@CoP/BGA) utilizing a combination of solvothermal, freeze-drying and vapor deposition techniques. Boron-doped graphene aerogel as flexible self-supporting positrode creates a unique three-dimensional porous interface with a larger specific surface area, which is conducive to exposing more active sites and avoids the additional process of adding binders and conductive agents. The heterointerface engineering of CoP epitaxial growth on CoMoO4 can efficiently enhance electrolyte ions adsorption ability and fast reaction kinetics. As expected, the fabricated CoMoO4@CoP/BGA demonstrates a better specific capacitance of 3056.4F/g than that of CoMoO4/BGA (1582.4F/g) and pure CoMoO4 (669.2F/g), apart from retains the remarkable cyclic stability of 88.4 % after 10,000 cycles. Furthermore, a hybrid supercapacitor composed of CoMoO4@CoP/BGA and BGA can provide a high energy density of 50.2 Wh kg−1 at 800.0 W kg−1, and retains good capacitance retention of 95.6 % after 10,000 cycles, which can be attributed to the large specific surface of B doping 3D porous graphene aerogel and the rich strong coupling interface synergy between CoP and CoMoO4. More importantly, this work provides important guidance for the design of heterojunction electrodes based on heteroatom doped graphene aerogel and phosphides @ oxide based flexible energy storage devices.

Hydrogel Based UV Photodetector Based on Polarization of Free Water Molecules

AbstractConductive hydrogels retain the intrinsic softness and elasticity that are hallmark features of traditional hydrogels, and they also assimilate the electrical properties of the embedded conductive materials. Conventional epitaxial growth techniques for heterojunctions require precise lattice matching between adjacent materials, significantly limiting technological advancements in photodetector innovation. The integration of conductive hydrogels with semiconductors to fabricate photodetectors presents a feasible solution that bypasses the traditional lattice matching constraints inherent in the epitaxial growth of heterojunction semiconductor photodetectors. Herein, device integration is enhanced by developing a transparent conductive hydrogel, positioned between graphene and gallium nitride, which effectively mitigates the fluidity issues of the polar liquid complicating device encapsulation. A comprehensive analysis of the photodetection characteristics of the graphene/conductive hydrogel/gallium nitride heterostructure is conducted, thereby establishing its structural framework. Flexible transparent devices are further prepared based on PEDOT/Alg (Ca2+) and demonstrate their potential application in 360° omnidirectional photodetection. This work introduces a novel approach for the enhanced integration of hydrogel‐based spectrally selective and flexible transparent photodetectors.