Metalorganic Chemical Vapor Deposition Method Research Articles

Advancing Graphene Imaging for Clear Identification of Lattice Defects: The Application of Revolve Sphere Levelling to Scanning Tunnelling Microscopy Images.

Application of revolve sphere levelling (RSL) as a practical and effective image processing tool for enhancing scanning tunnelling microscopy (STM) images of graphene atomic lattices is presented. Low-cost, ambient, and non-invasive STM methods overcome limitations of traditional imaging methods like scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) that can introduce or alter defects in graphene. Utilizing high-quality graphene synthesized via Paragraf's patented Metal-Organic Chemical Vapor Deposition (MOCVD) method, RSL, which is easily implemented via the Gwyddion software package, effectively highlights the hexagonal lattice structure and specific defect structures. This provides clarity of the atomic structure that traditional methods struggle to achieve. This research emphasizes the utility of RSL in materials science for defect identification in graphene, and points to future research in optimizing RSL for a broader range of defects and applications in other 2D materials.

Phase-Selective Synthesis of Rhombohedral WS2 Multilayers by Confined-Space Hybrid Metal-Organic Chemical Vapor Deposition.

Rhombohedral polytype transition metal dichalcogenide (TMDC) multilayers exhibit non-centrosymmetric interlayer stacking, which yields intriguing properties such as ferroelectricity, a large second-order susceptibility coefficient χ(2), giant valley coherence, and a bulk photovoltaic effect. These properties have spurred significant interest in developing phase-selective growth methods for multilayer rhombohedral TMDC films. Here, we report a confined-space, hybrid metal-organic chemical vapor deposition method that preferentially grows 3R-WS2 multilayer films with thickness up to 130 nm. We confirm the 3R stacking structure via polarization-resolved second-harmonic generation characterization and the 3-fold symmetry revealed by anisotropic H2O2 etching. The multilayer 3R WS2 shows a dendritic morphology, which is indicative of diffusion-limited growth. Multilayer regions with large, stepped terraces enable layer-resolved evaluation of the optical properties of 3R-WS2 via Raman, photoluminescence, and differential reflectance spectroscopy. These measurements confirm the interfacial quality and suggest ferroelectric modification of the exciton energies.

Wafer scale and substrate-agnostic growth of MoS2 nanowalls for efficient electrocatalytic hydrogen generation in acidic and alkaline media

Emerging as a promising alternative to expensive platinum-based catalysts for electrocatalytic hydrogen evolution reaction (HER), molybdenum disulfide (MoS2) stands out for its favourable thermodynamic properties. However, the catalytic activity of MoS2 is mostly confined to its edges while the basal plane remains inactive, limiting practical applicability. Fabrication of stable MoS2 structures with enhanced active sites on a given surface area still remains a complex task. Here we introduce a substrate-agnostic, metal-organic chemical vapour deposition (MOCVD) method for large-area 3D dendritic nanostructures of 2D MoS2, termed as “MoS2 nanowalls”. Using scanning and transmission electron microscopy (SEM/TEM), we elucidate the growth mechanism of the MoS2 nanowalls and their branched dendritic structure. Even subjected to extreme pH environments (0 and 14) during the HER, the grown MoS2 nanowalls show remarkable stability even after >170 hours of continuous operation and exhibit excellent catalytic activity with 10 mAcm−2 current density achievable by applying low overpotentials (309±2 mV at pH = 0 and 272±2 mV at pH = 14). The presented large-area growth method for inexpensive MoS2 nanowall based catalyst can pave the way for practical applications of water electrolysis cells operating at low voltages (≤1.5 V).

Preparation and Electrical Performance Analysis of a GaAs‐Based Tritium Battery

A nuclear battery is a promising candidate for small power supply sources in the military and commercial fields, but its output power and energy conversion efficiency need to be improved. This paper mainly describes a design, preparation, and electrical performance analysis of a GaAs-based tritium battery. The design of the tritium battery uses a multistage process with Monte Carlo and Matlab simulations. A titanium tritide source was prepared by a high-temperature tritium absorption device, and a GaAs semiconductor transducer was developed using a metal-organic chemical vapor deposition method. The D/Ti ratio and T/Ti ratio of the deuterium/tritium titanium films were 1.9 and 1.7, respectively. Two kinds of GaAs-based PIN junction semiconductor transducers were proposed and irradiated with the prepared tritium source. Their electrical properties were measured in situ and analyzed qualitatively. Under the irradiation of a 0.61-Ci tritium source, the short-circuit current of the device was 0.3 to 0.38 μA, the open-circuit voltage was 35 to 63 mV, the peak power was 2.8 to 6.4 nW, and the energy conversion efficiency of the GaAs semiconductor transducer was about 1.86%. It was found that an air gap between the GaAs semiconductor transducer and the radioactive source caused serious loss of beta particle energy, resulting in low output power and low energy conversion efficiency of the nuclear battery. The open-circuit voltage of the devices with a SiO2 passivation layer on the surface decreased both in a dark environment and in light illumination, but SiO2 passivation did not reduce surface recombination as expected. The research work in this paper will provide some valuable reference for the preparation and performance optimization of nuclear batteries.

Compositional design of C-coated multi-elemental alloy nanoparticles for superior microwave absorption

Exploring improved electromagnetic wave absorption performance in magnetic components involves capitalizing on dual dielectric relaxation and multiple magnetic resonances, ultimately giving rise to a dielectric-magnetic matching effect. Herein, a one-step Metal-organic Chemical Vapor Deposition (MOCVD) method has been utilized to fabricate C-coated ternary and quaternary alloy nanoparticles (FeCoNi, FeCoNiCu, FeCoNiMg, and FeCoNiMn) with tunable magnetization. The results affirm that the capabilities of impedance matching and electromagnetic wave attenuation can be significantly regulated by compositing nanoparticles with different elements. The optimized C-coated FeCoNiMg nanoparticles demonstrate an exceptional capacity for electromagnetic wave attenuation, achieving a reflection loss value of −51.8 dB at a matched thickness of 2.9 mm. Moreover, the absorption bandwidth significantly broadens to 7.60 GHz (10.4 – 18.0 GHz) with a thickness of 2.7 mm. The exceptional performance in electromagnetic attenuation is credited to the dielectric loss and improved impedance matching characteristics induced by the C-coated FeCoNiMg nanoparticles. The findings from this study will be beneficial in the development of lightweight and highly effective electromagnetic wave absorbers using magnetic alloy nanoparticles.

Ultrastable and high-performance amplified spontaneous emission in centimeter-scale inorganic CsPbBr3 single-crystal thin films

In this work, high-quality and centimeter-scale CsPbBr3 perovskite single-crystal thin films were grown using the metal-organic chemical vapor deposition method. Extremely flat surfaces without distinct grain boundaries were observed throughout the thin films. A high polarized degree of approximately 0.52 was measured from the single-crystal thin films by analyzing the polarization of the photoluminescence emission spectra. Low threshold amplified spontaneous emissions with an optical gain coefficient of 1215 cm−1 under the excitation energy of 301 μJ/cm2 and the highest value of 2857 cm−1 under the excitation energy of 1.08 mJ/cm2 were investigated under the excitation of a nanosecond laser at 266 nm. The CsPbBr3 single-crystal thin films showed excellent stability where the optical gain coefficient could be maintained over 1800 cm−1 after exposure to air for ten months. The results of this study not only provide high-quality single-crystal films for laser applications but also put forward a feasible scheme to improve the stability of the materials.

Reduced Defect Density in MOCVD-Grown MoS2 by Manipulating the Precursor Phase.

Advancements in the synthesis of large-area, high-quality two-dimensional transition metal dichalcogenides such as MoS2 play a crucial role in the development of future electronic and optoelectronic devices. The presence of defects formed by sulfur vacancies in MoS2 results in low photoluminescence emission and imparts high n-type doping behavior, thus substantially affecting material quality. Herein, we report a new method in which single-phase (liquid) precursors are used for the metal-organic chemical vapor deposition (MOCVD) growth of a MoS2 film. Furthermore, we fabricated a high-performance photodetector (PD) and achieved improved photoresponsivity and faster photoresponse in the spectral range 405-637 nm compared to those of PDs fabricated by the conventional MOCVD method. In addition, the fabricated MoS2 thin film showed a threshold voltage shift in the positive gate bias direction owing to the reduced number of S vacancy defects in the MoS2 lattice. Thus, our method significantly improved the synthesis of monolayer MoS2 and can expand the application scope of high-quality, atomically thin materials in large-scale electronic and optoelectronic devices.

High Uniformity 6-Inch InGaP Epitaxial Growth

The growth of 6-inch In0.485Ga0.515P has been examined in this study. The effects of growth temperature, the V/III ratio, and the H2 total flow on solid composition, growth rate, and crystal quality have been systematically investigated and discussed. Additionally, the effect of growth conditions on doping efficiency has been investigated. Finally, the relationship between electrical uniformity, optical uniformity, and the growth conditions of the 6-in epitaxial layer is discussed. At a growth temperature of 600 °C and a V/III of 250, a high uniformity 6-in InGaP epitaxial layer with an electrical uniformity of 0.33% and optical uniformity of 0.03% was produced. InGaP was grown by the metal-organic chemical vapor deposition method in an Aixtron 2800G4 reactor. High resolution X-ray diffraction (HRXRD), photoluminescence (PL), sheet resistance, electrochemical capacitance-voltage (ECV), and the Hall effect were used to characterize the characteristics of InGaP epitaxial layers.

Various Types of Light Guides for Use in Lossy Mode Resonance-Based Sensors

A comparative study of figure-of-merit fiber sensors of the mass concentration of NaCl solutions based on single-mode and multi-mode fibers was carried out. Lossy mode resonance is realized on chemically thinned sections of optical fibers to various diameters (from 26 to 100 μm) coated with ZnTe. Thin-film coatings were applied using the method of metalorganic chemical vapor deposition (MOCVD). Samples of single-mode and multi-mode fiber sensors were created in such a way that the depth and spectral position of resonances in aqueous NaCl solutions coincided. Sensors implemented on a single-mode fiber have a higher sensitivity (5930 nm/refractive index unit (RIU)) compared to those on a multi-mode fiber (4860 nm/RIU) and a smaller half-width of the resonance in the transmission spectrum. According to the results of experiments, figure-of-merit sensors are in the range of refractive indices of 1.33–1.35 for: multi-mode fiber—25 RIU−1, single-mode fiber—75 RIU−1. The sensitivity of the resulting sensors depends on the surface roughness of the ZnTe coating. The roughness of films synthesized on a single-mode fiber is four times higher than this parameter for a coating on a multi-mode fiber. For the first time, in the transmission spectrum during the synthesis of a thin-film coating on a multi-mode fiber, the possibility of separating the first nine orders of resonances into electric and magnetic transverse components has been demonstrated. The characteristics of sensors with the operating wavelength range in the visible (500–750 nm) and infrared (1350–1550 nm) regions of the spectrum are compared. The characteristics of multi-mode lossy mode resonance sensors are demonstrated, which make them more promising for use in applied devices than for laboratory research.

Transparent and highly conductive La-doped strontium stannate thin film growth by PI-MOCVD method on LaAlO3 and MgO substrates of (100), (110), (111) orientations

Wide band gap (4.6 eV) lanthanum doped SrSnO3 semiconductors of high electron mobility have potential applications in transparent oxide semiconductor technologies. However, there is a lack of data on the electrical properties of La:SrSnO3 both in single crystals and in thin films with an orientation other than (001). We show that after applying a modified metalorganic chemical vapor deposition method it is possible to deposit relaxed thick (001), (110) and (111) oriented La:SrSnO3 films on LaAlO3 and MgO substrates. Thin films on LaAlO3 substrates exhibit comparatively high charge carrier mobility values: µ100=71 cm2V−1s−1, µ110=72 cm2V−1s−1 and µ111=67 cm2V−1s−1. In this article, using the results of X-ray diffraction and crystallite orientation distribution modeling, as well as scanning and transmission electron microscopy data, the link between the obtained charge carrier mobility (µ100 ≈ µ110 > µ111) and film's microstructure and morphology is discussed.

Stability of Graphene and Influence of AlN Surface Pits on GaN Remote Heteroepitaxy for Exfoliation.

Remote epitaxy is a promising technology that has recently attracted considerable attention, which enables the growth of thin films that copy the crystallographic characteristics of the substrate through two-dimensional material interlayers. The grown films can be exfoliated to form freestanding membranes, although it is often challenging to apply this technique if the substrate materials are prone to damage under harsh epitaxy conditions. For example, remote epitaxy of GaN thin films on graphene/GaN templates has not been achieved by a standard metal-organic chemical vapor deposition (MOCVD) method due to such damages. Here, we report GaN remote heteroepitaxy on graphene/AlN templates by MOCVD and investigate the influence of surface pits in AlN on the growth and exfoliation of GaN thin films. We first show the thermal stability of graphene before GaN growth, based on which two-step growth of GaN on graphene/AlN is developed. The GaN samples are successfully exfoliated after the first step of the growth at 750 °C, whereas the exfoliation failed after the second step at 1050 °C. In-depth analysis confirms that the pits in AlN templates lead to the degradation of graphene near the area and thus the alteration of growth modes and the failure of exfoliation. These results exemplify the importance of chemical and topographic properties of growth templates for successful remote epitaxy. It is one of the key factors for III-nitride-based remote epitaxy, and these results are expected to be of great help in realizing complete remote epitaxy using only MOCVD.

High-Performance Flexible GaAs Nanofilm UV Photodetectors

Flexible ultraviolet (UV) photodetectors have attracted extensive research interest due to their potential applications in personal UV monitoring, UV electronic eye, anti-UV gloves, and other wearable systems. In this study, we present a highly sensitive flexible UV photodetector based on an ultrathin GaAs layer that is produced using a metal–organic chemical vapor deposition method and epitaxial lift-off film transfer technology. Device performance analysis reveals that the photodetectors made of a 50 nm thick GaAs layer have excellent sensitivity to 365 nm light. Specifically, the responsivity, specific detectivity, and external quantum efficiency could reach as high as 99.01 A W–1, 1.51 × 1012 Jones, and 3.37 × 104%, respectively, at −2 V bias voltage, which are comparable to many UV photodetectors made from conventional wide band gap semiconductors. Such outstanding UV response properties are related to the very high absorption coefficient for UV light and carrier mobility of an ultrathin GaAs film. Equally importantly, the mechanical flexibility resulting from reducing the film thickness allows the device to maintain its photoelectric properties even after more than 500 bending cycles. The totality of these results suggests that our flexible UV photodetectors have strong potential applications in future wearable devices.

Luminescent iron disilicide film growth by metal–organic chemical vapor deposition

Semiconducting iron disilicide (β-FeSi2) films were epitaxially grown on the Ag-layer pre-coated Si(111) substrates with an initial β-FeSi2 layer by the metal–organic chemical vapor deposition method. These β-FeSi2 films had (101)-preferred orientations, and their crystal quality was improved as the growth temperature increased from 893 to 1093 K. The photoluminescence (PL) intensity of the (101)-oriented β-FeSi2 films grown at 973 K was larger than those of β-FeSi2 films at the other deposition temperatures, which indicates the decrease of the density of nonradiative recombination centers in β-FeSi2. A clear A-band emission originated from β-FeSi2 was observed for these films up to 285 K. This pronounced PL intensity enhancement from β-FeSi2 is attributed not only to the crystallinity evaluated by XRD measurement but also to the decrease in density of thermal equilibrium Si vacancy in β-FeSi2.

One ε-Ga2O3-based solar-blind Schottky photodetector emphasizing high photocurrent gain and photocurrent-intensity linearity

The ε-Ga2O3 thin film was grown on sapphire substrate by using metalorganic chemical vapor deposition (MOCVD) method, and then was used to fabricate a deep-ultraviolet (DUV) photodetector (PD). The ε-Ga2O3 thin film shown good crystal quality and decent surface morphology. Irradiated by a 254-nm DUV light, the photodetector displayed good optoelectronic performance and high wavelength selectivity, such as photoresponsivity (R) of 175.69 A/W, detectivity (D*) of 2.46 × 1015 Jones, external quantum efficiency (EQE) of 8.6 × 104% and good photocurrent-intensity linearity, suggesting decent DUV photosensing performance. At 5 V and under illumination with light intensity of 800 μW/cm2, the photocurrent gain is as high as 859 owing to the recycling gain mechanism and delayed carrier recombination; and the photocurrent gain decreases as the incident light intensity increases because of the recombination of photogenerated carriers by the large photon flux.

Electrical and Optical Properties of Boron Doped Zinc Oxide Thin-film Deposited by Metal-organic Chemical Vapour Deposition for Photovoltaic Application

Globally, there is a high demand for clean, sustainable and renewable energy for domestic and industrial use. Conventional photovoltaic cell technology relies heavily on crystalline silicon wafers which render silicon-based solar cells expensive due to the initial cost of production and required complex deposition methods. Due to these challenges, great research interest is now directed towards thin-film solar cells. In this work, the metal-organic chemical vapour deposition (CVD) method was chosen in the preparation of boron-doped zinc oxide (ZnO: B) thin film onto a glass slide substrate. The prepared ZnO: B thin films were characterized and optimized as a window layer for solar light trapping. The transmittance of the ZnO: B films varied between 70% and 81% for boron concentration ranging from 0.0 M to 0.06 M. With the increase in boron concentration, bandgap and resistivity of the ZnO: B varied from 2.96 to 3.72 eV and 120 Ω-cm to 58 Ω-cm, respectively. Based on the results obtained, we believe that ZnO: B is suitable as a window layer for solar light trapping in the fabrication of a photovoltaic cell.

The Effect of Ag-Doping on the Cytotoxicity of ZnO Nanostructures Grown on Ag/Si Substrates by APMOCVD

The search and development of new nanostructures and nanomaterials are very important for the progress of nanotechnology and modern microbiology. Due to the unique properties of silver and zinc oxide, these nanoparticles are the optimal basis for creating nanostructures with potential antiviral activity. An important issue in these studies is the establishment of cytotoxicity of these nanoparticles and their composites. Aim. To define the influence of substrate temperature and Ag concentration in ZnO lattice on the microstructure and cytotoxicity of zinc oxide nanostructures. Methods. Pure and Ag-doped ZnO nanostructures were grown on Ag/Si substrates by atmospheric pressure metalorganic chemical vapor deposition method using a mixture of zinc acetylacetonate and silver acetylacetonate powders as a precursor. Argentum thin films were deposited on Si substrates by a thermal evaporation method. MTT-assay was used for the analysis of MDBK and MDCK cell viability in the definition of zinc oxide nanostructure cytotoxicity. Results. Ag-doped zinc oxide nanostructures were grown and characterized by X-ray diff raction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. It was found that Si substrate and pure zinc oxide do not inhibit the cell viability of both epithelial cultures whereas Ag-doped ZnO nanostructures inhibit the cell viability because of all-time exposure in a sample without dilution. The cytotoxic effect was not observed at higher dilutions for Ag-doped zinc oxide nanostructures. Conclusions. The investigation of the effect of Ag-doping on the morphology and cytotoxicity of zinc oxide nanostructures is very important for implementing zinc oxide nanostructures into the current optoelectronics and photocatalysis.

Study of Defects and Nano-patterned Substrate Regulation Mechanism in AlN Epilayers.

The high crystal quality and low dislocation densities of aluminum nitride (AlN) grown on flat and nano-patterned sapphire substrate that are synthesized by the metal-organic chemical vapor deposition (MOCVD) method are essential for the realization of high-efficiency deep ultraviolet light-emitting diodes. The micro-strains of 0.18 × 10-3 cm-2 for flat substrate AlN and 0.11 × 10-3 cm-2 for nano-patterned substrate AlN are obtained by X-ray diffractometer (XRD). The screw and edge dislocation densities of samples are determined by XRD and transmission electron microscope (TEM), and the results indicate that the nano-patterned substrates are effective in reducing the threading dislocation density. The mechanism of the variation of the threading dislocation in AlN films grown on flat and nano-patterned substrates is investigated comparatively. The etch pit density (EPD) determined by preferential chemical etching is about 1.04 × 108 cm-2 for AlN grown on a nano-patterned substrate, which is slightly smaller than the results obtained by XRD and TEM investigation. Three types of etch pits with different sizes are all revealed on the AlN surface using the hot KOH etching method.

Atomic-scale insights of the effect of growth temperature on the migration behavior of Al atoms in InGaAs/AlGaAs multiple quantum wells

The growth temperature is a vital role in determining the overall crystal quality of the InGaAs/AlGaAs multiple quantum wells (MQWs) by regulating the migration of atoms at the interfaces, particularly the group (III) elements. In this study, two samples of InGaAs/AlGaAs MQWs were grown by the metal-organic chemical vapor deposition (MOCVD) method in the growth temperatures of 515°C and 650°C, respectively, and the effect of growth temperature on the interfacial structure of MQWs was investigated using high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and photoluminescence (PL). Moreover, the state-of-the-art aberration-corrected scanning transmission electron microscopy (Cs-STEM) technique is also used to observe the atomic structure at the well-barrier interface, particularly the migration behavior of Al atoms, to deeply discuss the effect of the growth temperature on the interface structure of InGaAs/AlGaAs MQWs, and subsequently the optical performance.

Structure Modification, Evolution, and Compositional Changes of Highly Conductive La:BaSnO3 Thin Films Annealed in Vacuum and Air Atmosphere.

Perovskite-type La:BaSnO3 (LBSO) has been drawing considerable attention due to its high electron mobility and optical transparency. Its thin film electrical properties, however, remain inferior to those of single crystals. This work investigates the thermal post-treatment process of films deposited using the metalorganic chemical vapor deposition method to improve the electrical properties of different stoichiometry films, and demonstrates the modification of thin film’s structural properties using short and excessive annealing durations in vacuum conditions. Using vacuum post-treatment, we demonstrate the improvement of electrical properties in Ba-rich, near-stoichiometric, and Sn-rich samples with a maximum electron mobility of 116 cm2V−1s−1 at r.t. However, the improvement of electrical properties causes surface morphology and internal structural changes, which depend on thin film composition. At temperatures of 900 °C–1400 °C the volatile nature of LBSO constituting elements is described, which reveals possible deterioration mechanisms of thin LBSO air. At higher than 1200 °C, LBSO film’s decomposition rate increases exponentially. Thin film structure evolution and previously unreported decomposition is demonstrated by Ba and La diffusion to the substrate, and by evaporation of SnO-SnOx species.

A COMPARATIVE STUDY OF MoS2 NANOSTRUCTURES GROWN ON GLASS AND ALUMINA SUBSTRATES BY METAL-ORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD) METHOD

MoS2 is one of the most attractive transition metal dichalcogenide (TMD) materials for electronic applications due to its unique properties. In this study, MoS2 nanoflakes were synthesized by the metal-organic chemical vapor deposition (MOCVD) method on various substrates including glass and alumina. The substrate effects on the morphology and structural properties of MoS2 nanostructures were evaluated by different techniques including field-emitting scanning electron microscope (FE-SEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD), photoluminescen and Raman spectra. The results indicate that MoS2 nanoflakes could be synthesized effectively on both glass (MoS2/glass) and alumina (MoS2/Al2O3) substrates. However, the morphology and many structural properties of MoS2 nanoflakes are modified significantly when changing the substrates. The Mo/S atomic ratio changes from 0.62 to 0.45 for MoS2/Al2O3 and MoS2/glass, respectively. In addition, MoS2/Al2O3 reveals a less uniform structure along with thicker edges of MoS2 nanoflakes in comparison to that of MoS2/glass. For application, as-synthesized MoS2 nanoflakes could be considered as a potential candidate for selective H2S detection owing to the presence of dangling bonds in MoS2 nanoflakes as well as the differences in the adsorption ability and charge transfer between target gases and MoS2.